EERI Special Earthquake Report — May 2012

Learning from Earthquakes

The March 20, 2012, , , Earthquake

The team that researched this re- are usually out of their houses at that Seismic History port was sponsored by the Universi- time of day, working in the farming dad Autónoma Metropolitana-Az- and agricultural activities. Seismic activity in southern Mexico capotzalco (UAM-A). It included Hu- (between longitudes -94° W and Ometepec (55,000+ inhabitants), Cua- gón Juárez García, Alonso Gómez -104° W) results primarily from sub- jinicuilapa (25,000+ inhabitants), and Bernal, José Luis Rangel Núñez, duction events along the Mexican San Juan Cacahuatepec (8,500+ Arturo Tena-Colunga, José Roldán Trench, where the Cocos Plate is inhabitants) are the towns in the epi- Islas, and Elsa Pelcastre Pérez being consumed under the south- central area; the epicentral region is (independent Structural Engineering ernmost parts of the North Ameri- largely comprised of small villages of consultant). Severiano Álvarez Cruz can Plate. Collisional velocities fewer than 2,000 inhabitants, some of of the Tecnológico Superior de la range from about 5.5 cm/yr at -104° them lacking basic services (INEGI, Costa Chica in Ometepec, , W to about 7.7 cm/yr at -94° W. The 2005). This earthquake was also also assisted the team. subduction zone is apparently well strongly felt in the cities of Chilpan- segmented, with some segments Unless otherwise indicated, photos cingo, Guerrero, and Mexico City. having numerous small events in this report were taken by team members. This report was edited by Sarah Nathe, EERI Newsletter Insert Editor.

Introduction On Tuesday, March 20th, 2012, at 12:02 p.m. local time (18:02 GMT), an Mw = 7.4 earthquake struck the area near the towns of San Juan Cacachuatepec, Oaxaca, and Ometepec, Guerrero, in the south- eastern region of Mexico (Figure 1). Table 1 shows the most important parameters of the main shock and the greatest aftershock. Epicenters of the March 2012 and the September 1995 Ometepec According to state and federal gov- Figure 1. earthquakes (Sordo et al., 1995 and USGS, 1995). ernment agencies, 19 municipalities in Guerrero and 28 in Oaxaca had heavy damage, and federal funds Table 1. Major parameters of the Ometepec earthquake of were deployed to help recover infra- March 20th, 2012, and the aftershock of April 2nd, 2012. structure and to support the affect- ed population (Diario Oficial de la Federación, 2012a and 2012b). Almost 2,000 houses collapsed or were judged to be total losses, over 3,000 houses sustained heavy dam- age, and over 3,000 were reported with minor damage. Also damaged to some extent were 22 schools, 42 government offices, and 24 histori- cal buildings (Oaxaca, 2012). How- ever, only two people were killed and few were injured, according to early local reports. Local residents

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spread Jurassic age granitic intru- sive rocks, locally intruded with Ter- tiary age granitic plutons. The older granitics are heavily weathered, while the younger are only moder- ately affected. The older bedrock consists of heavily decomposed granitics with considerable clays overlaid by thick laterite deposits where the terrain is gentle. Con- struction generally follows the exist- ing topography, but where fills are used, compaction or material suit- ability is not controlled (Sordo et al., 1995). There were some cases where fill softened due to ground motion and produced partial col- lapses of house foundations on Figure 2. Main shock and aftershocks of the March 20th, 2012, Ometepec, steep hill slopes. Guerrero, earthquake. Topography in the epicentral area is with short recurrence intervals, and 10-70 km, and focal mechanisms are gentle to moderate, with elevations others having great earthquakes both normal and thrust. Table 2 lists ranging from about 300m in Omete- with recurrence times of over 75 the major events on this segment pec and San Juan Cacahuatepec, years. A reasonable rupture history since 1882 (Sordo et al., 1995). to over 800m in the mountains just is known for the Mexican Trench for north of the city. Many of the vil- Seven asperities were identified on the last 140 years. Within this time lages are located on the highest this segment; the segments are as- period, some of the segments have portions of the hills, to avoid flood- sociated with earthquake events in endured less than two seismic cy- ing of the valleys in the rainy sea- Table 2 with a (*), (Núñez-Cornú, cles, e.g., the Michoacan segment son and to leave arable land for 1996). The epicenter location of the with one previous event in 1911, agricultural purposes. There are March 20th, 2012, before the great earthquake of Sep- earthquake is located tember 19th, 1985. The Guerrero Table 2. Largest Earthquakes in the Ometepec in the same region as seismic gap, roughly between the region since 1882, (González-Ruíz and McNally, the doublets in 1982. cities of and Petatlán, had 1988; Sordo et al., 1995; and Núñez-Cornú, 1996) We concluded that only one event in this time period. the earthquake A great earthquake is expected event of March 20th, along this segment of the subduc- 2012, and the sub- tion zone in the near future (Sordo sequent aftershocks et al., 1995). (including one of Mw The March 20th event struck on the = 6.0) are associat- Ometepec segment of the subduc- ed with the two as- tion zone. There were more than 50 perities and the aftershocks greater than magnitude doublets of 1982, 4.1 in the following month (Figure 2). thus indicating that This segment behaves differently the double events from those that produce great seis- might occur every mic events in Mexico: it ruptures 30 years. with events of about 7.0 to 7.5 mag- nitude and with a recurrence inter- Geology val of about 12 years. The previous and Soil event was on September 14th, 1995, Conditions with a magnitude of Mw = 7.3. With a depth of 21.8 km; it was given an Bedrock geology in MMI = 7 (Sordo et al., 1995). Re- the area with high- ported depths of the events on the est intensities is Ometepec segment range from composed of wide-

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Figure 3. Small debris displays with a solid line the loca- fall on Ometepec- tions we visited, and with a dotted Huanjintepec Road. line those locations for which we gathered data electronically. The Figure 4 shows the most important accelerographic isoseismal map for data was captured by Centro de the March 20th quake. Instrumentación y Registro Sísmico Through travel to (CIRES) in Mexico City. CIRES has damaged sites and recorded ground motions since analysis of media and 1988. In this event, 14 stations technical websites recorded the acceleration time (USGS and SSN), we histories in Mexico City: two in firm developed this map. It soil (zone I: TP13 and UI21); one some indications that topographic focusing occurred in some of the Table 3. MMI Intensity values observed in Guerrero and Oaxaca states. villages (Sordo et al, 1995). North and east of the epicentral area, the terrain is steep to very steep, and to the south are flood plains and the coastal plain, filled with young soft sediments (Sordo et al., 1995). No liquefaction or subsidence-related phenomena were reported or encountered. On the road between and Ometepec, approximately 10 km southwest of Ometepec, we ob- served minor landslides. On the roads around the epicenter area there were no slides (rotational or translational), lateral spreads, or in roads, but very small debris falls or debris topples were detected in short and large road cuts (Figure 3).

Ground Motion Maximum intensities were reported in Ometepec, Huixtepec, Huajinte- pec in Guerrero; and Buenavista and San Juan Cacahuatepec in Oaxaca, with values ranging from VII-VIII (see Table 3). Intensity levels decreased more rapidly along the coast than in the inland direction, as observed in previous earthquakes (Sordo et al., 1995; Juárez García et al., 1996 and 1997). Two small pockets of high intensity appear in Chilpancin- go and Mexico City, apparently in response to local topography and soil conditions. It is already known that seismic effects in the valley of Mexico City are amplified by soft soil and topography.

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Figure 5. Data acquisition zones for SAS and approximate distance from Mexico City (source: CIRES web page).

Figure 4. Isoseismal map of March 20th, 2012, Ometepec earthquake. The SAS project started in 1986 and was fully operational in 1991. The in a transition zone (zone II: DX37); April 1989 quake was in a different objective is to issue early warning and 11 in lakebed zones (zone III: zone in Guerrero, but it produced the reports to Mexico City, as most of 6 stations are located in Colonia largest PGA values in Mexico City the great earthquakes will strike on Roma CO56, LI58, EX12, CJ03, since the 1985 earthquakes. The PGA the Guerrero coast, 320 km away. CJ04 and CI05). values, in general, are very similar in The early warning system announ- the three earthquakes. ces the oncoming earthquake 60 Non-corrected maximum peak seconds before the arrival of the ground acceleration (PGA) values first seismic waves. A similar sys- recorded are shown in Table 4. Mexico City Early Warning tem has also been implemented on These PGA values are compared System the Oaxaca coast (SASO) (CIRES, with two different events: Sep- CIRES organizes and manages the 2012a). tember 14th, 1995 (Mw = 7.3), information and logistics of the Earth- The SAS system is comprised of and April 25th, 1989 (Mw = 6.9). quake Warning System (Sistema de 12 seismic-sonar stations along the The September 1995 quake was Alerta Sísmica — SAS). on the Ometepec segment; the Guerrero coast, which monitor the local seismic activity and issue a Table 4. Maximum non-corrected PGA values for three different earthquakes. warning radio signal to Mexico City receiver stations (Figure 5). In 18 years the SAS system has recorded over 2,000 earthquakes, with magnitudes ranging from 4-7.4; 53 have been classified as moderate, and 13 have been rated as strong events. For the moderate 53, a “preventive warning” code was issued; only government and emergency agencies (Protección Civil) received the warning, as the earthquake effects were estimated to be moderate; for the strong 13, “public warning code” were issued to the general public through local media. For both Ometepec events, the SAS issued warning signals: in 1995 a “public warning,” and in the 2012 main shock and aftershock a “preventive warning.” For the March 2012 quake, 7 out of 12 stations detected seismic activity forecast moderate effects (Table 5).

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Table 5. Data acquisition station status for SAS, March 20, 2012 with no lateral resistant elements. (source: CIRES web page). As was the case in 1995 (Sordo et al., 1995), adobe quality again played a key role in structural re- sponse: major damage was ob- served in weathered adobe blocks. Rain and other natural elements degrade the resistance and stiff- ness of adobe blocks. Three typical failures were detected: (1) inade- quate bonding in the adobe wall corners led to out-of-plane failures (Figure 7); (2) transverse wall fail- ure due to inertial forces induced by the main wooden beam of the roof system; and (3) stress concentra- tions at corners of openings in the walls (windows and doors). Confined masonry structures. In general, housing and commercial buildings are built with this tech- nique. We found structures that had redundant confined concrete ele- ments (“dalas” and “castillos” — Figure 8). We believe that this is a consequence of moderate earth- Due to the SAS notice, the Metro uated buildings, they filled streets and quakes in the past, local and state system and public buildings had 60 avenues that might have been need- seismic provisions, and the intuition seconds to stop operations and ed for emergency response vehicles. of the local construction workers. evacuate the facilities. Emergency The SAS earthquake warnings gave As the Guerrero area is hot and agencies have improved their evac- 60 seconds to the Metro system to humid, interstory heights range uation procedures, and every work- stop operations, and also gave oper- from 4-5m, and hence they place er is accounted for; however, there ators and people time to evacuate. concrete elements every 1.5m or are still some issues for emergency No damage or injuries were account- less, as illustrated in Figure 9. Even agencies and local authorities to re- ed for, although Line A of the Metro though the materials used for the solve, for example, as people evac- station sustained damage to the rail clay bricks and concrete elements structures, probably due to presence might not be of the highest quality, of large surface waves (Figure 6). these structures exhibited no However, 24 hours later, line A was damage. repaired and fully functional. Concrete structures. The two- story reinforced concrete frame Structural Response Unreinforced adobe masonry structures. Typical villages in the epicentral region are small rural communi- ties lacking adequate roads and communica- tion systems. Inhabit- ants build their own one-room houses with Figure 6. Damage to Metro´s rail load-bearing walls Figure 7. Out-of-plane failure in adobe walls due to system (source: Twitter, user made of adobe blocks, inadequate bonding. Adan4xinhua?).

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Figure 8a and b. Redundancy of concrete elements in new reinforced masonry structures.

resistance and was inadequate and was filled up stiffness of the with rocks and stones; this led to structural and pounding, causing damage at the nonstructural joints of the beams and columns systems, but no (Figure 11). damage was ob- At the Instituto Tecnologica Super- served in beams ior de las Costa Chice in Ometepec, and columns. there are ten reinforced concrete A two-story con- buildings, ranging from one to two crete and mason- stories. We observed light non- ry hotel in Omete- structural damage in two of the pec has a great buildings and light structural dam- wall density ratio, age in one. In Building C, a two- an irregular struc- story façade clay brick wall had an tural configura- out-of-plane failure. The heavy Concrete elements every 1.5m or less. Figure 9. tion, and inade- façade wall was about 90 cm thick, quate constructive with a height of about 10 m, and General Regional Hospital in Ome- joints, so cracking was visible in sev- was not properly anchored to the tepec suffered no structural dam- eral infill masonry walls and in places main structure. This wall did not age, though there was light to mod- where the constructive joints were cause collapse, but institute author- erate nonstructural damage in dry- located (Figure 10). ities decided to demolish it, in order walls and façade elements. The to prevent damage or casualties to A three-story concrete hotel with ma- main concrete columns were sur- the student population, rounded by dry wall casings that sonry infill walls also sustained light had cracks, due to the difference in to moderate structural damage in the The one-story reinforced concrete concrete frame. It was evident that the frame Library building had three separation from neighboring buildings bays in the longitudinal direction

Figure 10. Cracking at the con- structive joints and in infill masonry Figure 11a ▲and b. Pounding and walls. cracking in beam elements. ►

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ceramic tiles of the It is a three-story concrete frame ceiling. The window structure with masonry infill walls, and door frames with a 50 x 15 m plan dimension. were also bent, and The building was restricted on the some windows were ground level with a stair entrance, broken. which has two parallel walls in the center (Figure 13). This condition The two-story caf- restricted the oscillating movement eteria building with of the building in the short direction, RC frames and infill causing severe structural damage walls has a great in the ground and first levels. wall density at the first floor (adminis- Many buildings in Mexico City suf- trative offices), and fered light to moderate nonstruc- the cafeteria is lo- tural damage, such as broken glass, cated at the ground collapsed transformers on top of Figure 12. The concrete government building in down- floor. Cracks were light posts, and parapets fallen on town was damaged in the December 16th, visible in some of sidewalks. 2011, earthquake, and the March 20th, 2012, earth- the infill walls at quake increased the damage. the stairs structure, Lifelines located at one end (8.5 m), four bays in the transversal of the main structure. At the first floor In general, lifelines performed well direction (4 m), with RC columns some infill walls also exhibited crack- in the epicentral area, although in (45 x 30 cm), and an interstory ing, but the structural damage was rural villages, nearby water and height of about 5 m. It is considered considered light. electricity supplies were interrupted. a flexible structure with stiff non- Some electricity, telephone, and In Chilpancingo, Guerrero, geological structural elements (false ceiling and mobile phone service shut-offs conditions have significantly amplified large doors and windows). Non- were reported in Ometepec, but the ground motion in past earth- structural damage was observed in they were fully re-established within quakes (Gómez-Bernal et al., 1999). the false ceiling elements, the alu- a few hours. The March 2012 Ometepec earth- minum grid holding the false ceiling quake caused accumulation of dam- Local roads were damaged, but was not properly anchored at the age in some structures. The govern- they lack proper maintenance. edges of the building, and the verti- ment building located downtown, a Highway and related bridges suf- cal accelerations led to the failure few meters from the Cathedral, is a fered no damage, and they re- of the aluminum bars that hold the four-story concrete building with infill mained fully operational. Well- masonry walls, and an irregular struc- maintained highways and bridges tural configuration (soft first floor at retrofitted after previous quakes one end), had moderate nonstructural suffered no damage. damage and light structural damage In Mexico City, most damage oc- in the concrete columns (Figure 12). curred to the transportation system. A preparatory school built in the 1950s Line A of the metro sustained dam- reportedly sustained the most struc- age to its rails (Figure 6). Damage tural damage in the March quake. to the supporting system of a motor

Figure 13a-d. Structural damage to the first two levels of the building.

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Figure 14a and b. Left: vehicle bridge. Right: collapsed hand rail of a pedestrian bridge.

vehicle bridge caused damage to The media did not focus much at- is no subsequent coordination be- the deck (Figure 14 left). Part of a tention on the epicentral area or tween local and national authorities. pedestrian crossing bridge col- the rural communities; rather, they There are many accelerographic lapsed in a main street of northern reported on the SAS system and stations installed all over Mexico, Mexico City (Figure 14 right). Mexico City. As usual, the Mexican but only a few of them are fully army was the first group to help The new Deep Eastern Drainage operational. Fifteen years ago in people in the epicentral region. Tunnel System (TEO), currently Mexico, there was a will to estab- However, two weeks after the earth- under construction, and Line 12 of lish acceleration stations, and au- quake, most of the rural villages in the Metro both cross a variety of thorities and institutions were ready the area were still waiting for effec- soil deposits from soft clay to vol- to build them and to start recording tive government aid. canic tuff layers and sometimes seismic data, but equipment obso- pass through the water table bed. lescence, lack of technical exper- Both underground structures be- Conclusions tise, and human and economic haved well and were not damaged. The Ometepec earthquake shows resources have become serious problems in operating these sta- Some public hospitals in Mexico again the importance of good struc- tions. We also found a hesitancy City reported nonstructural and tural configuration and adequate to share earthquake data; some structural damage. One of them, maintenance of buildings. We ob- data “owners” are not aware of the in the northern part of Mexico City, served that the construction prac- importance of having the seismic was totally evacuated and remained tice has improved; the affected pop- data available for research. closed for seven days. In the south- ulation has learned the lessons of ern part of the city, 60 m2 of façade having houses with poor adobe elements of another public hospital blocks or poor structural configura- References collapsed on April 13th, 2012. tion. People at the epicentral area Aguilar, J.; Juárez-García, H.; Or- are substituting confined masonry tega, R. and Iglesias, J., 1989. structures for unreinforced adobe The Mexico Earthquake of Sep- Social Impacts and structures. Emergency Response tember 19, 1985: Statistics of Emergency agencies have improved Damage and of Retrofitting Tech- There were only two people killed their evacuation procedures; how- niques in Reinforced Concrete by the earthquake; one of them as ever, there are still some problems. Buildings. Earthquake Spectra, a consequence of a heart attack. For example, as people evacuated Vol 5, Issue 1, pp. 145-152. Ometepec’s two new hospitals were public buildings, they invaded Alcocer, S. and Juárez García, H., operational, despite the light non- streets and avenues, which might 2003. Preliminary Observations structural damage, and treated all be compromised if they are turned on the Tecoman, Colima, Mexico casualties. From the 3,000 houses into Emergency Response Routes. Earthquake of January 21, 2003, reported as damaged, around Local authorities believe that re- Learning from Earthquakes, 10,000 people were affected. Many sponse is based on the way the EERI Special Earthquake Report, of these people still live in the dam- SAS functions and the organization EERI Newsletter, March, http:// aged houses or in their backyards in public buildings initiating the www.eeri.org/lfe/pdf/mexico_ with some sort of temporary shelter. evacuation process; however, there colima_EERI_preliminary.pdf

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Alcocer, S. and Klingner, R.E., 2006. Gómez-Bernal, A., 2002. “Interpreta- Oaxaca Government, 2012. Issue The Tecoman, Mexico, Earth- ción de los efectos del suelo en el 18910: http://www.oaxaca.gob. quake, January 21, 2003, An Valle de México empleando la red mx/?p=18910 acelerográfica de alta densidad,” EERI and SMIS Learning from Rangel, J.L.; Tena, A. and Gómez- Doctoral thesis, Facultad de Ingeni- Earthquakes Reconnaissance Bernal, A., 2011. Comportamien- ería, UNAM, (in Spanish). Report, Earthquake Engineering to observado en estructuras y Research Institute and Sociedad González-Ruíz, J.R. and McNally, subsuelo del valle y ciudad de Mexicana de IngenieríaSísmica. K.C. 1988. Stress Accumulation Mexicali durante el sismo El Centro de Instrumentación y Registro and Release since 1882 in Omete- Mayor-Cucupah del 4 de abril Sísmico A.C. (CIRES), 2012a. pec, Guerrero, Mexico: Implications del 2010. Revista Internacional http://www.cires.org.mx/sas_es.php for Failure Mechanisms and Risk de Ingeniería de Estructuras, Assessments of a Seismic Gap. Centro de Instrumentación y Registro Escuela Politécnica del Ejército, Journal of Gerophysical Research, Sísmico A.C. (CIRES), 2012b. Quito, Ecuador. Vol 16, no 1, Vol 93, no B6, pp. 6297-6317. http://www.cires.org.mx/sas_ 2011, pp. 39-68. historico_reporte_particular_es. Iglesias, J., 1989. The Mexico Earth- Sordo, E., Terán, A., Guerrero, J.J., php?fecha_aviso=2012-03- quake of September 19, 1985: Juárez-García, H. and Iglesias, J., 20&hora_aviso=12:03:09&tipo_ Seismic Zoning of Mexico City after 1989. The Mexico Earthquake of aviso=Preventiva®istros_ the 1985 Earthquake. Earthquake September 19, 1985: Ductility and aviso=7&fecha_sismo=2012-03- Spectra, Vol 5, Issue 1, pp. 257- Resistance Requirements Im- 20&hora_sismo=18:02:50&latitud_ 271. posed on a Concrete Building. sismo=16.42&longitud_sismo= Instituto Nacional de Estadistica y Earthquake Spectra, Vol 5, Issue 98.36&profundidad_sismo= Geografia (INEGI), 2005: http:// 1, pp. 41-50. 15.00&prel_sismo=7.8. cuentame.inegi.org.mx/default.aspx Sordo, E.; Gómez-Bernal, A.; Juár- Diario Oficial de la Federación, Juárez García, H.; Guerrero, J.J.; ez García, H.; Gama, A.; Guinto, 2012a. Issue 5241891: http://dof. Whitney, R. A.; Gama, A.; Vera, R.; E.R.; Whitney, R. A.; Vera, R.; gob.mx/nota_detalle.php?codigo= and Hurtado, F., 1996. Geological Mendoza, E. and Alonso, G., 1995. 5241891&fecha=03/04/2012 Aspects of the October 9, 1995 The September14, 1995 Omete- Diario Oficial de la Federación, Manzanillo, México Earthquake, pec, México, Earthquake, Learn- 2012b. Issue 5241892: http://dof. Learning from Earthquakes, EERI ing from Earthquakes, EERI gob.mx/nota_detalle.php?codigo= Special Earthquake Report, EERI Special Earthquake Report, EERI 5241892&fecha=03/04/2012 Newsletter, January, http://www. Newsletter, December, http:// eeri.org/lfe/pdf/mexico_manzanillo_ www.eeri.org/lfe/pdf/Mexico_ Gómez-Bernal, A.; Ortega, R.; eeri_preliminary_report.pdf Ometepec_Insert_Dec95.pdf Guerrero, J.J.; González, E.; Paniagua, J.P. and Iglesias, J., Juárez García, H.; Whitney, R. A.; 1989. The Mexico Earthquake of Gama, A.; Vera, R.; and Hurtado, September 19, 1985: Response F., 1997. The October 9, 1995 and Design Spectra Obtained Manzanillo, México Earthquake, from Earthquake-Damaged Build- Seismological Research Letters, V ings. Earthquake Spectra, Vol 5, 68, n 3 pp. 413 – 425. Issue 1, pp. 113-120. Juárez García, H.; Gómez Bernal, A.; Gómez-Bernal, A.; Juárez-García, H. Terán, A.; Sordo, E.; Arellano, E.; and Iglesias, J., 1991. “Intensida- Corona, M.; Perea, T.; Hernández, des y demandas de ductilidad de D.; Rangel, G.; Arzate, G.; Ramírez, sismos recientes en la ciudad de H. and Jara, M., 1999. “Intensidad- México”, Revista de Ingeniería es y daños asociados al sismo del Sísmica, SMIS, No. 43, pp. 3-18, 15 de junio de 1999.” Proceedings (in Spanish). of XII Congreso Nacional de Ingeni- ería Símica. Morelia, Mich, Vol II, Gómez Bernal, A.; Juárez García H. pp. 757-762 (in Spanish). and Corona, M., 1999. “Peligro sísmico en el valle de Chilpancin- Núñez-Cornú, F.J., 1996. A Double go, Guerrero”, Proceedings XII Seismic Front and Earthquake Cy- Congreso Nacional de Ingeniería cles along the Coast of Oaxaca, Sísmica, Morelia Mich. Mex. vol. 1 México. Seismological Research pp. 100-109, (in Spanish). Letters, V 67, n 6 pp. 33–39.

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