EERI Special Earthquake Report --September 1 999 20° , Guerrero, Oaxaca, Vera- cruz and Morelos. The state of 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- , 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 accel- eration (PGA) with distance is shown in Figure 6, which was ~~--~~..'!1~.~ "-~,~~" '~~~ ~..~."."~- ~.-L- developed using 18 rock, two tran- sition/soil, and nine soil site re- cordings 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 lo- cated 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 accel- erations 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 pri- mary distance parameter used in this attenuation relationship is the closest distance to the rupture; how- ever, 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 mo- tion, may help explain the concen- tration 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 ma- sonry 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. In other churches had a common orientation, vertical reinforcement. Roofs are states-including Mexico and its with the longitudinal axis running in commonly made of logs that support capital, -the damage the east-west direction, and the heavy clay tiles. A typical failure was light and mostly restricted to sanctuary located on the eastern end mode was the appearance of vertical nonstructural elements. Most of the observed damage in the cracks at the corners of the house churches corresponded to seismic leading to the out-of-plane failure of Historic Buildings: The largest and forces in the transverse direction of the walls (Figure 8). At the time of most prevalent damage was to over 700 historic buildings in the six states reporting damage. This region is rich in Catholic churches, convents, and palaces built between the 16th and 19th centuries. The structural system of most churches consists of a vault- ed masonry roof that is supported by side walls and stone arches span- ning the church width. Large interior pilasters and exterior buttresses Figure 8 -Damage to along the side walls support the arch- adobe housing units. es vertically and horizontally; At the (Photo: Iglesias-Jimenez crossing or bay in front of the sanc- et 81.) tuary, a masonry dome is located; at the rear of the church, opposite the sanctuary, there are usually one or two towers. The main floor is typi- cally constructed at grade.

All the building elements are very thick-the walls appeared to be as much as four feet thick-and con- structed of solid unreinforced stone

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~ EERI Special Earthquake Report --September 1999

Surveys of several reinforced con- crete structures revealed damage related to easily recognized defi- ciencies: captive columns, abrupt changes in the stiffness of the structures along their heights (soft stories), and poor detailing. In three buildings, severe structural damage was caused by the inter- action between columns and dis- continuous masonry walls. "Cap- tive columns" are common in school buildings, where the need for light and ventilation leads to the use of large windows that run from column to column in simple rein- forced concrete frames. Masonry Figure 9 -Soft story damage in Condominio 3 Oriente, Puebla. (Photo: S.Pujol) walls at the bottom of these win- dows confine the columns partially (Figure 11 ).

Two buildings of a Puebla condo- minium complex collapsed. Each building was supported by a rein- forced concrete frame with four stories and, in these two buildings, the first level was used for parking . Masonry walls used as partitions stiffened the structures in the up- per three stories. The "softer" first- story seems to have resulted in the structures' collapse (Figure 9).

Figure 10 -Column shear failure at the school of medicine of La Benemerita Universidad Autonoma de Puebla. (Photo: Alcocer et al.)

this report, 14,026 such units had this type showed severe damage associ- been reported as damaged in the ated with inadequate walls and poor states of Oaxaca and Puebla (2458 quality construction. units were considered a total loss). This damage resulted in the dis- Engineered Buildings: Relatively few placement of approximately 20,000 cases of major structural damage to people. engineered buildings were reported. However, over 1000 schools were Two-story brick masonry housing reported damaged in six states, with 12 units also suffered serious dam- of these schools severely damaged. age. These generally have small Nine hospitals were damaged, four in the vertical and horizontal, lightly rein- City of Puebla alone. Some had to be forced concrete elements ( col- closed due to nonstructural damage ~ ~ umns and beams) with infill walls of which, in some instances, led to a extruded brick masonry units. In significant down time for emergency Figure 11 -School damage in Ciudad Serdan, Puebla, 44 units of surgical and laboratory units. , Puebla. (Photo: $, Pujol)

6 EERI Special Earthquake Report --September 1999

~ EEAJ Special EarthQ~~~e Report --September 1999

Lessons Learned Alcocer, S. M., Aguilar, G., Duran, R., Flores. Nakamura, Y. (1989) "A Method for Dy- L., Lopez, 0., Pacheco, M., Uribe, C. M. namic Characteristics Estimation of the Five conclusions are presented be- (1999) "El Sismo de Tehuacan del15 de Junio Subsurface Using Microtremors on the de 1999," Reporte del Area de Ingenieria Es- low based on initial observations of Ground Surface," RTR1 30, Number 1 , tructural y Geotecnia, Sistema Nacional de Feb. the earthquake's effects. Proteccion Civil, Coordinacion General de Proteccion Civil, Centro Nacional de Preven- Pestana, j.M., Mendoza, M.j., Mayoral, 1) Because the affected region has cion de Desastres, June. j.M., Moss, R.E.S., Sancio, R.B., Seed, experienced frequent earthquakes R.B., Bray, j.D., Romo, M.P. (1999) "Geo- similar to or more intense than the Chavez-Garcia et al. (1994) "Microzonifica- technical Engineering Aspects of the june June 15, 1999 earthquake, it is es- cion Sismica de la Zona Urbana de la Ciudad 15 and june 21' 1999, Tehuacan, Mexico sential to enforce appropriate de- de Puebla." Informe Tecnico dellnstituto de Earthquakes." Web site: www.eerc. sign codes and other risk-reduction Ingenieria UNAM, 0996- T9111 , February. berkeley. ed u/mexi co/geotech/ce ntral- measures. This strategy is critical mexico-earthquake.html Eibenschutz, R., Dual1e, S. (1991) "Hacia una for buildings such as schools, Cultura para la Prevencion y Mitigacion de De- Singh, .S.K., Ordaz, M., Pacheco, J.F., where there are structural short- sastres." Memoria de el Simposio Interna- Quaas, R., Alcantara, L., Alcocer, S., comings, and hospitals, which de- cional Seguridad Sismica en la Vivienda. Mex- Gutierrez, C., Ovando, E. (1999) "A Pre- serve special attention to avoid the ico. CENAPRED/JICA. liminary Report on the Tehuacan, Mexico nonstructural damage that required Earthquake of June 15, 1999 (Mw= 7.0)," their evacuation in several cases. Iglesias-Jimenez, J., Iglesias-Villareal, J., UNAM and CENAPRED Seismology Ramirez-Centeno, M., Gomez-Gonzales, B., Group. 2) Site effects were found to have Guerrero-Correa, J., Ruiz-Acevedo, C., and Salgado-Salazar, I. (1999) "Tehuacan, Mexico, contributed to the damage resulting Toro, G.R., Abrahamson, N.A. and Earthquake of June 15, 1999," In preparation. Schneider, J .F. ( 1997) "Model of Strong from this earthquake. Ground Motions from Earthquakes in Macias, J. M. (1997) "La Sociedad y los Central and Eastern North America. 3) The damage to historical build- Riesgos Naturales (estudio de algunos Best Estimates and Uncertainties." ings was extensive; portions of efectos de los sismos recientes en Ciudad Seismological Research Letters, seis- them sometimes collapsed com- Guzman, Jalisco)," En: TerremotoySocie- mological Society of America, Volume 68 pletely. It is fortunate that the earth- dad di Pardo et al. Mexico, CIESAS, Cuader- Number 1, Jan/Feb. quake occurred at a time when nos de La Casa Chata, No.157 . many of these structures-espe- cially churches-were empty. The problems associated with either earthquake risk reduction or repair of such structures are complex and involve many sectors of society.

4) Failure in nonstructural elements such as unreinforced masonry parapets and walls caused a signif- icant percentage of the casualties, Figure 14- Reacti- injuries and economic loss related vated landslide on to this event. the road from Tehuacan to 5) The considerable damage to Huajapan de Leon. low-income housing units also (Photo: Pestana et al.) reveals the need to address this problem in a systematic manner

References

Alcantara Nolasco et al. (1999) "El Temblor de Tehuacan, Puebla del15 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.

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