EERI SPECIAL EARTHQUAKE REPORT -DECEMBER 1995 than in the inland direction, as illus- Table I. Largest Earthquakes in the Ometepec Region since 1882 (modified trated in Figure I. from Gonza1ez-Ruiz, I. R., and McNally, K. C., 1988, "Stress Two small pockets ofhigh intensity accumulation and release since 1882 in Ometepec...", Iournal of appear in Chilpancingo and Mexico Geophysical Research, v. 93, n. ---6297-6317). City , apparently in response to the particular characteristics of 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; significant amplifications of accelerations and long duration of ground motions recorded on soft deposits with respect to those on f1rIn soil were detected during the 1985 earthquake, and again during recent events. Likewise, the city of Chilpancingo has suffered in the past severe damage due to subduction earthquakes generated in the Mexican Pacific coast, like during the 1957 and 1985 earthquakes. Moreover, *Normal Mechanism accelerographic information from recent earthquakes indicates amplifica- segment undergoes a deep normal where fill softening due to rain, together tion effects in some zones of mechanism event followed several years with the earthquake motion, produced Chilpancingo. later by a shallow subduction thrust partial collapses of the foundation of The closest strong motion acceler- event. Table 1 lists the major events houses constructed on steep hill slopes. ometer to the epicentral area of the (M=6.5 or greater) which have affected Topography in the epicentral area is Ometepec earthquake is located in this segment since 1882. Normal gentle to moderate, with elevations Copala, Guerrero, at a distance of mechanism earthquakes are marked ranging from about 300 meters in approximately 80 km from the reported with an asterisk. Although the mecha- Ometepec to over 800 meters in the. epicenter. This station is part of the nism of the 1995 event has not been mountains just north of the city .Many of Guerrero Array installed by the determined, if the event follows the villages are located on the highest Instituto de Ingenieria de la Universidad theseismic pattern for this segment, the portions of the hills, to avoid flooding of Nacional Aut6noma de Mexico mechanism will be normal. the valleys in the rainy season and to (UNAM) and the Institute of Geophys- leave arable land for agricultural pur- ics and Planetary Physics of the poses. There are some indications that University of California at San Diego. Geology and soil conditions topographic focusing occurred in some of The peak ground acceleration at this The region which experienced the the villages. station is reported as 77 gals. highest intensities lies a few kilometers North and east of the epicentral area The amplification effects in north of the town of Ometepec (Figure the terrain is steep to very steep. To the Chilpancingo and Mexico City are 1). Bedrock geology in this area is west the terrain is equivalent to the illustrated in Figure 2, where recorded composed oIwidespread Jurassic age Ometepec area and to the south are flood ground accelerograms are indicated for granitic intrusive rocks, locally intruded plains and the coastal plain, filled with both fIrm (CA and DA) and valley soft with Tertiary age granitic plutons. The young soft sediments. No liquefaction or (CC and DX) soil sites at stations of the older granitics are heavily weathered subsidence related phenomena were array installed by the Red Inter- while the younger are only moderately reported or encountered. universitaria de Instrumentaci6n affected. The older bedrock consists of Sismica (RlIS). heavily decomposed granitics with a The earthquake alarm ofMexico considerable content of clays overlaid Ground Motion City announced the event 70 seconds by thick laterite deposits where the A schematic distribution of the reported prior to the strong motion arrival in the terrain is gentle. Locally this is used to modified Mercalli intensities is presented city, allowing enough time for evacua- form poor to average quality adobe in Figure I. Maximum intensities were tion of some schools and official blocks for house construction. con- reported in Ometepec and surrounding buildings where the alarm is currently struction generally follows the existing villages, like Igualapa, La Soledad, Santa installed. topography, but where fills are used, Maria, Azoyu and Chacalapa, with values compaction or material suitability are ranging from VII to VIII. Intensity level not controlled. There were some cases decreases more rapidly along the coast 2 !!:!!:.&I~EC~ EARmQuA¥;.E REPORT -DECEMBER 1995 I Chilpancingo -EW ' I Station CA ";' 50 - PGA=46 gals = ~ U "-' = ! = -50 ~tation CC ~ PGA= 84 gals 501 ~ I ~ 01 CJ : = -50 10 30 40 50 60 time (segs) I Mexico City -EW ..Station DA ::;- 251 PGA= 23 gals '-"~ u -c,.-::::::.-;-:..."-Jo~~~';#~ § -25 .~ PGAStation = 44 DXgals - ~ 25 Figure 4 Roof configuration in ~ adobe houses. ~ O CJ = -25 with a particular roof system that is typically utilized in the area. As illustrated in Figure 4, a light wooden 20 40 60 80 100 120 time (segs) net of ceiling joists, resting on the walls, holds the clay roof tiles. At the Figure 2 Ground motion records atfirm (CA, DA) and soft (CC, DX) soil top vertex of the roof, this net is sites in Chilpancingo and Mexico City (stations operated by RIIS). supported by a main longitudinal wooden beam held by the transverse Structural Response detected from the severely damaged walls and three or four secondary Unreinforced Adobe Masonry houses. The f1fSt one is the result of interior horiZontal beams resting Structures inadequate bond in the adobe wall directly on the longitudinal walls. a) Villages corners, leading to comer cracks and Earthquake motions generated inertial Typical villages at the epicentral region eventually to an outward failure of the forces at the roof which were transmit- in southeast Guerrero are small rural walls (Figure 3). ted through the top longitudinal beam communities wich lack adequate roads directly to the transverse walls, leading and communications. At these locations, The second failure mode is associated to a critical out of plane flexural inhabitants build their own one-room failure (Figure 5). Lack of cross- houses with load bearing walls made of adobe blocks, with no specific lateral resistant elements to account for seismic induced forces. Adobe quality has been shown to playa key role in the struc- tural response, and major damage has been detected in the small villages where adobe blocks are directly exposed to weather conditions. Inten- sive rains were reported in the area prior to the earthquake occurrence, degrading the resistance and stiffness characteris- tics of the adobe blocks. In these villages, an average of 20% of the adobe houses collapsed, 75% were heavily damaged and only 5% received light, if any, damage. The least damaged houses are relatively new constructions with good adobe quality Figure 5 Transversalwallfailure in terms of maintenance and mixture. Figure 3 Lack ofbondingfailure due to inertialforce concentration Three typical failure modes can be in adobe house walls. in ridge longitudinal beam. 3 EERI SPECIAL EARTHQUAKE RE~~T -DECEMBER 1995 bracing in the roof system was a main unreinforced adobe structures were cause for this type of failure. It is worth heavily damaged, and about 70% suffered noting that in some cases, roof tiles fell light cracking. In addition to the failure down during the earthquake motion patterns described above, typical diagonal becausethey were not adequately shear cracks in walls were also observed connected to the roof support system. in this city. In spite of the obvious danger of this situation for the physical integrity of Reinforced Adobe Masonry Structures the inhabitants, it was beneficial from Although traditional construction in the structural point of view for the southeastern Guerrero is based on Figure 8 1Ypical castillos and houses; it acted as a seismic fuse by unreinforced adobe blocks, it is not cadenas system minimizing inertial forces at the roof uncommon to find adobe houses having level and, therefore, reducing damage rudimentary reinforced concrete elements In Ometepec, many adobe houses to the construction. that somehow improve lateral structural are also provided with this system The third main failure mode is the behavior. In many cases, it is clear that minimizing earthquake damage to result of stress concentrations at the these elements were added after the minor diagonal cracks. comers of openings in the walls occurrence of earlier earthquakes that (windows or doors), as illustrated in caused damage to the construction. Steel Structures (Schools) Figure 6. Owner's intuition and experience are Steel structures in the epicentral area usually the main guides for these retrofit- are not used, except for the relatively ting techniques, like the reinforcement new school facilities. Most of the local illustrated in figure 7, where reinforced schools follow a standard structural concrete elements were supplied because configuration based on a light structure of previous earthquake damage. In with thin-walled steel section frames general, supporting a zinc plate roof. Typical failure in these schools was the loosen- ing of non-structural dividing walls (Figure 9), and no serious structural damage was reported in these small Figure 6 Stress concentrationfailures buildings. at corner of door. A particular type ofunreinforced adobe masonry structures in the area are local churches, usually the largest adobe structures in the area. Most of them suffered extensive cracking Figure 7 Adobe house retrofitted after during the earthquake, but few of them past earthquakes collapsed. From an inspection of these structures, one can easily see damage any type of reinforced concrete retrofit- associatedwith previous earthquakes ting in adobe houses inlproved substan- Figure 9 Nonstructural dividing wall that was repaired by simply filling the tially the lateral resistance of the struc- failures in elementary school cracks with some mortar or clay.