Purpose The Copala earthquake produced a strong motion accelerograms at distances close to the source. Since these data have not yet been widely disseminated, we would like to invite people who are involved in prediction of strong motions to submit blind prediction of the ground motions. We ask that persons participate on an honor system, and describe as part of their submission all information that they may have heard about the strong motions, since some information may have been distributed by "word of mouth" - we have not made a systematic effort to avoid its circulation. Predictions are due by 8:00 AM on Monday, October 23, at which time I expect to have printed copies of a report on the earthquake and to put them into the mail to those on the GAA mailing list*. Data will be put on the UNR computer system shortly after that, and will become available over the WWW or by anonymous FTP. Our intention is to compare predictions in a way similar to the method used by SCEC to validate synthetic seismogram computation. We will submit a proposal for a workshop to compare results. As a part of that proposal, we would like to pay participants to prepare additional blind synthetic predictions for the strong motions from the eventual failure of the Guerrero gap. The following sections provide the following information: 1. A condensed copy of a paper that will appear in Seismological Research Letters on this earthquake, in the November-December, 1995 issue. 2. Additional information about the source of the earthquake that can be used for modeling purposes. 3. Relevant references. Description of the earthquake and the data. The text in the following sections has been borrowed and severely condensed from a paper by Anderson and others, to be published in Seismological Research Letters. John Anderson, Roberto Quaas, Shri Krishna Singh, Juan Manuel Espinosa, Alejandro Jimenez, Javier Lermo, Julio Cuenca, Francisco Sanchez-Sesma, Roberto Meli, Mario Ordaz, Sergio Alcocer, Bertha Lopez, Leonardo Alcantara, Enrique Mena, Clara Javier (1995). The Copala, Guerrero, Mexico Earthquake of September 14, 1995 (MW=7.4): A Preliminary Report. Introduction The Copala, Guerrero, Mexico earthquake is the largest earthquake to occur in Mexico since the destructive 1985 events. It occurred at a time when earthquake awareness in Mexico was heightened, as the country prepared for numerous commemorative activities of the 1985 main shock, which were to occur five days later. The strong motion data from this earthquake highlights the enormous progress in instrumentation that has occurred over the past ten years. Table 1 lists hypocentral parameters for the earthquake. The UNAM epicenter is shown in Figure 1. All the hypocenters derived from teleseismic data are demonstrably less reliable. The UNAM hypocenter listed here is consistent with observations that the S-P time at the Copala strong motion station is 3.0 seconds and that the hypocenter is southeast of Copala based on first motions (Anderson et al., 1995). For the teleseismic depth of about 20 km, this implies an epicentral distance of 13 km from Copala, a constraint that plays an important role in the UNAM location. We believe that the fault plane involved is the shallow-dipping plane (Table 1), which strikes parallel to the coast and the subduction zone and dips towards the north. This mechanism is consistent with focal mechanisms that have been observed in numerous previous earthquakes (Suarez et al., 1994) and it is consistent with observed locations of microearthquakes which define a shallow dip to the subduction thrust. Figure 1 shows several aftershock locations, obtained from the US Geological Survey Preliminary Determination of Epicenters. Although locations in this region based on teleseisms are likely to have very large uncertainties, the locations are likely to be reasonably well constrained along the direction parallel to the coast. Thus, the aftershocks can be used to estimate the possible extent of rupture during this event. They suggest that the epicenter was near the northwest limit of rupture and that the rupture may have extended 25-30 km towards the southeast. Field studies by the Instituto de Geofisica and the Instituto de Ingenieria at UNAM will eventually provide an improved description of aftershock locations. The Copala earthquake and aftershocks on Figure 1 are in the Ometepec seismic gap (Singh et al, 1991) and appear to be a success for the seismic gap hypothesis. A description of the evolution of seismic gaps in Guerrero is presented by Anderson et al. (1994). Nishenko and Singh's (1987) best estimate for the probability that an earthquake would recur in this region between 1986 and 1996 was 83% (55%-96%). Previous earthquakes that possibly ruptured within the gap include 1890 (7.5), 1907 (7.9), 1937 (7.5), 1950 (7.3), and 1982 (6.9, 7.0). Seismic Alert One special supplement to the southern Mexican accelerograph network is the seismic alert system. The main source of damaging earthquakes for Mexico City is the Mexican subduction thrust, which is more than 250 km away at closest approach. Since shear waves travel at about 3.5 km/sec, they take at least 70 seconds to reach Mexico City from the subduction zone. Considering this, seismologists in Mexico have long recognized that if a system on the coast of Mexico could be established to quickly recognize that a strong earthquake had begun, then some warning could be transmitted to Mexico City allowing time for some evasive actions. This concept was implemented in 1991 by the Centro de Instrumentacion y Registro Sismico (CIRES), under the auspices of Fundacion Javier Barros Sierra and under the leadership of Juan Manuel Espinosa-Aranda. The project was sponsored primarily by the Departamento del Districto Federal (Federal District). The seismic alert system is described in some detail by Espinosa et al (1989), Espinosa (1995), and Espinosa et al (1995). One aspect of the system is that the first 10 seconds of accelerograms to cause a trigger are saved on disk at the remote stations and can be analyzed at a later time. From the Copala earthquake, the initial parts of seismograms at Marquelia and Huehuetan are consequently available. Strong Motion Data Preliminary reports on strong motion data, as of September 21, included Almora et al. (1995), Alcantara et al. (1995), Anderson et al (1995), Guevara et al. (1995), Gonzalez et al. (1995), Quaas et al. (1995b), and Centro de Instrumentacion y Registro Sismico, A.C. (1995a,b). The first report by CENAPRED (Guevara et al., 1995) was published the same day of the earthquake. Table 2 lists locations of accelerograph stations within 150 km of the UNAM hypocenter. This represents only a small fraction of the total amount of data that will eventually be gathered from an estimated 300 to 400 stations. Considering the number of stations, coupled with the difficulty and expense of field work, it will probably take the rest of the year to visit all of the stations and to compile a complete picture of the strong motion data. Consequently, Table 2 also lists some stations for which data is not yet available. Guerrero Accelerograph Network We begin with the Guerrero accelerograph network, since it has stations closest to this epicenter. The network has been described by Anderson et al., (1994), and in more detail by Anderson and Quaas (1994). Past data from this network is cataloged in a series of reports (GAA-1 to GAA-17), available from either J. Anderson or R. Quaas. Figure 7 shows the distribution of stations in the Guerrero network. Stations from which accelerograms had been recovered by September 17 are shown with solid symbols. Open circles with dots inside had not been checked but are close enough that they might have triggered. Broadband Stations from the Instituto de Geofisica The Instituto de Geofisica is in the process of installing a network of about 25 broadband, high-dynamic-range stations throughout Mexico. These stations have both high gain velocity recorders and lower gain accelerometers. Twelve of these are operating in the field, and at least eight recorded the Copala earthquake. The nearest of these, at Pinotepa Nacional (Figure 1), is only slightly farther from the rupture zone than the GAA station at Copala. Centro Nacional de Prevencion de Desastres Network Data from this network of stations are described by Guevara et al. (1995) and Gonzalez et al. (1995). All of the stations in the attenuation array (Table 2) are sited on rock. The CENAPRED network in Mexico City has a vertical array of sensors at each of its stations. Instituto de Ingenieria Networks Some of the preliminary data recovered by the Instituto de Ingenieria are described in reports by Almora et al. (1995), Alcantara et al. (1995), and Quaas et al. (1995). The station distribution is described in Quaas et al (1993). Table 2 lists the station locations in Acapulco, which is within the 150 km radius selected for this study, and site condition where known. The Instituto de Ingenieria also maintains arrays that instrument several major buildings in Mexico City. Fundacion Javier Barros Sierra Network in Mexico City The Fundacion Javier Barros Sierra operates 86 stations in Mexico City. Preliminary data are illustrated by reports by Centro de Instrumentacion y Registro Sismico, A.C. (1995a,b). The stations are distributed widely throughout Mexico City. These stations however are not listed in Table 2 since all are greater than 150 km from the epicenter. Comision Federal de Electricidad Network The Comision Federal de Electricidad (CFE) operates 67 accelerograph stations at its most important hydroelectric power facilities. The only one that recorded the Copala earthquake was the network of stations at the El Caracol dam, about 145 km from the epicenter (Javier and Andrade, 1995). There is a free field station at this dam, with coordinates as listed in Table 2. Note that we do not yet have a digital file with any of the accelerograms from this site. Conclusions Figures Table 1. Hypocentral Parameters My suggested locations, etc. This is west of the UNAM epicenter, and moved there to match the observed S-P time at Copala. Location: 16.54o N 98.87o W 20 km depth Center of 16.52o N fault 98.60o W 20 km depth radius: 17.5 km Institute of Geophysics, UNAM Location: 16.54o N 98.73o W 20 km depth U. S. Geological Survey: Preliminary Determination of Epicenters Origin September 14, 1995 Time 14:04:31.7 GMT 8:04 AM local time Location 16.830 N 98.647 W 21 km depth Size mb=6.5 MS=7.2 Harvard Centroid 16.82 N Location 98.64 W 15 km depth (Fixed) Moment Best Double Couple: 1.8 * 10**27 dyne-cm Tensor Nodal Plane 1: strike 289, dip 11, slip 85 Nodal Plane 2: strike 114, dip 79, slip 91 Magnitude MW=7.4 University of Tokyo Centroid 16.47 N Location 98.34 W 17.8 km depth Moment Best Double Couple: 1.29 * 10**27 dyne cm Tensor Nodal Plane 1: strike 316.2, dip 8.5, slip 103.9 Nodal Plane 2: strike 122.2, dip 81.7, slip 87.9 Magnitude MW=7.3 University of Michigan Location 16.7 N 98.5 W 23 km depth Focal Strike 115.5, dip 77.4, rake 87.1 Mechanism Source Total moment: 1.11 * 10**27 dyne-cm Time Duration of pulse with this moment: 15 seconds Function Preferred fault dimention and geometry: Fault corner: 16.497 -98.913 16.320 -98.608 16.576 -98.449 16.753 -98.755 Dip: 11.0 Depth: 20 km Epicenter: 16.540 -98.870 Table 2 Strong Motion Stations within 150 km of the UNAM epicenter Station Operator Latitude Longitude Dist. Brief site condition Copala GAA 16.610 98.980 23 Rock Marquelia Alert 16.58 98.56 Soil Huehuetan Alert 16.61 98.81 Soil Pinotepa Nacional Geofis. 16.395 98.127 Rock Las Vigas GAA 16.758 99.230 54 Rock Acapulco LaSalle GAA 16.866 99.862 120 Rock Acaupuco Diana II 16.867 99.880 122 Soil Ocotillo GAA 17.036 99.880 129 Rock Pozuelos GAA 17.10 99.62 110 Rock Ocotito GAA 17.246 99.507 112 Rock Chilpancingo CENAPRED 17.446 99.452 127 Rock El Caracol CFE 17.95 99.967 145 Rock & dam Analysis of Guerrero data The following is a list of some of the papers that have presented analysis of some of the Guerrero data. Anderson J. G., and Q. Chen (1995a). Beginnings of Earthquakes in the Mexican subduction zone on strong-motion accelerograms. Anderson J. G., and Q. Chen (1995b). Illustrations of dependence of strong ground motions on magnitudes and hypocenter distances. Anderson J. G. and Y. Lei, (1994). Nonparametric description of peak acceleration as a function of magnitude, distance, and site in Guerrero, Mexico. Anderson, J. G. and R. Quaas (1988). Effect of magnitude on the character of strong ground motion: an example from the Guerrero, Mexico, strong motion network. Anderson, J. G. and R. Quaas, editors (1994a). Guerrero, Mexico, Accelerograph Network: Network Description, Report GAA 15, Seismological Laboratory, University of Nevada, Reno, 90 pages. Anderson, J.G., P. Bodin, J.N. Brune, J. Prince, S.K. Singh, R. Quaas, M. Onate (1986). Strong ground motion from the Michoacan, Mexico earthquake. Anderson J. G., J. N. Brune, J. Prince, R. Quaas, S. K. Singh, D. Almora, P. Bodin, M. Onate, R. Vasquez, and J. M. Velasco (1994a). The Guerrero accelerograph network, Geofisca Internacional. Castro, R. R., J. G. Anderson, and S. K. Singh (1990). Site response, attenuation and source spectra of S waves along the Guerrero, Mexico, subduction zone. Humphrey, J. R. Jr. and J. G. Anderson (1992). Shear wave attenuation and site response in Guerrero, Mexico. Humphrey, J. R. Jr. and J. G. Anderson (1994). Seismic source parameters from the Guerrero subduction zone. Ordaz, M. and S. K. Singh (1992). Source spectra and spectral attenuation of seismic waves from Mexican earthquakes, and evidence of amplification in the hill zone of Mexico City. Singh, S.K., L. Astiz, and J. Havskov (1981). Seismic gaps and recurrence periods along the Mexican subduction zone: a re-examination. Singh, S. K., M. Ordaz, J. G. Anderson, M. Rodriguez, R. Quaas, E. Mena, M. Ottaviani and D. Almora (1989a). Analysis of near-source strong motion recordings along the Mexican subduction zone. Singh, S. K., E. Mena, J. G. Anderson, R. Quaas, and J. Lermo (1990). Source spectra and RMS acceleration of Mexican subduction zone earthquakes. Singh, S. K., and M. Ordaz (1994). Seismic energy release in Mexican subduction zone earthquakes. Singh, S. K., R. Quaas, M. Ordaz, F. Mooser, D. Almora, M. Torres, and R. Vasquez (1995). Is there truely a "hard" rock site in the Valley of Mexico. Additional information Table 3. A velocity model The following velocity model has been calibrated for the path from the coast of Mexico to Mexico City. Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Thickness 2.0 2.3 4.70 36.0 1000.0 P-velocity 2.9 4.90 5.82 6.55 8.10 Q-P 200.0 200.0 400.0 400.0 800.0 S-velocity 1.60 2.85 3.38 3.81 4.70 Q-s 100.0 100.0 200.0 200.0 400.0 density 2.28 2.68 2.86 3.03 3.32 Site effects The following paper is the most recent description of site conditions in Guerrero stations. Humphrey, J. R. Jr. and J. G. Anderson (1992). Shear wave attenuation and site response in Guerrero, Mexico. According to Humphrey and Anderson, the following characteristics apply to the nearest sites. The site response is simplified. Residual amplitude is in log10 (amplitude at station/amplitude of average site). Station kappa Other comments (sec) Copala 0.0346 Weathered granitic gneiss. Average residual site response (kappa removed) is nearly flat, at an average amplitude of -0.1 log units (Fig 7a). Humphrey and Anderson found a narrow spectral hole at f=-0.2 Hz. Las Vigas 0.0323 Quartz monzonite. Average residual site response is not flat. The following points characterize the curve: log f Residual -0.5 0.1 0.25 -0.1 1.0 0.40 1.5 0.40 1.7 -.20 References Almora D., L. Alcantara, R. Quaas, M. Torres, R. Vazquez, J.M. Velasco, C. Perez, M. Macias, G. Castro, J. Ylizaliturri, H. Sandoval, (1995). El temblor del 14 de septiembre (Mw=7.3) regsitrado por la red del Instituto de Ingenieria. (Informe Preliminar), Instituto de Ingenieria, UNAM, septiembre de 1995. Alcantara L, D. Almora, G. Castro, M. Macias, H. Sandoval, J. Ylizaliturri, (1995), El sismo del 14 de septiembre de 1995 (ME=7.3) en el edificio Plaza Cordoba, acelerogramas obtenidos. Instituto de Ingenieria, UNAM, septiembre de 1995. Anderson J. G., and Q. Chen (1995a). Beginnings of Earthquakes in the Mexican subduction zone on strong-motion accelerograms, Bull. Seism. Soc. Am., 85, 1107-1115. Anderson J. G., and Q. Chen (1995b). Illustrations of dependence of strong ground motions on magnitudes and hypocenter distances, Submitted to Tectonophysics. Anderson J. G. and Y. Lei, (1994). Nonparametric description of peak acceleration as a function of magnitude, distance, and site in Guerrero, Mexico, Bull. Seism. Soc. Am., 84, 1003-1017. Anderson, J. G. and R. Quaas (1988). Effect of magnitude on the character of strong ground motion: an example from the Guerrero, Mexico, strong motion network, Earthquake Spectra 4, 635-646. Anderson, J. G. and R. Quaas, editors (1994a). Guerrero, Mexico, Accelerograph Network: Network Description, Report GAA 15, Seismological Laboratory, University of Nevada, Reno, 90 pages. Anderson, J.G., P. Bodin, J.N. Brune, J. Prince, S.K. Singh, R. Quaas, M. Onate (1986). Strong ground motion from the Michoacan, Mexico earthquake, Science 233, 1043-1049. Anderson J. G., J. N. Brune, J. Prince, R. Quaas, S. K. Singh, D. Almora, P. Bodin, M. Onate, R. Vasquez, and J. M. Velasco (1994a). The Guerrero accelerograph network, Geofisca Internacional, Vol. 33, pp. 341-371. Mexico city, Mexico. Anderson, J. G. et al. (1995). Accelerograms from the Guerrero, Mexico, strong motion array for the September 14, 1995 earthquake (Mw=7.4): A preliminary report. Castro, R. R., J. G. Anderson, and S. K. Singh (1990). Site response, attenuation and source spectra of S waves along the Guerrero, Mexico, subduction zone, Bulletin of the Seismological Society of America 80, 1481-1503. Centro de Instrumentacion y Registro Sismico, A.C. (1995). Informe preliminar sobre las aceleraciones captadas por la Red Basica de Acelerografos del D.F. Sismo de septiembre 14 de 1995, Informe RADF-023A, Mexico, septiembre de 1995. Centro de Instrumentacion y Registro Sismico, A.C. (1995). Sismo de septiembre 14 de 1995. Informe preliminar de las aceleraciones captadas por la Red de Acelerografos Kinemetrics., Informe RA-ICA-022A, Mexico, septiembre de 1995. Espinosa-Aranda, J. M. (1995). Sistema de alerta sismica, Ingenieria Civil 317, Septiembre 1995, 38-40. Espinosa-Aranda, J. M., B. Frontana, S. Maldonado, G. Legaria, M. Median, A. Uribe (1989). Sistema de control para el disparo de una alarma sismica, VIII Congreso Nacional de Ingenieria Sismica y VII Congreso Nacional de Ingenieria Estructural, Acapulco, Gro., Mexico. Espinosa-Aranda, J. M., A. Jimenez, G. Ibarrola, F. Alcantar, A. Aguilar, M. Inostroza, and S. Maldonado (1995). Mexico City seismic alert system (this issue). Gonzalez R., B. Lopez, Guevara E., R. Quaas, M. Legorreta, A. Flores, A. Mirano, A. Montalvo, R. Vazquez, M.A. Franco, J.M. Velasco, L. Gaytan, G. Espitia, J. Saucedo (1995). Reporte preliminar de los registros del temblor del 14 de septiembre de 1995 (M=7.2) obtenidos por la red de observacion sismica del Cenapred (2a. Parte), Centro Nacional de Prevencion de Desastres, Mexico, 21 de septiembre de 1995. Guevara E., B. Lopez, R. Quaas, M. Legorreta, A. Montalvo, A. Flores, R. Gonzalez, A. Mirano, R. Vazquez, M. Franco, J.M. Velasco, L. Gaytan, G. Espitia, J. Saucedo (1995). Reporte preliminar de los registros del temblor del 14 de septiembre de 1995 (M=7.2) obtenidos por la red de observacion sismica del Cenapred. (1a Parte), Centro Nacional de Prevencion de Desastres, Mexico, 14 de septiembre de 1995. Humphrey, J. R. Jr. and J. G. Anderson (1992). Shear wave attenuation and site response in Guerrero, Mexico, Bull. Seism. Soc. Am., 82, 1622-1645. Humphrey, J. R. Jr. and J. G. Anderson (1994). Seismic source parameters from the Guerrero subduction zone, Bull. Seism. Soc. Am., 84, 1754-1769. Javier C., E. Andrade (1995). El sismo del 140995 (Mw=3D7.3) en Guerrero. Acelerogramas obtenidos en la Presa Caracol. Informe 841.01-050/95, GEIC, Comision Federal de Elecricidad, Mexico. Nishenko, S. P. and S. K. Singh (1987). Conditional probabilities for the recurrence of large and great intraplate earthquakes along the Mexican subduction zone, Bull Seism. Soc. Am. 77, 2095-2114. Ordaz, M. and S. K. Singh (1992). Source spectra and spectral attenuation of seismic waves from Mexican earthquakes, and evidence of amplification in the hill zone of Mexico City, Bull. Seism. Soc. Am. 82, 24-43. Quaas, R., J. A. Otero, S. Medina, J. M. Espinosa, H. Aguilar and M. Gonzalez (1993). Base Nacional de Datos de Sismos Fuertes, Catalogo de Estaciones Acelerograficas 1960-1992, Sociedad Mexicana de Ingenieria Sismica, Mexico, D. F., Mexico. Quaas R., S. Medina, L. Alcantara, C. Javier, J.M. Espinosa, E. Mena, J.A. Otero, O. Contreras, L. Munguia (1995a). Base Nacional de Datos de Sismos Fuertes. Catalogo de Acelerogramas 1960- 1993, Sociedad Mexicana de Ingenieria Sismica, Mexico. Quaas, R., D. Almora, L. Alcantara, (1995b). Acelerogramas del sismo del 14 de septiembre de 1995 (M=7.2) obtenidos en el edificio Jalapa, Instituto de Ingenieria, UNAM, Mexico, 16 de septiembre de 1995. Singh, S.K., L. Astiz, and J. Havskov (1981). Seismic gaps and recurrence periods along the Mexican subduction zone: a re-examination, Bulletin of the Seismological Society of America 71, 827-843. Singh, S. K., M. Ordaz, J. G. Anderson, M. Rodriguez, R. Quaas, E. Mena, M. Ottaviani and D. Almora (1989a). Analysis of near-source strong motion recordings along the Mexican subduction zone, Bulletin of the Seismological Society of America 79, 1697-1717. Singh, S. K., E. Mena, J. G. Anderson, R. Quaas, and J. Lermo (1990). Source spectra and RMS acceleration of Mexican subduction zone earthquakes, PAGEOPH (Pure and Applied Geophysics) 133, 447-474. Singh, S. K., and M. Ordaz (1994). Seismic energy release in Mexican subduction zone earthquakes, Bull. Seism. Soc. Am., 84, 1533-1550. Singh, S. K., R. Quaas, M. Ordaz, F. Mooser, D. Almora, M. Torres, and R. Vasquez (1995). Is there truely a "hard" rock site in the Valley of Mexico, Geophys. Res. Lett. 22, 481-484. Seismological Laboratory, MS 174, University of Nevada, Reno, Nevada 89557. (702) 784-4265; jga@seismo.unr.edu Persons who wish to be on this mailing list should inform me with their mailing address at their convenience.