UNCORRECTED PROOF 2012 1957 2013 1958 2014 1959 2015 1960 Fig

ARTICLE IN PRESS

1 57 2 58 3 59 Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 4 60 www.elsevier.com/locate/soildyn 5 61 6 62 7 63 8 The El Salvador earthquakes of January and February 2001: 64 9 65 10 context, characteristics and implications for seismic risk 66 11 67 12 J.J. Bommera,*, M.B. Benitob, M. Ciudad-Realc, A. Lemoined, M.A. Lo´pez-Menjı´vare, 68 13 d f g h 69 14 R. Madariaga , J. Mankelow ,P.Meńdez de Hasbun , W. Murphy , 70 e i j 15 M. Nieto-Lovo , C.E. Rodrı´guez-Pineda , H. Rosa 71 16 72 a 17 Department of Civil Engineering, Imperial College, London SW7 2BU, UK 73 bUniversidad Politećnica de Madrid, Spain 18 74 cKinemetrics, 222 Vista Avenue, Pasadena, CA 91107, USA 19 75 dEcole Normale Supe´rieure, Paris, France 20 eEscuela de Ingenierıá Civil, Universidad de El Salvador, San Salvador, El Salvador 76 21 fBritish Geological Survey, Keyworth, UK 77 22 gDpto. Mecańica Estructural, Universidad Centroamericana “Jose´ Simeoń Canãs”, San Salvador, El Salvador 78 h 23 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK 79 i 24 Facultad de Ingenierıá, Universidad Nacional de Colombia, Santafe´ de Bogota´, Colombia. 80 jFundacioń PRISMA, San Salvador, El Salvador 25 81 26 82 27 Abstract 83 28 The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last 84 29 hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes Mw 7.7 and 6.6, respectively. The two 85 30 events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known 86 31 precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses 87 32 and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of 88 33 increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by 89 34 deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very 90 weak and present a major obstacle to risk mitigation. q 2002 Elsevier Science Ltd. All rights reserved. 35 91 36 Keywords: El Salvador; Earthquakes; Strong motion; Landslides; Seismic risk; Vulnerability 92 37 93 38 94 39 1. Introduction the GDP of the previous year [1]. Estimates by the IMF and 95 40 the World Bank give a higher figure of US$ 1.9 billion [2]. 96 41 The earthquake of 13 January 2001 that struck El This paper presents the context in which these earth- 97 42 Salvador was the first major seismic disaster of the third quakes occurred, including their precedent in the seismic 98 43 millennium and the fifth destructive earthquake to affect the history of El Salvador, and describes the characteristics of 99 44 small Central American republic in 50 years. The earth- the events and their impact on the built and natural 100 45 quake was followed exactly 1 month later by a second event, environments, and on the population. The primary objec- 101 46 of different tectonic origin, on 13 February, which tives of the paper are firstly to provide an overview of the 102 47 compounded the destruction. These two earthquakes characteristics and effects of the earthquakes, and secondly 103 48 claimed almost 1200 lives. In addition, 20% of houses to assess the relative importance of the different factors, 104 49 were damaged, with 12% either completely destroyed or physical and social, which have been demonstrated as 105 50 declared uninhabitable. Economic losses were estimated by contributing to the high level of seismic risk in El Salvador. 106 51 the UN Economic Commission for Latin America (ECLA/- 107 52 CEPAL) at US$UNCORRECTED 1.6 billion, which is equivalent to 12% of PROOF 108 53 2. Geophysical, geological and geographical contexts 109 54 * þ þ 110 Corresponding author. Tel.: 44-171-594-5984; fax: 44-171-225- 2 55 2716. With an area of just over 20,000 km El Salvador is the 111 56 E-mail address: [email protected] (J.J. Bommer). smallest of the Central America republics, located on the 112

2 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

113 169 114 170 115 171 116 172 117 173 118 174 119 175 120 176 121 177 122 178 123 179 124 180 125 181 126 182 127 183 128 184 129 185 130 186 131 187 132 188 133 189 134 190 135 191 136 192 137 193 138 194 139 195 140 196 141 197 Fig. 1. El Salvador. 142 198 143 Paciﬁc coast of the isthmus and bordered by Guatemala to 199 144 the west, and Honduras to the north and east (Fig. 1). 200 145 201 146 202 2.1. Tectonics, seismicity and seismic hazard 147 203 148 204 149 El Salvador is affected by earthquakes from two main 205 150 sources of seismicity. The largest earthquakes are generated 206 151 in the Benioff–Wadati zones of the subducted Cocos plate, 207 152 which is converging with the Caribbean plate in the Middle 208 153 America Trench (Fig. 2) at an estimated rate of 7 cm/year 209 154 [3]. The largest earthquake in this zone during the 20th 210 155 century, in the vicinity of El Salvador, occurred on 7 211 156 September 1915, with a reported magnitude of Ms 7.8 and a 212 157 focal depth between 45 and 60 km [4]. This earthquake 213 158 caused widespread destruction in western El Salvador, 214 159 affecting particularly the town of Juayu´a [5].Large 215 160 subduction earthquakes on 28 March 1921 (Ms 7.4) and 216 161 21 May 1932 (Ms 7.1) caused minor and moderate damage 217 162 in eastern and central El Salvador, respectively; the 218 163 relatively small impact of these earthquakes was probably 219 164 the result of their focal depths of 170 and 150 km, 220 UNCORRECTEDrespectively [4]PROOF. An earthquake on 19 June 1982, offshore 165 221 Fig. 2. Focal mechanisms determined for the earthquakes of 13 January and from western El Salvador, did cause widespread damage in 166 13 February 2001. Stars are epicentral locations from NEIC. Red line is the 222 167 Middle America Trench and yellow triangles are volcanoes. Locations of the southwest of the country, mainly in adobe (sun-dried 223 168 previous earthquakes are indicated by black stars. clay brick) and bahareque (wattle-and-daub) houses, and 224

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 3

225 triggered many landslides [6,7]. This earthquake shared than volcanic in origin, and are probably the result of a 281 226 many similarities with the earthquake of 13 January in terms right-lateral shear zone caused by an oblique component of 282 227 of mechanism and focal depth, although somewhat smaller the Cocos–Caribbean collision [15]. However, swarms, 283 228 with a magnitude of Mw 7.3. The damage patterns were which may have volcanic origin, are also relatively 284 229 similar to those of the January 2001 earthquake, but much frequent. In March and April 1999 an important seismic 285 230 less severe with a total of just eight fatalities. The worst swarm occurred in an area close to the San Vicente 286 231 destruction occurred in the town of Comasagua, which was (Chichontepec) volcano, with almost 1000 small earth- 287 232 also very severely affected by the January 2001 earthquake. quakes registered, none exceeding M 4.5, registered, and as 288 233 The second source of seismicity affecting El Salvador is a many as 160 occurring per day. A similar swarm had 289 234 zone of upper-crustal earthquakes that coincide with the affected approximately the same area in July 1975. The 290 235 Quaternary volcanoes that extend across the country from 1999 swarm, despite the size of the individual events, 291 236 west to east, forming part of a chain extending throughout caused minor to moderate damage to a number of adobe 292 237 the isthmus from Guatemala to Panama. Due to their houses and also the church in Apastepeque. The same area 293 238 shallow foci and their coincidence with main population was also affected by the earthquakes of January and 294 239 centres, these earthquakes (Fig. 2) have been responsible for February 2001; it is very likely that the level of damage 295 240 far more destruction in El Salvador, as in neighbouring was exacerbated by the damage inflicted during the 1999 296 241 Nicaragua, than larger earthquakes in the subduction zone swarm. 297 242 [8]. During the 20th century, such shallow focus earth- Major earthquakes also occur on the Motagua and 298 243 quakes caused destruction on at least seven occasions, Chixoy-Polochic faults that traverse Guatemala and mark 299 244 sometimes occurring in clusters of two or three similar the boundary between the Caribbean and North American 300 245 events separated by periods of minutes or hours. On 8 June plates, but they are sufficiently distant to not produce 301 246 1917 an earthquake occurred west of the capital, San damaging motions in El Salvador. The Ms 7.5 Guatemala 302 247 Salvador, assigned a magnitude Ms 6.7 by Ambraseys and earthquake of 4 February 1976 caused shaking that did not 303 248 Adams [4] and Ms 6.5 by White and Harlow [8], causing exceed MM intensity of V within El Salvador [16]. 304 249 destruction in Armenia, Ateos, Quetzaltepeque and other White [15] also describes a fourth source of seismicity as 305 250 towns. The earthquake was followed by an eruption of the a zone of tensional tectonics near the common borders of El 306 251 San Salvador volcano, which resulted in lava flows to the Salvador, Guatemala and Honduras, bounded by the 307 252 north. White and Harlow [8] report a second event of Ms 6.4, Motagua fault to the north, the volcanic chain to the south 308 253 on the eastern side of San Salvador, less than an hour later, and the Honduran Depression to the east. White [15] asserts 309 254 but this is contested by Ambraseys and Adams [4].On28 that an earthquake of Ms 7.5 occurred in this zone in June 310 255 April 1919 San Salvador was again damaged, this time by a 1765. The largest earthquake during the 20th century in this 311 256 shallow earthquake of Ms 5.9. On 20 December 1936, an zone was that of 29 December 1915 (Ms 6.4), for which 312 257 earthquake of Ms 6.1 caused very heavy damage to the town Ambraseys and Adams [4] relate press reports alleging two 313 258 of San Vicente, 40 km east of San Salvador, with more than deaths in San Salvador due the collapse of walls, although 314 259 100 deaths [9]. The 1936 earthquake is of particular interest the effects in El Salvador were clearly not overly important. 315 260 since the location was similar to that of the earthquake of 13 There have been a number of probabilistic seismic 316 261 February 2001. hazard assessments carried out for El Salvador [17–19] and 317 262 On 25 December 1937 an earthquake of Ms 5.8 near the for Central America [20,21]. The hazard maps produced for 318 263 Salvadorian border with Guatemala caused damage and a a 475-year return period, despite being based on generally 319 264 few deaths in the towns of Ahuachapań and Atiquizaya. A similar seismological and strong-motion data, differ signifi- 320 265 series of three destructive earthquakes occurred in eastern El cantly in terms of the geographical distribution of the hazard 321 266 Salvador on 6–7 May 1951, with magnitudes Ms 5.9, 6.0 and by more than a factor of three in terms of the maximum 322 267 and 5.5, destroying the towns of Jucuapa and Chinameca, ground accelerations [22]. Following the San Salvador 323 268 leaving about 400 dead [10]. earthquake of May 1965, Rosenblueth and Prince [23] 324 269 The capital city of San Salvador, amongst the Latin proposed two separate seismic zonations for El Salvador, 325 270 American city most frequently damaged by earthquakes, one for suduction earthquakes and one for upper-crustal 326 271 was badly hit on 3 May 1965 (Ms 5.9) and on 10 October seismicity. Although the application of this proposal has 327 272 1986 (Ms 5.4, Mw 5.7). The 1965 earthquake left about 120 been explored [24,25], it has not been incorporated into 328 273 dead [11] whereas the 1986, despite being of smaller seismic design codes. 329 274 magnitude, resulted in 1500 deaths and more than 100,000 330 275 homeless [12–14]. Many engineered structures that col- 2.2. Geology, geomorphology and landslide hazard 331 276 lapsed in 1986UNCORRECTED had been damaged by the 1965 earthquake PROOF 332 277 and possibly further weakened by the 1982 subduction El Salvador is made up of four morphological-geological 333 278 event. units, each of which forms an east–west strip across the 334 279 The shallow focus, moderate magnitude earthquakes that country parallel to the coast [26]. The northernmost unit, 335 280 occur along the volcanic chain are generally tectonic rather along the border with Honduras, is a mountain range 336

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

4 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

337 consisting mainly of plutonic rocks from the Tertiary. To the environments [29,31]. Historical evidences show that land- 393 338 south of these mountains is the Great Interior Valley that slides triggered by earthquakes in El Salvador occur as soil 394 339 forms the central area of the country; the southern part of the and rock slides on volcanic slopes but more abundantly as 395 340 valley includes the Salvadorian segment of the chain of soil falls and slides in slopes of pumitic volcanic ash [29, 396 341 Quaternary volcanoes, six of which are active. To the south 33]. Subduction earthquakes generally trigger landslides 397 342 of the valley are three coastal mountain ranges: Tacuba on over areas that are large compared to crustal earthquakes, 398 343 the western border with Guatemala; the Cordillera del which tend to concentrate landsliding around the epicentral 399 344 Ba´lsamo to the south and west of the capital; and the area. The 13 January and 13 February earthquakes have 400 345 Jucuarań range bordering the Gulf of Fonseca to the east. confirmed these trends. 401 346 Between the coastal ranges are two coastal plains, the larger Rymer and White [34] reviewed topography, lithology, 402 347 one, in the centre and east of the country, including the rainfall, seismic hazard and historical cases of earthquake- 403 348 estuary of the Rıó Lempa, El Salvador’s main river. induced landslides, and concluded that landslide hazard in 404 349 The geology of El Salvador is entirely volcanic. The El Salvador is high, the susceptible areas being the coastal 405 350 youngest and most commonly encountered volcanic soil is mountain ranges, the volcanic chain and the interior valley 406 351 the tierra blanca ash, which originates from eruptions in the areas. This evaluation has been confirmed by observations 407 352 Coatepeque and Ilopango calderas. The tierra blanca occurs during the 2001 earthquakes. 408 353 as silty sand or sandy silt, and is generally relatively well- Fig. 3 shows a landslide hazard map prepared by the 409 354 consolidated and stable only where it is thick [27]. The Planning Office for the Metropolitan Area of San Salvador 410 355 strength of the tierra blanca, which permits it to stand in (OPAMSS) as part of PLAMADUR in 1997. The map 411 356 near-vertical slopes of up to 15 m or more, derives from a identifies most of the metropolitan area of the capital as 412 357 combination of weak cementation, probably due to silica being of medium landslide hazard with several areas 413 358 gels, and negative pore water pressure, resulting from partial highlighted, in dark red, as being of high hazard: amongst 414 359 saturation, which have been measured in the range 400– these is the area affected by the catastrophic landslide at Las 415 360 500 kPa [28]. Colinas (Section 5.1). 416 361 Earthquake-induced landslides are common in Central 417 362 America although different mechanisms of slope failure 2.3. Demographic and socio-economic conditions 418 363 dominate in the northern countries of Guatemala and El 419 364 Salvador compared to southern countries of Costa Rica and The current population of El Salvador is about 6.3 420 365 Panama [29]. In the south, the most abundant and most million, very unevenly distributed throughout the national 421 366 damaging slope failures are translational soils in residual territory. There has been a steady trend for the population to 422 367 soils, whereas in the north soil and rock slides on volcanic concentrate in the south-western third of the country, which 423 368 slopes, and more commonly, soil falls and slides in steep was home to 53% of the population in 1971, a figure that had 424 369 slopes of pumitic ash, dominate. Although these volcanic risen to 64% by 1992 [35]. Probably three-quarters of the 425 370 ash deposits are able to form almost vertical slopes in population now live in the region west of Lake Ilopango and 426 371 incised ravines (barrancas ) and in road cuts, they are south of Santa Ana, which is also the area of greatest seismic 427 372 susceptible to sudden and catastrophic failure under hazard [36]. 428 373 sustained or intense rainfall and under earthquake shaking. The main agricultural export of El Salvador is coffee, 429 374 The numbers of landslides triggered by earthquakes in these having replaced anil (indigo) as the main cash crop at the 430 375 ash deposits tend to be disproportionately high compared to turn of the 20th century, following the introduction of 431 376 the numbers of landslides triggered by earthquakes of synthetic dyes in Europe. A large section of the rural 432 377 similar magnitude in other parts of the world. In the global population depends directly or indirectly on the cultivation 433 378 database of Keefer [30], the 1976 Guatemalan earthquake of coffee for its livelihood, often in precarious conditions. 434 379 stands out as having caused an order of magnitude more Even before the earthquakes of 2001, the coffee industry 435 380 landslides than any other earthquake of the same size; the was in a difficult situation as a result of low prices on the 436 381 1986 San Salvador stands out in the same way amongst the international market, partly as a result of a bumper crop in 437 382 cases in the database of Rodrı´guez et al. [31], which extends 1999–2000, and a delay in the previous year’s harvest due 438 383 the Keefer [30] database from 1980 to 1997. to particularly wet weather that affected Mexico, Guate- 439 384 The record of landslides induced by earthquakes in El mala, El Salvador and Honduras [37]. 440 385 Salvador dates back to 1576, when landslides in the Sierra During recent years the relative importance of the coffee 441 386 Los Texacuangos were reported to be triggered by an industry in El Salvador has declined, with its contribution to 442 387 earthquake [32]. Since then more than 20 earthquakes have the GDP dropping from close to 10% in the early 1980s to 443 388 been foundUNCORRECTED to cause widespread landsliding within the around 3% in recentPROOF years [38]. The main source of income 444 389 Salvadorian territory [33]. Areas affected by earthquake- to the Salvadorian economy is now the dollars sent back to 445 390 induced landslides in El Salvador are much higher than relatives by Salvadorians living, often illegally, in the USA. 446 391 those affected by earthquakes of comparable magnitude that The migration of Salvadorians to the United States was 447 392 occur in other geological, geomorphological and climatic accelerated by the civil war that engulfed the country from 448

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 5

449 505 450 506 451 507 452 508 453 509 454 510 455 511 456 512 457 513 458 514 459 515 460 516 461 517 462 518 463 519 464 520 465 521 466 522 467 523 468 524 469 525 470 526 471 527 472 528 473 529 474 530 475 531 476 532 477 533 478 534 479 535 480 536 481 537 482 538 483 539 484 540 485 541 486 542 487 543 488 544 489 545 490 546 491 547 492 548 493 549 494 550 495 551 496 552 497 553 498 Fig. 3. OPAMSS map of landslide hazard in the Metropolitan Area of San Salvador. 554 499 555 500 1980 to 1992, but poverty and increasing crime rates have US$ 1751 million in 2000, almost six times the total value 556 maintained theUNCORRECTED exodus since the peace accords were signed of coffee exports PROOF (US$ 298 million) and 2.7 times the net 501 557 502 between the Salvadorian government and the FMLN foreign exchange generated by the assembly (maquila ) 558 503 (Farabundo Martı´ National Liberation Front). Income industry (US$ 654 million). 559 504 from remittances (known in Spanish as remesas ) reached El Salvador is classiﬁed as a lower middle-income 560

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

6 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

561 Table 1 space with standard upper mantle wave velocities. We 617 562 Source parameters for 13 January 2001 earthquake modelled the direct waves (P and S) and the reflected phases 618 563 from free surface (pP, sP, sS, pS). In order to simulate 619 Time Epicentre Depth Magnitudes Agency p 564 (UTC) (km) seismic attenuation; we assumed t ¼ 11 s for P waves and 620 565 N8 W8 4 s for SH waves. We used a maximum likelihood principle 621 566 to obtain the source parameters that provide the best fit 622 567 17:33:32 13.049 88.660 60 Mw 7.7, Ms 7.8, mb 6.4 NEIC between observed and synthetic waveforms [40,41]. During 623 17:33:46 12.97 89.13 56 M 7.7, M 7.8, m 6.4 HRV 568 w s b the inversion, we solved simultaneously for focal mechan- 624 17:33:30 12.868 88.767 60 Mw 7.7 CASC 569 ism and source time function using the CMT solutions as a 625 570 priori models. We selected a set of teleseismic stations that 626 economy with an average GDP per capita of a little over 571 gave us the best azimuthal coverage as possible in order to 627 $2000, although the distribution of wealth is enormously 572 have a good constraint of the fault plane parameters. We 628 uneven. The economic fragility of the small republics of 573 used displacement seismograms, deconvolving them from 629 Central America when subjected to natural disasters is well 574 their instrumental response and then reconvolving each 630 established. Coburn and Spence [39] list the economic 575 signal to a common instrumental response. Band-passed 631 losses inflicted by major earthquakes from 1972 to 1990; the 576 filters were applied to the displacement records with a band- 632 three highest losses, when expressed as a percentage of the 577 pass Butterworth filter of order 3. 633 GNP for the year of the earthquake, of 40%, 18 and 31% 578 Fig. 4 shows body-waveform inversion results for the 634 were caused by the 1972 Managua, 1976 Guatemala and 579 earthquake of 13 January. For this event, P waves were 635 1986 San Salvador earthquakes, respectively. 580 relatively well fitted by our simple point source model. For 636 581 P- and S-waves the first arrivals were not very well 637 582 modelled because the S-waves—which are much stron- 638 583 3. Source characteristics of the 2001 earthquakes ger—dominate the inversion. The strong SH arrivals force 639 584 the source time function of the earthquake to start with a 640 585 The earthquake sequence that began on the 13 January very strong impulse. The peak appears to be much weaker 641 586 2001 lasted for a total of over 6 weeks. Within this sequence for P-waves. The depth we found was 50 km. The more 642 587 were distinct and tectonically separate main shocks, on 13 vertical fault plane was well constrained by the azimuthal 643 588 January and 13 February, whose characteristics are coverage. The source time function can be divided into two 644 589 described in the next sections. A third event that occurred sub-events: the first one had higher amplitude and a duration 645 590 on 17 February, with an epicentre on the western side of San of 22 s; the second sub-event occurred during 24 seconds. 646 20 591 Salvador, was sufficiently remote from the two main shocks The seismic moment was 5.54 £ 10 N m; it is difficult to 647 592 not to be considered as an aftershock of either. However, observe any directivity effect. However, if we compare 648 593 this was a small event, assigned a magnitude of ML 5.1 by signals from LBNH and KIP stations with their synthetics 649 594 the Centre for Geotechnical Investigations (CIG) of the (which were modelled with a point source), we observe a 650 595 Ministry of Public Works and mb 4.1 by NEIC, and caused possible directivity effect. If we assume that the fault plane 651 596 only very minor damage and caused less intense ground is the sub-vertical one, as is commonly observed for 652 597 shaking than many of the aftershocks of the 13 January intraplate events, there could be an upward rupture 653 598 event; its impact was primarily psychological, owing to the propagation (Fig. 4); more data is required in order to 654 599 understandably agitated state of the population after 5 weeks constrain this directivity. The Mw 7.7 earthquake of 13 655 600 of tremors. January 2001 was an intermediate depth earthquake that 656 601 occurred inside the down-going Cocos plate; its tension axis 657 602 3.1. The 13 January 2001 earthquake was sub-parallel to the dip direction of the descending slab. 658 603 Large magnitude, normal faulting earthquakes are not 659 604 The first earthquake struck just after 11:30 am (local unknown in subduction zones, indeed the El Salvador 660 605 time) on Saturday 13 January 2001. Table 1 gives the source earthquake of 19 June 1982 was of very similar rupture 661 606 parameters determined by different agencies, which in terms mechanism. The highly destructive Peru earthquake (Ms 662 607 of size and depth of the earthquake are remarkably 7.7) of 31 May 1970 was also associated with a normal 663 608 consistent. rupture, as was the large M 8.1 Chillan (Chile) earthquake in 664 609 We determined fault mechanism, depth, source time 1939. In the case of the Peruvian earthquake, the large-scale 665 610 function and seismic moment of earthquakes of 13 January extensional fracture in the underthrusting Nazca plate was 666 611 and 13 February using very broadband digital data. In order interpreted as being due to tensional stresses caused by the 667 612 to avoid multi-pathing,UNCORRECTED upper mantle and core arrivals, we denser descending PROOF plate [42]. In the case of the Cocos plate 668 613 only inverted body-waveforms from stations in the range in Central America, the cause of normal faulting may be 669 614 308,D , 908. We modelled the earthquakes as single both extensional stresses due to slab pull and flexural 670 615 point double-couple sources. The velocity structure near the stresses induced as the slab begins to descend at a greater 671 616 source and beneath the stations was approximated by a half dip angle inside the mantle [43]. 672

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 7

673 729 674 730 675 731 676 732 677 733 678 734 679 735 680 736 681 737 682 738 683 739 684 740 685 741 686 742 687 743 688 744 689 745 690 746 691 747 692 748 693 749 694 750 695 751 696 752 697 753 698 754 699 755 700 756 701 757 702 758 703 759 704 760 705 761 706 762 707 763 708 764 709 765 710 766 711 767 712 768 713 769 714 770 715 771 716 772 717 Fig. 4. Analysis of broadband body waves for the 13 January 2001 earthquake. Top: observed P-wave seismograms (solid lines) are in general fitted well by 773 718 synthetics (dashed lines) computed for a point source model with focal mechanism and source time function shown. Bottom: as above but for SH waves. 774 719 775 720 776 721 In view of the agreement that the focal depth was of and possibly due to the arrival of P-waves at the 777 722 the order of 50–60 km, the earthquake would not have surface. 778 723 been expected to generate tsunami, even though there The earthquake was felt from Mexico City in the north to 779 724 have been reports of a minor seismic sea wave [44]. Colombia in the south. Our field observations from extensive 780 Fig. 5 showsUNCORRECTED a tide gauge record from the port of travel throughout PROOF the interior valley and the coastal areas of El 725 781 726 Acajutla in which it can be seen that no tsunami Salvador suggest that MM intensities throughout the southern 782 727 occurred; the fluctuation in sea level at the time of the half of the country were between VI and VII with local pockets 783 728 earthquake was comparable with ambient noise levels, of higher intensity between VII and VIII. 784

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

8 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

785 841 786 842 787 843 788 844 789 845 790 846 791 847 792 848 793 849 794 850 795 851 796 852 797 853 798 854 799 855 800 856 801 857 802 858 803 859 804 860 805 861 806 862 807 863 808 864 809 865 810 866 811 867 812 868 813 869 814 Fig. 5. Tide gauge record from the port of La Libertad showing height above mean sea level (metres) from 12 to 15 January 2001. 870 815 871 3.2. The 13 February 2001 earthquake constrain the mechanism using the envelope of the signal. 816 872 The depth was 14 km and the seismic moment was 817 873 Aftershocks from the 13 January earthquake decayed 6.05 £ 1018 N m. The total source time function duration 818 874 approximately according to Omori’s law in the period up to 819 was 12 s. P and SH waves were very well fit even if at some 875 13 February and were gradually dying out when a second 820 stations P waves were very noisy (PAS, KDAK). In spite of 876 earthquake occurred. The parameters for Omori’s equation 821 these problems, the two fault planes were well determined. 877 for the aftershocks prior to the second major event were 822 The event of 13 February 2001 was totally different from 878 obtained as follows: 823 that of 13 January: it was a strike–slip event that took place 879 824 inside the upper continental plate, in the zone of weakness 880 log½NðtÞ ¼ 2:7 2 0:7 logðtÞ R2 ¼ 0:9 ð1Þ 825 of the volcanic axis. The fault plane must be the one sub- 881 826 parallel to the volcanic axis, i.e. sub-parallel to the 882 The source parameters for the second earthquake are listed subduction trench, which is confirmed by the distribution 827 in Table 2. Wave-form modelling was also carried out for 883 of aftershocks located by CIG. 828 this second event. Fig. 6 shows displacement seismograms 884 The 13 February earthquake, despite its size and 829 filtered between the same corner frequencies as the 13 885 relatively shallow focus, did not produce surface rupture, 830 January event. Signals were noisier but we managed to 886 831 although there are mapped faults to the east of Chichonte- 887 832 Table 2 pec volcano whose rupture would be compatible with the 888 833 Source parameters for 13 February 2001 earthquake fault plane solution [45]. An important issue in the 889 interpretation of these earthquakes is the focal depth of 834 Time Epicentre Depth Magnitudes Agency 890 835 (UTC) (km) the13Februaryearthquake,whichappearstobeofthe 891 836 N8 W8 order of 15 km from our well-determined solution. Focal 892 UNCORRECTEDdepth is the mostPROOF difficult seismic source parameter to 837 893 838 14:22:06 13.671 88.938 10 Mw 6.5, Ms 6.5, mb 5.5 NEIC determine reliably and seismograph coverage in Central 894 14:22:16 13.98 88.97 15 M 6.6, M 6.5, m 5.5 HRV 839 w s b America, although improved by recent regional collabor- 895 14:22:07 13.927 88.743 9.5 MC 5.9, ML 5.7 CASC 840 ations [46], is still limited, hence reported focal depths 896

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 9

897 953 898 954 899 955 900 956 901 957 902 958 903 959 904 960 905 961 906 962 907 963 908 964 909 965 910 966 911 967 912 968 913 969 914 970 915 971 916 972 917 973 918 974 919 975 920 976 921 977 922 978 923 979 924 980 925 981 926 982 927 983 928 984 929 985 930 986 931 987 932 988 933 989 934 990 935 991 936 992 937 993 938 994 939 Fig. 6. Analysis of body waves for the 13 February 2001 earthquake. As for Fig. 4 except that only P-waves are considered. 995 940 996 941 carry a considerable degree of uncertainty. A clear some correlation between magnitude and focal depth for 997 942 example of this is the earthquake sequence of Jucuapa– crustal earthquakes in the Central America region, with 998 943 Chinameca on 6–7 May 1951; contemporary catalogues events of this size occurring below the upper crust. 999 944 list the earthquakes with focal depths between 80 and Ambrasyes and Adams [4] report that the 20 December 1000 945 100 km, and re-determinations using teleseismic data by 1936 earthquake in the region of San Vicente, one of the 1001 946 Ambraseys and Adams [4] conﬁrms the intermediate focus towns most heavily affected by the 13 February event, was 1002 947 of the events. However, wave-form modelling, the of sub-crustal origin. The empirical relationship of Wells 1003 948 presence of well-developed surface waves on a seismo- and Coppersmith [47] for strike–slip faults yields a mean 1004 gram from GuatemalaUNCORRECTED City, and the distribution on damage value of 10.5 km PROOF for the rupture width of an earthquake of 949 1005 950 and intensity, all point compellingly towards very shallow this size; if the rupture did not advance more than 5–8 km 1006 951 focal depths, probably less than 10 km [10].Onthebasisof from the surface, this may at least partly explain why the 1007 952 the very limited evidence available, there does appear to be 13 February earthquake was less destructive than may have 1008

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

10 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

1009 been expected from an event of this size occurring so close 4.1. Characteristics of accelerograms 1065 1010 to population centres. 1066 1011 A preliminary isoseismal for the 13 February 2001 Tables 3 and 4 list the main characteristics of the 1067 1012 earthquake published by CIG reported a maximum MM accelerograph recordings of the 13 January and 13 February 1068 1013 intensity of VII–VIII in the area from Lake Ilopango to San earthquakes; the station locations are shown in Fig. 7.A 1069 1014 Vicente and VI in San Salvador. Our field observations major difficulty in performing detailed analysis and 1070 1015 suggest that these are overestimates and that the maximum interpretation of the recorded accelerograms is the lack of 1071 1016 intensity generally did not exceed VII. information about soil profiles at recording sites other than 1072 1017 An obvious question to be addressed is whether the 13 the CIG stations in San Salvador for which investigations 1073 1018 February earthquake was in some way triggered by the were carried out as part of a microzonation study following 1074 1019 subduction event a month earlier. Stress transfer due to the 1986 earthquake [51]. Nearly all of the stations that 1075 1020 relaxation of one crustal area leading to heightened tectonic recorded the two earthquakes are located on pyroclastic 1076 1021 stresses in an adjacent area has been clearly observed, for deposits such as tierra blanca and the older tobas color cafe´. 1077 1022 example, in the sequence of earthquakes from 1939 to 1999 Exceptions to this are the Presa 15 de Septiembre hydro- 1078 1023 along the North Anatolian fault in Turkey [48]. However, electric dam site (alluvium), La Libertad (alluvium) and 1079 1024 the situation in Central America is much less clear because Panchimaclo (volcanic rocks). It is very likely that the 1080 1025 the two earthquakes are of entirely different tectonic origin, ground motions at several of the recording sites are also 1081 1026 even though they are both ultimately the result of the same affected by topographical features: the Panchimalco station 1082 1027 general tectonic process. Lomnitz and Rodrı´guez Elizar- of the UCA network is located within a N–S trending 1083 1028 rara´s [44] report that normal faulting subduction earth- valley, whereas the San Pedro Nonualco station sits atop an 1084 1029 quakes in Mexico tend to be followed by either large thrust E–W trending narrow ridge. Recordings of distant subduc- 1085 1030 events or shallow intraplate events four or five years later. A tion events off the coast of Nicaragua have consistently 1086 1031 similar pattern may possibly exist in El Salvador, whereby produced relatively strong recordings at the latter site, 1087 1032 large magnitude subduction earthquakes in some way whereas at Panchimalco recordings have generally been 1088 1033 trigger crustal events within the Caribbean plate within weak, frequently below the instrument trigger level [50]. 1089 1034 similar, or in some cases much smaller, intervals. The large Regrettably several potential records were lost due to 1090 1035 subduction earthquake of 1915 in western El Salvador was malfunction of instruments. The CIG station at Santiago de 1091 1036 followed by crustal earthquakes in San Salvador and to the Marıá in eastern El Salvador did not trigger during the 13 1092 1037 west in 1917 and 1919; the subduction earthquake of 1932 January event; the instruments in the north-western towns of 1093 1038 offshore of central El Salvador was followed by the crustal Santa Ana and Metapań also failed to produce records, 1094 1039 earthquake in San Vicente in 1936; the 1982 subduction although it is not clear whether this was due to malfunction 1095 1040 earthquake was followed in 1986 by the San Salvador or due to accelerations not reaching the triggering level, 1096 1041 upper-crustal earthquake. Therefore, the events of January which may have been the case at Metapań at least. It is clear 1097 1042 and February 2001 may be simply a highly accelerated case from comparison of Tables 3 and 4 that the functioning of 1098 1043 of a process that is characteristic of the region. Earlier the CIG instruments was not consistent. More important 1099 1044 studies have alluded to relationships between Quaternary cases of malfunction concerned the San Vicente instrument 1100 1045 faulting in the Caribbean plate and the nature of the of the UCA network, which did not record either of the 1101 1046 subducted Cocos plate [49] but the highly complex system earthquakes, and the San Pedro Nonualco station that would 1102 1047 of stress transfer and the exact nature of the plate have produced the most important recording of the 13 1103 1048 interactions are not sufficiently well known to infer any February earthquake. The station operated by GESAL at the 1104 1049 definitive model at this stage. Berlıń geothermal energy plant also failed to yield an 1105 1050 accelerogram of the 13 February earthquake. 1106 1051 1107 1052 4. Strong ground-motion 4.2. Comparisons of strong-motion parameters with 1108 1053 predictions 1109 1054 Both the 13 January and 13 February 2001 earthquakes 1110 1055 were well recorded by three accelerograph networks in For earthquakes of magnitude greater than about 6, for 1111 1056 operation in El Salvador: a network of SMA-1 analogue which the source dimensions are of the order of more than a 1112 1057 instruments operated by the CIG, a network of digital and few kilometres, the use of epicentral distance can seriously 1113 1058 analogue instruments operated at geothermal and hydro- overestimate the separation of the site and the source of 1114 1059 electric plants by GESAL, and the TALULIN network of energy release. For the 13 January earthquake, distances 1115 1060 digital SSA-2 instruments operated by the Universidad have been measured from the assumed fault rupture, since 1116 UNCORRECTED´ ´ PROOF 1061 Centroamericana (UCA) ‘Jose Simeon Canãs’ [50]. Records this is the distance measure proposed by Youngs et al. [52] 1117 1062 were also obtained from the network of INETER in for subduction zone earthquakes. The actual location of the 1118 1063 Nicaragua. The records from the CIG network were fault rupture has been fixed by the angle of dip of the fault, 1119 1064 digitised and processed by the USGS. which coincides with the angle of dip of Cocos plate as 1120

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 11

1121 Table 3 1177 1122 Strong-motion records of 13 January 2001 earthquake 1178 1123 a 1179 Network Station Location drup (km) PGA (g) PGV (cm/s) 1124 1180 1125 N8 W8 N–S E–W V N–S E–W V 1181 1126 1182 GESAL Berlıń Geoth. 13.50 88.53 54 0.459 0.370 0.235 21.3 24.0 12.3 1127 1183 UCA Armenia 13.744 89.501 93 0.601 0.454 0.223 28.8 29.4 19.6 1128 UCA La Libertad 13.468 89.327 60 1.113 0.575 0.617 53.2 35.5 16.0 1184 1129 UCA Panchimalco 13.614 89.179 75 0.177 0.154 0.089 9.2 9.4 7.3 1185 1130 UCA San Bartolo 13.705 89.106 85 0.157 0.199 0.166 25.2 31.2 15.2 1186 1131 UCA S Pedro Nonualco 13.602 88.927 50 0.580 0.488 0.439 37.5 26.4 18.2 1187 UCA San Salvador ESJb 13.707 89.201 85 0.301 0.278 0.154 25.4 17.4 11.9 1132 1188 UCA Santa Tecla 13.671 89.279 83 0.496 0.243 0.487 57.0 34.2 18.5 1133 UCA Tonacatepeque 13.778 89.114 93 0.234 0.205 0.263 23.1 23.2 9.8 1189 1134 UCA Zacatecoluca 13.517 88.869 47 0.260 0.314 0.253 12.3 21.9 10.4 1190 1135 CIG Ahuachapań 13.925 89.805 123 0.146 0.214 0.124 14.9 16.6 10.8 1191 1136 CIG Acajutla 13.567 89.833 95 0.098 0.108 0.050 14.6 18.6 4.2 1192 CIG Cutuco 13.333 87.817 125 0.078 0.079 0.063 13.8 8.6 4.0 1137 1193 CIG Presa 15 de Sept.c 13.616 88.550 66 0.152 0.187 0.122 23.5 16.0 10.2 1138 CIG San Salvador DBd 13.733 89.150 84 0.225 0.250 0.160 23.2 19.2 11.3 1194 1139 CIG San Salvador REe 13.692 89.250 83 0.304 0.323 0.329 22.9 27.6 15.3 1195 1140 CIG San Miguel 13.475 88.183 107 0.136 0.120 0.089 12.8 12.1 6.0 1196 1141 CIG Sensuntepeque 13.867 88.663 81 0.082 0.061 0.058 8.5 9.1 6.2 1197 INETER Boaco 12.473 85.658 336 0.004 0.003 0.002 0.5 0.5 0.4 1142 1198 INETER Chinandega 12.632 87.133 175 0.090 0.070 0.042 6.3 4.6 2.1 1143 INETER DEC 12.124 86.267 276 0.045 0.044 0.028 3.1 3.3 1.7 1199 1144 INETER Estelı´ 13.092 86.355 263 0.014 0.011 0.009 2.3 2.5 0.9 1200 1145 INETER Granada 11.937 85.976 312 0.009 0.009 0.006 1.7 1.3 0.9 1201 1146 INETER Jinotega 13.086 85.995 302 0.006 0.005 0.004 0.7 0.9 0.5 1202 INETER Juigalpa 12.107 85.372 371 0.003 0.003 0.002 0.6 0.6 0.5 1147 1203 INETER Leoń 12.117 86.266 276 0.040 0.037 0.026 2.3 2.6 1.4 1148 INETER Managua (ESSO)f 12.144 86.320 270 0.057 0.045 0.022 3.8 3.9 1.5 1204 1149 INETER Managua (INET)g 12.149 86.248 277 0.034 0.041 0.014 2.6 2.7 1.1 1205 1150 1206 a Distance from fault rupture as defined by Youngs et al. [52]. 1151 1207 b Externado de San Jose´. 1152 c Ground level instrument adjacent to dam. 1208 1153 d Ciudadela Don Bosco. 1209 e 1154 Ministerio de Relaciones Exteriores; there are two accelerographs at the this site, the reported values are from the instrument at ground level, the other is at 1210 1155 the base of a well. 1211 f ESSO Refinery. 1156 1212 g INETER. 1157 1213 1158 proposed by Burbach et al. [43]. Taking account of the focal February earthquake were determined by modelling the 1214 1159 depth of the main shock, the seismic moment and the spatial fault as a line striking N948E, extending from (13.668N, 1215 1160 distribution of aftershocks, the fault plane is modelled as a 89.08W) to (13.638N, 88.618W). This places the fault 1216 1161 fault plane with a strike of 3008 dipping 558 to the NE, rupture as extending eastwards from the western shore of 1217 1162 which corresponds to a plane sub-parallel to the subduction Lake Ilopango for about 42 km; this is longer than would be 1218 1163 trench. The dimensions of the rupture plane were con- expected from the relationships of Wells and Coppersmith 1219 1164 strained by the distribution of aftershock hypocentres from [47], which may indicate a narrow rupture and hence the 1220 1165 13 January until the end of August, concentrated at focal effective depth of the source that may explain the relatively 1221 1166 depths between 20 and 40 km. The dimensions of the low amplitudes recorded. This rupture plane was con- 1222 1167 inferred fault rupture plane are 65 km in length and 55 km in strained from aftershock distributions from 13 February 1223 1168 width. The uppermost part of the rupture is assumed to until the end of August with depths up to 15 km. Seismic 1224 1169 extend to a depth of 20 km and extends from (12.958N, activity west of Ilopango has been reported after the 13 1225 1170 89.258W) in the west to (12.678N, 88.748W) in the east. The February earthquake but it is probably related to the 17 1226 1171 distances from this assumed rupture are given in Table 3. February event near San Salvador. The calculated distances 1227 1172 For the crustalUNCORRECTED earthquake of 13 February, a more from this assumed PROOF source are presented in Table 4; since it is 1228 1173 appropriate measure of the source-to-site distance is that possible that the length of the fault rupture has been 1229 1174 proposed originally by Joyner and Boore [53], namely the overestimated, there is the possibility that some of the 1230 1175 shortest distance from the surface projection of the fault distances are underestimated. The uncertainty, however, lies 1231 1176 rupture. The distances from the fault rupture for the 13 mainly in the eastward extension of the fault rupture, which 1232

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

12 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

1233 Table 4 1289 1234 Strong-motion records of 13 February 2001 earthquake 1290 1235 a 1291 Network Station Location drup (km) PGA (g) PGV (cm/s) 1236 1292 1237 N8 W8 N–S E–W V N–S E–W V 1293 1238 1294 GESAL Berlıń (town) 13.50 88.53 17 0.032 0.070 0.031 4.1 6.0 2.9 1239 1295 UCA Armenia 13.744 89.501 55 0.029 0.037 0.026 4.0 2.3 1.3 1240 UCA La Libertad 13.468 89.327 41 0.091 0.093 0.037 4.7 4.5 3.1 1296 1241 UCA Panchimalco 13.614 89.179 20 0.185 0.106 0.045 9.4 4.6 2.0 1297 1242 UCA San Bartolo 13.705 89.106 13 0.106 0.141 0.123 25.6 22.3 6.9 1298 b 1243 UCA San Salvador ESJ 13.707 89.201 22 0.124 0.099 0.052 18.3 6.6 2.7 1299 UCA Santa Tecla 13.671 89.279 30 0.047 0.040 0.023 6.4 4.8 2.0 1244 1300 UCA Tonacatepeque 13.778 89.114 18 0.345 0.251 0.240 30.0 24.7 10.5 1245 UCA Zacatecoluca 13.517 88.869 18 0.408 0.305 0.262 20.1 20.4 9.6 1301 1246 CIG Presa 15 de Sept.c 13.616 88.550 7 0.020 0.026 0.017 6.4 5.0 2.4 1302 1247 CIG S. Salvador CIGd 13.698 89.173 19 0.138 0.071 0.059 19.9 8.4 3.8 1303 e 1248 CIG San Salvador DB 13.733 89.150 18 0.100 0.094 0.055 14.8 12.2 4.6 1304 CIG S. Salvador DUAf 13.737 89.209 24 0.077 0.059 0.046 8.2 8.7 3.5 1249 1305 CIG S. Salvador OBSg 13.681 89.198 22 0.107 0.104 0.068 6.7 13.9 3.3 1250 CIG San Salvador REh 13.692 89.250 27 0.058 0.063 0.034 3.9 8.1 2.2 1306 1251 CIG S. Salvador SEMi 13.705 89.225 25 0.065 0.071 0.044 5.7 10.8 2.6 1307 1252 CIG S. Salvador UCAj 13.677 89.236 26 - 0.058 0.040 - 8.5 2.1 1308 1253 CIG Santa Tecla 13.675 89.300 32 0.039 0.042 0.019 6.4 7.4 2.2 1309 1254 a Distance from fault rupture as defined by Joyner and Boore [53]. 1310 1255 b Externado de San Jose´. 1311 c 1256 Ground level site adjacent to dam. 1312 d 1257 Centro de Investigaciones Geotećnicas. 1313 e Ciudadela Don Bosco. 1258 1314 f Viveros, Direccioń de Urbanismo y Arquitectura; there are two accelerographs at the this site, the reported values are from the instrument at ground level, 1259 the other is at the base of a well. 1315 1260 g Observatorio Sismolo´gico. 1316 h 1261 Ministerio de Relaciones Exteriores, ground-level instrument. 1317 i 1262 San Jose´ de la Montanã Seminary, ground-level instrument. 1318 j Universidad Centroamericana. 1263 1319 1264 1320 would only affect the distances to the stations at Berlıń and There are shortcomings in all of the above attenuation 1265 1321 the 15 de Septiembre dam. relationships in terms of applicability to El Salvador, either 1266 1322 Algermissen et al. [18] derived an attenuation relation- because they do not discriminate between subduction and 1267 1323 1268 ship from 82 recordings of strong-motion obtained in the crustal earthquakes, or because they are based on insuffi- 1324 1269 vicinity of San Salvador, without distinguishing between cient datasets. The equations of Schmidt et al. [56] are the 1325 1270 subduction and crustal earthquakes. Taylor Castillo et al. only exceptions, but there are important tectonic and 1326 1271 [54] derived an equation from 89 records from Costa Rica, geologic differences between Costa Rica and El Salvador, 1327 1272 El Salvador and Nicaragua, again combining crustal and on the one hand, and on the other they make use of 1328 1273 subduction earthquakes. Dahle et al. [55] subsequently epicentral and hypocentral distance, which are unsuitable 1329 1274 produced attenuation equations for response spectral for large events as was noted previously. For these reasons, 1330 1275 ordinates, using a database of 280 records, including 157 comparisons have been made with predictions from 1331 1276 from Costa Rica and more than 60 from Mexico, and relationships derived for other regions. For the subduction 1332 1277 making no distinction between different sources of seismi- earthquake of 13 January, the most appropriate attenuation 1333 1278 city. Some other studies have separated subduction zone and relationships are those of Youngs et al. [52] derived from 1334 1279 crustal earthquakes: Alfaro et al. [17] derived two separate regressions on almost 500 accelerograms from Alaska, 1335 1280 equations for PGA, but used only 20 records for each. Chile, Cascadia, Japan, Mexico, Peru and the Solomon 1336 1281 Schmidt et al. [56] have derived equations for spectral Islands. These equations have been proposed for intra-slab 1337 1282 ordinates from a database of 200 accelerograms recorded in and interface subduction earthquakes, for events larger than 1338 1283 Costa Rica, presenting coefficients for the entire dataset and Mw 5 and distances from the fault rupture between 10 and 1339 1284 for subduction and crustal sub-sets. Climent et al. [57] 500 km, making them ideally suited to this situation. The 1340 derived spectralUNCORRECTED acceleration equations for Central America recorded PGA PROOF values are compared with those predicted by 1285 1341 1286 using 280 records from Costa Rica, Mexico, Nicaragua and the intra-slab equation of Youngs et al. [52] in Fig. 8; 1342 1287 El Salvador; these relationships also did not separate crustal ground conditions corresponding to more than 20 m of soil 1343 1288 and subduction events. overlying rock have been assumed. The equation appears to 1344

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 13

1345 1401 1346 1402 1347 1403 1348 1404 1349 1405 1350 1406 1351 1407 1352 1408 1353 1409 1354 1410 1355 1411 1356 1412 1357 1413 1358 1414 1359 1415 1360 1416 1361 1417 1362 1418 1363 1419 1364 1420 1365 1421 1366 1422 1367 1423 1368 1424 1369 1425 1370 1426 1371 1427 1372 1428 1373 1429 1374 1430 1375 Fig. 7. Location of strong-motion recording stations in El Salvador. 1431 1376 1432 ﬁt the data well at distances of less than 300 km, with values and long periods would have been expected. Similar 1377 1433 from greater distances being overestimated. It is worth features have been observed in accelerograms from large 1378 1434 noting that in the distance range from 50 to 130 km, the subduction zone earthquakes in Japan and also in Peru [58, 1379 1435 values obtained from the CIG network are consistently 59]. Since high-frequency ground motions were recorded in 1380 1436 lower than those from the UCA network. both the 1966 and 1970 Peruvian earthquakes, which were, 1381 1437 1382 One particularly interesting feature of the motions respectively, associated with thrust and normal ruptures 1438 1383 recorded during the 13 January earthquake is the fact that [42], it would appear that this feature may not be exclusively 1439 1384 the response spectra are rich in high frequencies whereas for a function of source mechanism. Nonetheless, Prvance and 1440 1385 such a large magnitude event greater energy at intermediate Anderson [60] identify normal faulting earthquakes in the 1441 1386 Mexican subduction zone as producing consistently more 1442 1387 high-frequency radiation than thrusting events. There is 1443 1388 evidence that the recording from La Libertad (Fig. 9), where 1444 1389 PGA exceeds 1 g, displays strong site effects at a period of 1445 1390 about 0.2 s (Fig. 10). This is visible on many recordings 1446 1391 from this station from previous smaller or more distant 1447 1392 earthquakes and would be consistent with a relatively thin 1448 1393 layer (,10 m) of alluvium overlying bedrock (lavas). 1449 1394 For the crustal earthquake of 13 February, one possibility 1450 1395 would be to use the relationships derived by Spudich et al. 1451 1396 [61] for zones of extensional tectonics, but the two 1452 UNCORRECTEDrecordings from thePROOF 1986 San Salvador earthquake used in 1397 1453 1398 that study were found to be outliers whose amplitudes were 1454 1399 Fig. 8. Recorded PGA values from the 13 January earthquake compared signiﬁcantly underestimated by the median predictions. 1455 1400 with predictions from the attenuation relationships of Youngs et al. [52]. Other candidate equations derived from crustal recordings 1456

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

14 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

1457 1513 1458 1514 1459 1515 1460 1516 1461 1517 1462 1518 1463 1519 1464 1520 1465 1521 1466 1522 1467 1523 1468 1524 1469 1525 1470 1526 1471 1527 1472 1528 1473 1529 1474 1530 1475 1531 1476 1532 1477 1533 1478 1534 1479 1535 1480 1536 1481 Fig. 9. Accelerogram recorded at La Libertad during 13 January 2001 earthquake. 1537 1482 elsewhere in the world would include the western USA 1538 1483 relationships of Boore et al. [62], the global relationships of 1539 1484 Abrahamson and Silva [63], the European relationships of 1540 1485 1541 Ambraseys et al. [64] and the Italian relationships of Sabetta 1486 1542 and Pugliese [65]; the latter may be particularly suitable 1487 1543 since large areas of Italy are also volcanic. Fig. 11a 1488 1544 compares the recorded PGA values with the median values 1489 1545 from these relationships, in which the equations seem to 1490 1546 consistently overestimate the observed values beyond about 1491 1547 20 km. Fig. 11b conﬁrms that most of the recorded PGA 1492 1548 values are within the ^s values predicted by Ambraseys 1493 1549 1494 et al. [64]. Worthy of particular note in this ﬁgure are the 1550 ´ 1495 low PGA values obtained at Berlın in this event, which 1551 1496 raises questions about the reliability of this particular 1552 1497 recording: GESAL operates digital accelerographs in the 1553 1498 1554 1499 1555 1500 1556 1501 1557 1502 1558 1503 1559 1504 1560 1505 1561 1506 1562 1507 1563 1508 1564 UNCORRECTEDFig. 11. (a) Recorded PROOF PGA values from the 13 February earthquake 1509 1565 compared with median values predicted by the equations of Abrahamson 1510 and Silva [63], Boore et al. [62], Ambraseys et al. [64] and Sabetta and 1566 1511 Fig. 10. Absolute acceleration response spectrum (5% damping) of record Pugliese [65]. (b) Recorded values of PGA compared with the 16, 50 and 1567 1512 in Fig. 9. 84% predictions from Ambraseys et al. [64]. 1568

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 15

1569 introduced in 1966 following the San Salvador earthquake 1625 1570 of the previous year; this code was adapted for El Salvador 1626 1571 from the code from Acapulco, Mexico [23]. A revised code 1627 1572 was drafted by the Salvadorian Association of Engineers 1628 1573 and Architects (ASIA) in 1989, issued as an emergency 1629 1574 regulation following the 1986 San Salvador; the design 1630 1575 spectra in this code took account of the nature of the ground 1631 1576 motions recorded in the earthquake. The current seismic 1632 1577 code, published in 1994, forms part of a comprehensive set 1633 1578 of regulations for building and civil works produced by the 1634 1579 Ministry of Public Works. The current code has several 1635 1580 Fig. 12. Envelope of horizontal acceleration spectra from the 13 January merits, including the fact that it is the first to have involved a 1636 1581 recording at La Libertad compared with the elastic spectra from the 1994 probabilistic assessment of seismic hazard in El Salvador 1637 1582 design code for soil class S3 and importance categories I and III. [19]. Furthermore, the regulations cover a wide range of 1638 1583 practices, including geotechnical works, and also provides 1639 town of Berlin and at the nearby geothermal energy plant, 1584 guidance on construction using adobe despite initial 1640 with records having been obtained only from the latter 1585 opposition from contractors who were concerned that 1641 instrument on 13 January and reportedly from the former on 1586 promotion of vernacular building techniques would be 1642 13 February. The even lower amplitudes recorded at the 15 1587 detrimental to their business. 1643 de Septiembre dam may point to the fault rupture being 1588 The elastic spectra in the current code appear to be 1644 shorter than 42 km and not extending so far east as has been 1589 sufficient for most of the ground motions recorded in these 1645 assumed. 1590 earthquakes. The somewhat exceptional record of 13 1646 1591 January at La Libertad exceeds the code spectrum 1647 1592 4.3. Implications of recorded motions for seismic design (Fig. 12), but it would not seem reasonable to increase the 1648 1593 code code spectrum to a maximum level of 5g just to 1649 1594 accommodate the narrow-band amplification due to specific 1650 1595 The first seismic design code in El Salvador was site effects at this location. The strongest recording from the 1651 1596 13 February earthquake, obtained at Zacatecoluca, is 1652 1597 covered by the spectral ordinates specified in the code, as 1653 1598 shown in Fig. 13a. Fig. 13b shows the spectrum at the same 1654 1599 station from the 13 January earthquake, which is also 1655 1600 adequately covered by the code spectrum. In passing it can 1656 1601 be noticed that as at other stations [66], the shape of the 1657 1602 spectra from the two earthquakes are generally similar, 1658 1603 confirming the importance of site effects in determining the 1659 1604 nature of the ground motion. 1660 1605 1661 1606 1662 1607 5. Effects of the earthquakes 1663 1608 1664 1609 The impact of the January and February 2001 earth- 1665 1610 quakes was strong in many parts of the south of the country, 1666 1611 particularly the coastal cordilleras and locations around the 1667 1612 volcanic centres. The area around the San Vicente volcano, 1668 1613 where buildings had been weakened by the 1999 swarms, 1669 1614 and where both the 13 January and 13 February earthquakes 1670 1615 caused strong shaking, was particularly affected. The 1671 1616 patterns of damage, however, were very uneven and the 1672 1617 capital city, San Salvador, was largely unaffected. None- 1673 1618 theless, the overall impact was devastating to the fabric of 1674 1619 the country, with an estimated 40% of the health service and 1675 1620 Fig. 13. (a) Envelope of horizontal acceleration spectra from the 13 30% of schools severely damaged. 1676 February recordingUNCORRECTED at Zacatecoluca compared with the elastic spectra from PROOF 1621 The death tolls due to the two earthquakes have been 1677 the 1994 design code for soil class S3 and importance categories I and III. 1622 (b) Envelope of horizontal acceleration spectra from the 13 January reported as 844 and 315, respectively, with the majority of 1678 1623 recording at Zacatecoluca compared with the elastic spectra from the 1994 the casualties, particularly in the 13 January event, being 1679 1624 design code for soil class S3 and importance categories I and III. due to landslides. It is worth highlighting here that the loss 1680

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

16 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

1681 1737 1682 1738 1683 1739 1684 1740 1685 1741 1686 1742 1687 1743 1688 1744 1689 1745 1690 1746 1691 1747 1692 1748 1693 1749 1694 1750 1695 1751 1696 1752 1697 1753 1698 1754 1699 1755 1700 1756 1701 1757 1702 1758 1703 1759 1704 1760 1705 1761 1706 1762 Fig. 14. Single block fall, triggered by an aftershock of the 13 January earthquake, on the coastal motorway near La Libertad. 1707 1763 1708 of life in these earthquakes underestimates their impact; area and ranged from single block falls (some of which were 1764 1709 1765 more people were killed by the Mw 5.7 San Salvador up to 3 m in diameter, Fig. 14) to the collapse of slopes cut 1710 earthquake of October 1986. The death toll is also small in pyroclastic ashfall deposits, which exist as a result of 1766 1711 compared with the 75,000 lives lost in the fratricidal war weak cementation and high negative pore pressure [28,68]. 1767 1712 from 1980 to 1992, and indeed when compared with the Such failures were largely independent of lithology, but 1768 1713 numbers of victims of violent crime, which has grown to occurred only on steep slopes. Individual block falls were 1769 1714 epidemic proportions in recent years. The true impact of the more common in the rocks of the Ba´lsamo Formation 1770 1715 earthquakes is more accurately represented by the fact, because of the prevalence of persistent discontinuities in the 1771 1716 mentioned previously, that a significant proportion of the form of bedding and cooling joints. Highly altered layers of 1772 1717 population was either made homeless or suffered substantial volcanic rock also acted as aquitards. 1773 1718 damage to their housing. Even before the earthquakes it had The occurrence of rock and debris falls in the tierra 1774 1719 been estimated that El Salvador had a housing deficit of blanca affected an extensive area around the Comasagua 1775 1720 more than half a million dwellings. Road on the Ba´lsamo Ridge. Here the steep slopes which 1776 1721 were formed as part of the road construction and also by 1777 1722 quarrying failed in spectacular manner giving rise to an 1778 1723 5.1. Landslides and liquefaction 1779 extensive area of slope instability which extended from the 1724 1780 landslide at Las Barrioleras to the Jayaque–Comasagua 1725 The 13 January earthquake triggered more than 500 1781 Junction (Fig. 15) and in Comasagua itself (Fig. 16). This 1726 landslides across in El Salvador and a further 70 occurred as 1782 formed a classic shattered ridge. In many cases, it was 1727 a result of the 13 February earthquake. Landslides were an 1783 impossible to tell where one landslide ended and another 1728 extensive secondary effect of the earthquake of 13 January 1784 began. 1729 2001. The landslides could be divided into three broad types 1785 Large landslides were observed along roads to Comasa- 1730 according to the classification scheme proposed by Dikau 1786 gua, Talnique, Jayaque, Tepecoyo and Sacacoyo. The 1731 et al. [67]. These were rock and debris falls, slides and 1787 1732 debris flows. Many of the landslides occurred on the slopes principal cases were reported along the road between 1788 of volcanoesUNCORRECTED that are used for coffee cultivation; coinciding Nueva San Salvador PROOF and Comasagua on slopes of volcanic 1733 1789 1734 with the time of the coffee harvest, many coffee pickers ashes mainly tierra blanca. The Pan-American Highway 1790 1735 were killed by these slides. was blocked between Los Chorros and Coloń by landslides 1791 1736 Rockfalls and debris falls were common throughout the to the west of San Salvador. At the Las Leonas location, to 1792

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 17

1793 1849 1794 1850 1795 1851 1796 1852 1797 1853 1798 1854 1799 1855 1800 1856 1801 1857 1802 1858 1803 1859 1804 1860 1805 1861 1806 1862 1807 1863 1808 1864 1809 1865 1810 1866 1811 1867 1812 1868 1813 1869 1814 1870 1815 1871 1816 1872 1817 1873 1818 1874 1819 Fig. 15. Debris fall from the Tierra Blanca near Comasagua. 1875 1820 1876 1821 the east of San Salvador, this road was blocked by a large devastating impact on population. This slope failure buried 1877 3 1822 slide of approximately 500,000 to 700,000 m of rock and as many as 500 people. This slide was approximately 790 m 1878 1823 soil debris. Roads to San Agustıń, Santiago de Marıá and long, 150 m wide and left a scarp 50 m high. The total 1879 3 1824 Berlıń were also blocked. Several landslides were also volume of the slide was approximately 200,000 m . The 1880 1825 observed along the road between Cojutepeque and Santiago slide affected a part of the northern flank of the Ba´lsamo 1881 1826 Texacuangos, and around Lake Ilopango. Ridge composed of the Ba´lsamo Formation. This formation 1882 1827 The most important group of landslides, which were is formed mainly of andesitic cinders and some interbedded 1883 1828 triggered by the earthquake, were the debris flows. These tephra. Extensive cracking was observed on the ridge crest 1884 1829 landslides were responsible for more than half the deaths in areas that did not slide, which was cause for additional 1885 1830 during the 13 of January earthquake. The two most concern. Some authors attributed this slide to liquefaction of 1886 1831 important of these slides were at Las Colinas (Fig. 17) saturated tierra blanca deposits [70–72], however, a 1887 1832 and Las Barioleras. These landslides, which occurred in the rotational slope failure of the upper part of the slope has 1888 1833 Tierra Blanca showed significant travel distances of 735 m also been attributed as initial failure mechanism [73]. 1889 1834 and c. 1140 m, respectively. The former of these two slides Failure has been found to be related to high water content of 1890 1835 destroyed part of the residential area at Las Colinas, while the lower part of the slope, which has been attributed to 1891 1836 the latter killed many people who were working on the natural drainage blockage by a retaining wall observed on 1892 1837 coffee plantations and travelled onto the Pan-American the bottom of the slope [72], although a perched aquifer on 1893 1838 Highway. The long travel distances of these landslides the slope due to the impermeable nature of the Ba´lsamo 1894 1839 indicate low coefficients of internal friction of between 6 Formation has also been proposed as the cause of this high 1895 1840 and 98, inferred from the ratio of slope height to run-out water content. Soil saturation was observed only locally, 1896 1841 length of the slide [69]. This indicates a significant drop in due mainly to the earthquake occurring after 5 months of 1897 1842 frictional strength from the undisturbed state, which may be dry season. The destructiveness of the landslide may have 1898 1843 as high as 388. Debris flows were common throughout the been due to its high mobility, which may have been the 1899 1844 Cordillera delUNCORRECTED Balsamo resulting from the steep terrain result of an unfavourable PROOF combination of high water content 1900 1845 mantled with weak volcanic debris and the presence of and material brittleness. 1901 1846 aquitards in the underlying Ba´lsamo Formation. Harp and Wilson [74] have identified Arias intensity 1902 1847 The Las Colinas landslide in Santa Tecla was the most (sum of the two horizontal components) as a useful indicator 1903 1848 notorious slide triggered by the earthquakes due to its of the capacity of the ground shaking to trigger landslides. 1904

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

18 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

1905 1961 1906 1962 1907 1963 1908 1964 1909 1965 1910 1966 1911 1967 1912 1968 1913 1969 1914 1970 1915 1971 1916 1972 1917 1973 1918 1974 1919 1975 1920 1976 1921 1977 1922 1978 1923 1979 1924 1980 1925 1981 1926 1982 1927 1983 1928 1984 1929 1985 1930 1986 1931 1987 1932 1988 1933 1989 1934 1990 Fig. 16. Landslide damage in Comasagua. 1935 1991 1936 1992 1937 1993 1938 1994 1939 1995 1940 1996 1941 1997 1942 1998 1943 1999 1944 2000 1945 2001 1946 2002 1947 2003 1948 2004 1949 2005 1950 2006 1951 2007 1952 2008 1953 2009 1954 2010 1955 2011 1956 UNCORRECTED PROOF 2012 1957 2013 1958 2014 1959 2015 1960 Fig. 17. The Las Colinas landslide, Santa Tecla. 2016

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 19

2017 2073 2018 2074 2019 2075 2020 2076 2021 2077 2022 2078 2023 2079 2024 2080 2025 2081 2026 2082 2027 2083 2028 2084 2029 2085 2030 2086 2031 2087 2032 2088 2033 2089 2034 2090 2035 2091 2036 2092 2037 2093 2038 2094 2039 2095 2040 2096 2041 2097 2042 2098 2043 2099 2044 2100 2045 2101 2046 2102 2047 2103 Fig. 18. Distribution of areas affected by landslides (solid line), by concentrated landslides (small dashes) and by liquefaction (large dashes). 2048 2104 2049 From studies of the 1987 Superstition Hills and Whittier slides to those reported by the 13 January event. Along the 2105 2050 Narrows earthquakes in California, Harp and Wilson [74] Pan-American Highway new landslides were observed at 2106 2051 proposed thresholds of about 0.25–0.3 m/s for landslide Las Leonas and adjacent locations. A large landslide was 2107 2052 triggering. The values of Arias intensity for the records of reported in the water head part of the Rio Jiboa; it was 2108 2053 the 13 January earthquake are as high as 14 m/s and in all estimated that volume of sediments yielded in this area 2109 2054 cases (excluding the Nicaraguan records), exceed this reaches between 10 and 15 £ 106 m3 of debris, mainly 2110 2055 threshold [75]. tierra blanca [77]. This landslide blocked the river course 2111 2056 In terms of geographical distribution, landslides were for 600–700 m causing an artificial lake to be formed. 2112 2057 triggered across most of the southern half of El Salvador, Another large landslide blocked the course of Rio El 2113 2058 with a particular high concentration in the Cordillera del Desaguë; in this case a volume between 1 and 2 £ 106 m3 2114 2059 Ba´lsamo to the southwest of San Salvador, between Nueva was mobilised, consisting of andesitic breccia blocks of 2115 2060 San Salvador and Armenia (Fig. 18), affecting a much larger around 0.5–2 m in diameter embedded into a tierra blanca 2116 2061 area than in previous earthquakes [29]. In a general way, the matrix [77]. 2117 2062 geographical distribution of landslides roughly corresponds On the slopes of the San Vicente volcano landslides were 2118 2063 to the distribution of young ash, tuff, and tephra deposits on reported along the El Muerto and El Blanco creeks. The El 2119 2064 2120 steep slopes, incised valley walls and river channels. Muerto landslide was estimated to have mobilised around 2065 2121 Landslides were also reported to occur in Guatemala [76]. 700,000–800,000 m3 of andesitic rock blocks, whereas the 2066 2122 Slides blocked roads between Quesada and Monte Verde El Blanco landslide mobilised silty and sandy gravels and 2067 2123 and between Moyuta and El Obraje in the Jutiapa District. blocks coming from pyroclastic flows. This slide becomes a 2068 2124 Landslides wereUNCORRECTED also reported along the Guatemala-Mexico latent hazard against PROOF the Tepetitan town, which was flooded 2069 2125 and Quetzaltenango–Retalhuleu roads, and along the road in 1930 by a mudflow resulting in four deaths. New 2070 2126 to Ixtahuacan, Solola. landslides were also reported around the Lake Ilopango 2071 2127 The 13 February earthquake triggered additional land- [77]. 2072 2128

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

20 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

2129 Table 5 2185 2130 Annual average rainfalls (mm) at selected meteorological stations 2186 2131 Year Ilopango Santiago de Marıá La Unioń San Miguel Ahuachapań Acajutla Puente Cuscutlań 2187 2132 2188 2133 1998 1958 2338 2123 1648 1623 2280 2037 2189 2134 1999 1504 1902 1859 1470 1554 1953 1303 2190 2135 2000 1454 1890 1783 1543 1052 1761 1637 2191 2136 2192 2137 Analysis of SPOT image data after the earthquake of 13 damage. The most serious effects of lateral spreading 2193 2138 of January 2001 with 10 m ground resolution, reveals many occurred on the banks of the Lempa River at San Nicolas 2194 2139 flowslides in the Balsamo Cordillera. A similar scene Lempa that resulted in collapse of a railway bridge (Figs. 19 2195 2140 collected after the earthquake of 13 February shows that and 20). 2196 2141 many of these landslides have expanded in size either as a 2197 2142 result of aftershocks from the first earthquake, or from the 5.2. Damage to housing 2198 2143 effects of the second, much closer, event. 2199 2144 The susceptibility of slopes to earthquake-induced The initial estimates by the Committee for National 2200 2145 instability has been shown to be strongly dependent on the Emergency (COEN) of the Salvadorian government of the 2201 2146 rainfall in the months and weeks prior to the seismic event number of homes destroyed by the earthquake was about 2202 2147 [33]. Although ACPC [37] reports that the 2000/2001 coffee 150,000, with another 185,000 damaged. There has been 2203 2148 harvest was delayed due to wet weather, average annual some debate regarding the damage statistics, with reports 2204 2149 precipitations reported by the Meteorlogy Department of the that the COEN figures are overestimated [2], but all sources 2205 2150 Salvadorian Ministry of Agriculture (MAG) indicate that agree that more than one million people were made 2206 2151 rainfalls for the year 2000 were in fact slightly low in many homeless by the earthquakes. The overwhelming majority 2207 2152 parts of the country, at least compared to the previous 2 of the damaged houses were of adobe and bahareque, with 2208 2153 years (Table 5), although it should be noted that 1998 was an the former being the most susceptible type of housing. 2209 2154 exceptional year because of Hurricane Mitch. Timber frames and reinforced masonry houses performed 2210 2155 The hazard of rainfall-induced landslides in the rainy significantly better and it was not uncommon to visit 2211 2156 season (normally starting in April or May) following the locations where most adobe houses were in a state of at least 2212 2157 earthquakes became a major concern. On 19 September partial collapse whereas reinforced masonry houses were 2213 2158 1982, after a rainfall of 223 mm in less than 2 days, a practically unscathed. 2214 2159 landslide began to move on the slopes of San Salvador In the rural area of El Salvador the dwelling construction 2215 2160 Volcano (El Picacho) and then descended rapidly into the types mostly used are adobe, bahareque, reinforced brick 2216 2161 densely populated neighbourhood of Montebello. The slide masonry (mixto ), wood frames cover by thin metal sheets 2217 3 2162 had an estimated volume of 200,000 m and killed an (lamina ), and wood frames cover by palm fronds (ranchos ). 2218 2163 estimated 500 people, leaving another 2400 homeless Other building practices, which are less widely used, 2219 2164 (CEPRODE, 1994). This slide happened exactly three include concrete and soil-cement block masonry using 2220 2165 months after an Mw 7.3 subduction earthquake, which is soil-cement blocks, and steel frames cover by precast walls 2221 2166 reported to have caused extensive cracking on slopes. (Fig. 21). 2222 2167 Extensive cracking along ridges, especially along the road Roofs of adobe houses may be of metal sheets and/or 2223 2168 to Comasagua in the Cordillera del Ba´lsamo, caused by the clay tiles supported by wood trusses or thatched roof 2224 2169 13 January earthquake led to concerns that a similar supported on wood timber purlins. Load transfer between 2225 2170 sequence of events might follow in the 2001 rainy season. the roof and walls, or between walls, is often not effective. 2226 2171 However, the hazard did not materialise during the first This building system has high mass and stiffness but low 2227 2172 months of the rainy season since rainfall levels were strength. 2228 2173 exceptionally low, to the point of creating drought and Bahareque consists of timber vertical elements and 2229 2174 consequently severe problems with water supply and horizontal timber, cane or bamboo elements, infilled with 2230 2175 agriculture. Nonetheless, heavy rainfalls have occurred mud and finished with plaster. The seismic resistance of 2231 2176 since the earthquakes and a large mud and debris flow was bahareque depends primarily on the condition of the timber 2232 2177 triggered on the lower slopes of the San Vicente and cane elements, having low vulnerability when carefully 2233 2178 (Chichontepec) volcano on 15 September 2001. maintained. Bahareque is a more expensive building system 2234 2179 Liquefaction was observed at various locations along the than adobe. Roofs are similar to those for adobe and show the 2235 2180 coast in centralUNCORRECTED and eastern El Salvador, accompanied by same problems. PROOFMixto is composed of fired clay bricks with 2236 2181 lateral spreading and consequent damage to some houses. mortar and slender elements of concrete with thin steel 2237 2182 Similar observations were made on the shores of Lake reinforcement, or the same thickness as the wall, which are not 2238 2183 Ilopango, where lateral spreading was significant and some properly reinforced concrete and are known as nervios (nerves 2239 2184 houses were rendered uninhabitable due to foundation or tendons). This system, in which the load bearing system is 2240

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 21

2241 2297 2242 2298 2243 2299 2244 2300 2245 2301 2246 2302 2247 2303 2248 2304 2249 2305 2250 2306 2251 2307 2252 2308 2253 2309 2254 2310 2255 2311 2256 2312 2257 2313 2258 2314 2259 2315 2260 2316 2261 2317 2262 2318 2263 2319 2264 2320 2265 2321 2266 2322 2267 2323 2268 2324 2269 2325 Fig. 19. Lateral spreading on river bank at San Nicolas Lempa. 2270 2326 2271 2327 2272 2328 2273 2329 2274 2330 2275 2331 2276 2332 2277 2333 2278 2334 2279 2335 2280 2336 2281 2337 2282 2338 2283 2339 2284 2340 2285 2341 2286 2342 2287 2343 2288 2344 2289 2345 2290 2346 2291 2347 2292 UNCORRECTED PROOF 2348 2293 2349 2294 2350 2295 2351 2296 Fig. 20. Collapse of railway bridge due to lateral spreading on banks of the Lempa River. 2352

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

22 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

2353 2409 2354 2410 2355 2411 2356 2412 2357 2413 2358 2414 2359 2415 2360 2416 2361 2417 2362 2418 2363 2419 2364 2420 2365 2421 2366 2422 2367 2423 2368 2424 2369 2425 2370 2426 2371 2427 2372 2428 2373 2429 2374 2430 2375 2431 2376 2432 2377 2433 2378 2434 2379 Fig. 21. Collapsed adobe house in San Agustıń, of which only the door remains standing after the 13 January earthquake; behind are houses of mixto and 2435 2380 lamina, which have survived the earthquake. 2436 2381 2437 provided by the masonry walls, has relatively good seismic located in the metropolitan area of San Salvador, and mainly 2382 2438 resistance but is considerably more expensive than both adobe in the cities of San Salvador and Nueva San Salvador (Santa 2383 2439 and bahareque. Lamina is the name given to buildings of Tecla). When seismic design has been considered at all, the 2384 2440 timber or metal frames covered by thin metal sheets, usually approach has generally been focused on the ultimate limit 2385 2441 founded on a 50 cm high block wall. Lamina has good seismic state. Most damage in engineered structures due to the 13 2386 2442 resistance due to its low weight and mass. Wood frames 2387 January earthquake was non-structural, with damage to 2443 covered by palm fronds have excellent seismic response 2388 partition walls, ceilings, A/C ducts and windows. Many 2444 characteristics but this building system is rapidly disappearing 2389 major hospitals were put out of service because they lost 2445 2390 due to scarcity of materials. The construction systems most their functionality and non-structural damage needed to be 2446 2391 severely affected by both the January and February earth- repaired before they could be used again. Several buildings 2447 2392 quakes were adobe and bahareque. that suffered light damage in the 1986 earthquake and were 2448 2393 The damage patterns clearly revealed the social vulner- not adequately repaired suffered some damage due to the 2449 2394 ability of poor forced to live in susceptible locations and 2001 earthquakes. One structure that had been badly 2450 2395 vulnerable houses. Small towns such as San Agustıń(Fig. damaged in 1986, a building housing a hair salon next to 2451 2396 20), where 80% of the houses were made from adobe, were the Externado de San Jose´ High School (itself destroyed in 2452 2397 particularly hard hit. The same pattern was visible in small the 1986 earthquake and subsequently rebuilt), withstood 2453 2398 hamlets and villages, where adobe was even more dominant the shock without collapse, although the owners have since 2454 2399 and where the quality of construction was generally poor. taken the decision to demolish. 2455 2400 Even in San Salvador, where damage to engineered There are only a few buildings in San Salvador that stand 2456 2401 structures was very limited, extensive damage was observed more than 70 m height, most of which behaved very well 2457 2402 in shanty dwellings such as in the Jose´ Cecilio del Valle area during the earthquakes. One of these, the Torre Cuscutlań 2458 2403 adjacent to the exclusive Escaloń neighbourhood, due to (formlerly known as Torre de Democracia), located in the 2459 2404 both shaking and to movements on slopes. south west of San Salvador, is an irregular tower with 2460 UNCORRECTEDexternal glass walls;PROOF none of these were broken. One high- 2405 2461 2406 5.3. Damage to engineered structures rise structure did suffer some internal damage, the 2462 2407 Ministerio del Interior building in the centre of San 2463 2408 The majority of engineered structures in El Salvador are Salvador, which was also damaged by the 1986 earthquake. 2464

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 23

2465 2521 2466 2522 2467 2523 2468 2524 2469 2525 2470 2526 2471 2527 2472 2528 2473 2529 2474 2530 2475 2531 2476 2532 2477 2533 2478 2534 2479 2535 2480 2536 2481 2537 2482 2538 2483 2539 2484 2540 2485 2541 2486 2542 2487 2543 2488 2544 2489 2545 2490 2546 Fig. 22. Guadelupe following the 13 February earthquake, contrasting total collapse of adobe houses with the almost undamaged state of the reinforced 2491 2547 masonry building to the right. 2492 2548 2493 There are a few other examples of structures having suffered earthquake. In the towns of Guadalupe (Fig. 22), Verapaz 2549 2494 severe damage, such the Regis Condominium in the San and Santa Cruz Analquito, which are located very close to 2550 2495 Jacinto area of south San Salvador. Most structures that the assumed fault rupture, there was total collapse of many 2551 2496 suffered some damage are of reinforced concrete; steel houses built from adobe and bahareque, but mixto 2552 2497 structures behaved well. (reinforced masonry) constructions generally survived 2553 2498 Outside the capital there are few engineered structures intact. Even structures that had been weakened by both 2554 2499 compared to San Salvador. The hospitals in Usulutań and the 1999 seismic swarm and the 13 January earthquake, 2555 2500 2556 San Miguel were left non-operational due to non-structure such as the church in the town of San Estebań Catarina, did 2501 2557 damage. Public buildings erected in the 1960’s were not suffer as much damage as might have been expected. In 2502 2558 severely damaged in Santiago de Marıá. Another case of the town of Apastepeque, close to the source of the 13 2503 2559 damage outside of the capital city was El Salvador’s February earthquake and badly affected by the 1999 swarm, 2504 2560 International Airport located near Comalapa on the coastal residents reported that the most severe effect of the 2505 2561 plain. The airport suffered important non-structural damage earthquake was to dislodge roof tiles. That a crustal 2506 2562 including cracking of infill walls, breakage of windows and earthquake of M 6.6 did not cause greater levels of 2507 w 2563 collapse of ceilings, as well as some minor cracking in damage in reinforced masonry nor in some cases in 2508 2564 2509 columns and beams in the older sections of the airport weakened adobe buildings, suggests that the earthquake 2565 2510 buildings. was less superficial than is typical of volcanic chain 2566 2511 The 13 February earthquake damaged some engineered earthquakes such as those in Jucuapa–Chinameca in 1951 2567 2512 structures mainly in the central area of El Salvador. The city and in San Salvador in 1986. 2568 2513 that shows most of this kind of destruction is San Vicente, In terms of the cultural heritage of El Salvador, there are 2569 2514 where at least two schools, one of them built in the early relatively few examples of colonial architecture surviving in 2570 2515 1970s, have suffered severed damage; one branch of the many parts of the country, indeed in San Salvador all 2571 2516 Universidad de El Salvador suffered non-structural damage buildings from the colonial period have been destroyed by 2572 that limited itsUNCORRECTED function. In addition, this earthquake fire or earthquake. PROOF The earthquakes caused damage to more 2517 2573 2518 damaged the Zacatecoluca hospital, which is a twin than 400 churches in El Salvador. 2574 2519 structure of the Usulutan Hospital. However, in general An important question that immediately presents itself is 2575 2520 damage levels were low for the size and location of the why the damage to engineered structures, particularly 2576

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

24 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

2577 during the 13 January earthquake, was so limited? Even at country, used predominantly for transportation of cargo 2633 2578 the Health Centre in La Libertad, where the maximum 5% rather than passengers. The eastern line connecting Cutuco 2634 2579 damped spectral acceleration exceeded 4.5g, damage was has not been operational for many years. The only damage 2635 2580 limited to the fall of part of the ceiling (non-structural) and to the railway system was the collapse of the steel arch truss 2636 2581 minor cracks in an external wall. Comparison of accel- bridge at San Nicolas Lempa due to lateral spreading. 2637 2582 erograms obtained in San Salvador during the 1982 The two main highways in El Salvador run across the 2638 2583 subduction-zone and 1986 upper-crustal earthquakes pro- country from east to west. The Panamerican Highway 2639 2584 vides insight into possible reasons, since the latter event (CA-1) runs along the Great Interior Valley; it was 2640 2585 caused significantly higher levels of damage in engineered originally constructed to serve the coffee industry. The 2641 2586 structures, despite the fact that the response spectral shapes second major artery is the coast road (CA-2) whose original 2642 2587 were not very dissimilar, hence the frequency content of the purpose was to serve the cotton plantations that previously 2643 2588 motions is unlikely to provide the explanation. The 1982 occupied the coastal plains. Transport on both roads was 2644 2589 and 1986 accelerograms were found, however, to contain severely disrupted by landslides. The coast road between the 2645 2590 almost identical levels of energy, as measured by the Arias ports of La Libertad and Acajutla in the west was partially 2646 2591 intensity but with very different durations, so that the rate of blocked by a number of rock falls and relatively small 2647 2592 energy input was an order of magnitude greater in the 1986 landslides; the five tunnels on this section of motorway were 2648 2593 earthquake [50]. The total energy input, which was actually undamaged apart from minor cracks in their lining. The 2649 2594 higher in the January 2001 earthquake than for the 1982 and Panamerican Highway was completely blocked by major 2650 2595 1986 records, is a good indicator of the damage potential in landslides both east and west of San Salvador for several 2651 2596 brittle and degrading materials such as adobe and volcanic weeks. To the west, major slides at Los Chorros blocked the 2652 2597 soils. It would appear that for damage to be inflicted on road and even after several weeks traffic was only able to 2653 2598 engineered structures it is necessary that the motion has both circulate in one direction, with vehicles entering the capital 2654 2599 a high energy content and a high rate of energy input, as in the morning and leaving in the afternoon. East of San 2655 2600 indicated by the root-mean-square acceleration. Salvador the highway was completely blocked in both 2656 2601 directions by the huge slide, re-activated by the 13 February 2657 2602 5.4. Performance of lifelines earthquake, at Las Leonas, obliging traffic to use the old and 2658 2603 practically abandoned road running approximately parallel 2659 2604 The performance of lifelines in the two earthquakes has to the north. 2660 2605 been reported in detail by Lund [79] and EERI [80]. The motorway joining San Salvador and the international 2661 2606 Telecommunications were not seriously affected and service airport at Comalapa on the coastal plain was damaged by 2662 2607 was fully restored in the capital within one day of the first cracks at several locations and during several weeks traffic 2663 2608 event. Electricity generation was not seriously affected but was reduced to a single lane in each direction over part of 2664 2609 the distribution system was affected by a large number of the road. The airport was closed for one day following the 2665 2610 transmission lines broken by landslides. There are no gas 13 January earthquake to allow clearing up of debris and 2666 2611 distribution lines in El Salvador since all household use inspections of buildings and runways. 2667 2612 imported propane distributed in canisters. The diesel and 2668 2613 petrol refinery in the port of Acajutla was not damaged and 2669 2614 production was not interrupted by the earthquakes. 6. Implications for seismic risk: physical, social and 2670 2615 The distribution of potable and waste water in El institutional vulnerability 2671 2616 Salvador is managed by the state-owned company ANDA. 2672 2617 The earthquakes caused disruption to the water distribution The 2001 earthquakes have revealed the extreme levels 2673 2618 system but breakage of pipes was limited; for example, only of vulnerability to natural hazards that exist in El Salvador. 2674 2619 three repairs in the northern area supply line in San Salvador Moreover, the failure to mitigate earthquake risk in El 2675 2620 were reported by ANDA. The most serious disruption to the Salvador is a reflection of institutional vulnerabilities that 2676 2621 water distribution system was the damage caused by the 13 have not been addressed; chief among these are the 2677 2622 February earthquake to the treatment plant at Cacahuatal capacities for emergency response, monitoring of natural 2678 2623 that supplies the San Vicente area. Although the disruption hazards, land-use planning, and seismic design and its 2679 2624 to the water distribution system by the earthquakes was enforcement. 2680 2625 limited, it is worth noting that even under normal conditions 2681 2626 there are problems with water distribution in El Salvador, 6.1. Emergency response 2682 2627 with chronic shortages and few households, even in urban 2683 2628 areas, have uninterruptedUNCORRECTED water supply 24 h a day. Some observers PROOF have claimed that the government 2684 2629 The most seriously affected lifelines were transport lines. response to the disaster in El Salvador has been poorly 2685 2630 There are three railway lines in El Salvador, connecting the organised and in particular that the lessons from Hurricane 2686 2631 ports of Acajutla and Cutuco (La Unioń) and the cement Mitch were clearly not learnt [81]. Although this study is not 2687 2632 production plants in Metapań in the northwest of the primarily concerned with emergency aid following the 2688

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 25

2689 earthquakes, there were some obvious shortcomings, at least minimal requirements in terms of additional costs and 2745 2690 in the initial phases of the response. For example, most aid building skills. There is clearly a need, however, for a 2746 2691 arriving at Comalapa International Airport, located on the transfer of this knowledge to the most isolated and vulnerable 2747 2692 coastal plain, was transported almost 30 km to the Feria rural communities where these forms of housing are most 2748 2693 Internacional in San Salvador for centralised logging and abundant and also where they are built with the highest levels 2749 2694 thence distributed to affected areas, several of which were of susceptibility. Amongst the many obstacles to this effective 2750 2695 within 1 hour’s drive of the airport. Our visits to badly mitigation are the relatively high rate of illiteracy in rural 2751 2696 affected rural areas generally indicated that government areas and the lack of confidence in adobe construction 2752 2697 assistance, in the first few weeks of the crisis, was not following its poor performance in the 2001 earthquakes. 2753 2698 getting through to many of the earthquake victims, 2754 2699 particularly in more remote rural areas. 6.3. Land use planning 2755 2700 The main response to the emergency seems to have been 2756 2701 provided by the affected people themselves, although The high level of landslide hazard in El Salvador makes 2757 2702 important contributions by NGOs and others, including land-use planning an issue of great importance. The high 2758 2703 contingents of the Venezuelan Armed Forces and, changing population density of El Salvador and the housing deficit 2759 2704 their historical role, the Salvadorian Armed Forces as well, also makes it a sensitive and controversial issue. There is 2760 2705 should not be overlooked. Despite the huge numbers of currently almost no effective control over land develop- 2761 2706 people made homeless by the earthquakes, there were very ment. It is interesting to note that the landslide hazard map 2762 2707 few examples of victims living in temporary shelters in the shown in Fig. 2 clearly indicates that the area affected by the 2763 2708 streets of the cities, as there were after the 1986 earthquake. catastrophic landslide at Las Colinas was identified as being 2764 2709 Most rural communities, except where affected by land- of high hazard. The hazard map was prepared some years 2765 2710 slides, appear to have remained to rebuild their homes and after Las Colinas was developed in 1985, but nonetheless no 2766 2711 continue with their lives. Middle class people made remedial action was taken to stabilise the slopes or to protect 2767 2712 homeless, such as those from Las Colinas and adjacent the area from landslides. The irrelevance of geohazards in 2768 2713 neighbourhoods, were either absorbed by relatives or added planning decisions is very clearly demonstrated by a recent 2769 2714 to the exodus to the USA. May 2001 saw the highest ever housing development to the north of San Salvador, called 2770 2715 influx of remesas into El Salvador, with a monthly total of Santı´sima Trinidad (Fig. 23). The development consists of 2771 2716 US$ 197.1 millions. several rows of four-storey apartment blocks built on 2772 2717 terraces on a natural slope with an inclination of about 2773 2718 6.2. Seismic design of buildings 358, above which three huge water tanks have been 2774 2719 constructed. The constructors apparently did not face any 2775 2720 Although damage to engineered structures was limited, serious obstacles in obtaining permission to build. 2776 2721 at least in terms of structural collapse, there is still a Despite the apparent lack of control and accountability in 2777 2722 significant danger of many large engineered structures land use planning, the earthquakes may lead to important 2778 2723 having been weakened by the earthquakes and therefore changes in this area. Following the earthquakes, 200 2779 2724 urgently requiring intervention. This is, in the majority of survivors from Las Colinas, supported by the Salvadorian 2780 2725 cases, which may be unlikely to happen given that seismic Foundation for the Application of Law (FESPAD) brought a 2781 2726 design requirements are not imposed even for new case against the State to the Supreme Court of Justice for 2782 2727 buildings. Lara [82] reports that prior to the 1986 earthquake their failure to prevent or mitigate the risk of landslides on 2783 2728 in San Salvador, the seismic design code was rarely applied, the slopes of Cerro La Gloria, which were well known and 2784 2729 and there is little evidence to suggest that the codes of 1989 identified in the PLAMADUR hazard map (Fig. 3). The case 2785 2730 and 1994 have been more widely implemented. Indeed, was unsuccessful, being dismissed by the Supreme Court; 2786 2731 although it has many technical merits, there is no effective similarly, the attempt by the Santa Tecla municipality to 2787 2732 mechanism for the imposition of the current code for prosecute the developers also failed [81]. 2788 2733 earthquake-resistant design in El Salvador [83]. More generally, after the earthquake there was a renewed 2789 2734 The current seismic design code in El Salvador has many interest in addressing hazard-related land use issues, not 2790 2735 technical merits but the lack of a credible system for its only in urban areas but in the country as a whole. However, 2791 2736 enforcement severely limits its effectiveness in mitigating it is not yet clear how this process will evolve. Certainly, 2792 2737 seismic risk. There are almost many aspects of seismic risk in both public and private sectors in El Salvador will need to 2793 2738 buildings that fall outside the remit of the code, one being make rapid advances in their risk management and 2794 2739 repair and strengthening. As noted previously, the code does evaluation practices if large-scale losses are to be avoided 2795 2740 include an appendixUNCORRECTED of guidelines for the improved earth- in future due to earthquakesPROOF or other natural hazards. 2796 2741 quake-resistant construction of adobe although this, logically, 2797 2742 does not form part of the actual regulations. These guidelines, 6.4. Seismic monitoring 2798 2743 and other publications [84], affirm that adobe buildings can be 2799 2744 constructed with a degree of earthquake resistance, with The monitoring of earthquakes, volcanoes and landslides 2800

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

26 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

2801 2857 2802 2858 2803 2859 2804 2860 2805 2861 2806 2862 2807 2863 2808 2864 2809 2865 2810 2866 2811 2867 2812 2868 2813 2869 2814 2870 2815 2871 2816 2872 2817 2873 2818 2874 2819 2875 2820 2876 2821 2877 2822 2878 2823 2879 2824 2880 2825 2881 2826 2882 2827 2883 2828 2884 2829 2885 2830 2886 2831 2887 2832 2888 2833 2889 2834 2890 2835 2891 2836 2892 2837 2893 2838 2894 2839 2895 2840 2896 2841 2897 2842 2898 2843 2899 2844 2900 2845 2901 2846 2902 2847 2903 2848 2904 2849 2905 2850 2906 2851 2907 2852 2908 Fig. 23. Santı´UNCORRECTEDsima Trinidad residential area north of San Salvador built on terraces along a steep slopePROOF (a) above which are three large water tanks (b). 2853 2909 2854 2910 2855 2911 2856 2912

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 27

2913 has traditionally been the responsibility of the Centre for America seems probable: it has been noted that the 2969 2914 Geotechnical Investigations (CIG), which is part of the subduction zone from central El Salvador to the northern 2970 2915 Ministry of Public Works. Monitoring capacities for natural Nicaragua has a far lower rate of moment release than the 2971 2916 hazards were severely weakened in the 1980s due to the war zones offshore from Guatemala and Nicaragua either side 2972 2917 and efforts to re-build and re-generate these activities since [88]. There is also evidence that destructive shallow-focus 2973 2918 have been limited. Some responses to this situation have earthquakes along the volcanic chain opposite the El 2974 2919 taken the form of independent initiatives by private Salvadorian section of the Middle America Trench are 2975 2920 institutions, a clear example of which is the digital more frequent than in Guatemala and Nicaragua. The exact 2976 2921 accelerograph network established in 1996 by the UCA. nature of the interaction and the mechanism of stress 2977 2922 The government of El Salvador has now responded to this transfer between the two seismogenic sources is, however, 2978 2923 situation by forming, in October 2001, SNET (National far from clear at this stage. 2979 2924 Service for Territorial Studies), which will unify and The large numbers of accelerograms recorded during the 2980 2925 strengthen current natural hazards monitoring capacities. two earthquakes provide a very useful basis for the 2981 2926 The structure of SNET includes four different national characterisation of strong ground-motion in Central Amer- 2982 2927 services: Geological Service (including earthquake, volca- ica, although the lack of any near-source recordings of the 2983 2928 noes and landslides); Meteorological Service; Hydrological 13 February earthquake—due to malfunction of the San 2984 2929 Service; and Risk Management Service. The United States Vicente and San Pedro Nonualco stations of the TALULIN 2985 2930 and Japan are expected to provide equipment and technical network—is an unfortunate gap in the data set. This is 2986 2931 assistance for SNET. The Spanish government has approved particularly the case because the indications from the 2987 2932 a project to expand and upgrade the existing strong-motion recorded motions of the second earthquake, and the 2988 2933 network previously managed by CIG and a convention has observed levels of damage, are that the ground motions 2989 2934 been agreed for collaboration amongst the three strong- generated were less intense than would be expected from a 2990 2935 motion networks in El Salvador. shallow earthquake of magnitude Mw 6.6, indicating either 2991 2936 very high attenuation with distance or a focus within the 2992 2937 lower part of the crust. Macroseismic observations and the 2993 2938 7. Discussion and conclusions limited strong-motion recordings from other earthquakes 2994 2939 point towards high rates of attenuation in the volcanic chain 2995 2940 The El Salvador earthquake of 13 January 2001 was the zone, as has been found elsewhere including the volcanic 2996 2941 first major earthquake disaster of the new millennium and region of the North Island of New Zealand [89]. 2997 2942 serves perhaps as a warning that in many countries of the Notwithstanding this observation, the 13 February earth- 2998 2943 Third World seismic risk is growing. The combination of quakes appears not to have been as shallow as other slightly 2999 2944 population expansion and increasing urbanisation, in the smaller but more destructive events along the volcanic chain 3000 2945 case of El Salvador with cities expanding in the zones of in El Salvador and neighbouring countries. There are 3001 2946 highest seismic hazard, together with the increasing several features of the ground motion that warrant further 3002 2947 susceptibility of the terrain to landslides, has led to research: 3003 2948 increased levels of risk both to lives and to the livelihood 3004 2949 of the country. † The differences between ground motions from crustal 3005 2950 The impact of the earthquake of 13 January was and subduction events in Central America, and the 3006 2951 compounded by the second event on 13 February, which development of separate predictive relationships for the 3007 2952 came as the aftershocks of the former event were beginning two sources of seismicity. 3008 2953 to diminish in frequency and intensity. The 13 February † The influence of site effects due to both surface geology 3009 2954 event was followed by many aftershocks, both around the and topographical features; the apparent predominance 3010 2955 crustal source of this earthquake but also offshore in the of these influences suggests that microzonation is a 3011 2956 subduction zone. This would tend to indicate interaction potentially very useful tool in El Salvador. 3012 2957 between the two earthquakes, a topic that will be the focus † The specification of earthquake loads for seismic design, 3013 2958 of future research. Interactions between earthquakes are taking account of both the different geographical 3014 2959 well recognised, with stress release in one location causing, distributions of the hazard from crustal and subduction 3015 2960 by transfer, stress increase in adjacent zones and hence earthquakes and the different natures of the resulting 3016 2961 inducing or accelerating rupture on adjacent faults or fault ground motions. 3017 2962 segments. The clearest example of such interaction is the † The relationship between the nature of the recorded 3018 2963 progression of earthquakes from 1939 to 1999 along the motion and its capacity to produce damage: it is 3019 2964 North AnatolianUNCORRECTED fault in Turkey [48,85,86]. Interactions abundantly clearPROOF that PGA is of very little significance 3020 2965 between different earthquakes has also been identified in this respect, and to some extent this is also true for 3021 2966 within subduction zones, as for example in the 1997–1998 spectral accelerations (whence the current trend towards 3022 2967 sequence in central Chile [87]. That there is interaction displacement-based approaches to assessment and 3023 2968 between subduction and crustal earthquakes in Central design). 3024

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

28 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

3025 The lack of extensive structural damage in reinforced earthquake risk reflects the fact that there are many urgently 3081 3026 concrete buildings due to these earthquakes should not be pressing needs on limited resources, exacerbated by the 3082 3027 interpreted as a vindication of the success of the current weakness of central and local government. A pessimistic 3083 3028 seismic design code in El Salvador nor its predecessors, view of the situation may conclude that earthquake risk 3084 3029 since regardless of their technical merits these codes have mitigation will only be possible following the solution of 3085 3030 generally been applied only sporadically. The lack of major other major social problems in El Salvador. An alternative 3086 3031 structural damages and collapse of large buildings appears view holds that recognition of the interaction of seismic 3087 3032 to be more closely related to the nature of the ground vulnerability with other features of vulnerability, including 3088 3033 motions generated than the quality of engineering design or institutional vulnerability, means that concerted programmes 3089 3034 construction. Particularly in San Salvador there is now a real of seismic risk mitigation could provide a vehicle and a 3090 3035 danger of complacency regarding the capacity of existing stimulus to the solution of many other issues, including the 3091 3036 buildings, despite the fact that it is widely known that many current concentration of more than half of the population in 3092 3037 buildings have been left damaged by the 10 October 1986 one-third of the national territory. El Salvador will need 3093 3038 earthquake and these may have been further weakened by external assistance, both in terms of material resources and 3094 3039 the 2001 earthquakes. Destructive moderate magnitude technology transfer, to make this vision a reality. 3095 3040 earthquakes occur in San Salvador on average every 20–25 3096 3041 years [14] and the next event could cause terrible damage 3097 3042 and loss of life in the overcrowded and expanding capital. 8. Uncited reference 3098 3043 The most devastating impact of the 2001 earthquakes has 3099 3044 been the triggering of hundreds of landslides in volcanic soils, [78]. 3100 3045 which have buried houses and blocked roads, causing most of 3101 3046 the deaths in these earthquakes and bringing massive 3102 3047 disruption: the Pan-American Highway remained closed for Acknowledgments 3103 3048 more than 10 months due to the landslide at Las Leonas. The 3104 3049 number of landslides triggered by these earthquakes, the size Field investigations of the 13 January earthquake were 3105 3050 of the slides and their geographical distribution, all indicate funded by the Natural Environment Research Council 3106 3051 increasing susceptibility of the terrain when compared to (NERC), the Royal Academy of Engineering and the 3107 3052 patterns in previous earthquakes, with no indication that this Universidad Nacional de Colombia. Records from GESAL 3108 3053 was due to precedent rainfall. The hazard of earthquake- and instrument in BerlıńwereprovidedbyJavierRivas, 3109 3054 rainfall-induced landslides in the volcanic soils that dominate Salvador Handal Candray and Jose´ Antonio Rivas; Griselda 3110 3055 much of El Salvador, and particularly the most densely Marroquıń kindly provided the records from Nicaragua. 3111 3056 populated areas, urgently requires attention. The identification Jose´ Mauricio Cepeda of the Universidad Centroamericana 3112 3057 of zones of high landslide hazard is an important component of provided useful assistance in deciphering incorrect time 3113 3058 any programme of mitigation, but relocation to lower hazard codes on records from the TALULIN network and in 3114 3059 zones will often not be an option in this densely populated providing additional information about the performance of 3115 3060 country with a long history of conflicts over land ownership. the network during the earthquakes. We are grateful to 3116 3061 Stabilisation measures cannot necessarily be imported from Dominic Dowling of the University of Technology, Sydney, 3117 3062 regions of the world with entirely different soil characteristics, for his very thorough review of the manuscript. We also 3118 3063 since one of the distinguishing features of volcanic soils such wish to express our sincere thanks to all the people in El 3119 3064 as tierra blanca is the complete loss of cementation at small Salvador who, despite the situation into which they had been 3120 3065 strains, followed by the collapse of its matrix structure and a thrown, generously assisted us in our field studies of the 3121 3066 drastic loss of strength [68]. One of the most important fields earthquakes: their fortitude, resilience and vitality are El 3122 3067 of research in El Salvador is the engineering characterisation Salvador’s most valuable resources in tackling the problems 3123 3068 of the tierra blanca, in order to model its behaviour in slopes posed by natural hazards. 3124 3069 subjected to rainfall and due to earthquake shaking, and 3125 3070 similarly to model its modified behaviour after the application 3126 3071 of different stabilisation techniques. References 3127 3072 Seismic risk in El Salvador clearly cannot be viewed in 3128 3073 complete isolation from other risks, including those due to [1] Zapata R. ECLAC: summary of the damage caused by the earthquakes 3129 3074 other natural hazards such as floods and volcanic eruption but of 13 January and 13 February in El Salvador. ISDR Informs 2001;3: 3130 3075 also anthropogenic risks such as pollution, deforestation, 12–17. 3131 3076 crime, poverty, disease and social conflict. The failure to [2] Acevedo C, Romano L. Economıá, Desastre y Desarrollo Sostenible. 3132 UNCORRECTEDSan Salvador, PROOF El Salvador: FLACSO; 2001. 3077 tackle the challenges of seismic risk, or even to hold back its 3133 [3] Dewey JW, Sua´rez G. Seismotectonics of Middle America. In: 3078 increasing levels, is not due to any lack of awareness amongst Slemmons, et al., editors. Neotectonics of North America. Geological 3134 3079 Salvadorians of the very high earthquake hazard that affects Society of America Decade Map, Vol. 1. GSA; 1991. p. 309–21. 3135 3080 their country. Rather the lack of effective measures against [4] Ambraseys NN, Adams RD. The Seismicity of Central America: A 3136

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx 29

3137 Descriptive Catalogue 1898–1995. London: Imperial College Press; [26] Weyl R. Geology of Central America. Berlin: Gebruder Borntraeger; 3193 3138 2001. 1980. 3194 [5] Larde´ J. El terremoto del 6 de Septiembre de 1915 y los dema´s [27] Schmidt-Thome´ M. The geology in the San Salvador (El Salvador, 3139 3195 terremotos de El Salvador. Obras Completas, San Salvador, El Central America), a basis for city development and planning. Geol Jb 3140 Salvador: Ministerio de Cultura; 1960. B 1975;13:207–28. Hannover, Germany. 3196 3141 [6] Alvarez S de J. Informe tećnico sobre apsectos sismolo´gicos del [28] Bommer J, Rolo R, Meńdez P. Propiedades mecańicas de la tierra 3197 3142 terremoto en El Salvador del 19 de Junio de 1982. San Salvador, El blanca y la inestabilidad de taludes. Rev ASIA 1998;129:15–21. San 3198 ´ 3143 Salvador: Centro de Investigaciones Geotećnicas, Ministerio de Obras Salvador, El Salvador: Asociacion Salvadorenã de Ingenieros y 3199 Pu´blicas; 1982. Arquitectos. 3144 3200 [7] Lara MA. The El Salvador earthquake of June 19, 1982. EERI [29] Bommer JJ, Rodrı´guez CE. Earthquake-induced landslides in Central 3145 Newslett 1983;17(1):87–96. Oakland, CA: Earthquake Engineering America. Engng Geol 2002;63(3/4):189–220. 3201 3146 Research Institute. [30] Keefer DK. Landslides caused by earthquakes. Geol Soc Am Bull 3202 3147 [8] White RA, Harlow DH. Destructive upper-crustal earthquakes of 1984;95:406–21. 3203 3148 Central America since 1900. Bull Seismol Soc Am 1993;83:1115–42. [31] Rodrı´guez CE, Bommer JJ, Chandler RJ. Earthquake-induced land- 3204 slides: 1980–1997. Soil Dynam Earthquake Engng 1999;18:325–46. 3149 [9] Levin SB. The Salvador earthquakes of December, 1936. Bull 3205 Seismol Soc Am 1940;30:1–45. [32] Montessus de Balore F. La Geólogie Sismologique: Les Tremble- 3150 3206 [10] Ambraseys NN, Bommer JJ, Buforn E, Udıás A. The earthquake ments de Terre. Paris: Libraire Armand Cilin; 1924. 3151 sequence of May 1951 at Jucuapa, El Salvador. J Seismol 2001;5(1): [33] Rodrı´guez CE. Hazard assessment of earthquake-induced landslides 3207 3152 23–39. on natural slopes. PhD Thesis, University of London; 2001. 3208 3153 [11] Lomnitz C, Schulz R. The San Salvador earthquake of May 3, 1965. [34] Rymer MJ, White RA. Hazards in El Salvador from earthquake- 3209 induced landslides. In: Brabb, Harrod, editors. Landslides: extent and 3154 Bull Seismol Soc Am 1966;56:561–75. 3210 [12] Bommer J, Ledbetter S. The San Salvador earthquake of 10th October economic significance. Rotterdam: Balkema; 1989. p. 105–9. 3155 3211 1986. Disasters 1987;11(2):83–95. [35] Rosa H, Barry D. Poblacioń, territorio y medio ambiente en El 3156 3212 [13] EERI, The San Salvador earthquake of October 10, 1986. Earthquake Salvador. Boletıń PRISMA 1995; No. 11, San Salvador, El Salvador: 3157 Spectra 1987;3(3):415–634. Programa Salvadorenã de investigacioń sobre Desarrollo y Medio 3213 3158 [14] Harlow DH, White RA, Rymer MJ, Alvadrado S. The San Salvador Ambiente. 3214 [36] Bommer JJ, McQueen C, Salazar W, Scott S, Woo G. A case study of 3159 earthquake of 10 October 1986 and its historical context. Bull Seismol 3215 the spatial distribution of seismic hazard (El Salvador). Natural Soc Am 1993;83(4):1143–54. 3160 Hazards 1998;18:145–66. 3216 [15] White RA. Tectonic implications of upper-crustal seismicity in 3161 [37] ACPC. Coffee Market Report Number 21, May, Association of Coffee 3217 Central America. In: Slemmons, et al., editors. Neotectonics of North Producing Countries; 2001. 3162 America,. Geological Society of America Decade Map, Vol. 1. GSA; 3218 [38] ACES, Monografıá del Cafe´: dos siglos de historia en la cafecultura de 3163 1991. p. 323–38. 3219 El Salvador. San Salvador, El Salvador: Asociacioń Cafetalera de El 3164 [16] Espinosa AF, editor. The Guatemala earthquake of February 4, 1976: 3220 Salvador; 1999. a preliminary report. US Geological Survey Professional Paper 1002; 3165 [39] Coburn, A., Spence, R., Earthquake Protection. Chichester, England: 3221 1976. 3166 Wiley, 1992. 3222 [17] Alfaro CS, Kiremidjian AS, White RA. Seismic zoning and ground 3167 [40] Na´be˘lek J. Determination of earthquake source parameters from 3223 motion parameters for El Salvador. Report No. 93, The John A. Blume inversion of body waves. PhD Thesis, MIT, Cambridge, MA; 1984. 3168 Earthquake Engineering Center, Stanford University; 1990. 3224 [41] Na´be˘lek J. Geometry and mechanism of faulting of the 1980 El 3169 [18] Algermissen ST, Hansen SL, Thenhaus PC. Seismic hazard evaluation 3225 Asnam, Algeria, earthquake from inversion of teleseismic body waves for El Salvador. Report for the US Agency for International 3170 and comparison with field observations. J Geophys Res 1985;90(12): 3226 3171 Development; 1988. 12713–28. 3227 [19] Singh SK, Gutierrez C, Arboleda J, Ordaz M, Peligro sı´smico en El 3172 [42] Abe K. Mechanisms and tectonic implications of the 1966 and 1970 3228 Salvador. Universidad Nacional Autońoma de Me´xico, Mexico DF; 3173 Peru earthquakes. Phys Earth Planetary Interiors 1972;5:367–79. 3229 1993. [43] Burbach GV, Frolich C, Pennington WD, Matumoto T. Seismicity and 3174 3230 [20] Lindholm C, Rojas W, Bungum H, Dahle A, Camacho E, Cowan H, tectonics of the subducted Cocos plate. J Geophys Res 1984;89: 3175 Laporte M. New regional seismic zonation for Central America. 7719–35. 3231 3176 Proceedings Fifth International Conference on Seismic Zonation, [44] Lomnitz C, Rodrı´guez Elizarrara´s S. El Salvador 2001: earthquake 3232 3177 Nice 1995;1:437–44. disaster and disaster preparedness in a tropical volcanic environment. 3233 [21] Villagran M. Problems related to seismic hazard in Central America: 3178 Seismol Res Lett 2001;72(3):346–51. 3234 magnitude estimation, attenuation of seismic waves, seismic hazard [45] IGN, Mapa Geolo´gico de la Repu´blica de El Salvador/Ame´rica 3179 3235 assessment for Guatemala city and site response. Master of Science Central 1:100,000. Bundesantalt fu¨r Bodenforschung, San Salvador, 3180 Thesis, Institute of Solid Earth Physics, University of Bergen, El Salvador: Instituto Geogra´fico Nacional; 1978. 3236 3181 Norway; 1995. [46] Alvarenga E, Barquero R, Boschini I, Escobar J, Fernańdez M, Mayol 3237 3182 [22] Bommer JJ, Hernańdez DA, Navarrete JA, Salazar WM. Seismic P, Havskov J, Ga´lvez N, Hernańdez Z, Ottemo¨ller L, Pacheco P, 3238 hazard assessments for El Salvador. Geofı´sica Internacional 1996; 3183 Redondo C, Rojas W, Vega F, Talavera E, Taylor W, Tapia A, 3239 35(3):227–44. Tenorio C, Toral J. Central America Seismic Center (CASC). Seismol 3184 3240 [23] Rosenblueth E, Prince J. El temblor de San Salvador, 3 de mayo 1965: Res Lett 1998;69(5):394–9. 3185 ingenierıásı´smica. Ingenierıá, vol. 36. Mexico DF: UNAM; 1966. p. [47] Wells DL, Coppersmith KJ. New empirical relationships among 3241 3186 31–58. magnitude, rupture length, rupture width, rupture area, and surface 3242 3187 [24] Bommer J. Cargas sı´smicas para el disenõ estructural en El Salvador. displacement. Bull Seismol Soc Am 1993;83(4):974–1002. 3243 3188 Rev ASIA 1999;132:15–25. San Salvador, El Salvador: Asociacioń [48] Stein RS, Barka AA, Deitrich JH. Progressive failure on the North 3244 SalvadorenãUNCORRECTED de Ingenieros y Arquitectos. Anatolian fault sincePROOF 1939 by earthquake stress triggering. Geophys J 3189 3245 [25] Bommer JJ, White N. Una propuesta para un me´todo alternativo de Int 1997;128:594–604. 3190 zonificaciońsı´smica en los paı´ses de Iberame´rica. Proceedings of [49] Carr MJ. Underthrusting and Quaternary faulting in northern Central 3246 3191 Segundo Congreso Iberoamericano de IngenierıáSı´smica, Madrid; America. Geol Soc Am Bull 1976;87:825–9. 3247 3192 2001. [50] Bommer JJ, Udıás A, Cepeda JM, Hasbun JC, Salazar WM, Sua´rez A, 3248

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5 ARTICLE IN PRESS

30 J.J. Bommer et al. / Soil Dynamics and Earthquake Engineering xx (xxxx) xxx–xxx

3249 Ambraseys NN, Buforn E, Cortina J, Madariaga R, Meńdez P, [70] Devoli G, Egger C, Ferres D, Rubio J. Estudio geolo´gico preliminary 3305 Mezcua J, Papastamatiou D. A new digital accelerograph network for 3250 de la ladera norte de la Sierra del Balsamo, Santa Tecla, Departamento 3306 El Salvador. Seismol Res Lett 1997;68(3):426–37. de La Libertad, El Salvador. Geo´logos del Mundo; 2001. 3251 3307 [51] Faccioli E, Battistella C, Alemani P, Tibaldi A. Seismic microzoning [71] Jibson RW, Crone AJ. Observations and recommendations regarding 3252 investigations in the metropolitan area of San Salvador, El Salvador, landslide hazards related to the January 13, 2001 M 7.6 El Salvador 3308 3253 following the destructive earthquake of October 10, 1986. Proceed- earthquake. US Geological Survey Open-File Report 01-141; 2001. 3309 3254 ings International Seminar on Earthquake Engineering, Innsbruck; [72] Mendoza MJ, Dominguez L, Melara EE. Deslizamientos y flujo de 3310 3255 1988. p. 28–65. tierras en la ladera Las Colinas, Nueva San Salvador, El Salvador, 3311 [52] Youngs RR, Chiou SJ, Silva WJ, Humphrey JR. Strong ground motion C.A., disparado por el sismos del 13 de Enero de 2001. Proceedings of 3256 3312 attenuation relationships for subduction zone earthquakes. Seismol the Second Iberoamerican Conference on Earthquake Engineering, 3257 3313 Res Lett 1997;68(1):58–73. Madrid; 2001. p. 771–80. 3258 [53] Joyner WB, Boore DM. Peak horizontal acceleration and velocity [73] Bernal A. Establidad de taludes en terremotos: el deslizamiento de Las 3314 3259 from strong-motion records including records from the 1979 Imperial Colinas, El Salvador, en el terremoto del 13 de Enero de 2001. 3315 Valley, California, earthquake. Bull Seismol Soc Am 1981;71: 3260 Proceedings of the Second Iberoamerican Conference on Earthquake 3316 2011–38. Engineering, Madrid; 2001. p. 781–91. 3261 ´ 3317 [54] Taylor Castillo W, Santos Lopez P, Dahle A, Bungum H. Digitization [74] Harp EL, Wilson RC. Shaking intensity thresholds for rock falls and 3262 of strong-motion data and estimation of PGA attenuation. NORSAR 3318 slides: evidence from Whittier Narrows and Superstition Hills 3263 Technical Report 2–4; 1992. 3319 earthquake strong-motion records. Bull Seismol Soc Am 1995;85: [55] Dahle A, Climent A, Taylor W, Bungum H, Santos P, Ciudad Real M, 3264 1739–57. 3320 Lindholm C, Strauch W, Segura F. New spectral strong motion 3265 [75] Murphy W, Petley DN, Bommer JJ, Mankelow JM. Uncertainty in 3321 attenuation models for Central America. Proceedings of the Fifth ground motion estimates for the evaluation of slope stability during 3266 International Conference on Seismic Zonation, Nice 1995;II:1005–12. 3322 earthquakes. Q J Engng Geol Hydrogeol 2002;35:71–8. 3267 [56] Schmidt V, Dahle A, Bungum H. Costa Rican spectral strong motion 3323 [76] CEPREDENAC. Informe sobre terremotos en El Salvador.http:// 3268 attenuation. Norway: NOSAR; 1997. 3324 www.cepredenac.org; 2001. [57] Climent A, Taylor M, Ciudad Real M, Strauch W, Villagran M, Dahle 3269 ´ 3325 A, Bungum H. Spectral strong motion attenuation in Central America. [77] Baum RL, Crone AJ, Escobar D, Harp EL, Major JJ, Martınez M, 3270 3326 Technical Report No. 2–16, Norway: NORSAR; 1994. Pullinger C, Smith ME. Assessment of landslide hazards resulting 3271 [58] Cloud WK, Perez V. Unusual accelerograms recorded at Lima, Peru. from the February 13, 2001. El Salvador earthquake. US Geological 3327 3272 Bull Seismol Soc Am 1971;61:633–40. Survey Open-File Report.01-119; 2001. 3328 3273 [59] Knudson CF, Perez V. Accelerograph records from Lima, Peru. [78] CEPRODE, Caracterizacioń de los desastres en El Salvador: tipologıá 3329 y vulnerabilidad socioecono´mica. San Salvador, El Salvador: Centro 3274 Proceedings of the Sixth World Conference on Earthquake Engin- 3330 eering, New Delhi 1977;1:338–44. de Proteccioń para Desastres; 1994. 3275 3331 [60] Purvance MD, Anderson JG. The behavior of spectral decay in strong [79] Lund LV, editor. Lifeline performance in the El Salvador earthquakes 3276 3332 ground motion accelerations recorded in Guerrero, Mexico. Proceed- of January 13th and February 13th, 2001: preliminary reconnaissance 3277 ings of the Seventh US National Conference on Earthquake survey. Technical Council on Lifeline Earthquake Engineering, 3333 3278 Engineering, Boston; 2002. ASCE; 2001. 3334 3279 [61] Spudich P, Joyner WB, Lindh AG, Boore DM, Margaris BM, Fletcher [80] EERI, Preliminary observations on the El Salvador earthquakes of 3335 JB. SEA99: a revised ground motion prediction relation for use in January 13 and February 13, 2001. EERI Newslett 2001;35(7):12. 3280 3336 extensional tectonic regimes. Bull Seismol Soc Am 1999;89(5): [81] Wisner B. Risk and the neoliberal state: why post-Mitch lessons didn’t 3281 1156–70. reduce El Salvador’s earthquake losses. Disasters 2001;25(3):251–68. 3337 3282 [62] Boore DM, Joyner WB, Fumal TE. Equations for estimating [82] Lara MA. The San Salvador earthquake of October 10, 1986—history 3338 3283 horizontal response spectra and peak accelerations from western of construction practices in San Salvador. Earthquake Spectra 1987; 3339 3284 North American earthquakes: a summary of recent work. Seismol Res 3(3):491–6. 3340 Lett 1997;68(1):128–53. 3285 [83] Bommer J. Terremotos, urbanizacioń y riesgo sı´smico en San 3341 [63] Abrahamson NA, Silva WJ. Empirical response spectral attenuation Salvador. Boletıń PRISMA 1996; No. 18. San Salvador, El Salvador: 3286 3342 relations for shallow crustal earthquakes. Seismol Res Lett 1997; Programa Salvadorenã de investigacioń sobre Desarrollo y Medio 3287 68(1):94–127. Ambiente. 3343 3288 [64] Ambraseys NN, Simpson KA, Bommer JJ. Prediction of horizontal [84] Asociaioń Equipo Maı´z, La Casa de Adobe Sismorresistente. San 3344 3289 response spectra in Europe. Earthquake Engng Struct Dynam 1996; Salvador, El Salvador: Associacioń Equipo Maı´z; 2001. 3345 25:371–400. 3290 [85] Barka AA. Slip distribution along the North Anatolian Fault 3346 [65] Sabetta F, Pugliese A. Estimation of response spectra and simulation 3291 associated with large earthquakes of the period 1939 to 1967. Bull 3347 of nonstationary earthquake ground motions. Bull Seismol Soc Am Seismol Soc Am 1996;86:1238–54. 3292 3348 1996;86(2):337–52. [86] Parsons T, Toda S, Stein RS, Barka A, Dietrich JH. Heightened odds 3293 [66] Lo´pez Casado C, Benito B, Bommer JJ, Ciudad Real M, Pelaéz JA. of large earthquakes near Istanbul: an interaction-based probability 3349 3294 Ana´lisis de los acelerogramas registrados en los terremotos de El calculations. Science 2000;288:661–5. 3350 Salvador de 2001. Proceedings of the Second Iberoamerican 3295 [87] Lemoine A, Madariaga R, Campos J. Evidence for earthquake 3351 Conference on Earthquake Engineering, Madrid; 2001. p. 771–80. interaction in Central Chile: the July 1997–September 1998 3296 [67] Dikau R, Brunsden D, Schrott L, Ibsen M-L. Landslide recognition. 3352 sequence. Geophys Res Lett 2001;28(14):2743–6. 3297 Chichester: Wiley; 1996. 3353 [88] Ambraseys NN, Adams RD. Large Central American earthquakes 3298 [68] Bommer J, Rolo R, Mitroulia A, Berdousis P. Geotechnical properties 3354 1898–1994. Geophys J Int 1996;127:665–92. and seismic slope stability of volcanic soils. Proceedings of the 12th 3299 [89] Cousins WJ, Zhao JX, Perrin ND. A model for the attenuation of 3355 European Conference on Earthquake Engineering, London; 2002, 3300 peak ground acceleration in New Zealand earthquakes based on 3356 Paper No. 695. UNCORRECTEDseismograph andPROOF accelerograph data. Bull NZ Soc Earthquake Engng 3301 3357 [69] Hsu KJ. Catastrophic debris streams (sturzstroms) generated by 1999;32(4):193–220. 3302 rockfalls. Geol Soc Am Bull 1975;86:129–40. 3358 3303 3359 3304 3360

SDEE 2592—10/5/2002—AWINDOW—47521 – MODEL 5