INTRODUCTION

        A seismic event of moment magnitude 7.0 (USGS) struck the central region of México on June 15, 1999, at 15:42 hours (20:42 GMT). The epicenter of the earthquake was located near the border of the states of Puebla and Oaxaca. A second event of moment magnitude 6.3 (USGS) was recorded on June 21, 1999, at 12:43 hours (17:43 GMT), also affecting the central region of México.

        Figure 1.1 shows the location of the epicenters of these two seismic events, which together affected cities in the states of Guerrero, Hidalgo, México, Michoacán, Morelos, Oaxaca, Puebla, Tlaxcala, Veracruz and the Federal District.
 

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        The first event was felt in the Federal District (México City), approximately 250 km from the epicenter, where many people rushed into the streets alarmed by the vibrations felt in the buildings. More than 400 people required attention due to nervousness or anxiety, and two fatalities were reported due to heart attacks. The electric power was interrupted in some sectors of the city. The conventional telephone service was sporadically interrupted as well as the cellular phone service, but both were completely reestablished fifteen minutes later. Many pipe breaks and leaks were reported in México City.

        Puebla City, located approximately 125 km from the June 15 epicenter, has a population of about 5 million and is home to more than 2,000 churches and temples. More than 100 of these structures were damaged by the seismic events. A four story building complex located in the downtown area was severely damaged, in an area where local soil conditions had led to prediction of significant amplification in earlier microzonation studies. The death toll in this city, the highest, was five.

        In the states of Morelos and Hidalgo the damage was minor and mainly affected churches and historical monuments built in the 16^th and 17^th centuries. In the state of Tlaxcala only one fatality was reported; this due to a heart attack. The damage here was also minor and consisted of small landslides along road cuts, cracking of the old churches and houses, and a fire in a textile factory that resulted in one injury.

        In the area investigated by the authors the most notable effects were concentrated in unreinforced masonry (URM) structures, such as churches and small adobe and cane houses, of which more than 7,000 were damaged.

        The towns in the area surrounding the rupture plane are mostly small agricultural and mining towns where the damage was concentrated predominantly in URM structures, such as churches, houses, and rock walls. The bigger towns of Tehuacán, the closest to the epicenter, and Acatlán de Osorio, located approximately 60 km to the east of the epicenter, were affected in a similar manner.

        According to the Attorney General of the State of Puebla, Carlos Arredondo Contreras, there were 15 fatalities due to the first event. The second earthquake caused no deaths. Table 1.1 presents the summary of damage induced by the earthquakes of June 15 and June 21, detailed per state.

TABLE 1.1 Summary of the damage in each state caused
by the seismic events of June 15 and June 21, 1999.
 


 

State	Damaged Houses	Deaths	Injuries
Distrito Federal	Unreported	2	Unreported
Guerrero	Unreported	1	Unreported
Oaxaca	3,476	Unreported	Unreported
Puebla	4,299	15	188
Tlaxcala	Unreported	1	1
Veracruz	70	2	5

        A research team consisting of graduate students and faculty from the University of California at Berkeley (UCB) and the National Autonomous University of México (UNAM) performed a preliminary field investigation of seismic geotechnical issues under the sponsorship of the National Science Foundation (NSF) and the Pacific Earthquake Engineering Research Center (PEER). In Figure 1.2, the route followed by the research team is shown, with points of interest and strong-motion stations indicated.
 

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SEISMOLOGICAL AND GEOLOGICAL OBSERVATIONS

        According to the National Seismological Service of México (SSN), the focus of the seismic event registered on June 15, 1999, was located at 18.20° north latitude and 97.47° west longitude at a depth of 92 km. The epicenter was calculated at 30 km south southeast of the city of Tehuacán in the state of Puebla, approximately 55 km northeast of the city of Huajuapan de León in the state of Oaxaca. The United States Geological Survey (USGS) located the epicenter at 18.40° north latitude and 97.45° west longitude at a depth of 71 km. This discrepancy between SSN and USGS data made determining the distance to the rupture surface for attenuation calculations difficult.

        A second strong event with a moment magnitude of 6.3 (USGS) occurred on June 21 at 12:43 hours (17:43 GMT). According to SSN the focus was located at 18.09° north latitude and 101.78° west longitude at a depth of 42 km. The USGS published data locating the second event at 18.34° north latitude and 101.49° west longitude with a depth of 50 km.

        The June 15 event occurred in a complex region of normal and reverse faults with a regional tectonic mechanism associated with the subduction of the Cocos plate under the North American plate. The Cocos plate moves towards the northeast and subsides under the Pacific Coast of México, producing events like the September 19, 1985, earthquake that caused significant damage in México City. This plate subduction also leads to the formation of the Trans-Mexican Volcanic Belt.

        The hypocenter determined by the USGS was used to locate the hypocenter-to-site (i.e. source-to-site) distance of the June 15, 1999 event.  The rapid moment-tensor solutions for this earthquake suggest that this event was complex, consisting of two sub-events, separated by several seconds (e.g. <http://wwwneic.cr.usgs.gov/neis/FM/previous/9906.html>).  The fault mechanism was that of a normal fault.  The strike of the potential rupture plane was approximately 310 degrees and dipped to the east or west at about 45 degrees.  This type of fault rupture mechanism is not unusual for this area of Central Mexico.

        The following regional geology descriptions correspond to the central and southern portions of the state of Puebla that were surveyed by the research team. The central region, in which Puebla City is located, can be characterized by the predominance of relatively young (upper tertiary and quaternary) volcanic structures of diverse types and textures such as basaltic flows, tuffs and ashes created successively by many volcanoes during the Cenozoic period. The southern portion of the state, in which the June 15 focus was located, and in which the cities of Tehuacán and Acatlán de Osorio are located, is composed of a combination of metamorphic, igneous and sedimentary rocks affected by intense erosion. The structure of this zone is characterized by normal and reverse faulting with a predominant northwest strike and a predominant dip toward the northeast.

STRONG GROUND MOTION

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4.1 Effects of Ground Motions on Some Structures:

        Collapse of a four-story building, shown in Figure 4.1, occurred in downtown Puebla. The collapse of this structure may be attributed, in part,  to local soil conditions and site amplification as discussed in Section 3.4. An adjacent building was severely affected as well, as shown in Figure 4.2. This structure had a street level garage that collapsed due to column shear; apparently no punching or settlement occurred. These buildings are located in the double hatched portion of the microzonation map (Figure 3.3) for which significant amplification of ground motion was predicted. They are also located within 1 km of the PHPU station that recorded amplified ground motions in Puebla. These buildings were part of a four building, four story complex of which two were completely leveled, one was sheared without collapse, and the remaining building showed little damage. No one was killed in these buildings.

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Figure 4.1 Collapse of a four story structure in downtown Puebla in the zone of predicted maximum amplification.
Figure 4.2  Adjacent to structure in Figure 4.1, this building apparently collapsed due to column shear and/or buckling, not foundation punching or settlement.

 

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        Figure 4.3 shows a 5 m deep, unbraced open excavation for the placement of a sewer line. This site is also located in the double hatched zone on the microzonation map, but did not show any evidence of failure or displacement. At the time of the June 15 earthquake, workers were inside the excavation.
 

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Figure 4.3  Unaffected 5 m deep open excavation in Puebla City.
Figure 4.4  Cracked dome arches in the old Carolino building, downtown Puebla City.

 

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        Old structures were particularly hard hit during the events of June 15 and June 21. Figure 4.4 shows the Carolino building of downtown Puebla. The interior structural damage seen is indicative of damage that most URM structures in the Puebla and Oaxaca regions experienced. Figures 4.5 and 4.6 show the typical damage that many of the churches in the affected region experienced. In nearly every town, regardless of how small the population is, there is a church similar to the churches seen in these two figures. With unerring regularity, these churches were damaged during the strong ground shaking. Even in the towns least affected by the tremors, where all other structures emerged unscathed, the churches often had some sort of damage.
 

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Figure 4.5  Collapsed Tower of Saint Augustine church, Puebla City.  Together with the Municipal Palace, this was on of the most affected old structures.
Figure 4.6  Cracked church dome in Acatlán de Osorio, typical of observed damage.

 

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        An interesting example of "local site effects" is the damage to the church in the town of Cholula, approximately 10 km west of Puebla City. Figures 4.7 and 4.8 show the damage that occurred to this 16^th century church that is built on an ancient stone pyramid. The severe damage may be attributed, at least in part, to topographic amplification of ground motions due to the pyramid.
 

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Figure 4.7  Church in Cholula founded on the top of an ancient stone pyramid.
Figure 4.8  Damage to the Cholula church due, in part, to topographic amplification of ground motions.

 

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        The most severely affected town that the authors visited was Acatlán de Osorio, located approximately 50 km west of the June 15 epicenter. Site effects were prominent in this town, with the most damage occurring to structures in the sediment filled valley and little or no damage to the structures located on rock in the foothills. Figure 4.9 shows the type of damage that was common in the downtown region. On certain streets many houses were reduced to rubble such as the one shown. Figure 4.10 shows a relatively modern two-story structure that was destroyed.
 

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Figure 4.9  Demolished URM structure in Acatlán de Osorio.
Figure 4.10  Damaged modern reinforced concrete structure in Acatlán de Osorio.

 

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        Damage to a bridge, shown in Figures 4.11 and 4.12, was observed on Highway 135 north of Tehuacán. Four bridges are located within 3 km of each other along this stretch of highway. The only bridge to be structurally damaged was one supported by three columns. This bridge experienced 15 cm of displacement in the lateral and longitudinal directions. Settlement of the approach embankments was identified by large tension cracks in the embankment shoulders. The three nearby undamaged two column bridges had the same orientation and were located in similar topography, yet experienced negligible damage.
 

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Figure 4.11  Damaged bridge on Highway 135 north of Tehuacán.
Figure 4.12  This  three column bridge experienced both lateral and longitudinal displacement.

 

4.2 Landslides:

        Figure 4.13 shows shallow landsliding that occurred on the slopes of Cerro el Pinal. Larger landslides occurred on the flanks of the extinct volcano, Cerro la Malinche. These slides damaged the aqueduct that supplies water to the town of Ixtenco in the State of Tlaxcala, effectively cutting off the water supply for several days.
 

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Figure 4.13  Landslide scars on Cerro el Pinal.  Similar landslides on Cerro la Malinche disrupted an aqueduct that supplied water to surrounding towns.
Figure 4.14  Reactivated landslide on the road from Tehuacán to Huajuapan de Leon.

 

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        A series of small rock falls occurred along road cuts in the proximity of Tehuacán. Small slides were common along the cut slopes adjacent to roads in the vicinity of the epicenter. These failures included rockslides, rock toppling, block sliding of both earth and rock, and surficial raveling. In the states of Puebla and Oaxaca, sliding deposited debris on the roadways causing traffic delays and road closures. Figure 4.14 shows a larger reactivated slide located on the route from Tehuacán to Huajuapan de León. Figure 4.15 shows tension cracks parallel to the mountainous road that extends from the city of Acatlán de Osorio to Izucar de Matamoros.
 

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Figure 4.15  Slumping and cracking of soil near a steep slope adjacent to the highway at Puerto el Gato.
Figure 4.16  Lateral extension cracks in soil, Acatlán de Osorio.

 

4.3 Ground Failure:

        Surface expression of fault rupture was not seen along the path followed by the reconnaissance team. Lateral cracks in the earth, not associated with surface rupture, were observed in the backyard of a house in the city of Acatlán de Osorio in the state of Puebla, as shown in Figure 4.16. These appeared to be related to poor performance of relatively loose surficial soil or fill.

4.4 Soil Liquefaction:

        Liquefaction occurred in the highlands ("altiplano") of the State of Tlaxcala, about 20 km northwest of the City of Puebla. Sand boils, shown in Figure 4.17, were observed throughout a large area in a cornfield. The liquefied soil was volcanic in origin, a non-plastic silty sand with 28% passing the #200 sieve. There were no observations of significant soil liquefaction affecting structures, which is not surprising given the dry season and relatively moderate levels of shaking. Several cases of poor structural performance, apparently due to cyclic densification/settlement of loose surficial soils or fills were noted.
 

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Figure 4.17  Sand boil in volcanic non-plastic silty sand, located in a corn field 20 km northwest of  Puebla.

 

4.5 Performance of Earth Structures:

        Not many earth dams are built in the state of Puebla; the water demand is mainly supplied by groundwater in underlying aquifers. "Valsequillo" is a small rockfill dam located to the south of the city of Puebla, as shown in Figure 1.2. The embankment has a maximum crest height of approximately 25 m. Inspection of the 2H:1V upstream and downstream slopes of the embankment showed no evidence of cracking or displacement.

        The solid-waste of many of the cities and towns in the proximity of the epicenter is eliminated by incineration in the open field. The solid-waste landfill of the city of Puebla, Chiltepeque, is a modern facility with side slopes of 3H:1V and a base liner system. During the landfill inspection of this site no signs of either major instability or localized failure were observed in the slopes. The solid-waste landfill of the City of Tehuacán also showed no evidence of seismically-induced deformations

4.6 Miscellaneous:

        According to the Red Cross, 35 people were trapped and one died in a mine at San Antonio Atexcac, in the State of Veracruz. In Tehuacán, another report claims that several people were trapped in a quarry due to a landslide. Many schools reported damage, but the authors saw very little in the way of geotechnical or structural related issues associated with these reports.
 

ACKNOWLEDGMENTS

        The authors would like to extend their thanks to the National Science Foundation and the Pacific Earthquake Engineering Research Center for funding the reconnaissance trip. Special thanks to Dr. Cliff Astill, Dr. Jack Moehle, Dr. Sergio Alcocer, Dr. Mario Ordaz, Dr. Douglas Dreger, Dr. Lind Gee, Dr. Raúl Castro, Dr. Walt Silva, Dr. Gabriel Toro, Joe Whartman, Gerardo Aguilar, Francisco Hernández, Oswaldo Flores, Loren Bedolla, Helmuth Lau, Guillermo González, Alberto Altamirano, Guillermo Melgarejo, Carlos Hernández, and Liliana Ramírez for their contributions and assistance.
 

REFERENCES

• Aguirre et al. (1999) "Reporte Preliminar del Temblor de Tehuacán, Puebla, del 15 de Junio de 1999 (M = 6.7)." Aspectos Sismológicos y de Movimientos Fuertes. Unpublished.

• Alcántara et al. (1999) "El Temblor de Tehuacán, Puebla del 15 de junio de 1999 (M=6.7) registrado por lared de Acelerografos de la Ciudad de Puebla (RACP)." UNAM internal report, RACP-II/BUAP-04, June.

• Chávez-García et al. (1994) "Microzonificación Sismica de la Zona Urbana de la Ciudad de Puebla." Informe Técnico del Instituto de Ingeniería UNAM, 0996-T9111, Feb.

• Chávez-García et al. (1995) "Seismic microzonation of the city of Puebla, Mexico." Third International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Pages 545-548, Vol. II.

• Figueroa, J. (1974) Sismicidad en Puebla: macrosismo del 28 de agusto de 1973. Instituto de Ingenieria, UNAM, informe No. 343, México, D.F., Octubre.

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• http://www.infosel.com/noticias/articulo/016720/pagina12.htm

• http://wwwneic.cr.usgs.gov/neis/bulletin/99_EVENTS/990615204206/990615204206.HTML

• Idriss, I. M. and Sun, J. I. (1992). "SHAKE91: a computer program for conduction equivalent linear seismic response analyses of horizontally layered soil deposits." User’s Guide, University of California, Davis, 13 p.

• Instituto Nacional de Estadística, Geografía e Informática (INEGI), Orizaba E14-6.

• International Conference of Building Officials (1997) "Uniform Building Code: Volume 2 – Structural Engineering Design Provisions," Chap. 16, Div IV – Earthquake Design.

• La Jornada, Various articles of the editions of June 16 through June 22, 1999.

• Nakamura, Y. (1989) "A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface." RTR1 30, Number 1, Feb.

• Ruiz, C. and Juarez, J. (1996). "Seismic zonation of Puebla, Mexico." Proceedings of the Eleventh World Conference on Earthquake Engineering, Pergamon, Elsevier Science Ltd., Disc 3, Paper No. 1647.

• Toro, G. R., Abrahamson, N. A., and Schneider, J. F. (1997) "Model of Strong Ground Motions from Earthquakes in Central and Eastern North America: Best Estimates and Uncertainties." Semoislogical Research Letters, Seismological Society of America, Volume 68, Number 1, Jan/Feb.

• Youngs, R.R., Chiou, S.-J., Silva, W. J., Humphrey, J. R., (1997) "Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes." Semoislogical Research Letters, Seismological Society of America, Volume 68, Number 1, pp. 58-73. Jan/Feb.