Mexico Tehuacan Eeri Prelimin

EERI Special Earthquake Report --September 1 999 20° Tlaxcala, Guerrero, Oaxaca, Vera- cruz and Morelos. The state of Puebla has an area of 33,997 km2 and a population of approximately 4.1 million. Geosciences 18° Large earthquakes in the subduct- ing Cocos plate result in significant damage to villages of the Mexican highlands. or altiplano. Between 1864 and 1999, there were ten earthquakes in this region with magnitudes greater than M=6.5, 16° and at least seven with a depth -100° -98° -96° -94° comparable to the June 15th Figure 2 -Broadband stations (seismic network of Servicio Sismologico Na- quake. The events are all associ- cional and Instituto de Geofisica) shown as triangles and the closest two ac- ated with the movement of the Co- celographic stations (CHFL and RABO) used to infer directivity. (Singh et al.) cos plate beneath this section of the North American continent. Be- celerograms in Figure 3 indicates that by Alcantara Nolasco et al. (1999) cause this type of earthquake is the duration of the intense part of the for the City of Puebla is given in relatively common in this part of ground motion at sites RABO, YAIG, and Table 1. Figure 5 shows the accel- Mexico, seismic-resistant designs PUG is smaller than at sites PNIG, OXIG, erograms for two stations: PHPU throughout the region ought to con- and UVIG. The duration at CHFL, the (soil), registering the highest peak sider the near certainty that struc- site closest to the epicenter, falls in ground accelerations (279 gals, or tures will experience such an event. between these two extremes. In general, cm/s/s) in Puebla, and BHPP the duration is short at stations located (rock). An indication of directivity of the NW of the epicenter, suggesting a NW source towards the NW was noted directivity. Geotechnical Aspects from the durations of the NS com- ponent of accelerograms recorded The acceleration data on hard (rock) The ear1hquake generated a num- at hard (rock) sites located within sites, available at the time of this report, ber of strong-motion recordings 230 km of the hypocenter (Figure yielded the isoacceleration map shown in over a variety of geologic site 2). A visual examination of the ac- Figure 4. The acceleration data collected conditions, including free-field soil .-- and rock, and from various instru- mented structures. Attenuation of ~~~, the horizontal peak ground acceleration (PGA) with distance is shown in Figure 6, which was ~~--~~..'!1~.~ "-~,~~" '~~~ ~..~."."~- ~.-L- developed using 18 rock, two transition/soil, and nine soil site recordings at strong-motion stations located throughout the affected re- CIfL SPN ~ 15(166), 1999 - gion. This information was pro- vided by UNAM, BUAP, and CEN- APRED, and compiled by Pestana et al (1999). The rock sites on which the strong- motion recording stations are located correspond to 1997 Unified Building Code (UBC) Site Classes B-C. Most rock sites appeared to be soft and weathered rock (Class Figure 3 -NS acceleration traces at close stations. (Singh et al- C). The soil sites correspond to EERI Special Earthquake Report --September 1999 UBC Site Classes C-D, with most being Class D. As shown in Figure 5, recorded peak horizontal accelerations were significantly higher on soil sites than on rock sites. The highest PGA was O.28g at the PHPU soil site. Figure 6 also shows the Youngs et al. 1997 intraslab attenuation rela- 17~ tionships for rock (UBC Class B) and soil (UBC Classes C and D) sites. These attenuation relation- 16 ships were developed for intraslab earthquakes associated with sub- duction zones using data from, among many others, several Cen- tral Mexico earthquakes. The primary distance parameter used in this attenuation relationship is the closest distance to the rupture; however, the hypocentral distance is corded at six strong-motion stations Preliminary analysis shows that the used when the fault plane geometry (Table 1 ). According to Chavez-Garcia significant duration, 05-95, of the is not available. Hypocentral dis- et al. (1995), the ground in this region PHPU soil record was about 40 sec- tance was used in Figure 6 because consists of a soft tuff interlayered with onds. The ratio of peak ground ve- the rupture plane geometry had not alluvial deposits. The predominant pe- locity to peak ground acceleration been clearly defined at the time of riod, obtained from microtremor stud- PGV/PGA was around 83 cm/s/g at this report. This distance definition ies and Nakamura's (1989) technique, the PHPU site. does not introduce a significant was approximately 0.8 seconds. The error due to the great source-to-site corresponding acceleration response When compared to neighboring rock distance of the recordings. spectra of the PHPU station record station recordings, significant site show energy concentration around a amplification was observed in the Site effects were evaluated for the period of 0.8 to 1.2 seconds (the pre- City of Puebla: PGA (soil)/PGA City of Puebla with the data -re- dominant period was 1.1 seconds). (rock) of about 4, e.g.using the Station Soil Station Peak Ground I Orientation Type Coordinates Ac~eleration~ (gal) C l-C2-C3 Code Name Lat. Lon. Cl C2 C3 N W BHPP Barranca Basaltic Scoria 98.23 58.8 33.4 58.3 N90W-V-NOOW Ronda - , CAPP Soil 19.09 I 98.19 44.9 72.7 103.2 V-NOOW-N90W Compressible Soil Med. 19.05 98.21 63.7 123.2 102.3 V-N90E-NOOE Compressibility Soil 19.04 98.17 56.1 104.5 279 V-N90E-NOOE SRPU Not Class. 18.97 98.26 71.3 216 131.3 V-N90E-NOOE UAPP Soil Low 19.00 94.9 Compressi~lity 98~~ lO~ ~-NOO~ Table 1 -City of Puebla peak ground acceleration data from records of the main shock (Alcantara Nolasco et al.) 3 EERI Special Earthquake Report September 1999 J.3 roadways in the states of Puebla 3.2 and Oaxaca, affecting traffic to :§ = varying degrees. 0 :>.1 ".g ... O ~ Some ground cracking was ob- "Q) u c:>.1 u served in the town of Acatlan de < ').2 Osorio and north along the road to Izucar de Matamoros. Liquefac- ').3 tion was observed in the altiplano 40 50 60 70 80 90 100 of the state of Tlaxcala, some 20 Time (8) km northwest of the City of Puebla. Sand boils were visible over part of a large cornfield. The liquefied soil was a non-plastic silty sand of vol- canic origin, with 28% passing the #200 sieve. However, there was little significant soil liquefaction, which is not surprising given the dry season and moderate levels of shaking. Several cases of poor structural performance were ap- parently due to settlement of loose superficial soils or fills. Time (5) Structural and Nonstruc- Figure 5 -Accelerograms for PUEBLA-PHPU station (soil) and for PUEBLA- tural Damage BHPP (rock). (Pestana etal.) The earthquake caused significant PHPU and PBPU stations (Alcan- duct and interrupted the water supply to damage in south Puebla, north tara et al., 1999). This site amplifi- the surrounding towns for several days. Oaxaca, and south Morelos. Struc- cation of ground motion amplitudes, Other slides deposited debris on the tural damage was observed in along with the relatively long dura- 10000I tion and periodic nature of the motion, may help explain the concentration of damage in Puebla, at an Mean + Standard epicentral distance of 110 km. Site Deviation effects were also observed in Acat- Mear Ian de Osorio, a town located ap- :§ 01000 proximately 50 km west of the epi- < e 0. Mean- Standard center. The damage occurred al- C Deviation .. most exclusively to structures in the Qj E sediment-filled valley; structures .~ founded on rock in the surrounding "'Gi E o foothills fared well. ~ 00100 There was landsliding throughout the region, with superficial sliding .Rock stations (UBC B-C) the primary mechanism. Other D Transition soil stations (UBC C) sliding mechanisms observed were OSoil stations (UBC C-D) rock toppling, rock sliding, block ooo~ sliding, and raveling. Large land- 10 100 1000 slides occurred on the slopes of Focal distance (km) .. "Cerro el Pinal" and "Cerro la **Youngs et al. (1997) rock relationship uses the closest distance to the rupture. which for this purpose Malinche," approximately 120 km can be closely approximated by focal distance northwest of the epicenter. The La Figure 6 -Peak ground acceleration vs. distance for the 6115/99 seismic event Malinche slide disrupted an aque- and the Youngs et al. (1997) rock attenuation relationship. (Pestana et al.) 4 EERI Special Earthquake Report 00 September 1999 the church, or the north-south direc- tion. There was generally damage at the upper level of the towers, cracking between the towers and adjacent walls (Figure 7), longitudinal cracking on the underside of the vaulted masonry roofs and masonry arches, and dome cracking. In some cases there was complete collapse of the domes, vaults and bell towers. The churches performed poorly relative to other types of buildings, considering the moderate ground accelerations of this earthquake. Damage observed in palaces and other historical monuments was no ~ ~ doubt worsened by the lack of ade- quate maintenance on these buildings Figure 7- Nuestra Senora de Los Remedios Church in Cholula, Puebla. (Photo: J. Ramirez) Housing Units: Most significant three categories of buildings: (1 ) or brick masonry. In general, the residence damage was observed in historic buildings (churches, con- I mortar was hard and appeared to be adobe houses that usually have walls vents, monuments); (2) houses; and in good condition. Several of the without any type of horizontal or (3) engineered buildings.

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