Bulletin of the Seismological Society of America, Vol. 84, No. 6, pp. 1754-1769, December 1994

Seismic Source Parameters from the Guerrero Subduction Zone

by James R. Humphrey, Jr. and John G. Anderson

Abstract A method to systematically determine seismic moment and comer frequency from shear-wave spectra has been applied to strong-motion data re- corded in Guerrero, Mexico. The method determines a model spectrum that minimizes the misfit from the observed spectrum, after a correction for fre- quency-dependent site response has been applied. The calculated moments for larger events in Guerrero are consistent with moments determined from telese- ismic body waves adjusted for differences in focal depth and consequent as- sumed differences in the velocity and density structure in the source region. The average stress drop for the 82 earthquakes with magnitudes from 3 to 7 is 150 bars. The spatial distribution of the stress drops shows a distinctive pattern. At the northwest end of the Guerrero gap, adjacent to the aftershock zones of large earthquakes in 1979 and 1985, there is a mix of low- and high-stress-drop events. At the southeast end of the Guerrero gap, where the most recent large earth- quake occurred in 1957, there are predominantly high-stress-drop events. This pattern might be caused by the different relative positions of these two active regions in the earthquake cycle, although several other explanations cannot be ruled out.

Introduction

The Pacific coast of southern Mexico is the site of northeastward subduction of the Cocos Plate under the North American Plate via the Middle America Trench. In the past 200 yr this region has been the site of nu- merous large and great interplate earthquakes, including the Ms 8.1 Michoacan earthquake in September 1985. The short recurrence intervals of Guerrero earthquakes (30 to 60 yr; Singh et al. , 1981) led to the development of the Guerrero Accelerograph Array to record the strong ground motion from future major earthquakes (Anderson et al., 1991a). The short recurrence intervals and the high rate of seismicity for smaller events provide the oppor- tunity to observe the evolution of the seismic cycle of a major plate boundary. Nishenko and Singh (1987) have inferred segmentation of the Mexican portion of the Middle America Trench from rupture zones of the most recent earthquakes. Two of the segments along the Guer- rero coast have high probabilities for recurrence of M > 7.0 events. Similarly, Anderson et al. (1989b) have identified the Guerrero region as one most likely to ex- perience a great earthquake or several large events, based on calculations of seismic strain release.

The Guerrero Accelerograph Array (GAA) is a 30 station network of strong-motion accelerographs de- ployed in and around the state of Guerrero, Mexico (Fig. 1). It is currently operated as a joint project of the In- stituto de Ingenieria of the Universidad Nacional Auton-

oma de Mexico (UNAM), in Mexico City, and the Seis- mological Laboratory, Mackay School of Mines, of the University of Nevada, Reno (UNR). The purpose of the array is to monitor the Guerrero subduction zone and provide close-in recordings of the anticipated rupture of the seismic gap. The Guerrero seismic gap is given a conditional probability of occurrence of 56 to 79% by Nishenko and Singh (1987) for the period 1986 to 1996. Between 1985, when the GAA was installed, and 1992, over 927 accelerograms were recorded from 385 earth- quakes ranging in magnitude from less than 3 to greater than 8 (Anderson et al. , 1991a; 1994). The large volume of data available and the spread in magnitude ranges al- low comprehensive investigations of source spectra, site response, attenuation, and earthquake source parame- ters. Humphrey and Anderson (1992) describe an inves- tigation into the spectral shape and site response at each GAA station. This article continues that research with an analysis of the seismic moment and stress drop of these earthquakes. The result is a picture of the spatial distri- bution of stress drops over a 5-yr interval in the vicinity of a seismic gap.

Guerrero Subduction Zone

Tectonics

The western coast of Mexico has been a major plate boundary for at least 100 Ma and possibly longer (Karig

1754

Seismic Source Parameters from the Guerrero Subduction Zone 1755

et al., 1978). The subduction zone may, however, have been in its present location for as little as 10 Ma, since there is evidence for truncation of the continental margin prior to the late Miocene (Karig et al., 1978; Pindell and Dewey, 1982). The oceanic crust being subducted in Mexico is relatively young and buoyant. This may cause the shallow dip (10 ° to 15 °) of the Benioff zone observed along the Guerrero coast (Suarez et al., 1990). This buoyancy of the oceanic plate may explain the flattening of the thrust interface beyond about 150 km from the trench axis (Suarez et al., 1990), in contrast to the sub- duction geometry along the South American coast, where the older oceanic crust has a much steeper dip.

The long history of subduction in western Mexico introduces the possibility of complexities in geologic structure on virtually all length scales. In spite of this, all indications from the surficial geology suggest that the upper crust near the coast is predominantly composed of highly metamorphosed Precambrian and/or Paleozoic rocks and granitic intrusives rocks of various ages (de Cserna, 1989). The northern part of the Guerrero net- work (north of 18 ° N) is closer to the trans-Mexican vol- canic belt, and some of these stations are located on Ter-

tiary extrusive volcanics that may overlie Mesozoic sedimentary rocks. Most of the seismic data collected to date, however, are from the southern part of the array. Thus, most of the stations have a continuous crystalline propagation path from the seismic sources. Variations in rock properties near the stations do cause path and/or site effects that complicate the recovery of source pa- rameters from Fourier spectra. This point will be dis- cussed further in a later section.

Seismicity

The historical record for large earthquakes along the Guerrero subduction zone (Fig. 1) is relatively complete back to 1806 (Singh et al., 1981; Anderson et al., 1989b). This record is mainly comprised of thrusting events near the coast, although several large inland events, assumed to be located within the subducting plate, have also oc- curred. The average repeat time for. the large thrust events is between 30 and 60 yr (Kelleher et al., 1973; Singh et al., 1981; McNally and Minster, 1981), but there is con- siderable variation from this average. Kelleher et al. (1973) first identified several seismic gaps along the

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1756 J.R. Humphrey, Jr. and J. G. Anderson

Middle America Trench. One of these gaps, the Mi- choacan seismic gap, ruptured in September 1985, in two earthquakes (Ms 8.1, Ms 7.6). Another gap identified in their study was the Guerrero seismic gap, which extends from near the southeastern limit of rupture of the Mi- choacan event to south of Acapulco (Singh et al., 1981). Anderson et al. (1989b) calculated that a seismic mo- ment deficit of 20 x 1027 dyn-cm has accumulated in this gap since the last rupture sequence between 1899 and 1912. This study examines earthquake source pa- rameters in the vicinity of the Guerrero gap that are re- lated and may provide predictive insight into the next great earthquake in Guerrero.

The earthquakes in the region surrounding the Guer- rero gap recorded by the Guerrero accelerograph net- work from January 1985 to January 1990 are shown in Figure 2. The events range in magnitude from 2.2 to 8.1. The Guerrero gap is manifest on this map as a re- gion with reduced numbers of moderate-sized earth- quakes. There is clustering of seismicity at either end of the gap. The region northwest of the gap coincides with the southeastern extent of the Ms 7.6 Michoacan after- shock in 1985. This article will refer to this northwest cluster of earthquakes as the Petatlan active region. The

cluster of seismicity southeast of the gap is near or north- west of the rupture zone of the 1957 Acapulco earth- quake (M 7.5), and northwest of the rupture zone of the 1989 San Marcos earthquake (Ms 6.9). This area will be referred to as the Acapulco active region. The gap itself is not completely aseismic, but rather has significant mi- croearthquake seismicity (Suarez et al., 1990). Inland of the coastal band of seismicity are events which, when accurate locations are available, have focal depths be- tween 30 and 80 kin.

A bimodal distribution of focal mechanisms, con- sisting of thrusting along the plate interface and downdip extension in the subducted crust, has been observed along the Middle America Trench, including Guerrero, from teleseismic studies (Dean and Drake, 1978; LeFevre and McNally, 1985). Suarez et al. (1990), using information from a microearthquake network operating in the Guer- rero gap, show that these two classes of mechanisms are spatially distinct. The thrust faulting zone starts about 50 km from the trench axis and is approximately 35-km wide. The focal depths range from 10 to 25 km, and composite focal mechanisms demonstrate thrust faulting occurring along the plate interface. This band results from a zone of strong seismogenic coupling between the two

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Seismic Source Parameters from the Guerrero Subduction Zone 1757

plates. The coastal normal faulting zone of seismicity is located inland of the first band, with a clear aseismic zone separating the two. This band has focal depths be- tween 32 and 42 km, and composite focal mechanisms suggest normal faulting as the dominant mode of stress release. The orientations of the tension axes in these nor- mal solutions are more or less horizontal and perpendic- ular to the subduction zone. This is interpreted as a zone of intraplate (Cocos) deformation associated with bend- ing of the slab to a subhorizontal position. The relatively restricted downdip extent of the coastal zone of thrust faulting in the Guerrero gap is similar to that determined in the Michoacan area from aftershock studies of the 1985 sequence (UNAM Seismology Group, 1986; McNally et al., 1986). Farther inland from the region of coastal nor- mal faulting, again separated by another aseismic zone, there is an inland zone of normal faulting that includes many of the earthquakes studied teleseismically by Le- fevre and McNally (1985). The deeper focal depths (be- tween 40 and 60 km) and the subhorizontal tension axes suggest deformation in the bouyant subducted plate.

Several prior studies have estimated stress-drop pa- rameters. Most previous large events have had low stress drops, based on teleseismic analysis. Singh et al. (1981) and Chael and Stewart (1982) found static stress drops below 10 bars for several large interplate events with Ms --- 7.1. The 19 September 1985 earthquake (Ms = 8.1) had a stress drop of 19 bars (Anderson et al., 1986; Priestley and Masters, 1986), and the 21 September 1985 earthquake (Ms = 7.6) had stress drop of 33 bars (Priest- ley and Masters, 1986). The 25 October 1981 Playa Azul earthquake (Ms = 7.3) was apparently higher with a stress drop of about 45 bars (LeFevre and McNally, 1985). Based on analysis of moderate to large events recorded at shorter distances, prior studies have obtained mixed results. Castro et al. (1990) obtained low stress drops (under 20 bars) for four events with Ms between 4.2 and 6.9, based on the Brune (1970) model, but Rebollar et al. (1991) reported stress drops averaging about 100 bars (ranging from 15 to 250 bars) for seven events in Oax- aca, immediately southeast of Guerrero, with magni- tudes between 3.6 and 4.8. Singh et al. (1989a, 1990) interpreted peak accelerations and acceleration spectra for events with Ms >= 6.0 as requiring an rms drop of 40 to 100 bars. Below, we apply a consistent methodology to measure stress-drop variation across the entire region.

Data Acquisition

Instrumentation

The GAA consists of Kinemetrics DSA-1 and PDR- 1 and Terra Tech DCA-333 accelerographs with a full scale range of +2 g (Quaas et al., 1987). The majority of the stations are located on exposures of crystalline rock. The location of the sites and the type of rock pres-

ent at the surface are given by Anderson et al. (1994). The accelerographs are installed on low concrete piers sited or the least weathered rocks available at the site. All the sites have Omega navigation signals for accurate timing of events. Accurate timing improves location ac- curacy, since the accelerographs are often the closest seismographs to earthquakes from the Guerrero region. Detailed information on the operation of the array, in- cluding instrument parameters, calibrations, and the re- corders at each site, is in Anderson et al. (1994) and Quaas et al. (1987).

Earthquake Locations

All events that triggered at least one Guerrero ac- celerograph are located using phase arrival times from the Mexican national array, SISMEX, and phase arrivals obtained from the accelerograms. The locations are cal- culated with the computer program HYPOINVERSE (Klein, 1979) assuming a regional velocity model (Anderson et al., 1991b). Since the strong-motion stations are usually the closest recorders to the epicenter, the addition of these local data is crucial in constraining the hypocentral so- lutions. Details of this analysis and complete listings of the hypocentral solutions, peak acceleration data, and plots of the accelerograms are given in the GAA annual data summary reports available on request from the Seis: mological Laboratory at UNR (Anderson et al. 1987a, b, 1988, 1989a, 1990a, b, 1991a, b,c, Quaas et al., 1988, 1990).

Table 1 lists the locations and magnitudes of the earthquakes used in this study, along with the vertical and horizontal location errors. When available, both P and S phase arrivals are used in the locations, but often for events where the S wave triggers the accelerograph, the S-P time exceeds the length of the pre-event memory and the P arrival is not recorded. There are some events for which the location resolution is poor. This is usually the case when only one GAA station triggers find the hy- pocentral solution must be constrained using the more distant SISMEX stations. This was most common in 1985 and 1986, when the array triggering parameters were being refined. These early events are considered important be- cause of their proximity to the 1985 Michoacan rupture zone, and they are included in this study if their location appears reasonable. About 10% of the events originally analyzed were later rejected for poor location quality. Table 2 gives the number of events recorded by the GAA in the active regions defined in this article, and those events used in the spectral analysis.

Figure 3 shows the depth distribution of the earth- quake locations. The majority of the events, including the three largest events, are located along the coast, be- tween 10 and 25 km below the surface. A few events in the interior are located deeper than 30 kin.

1758 J.R. Humphrey, Jr. and J. G. Anderson

Table 1 Hypocentral Location of Guerrero Events

Date Origin Lat. Long. Depth rms ERH ERZ Gap Event (m-d-yr) (hr) (sec) (°N) (°W) (km) (sec) (kin) (kin) (°) MjI* m j Ms ~

8 5 0 6 0 7 - 0 2 - 8 5 0 1 2 5 4 7 . 8 2 1 6 . 6 5 2 9 9 . 2 1 0 28 .1 0 . 6 8 11.4 16.3 248 3 .7 - - - -

8507 0 7 - 0 4 - 8 5 0851 3 7 . 2 2 1 7 . 5 6 4 9 6 . 9 7 1 4 4 . 8 0 . 8 4 2 .6 9 . 6 134 4 . 0 - - - -

8 5 1 0 0 7 - 0 7 - 8 5 1827 5 7 . 1 0 16 .791 1 0 0 , 4 3 7 10.5 3 .07 6 3 . 9 7 5 . 7 275 3 .9 - - - -

8511 0 7 - 1 9 - 8 5 2 2 2 5 3 8 . 5 6 16 ,933 9 8 . 1 3 6 19.8 0 .95 8 .8 7 3 . 4 228 4 . 0 - - - -

8 5 1 2 0 8 - 2 1 - 8 5 0 5 1 4 4 7 . 7 9 17 .544 100 .771 13.9 0 . 0 3 4 1 . 8 34 .8 195 3 .6 - - - -

8513 0 8 - 2 2 - 8 5 1950 5 3 , 3 3 1 7 . 2 1 0 1 0 1 . 1 0 4 18.3 0 . 0 6 9 9 . 0 1,0 358 3 .3 - - - -

8 5 1 4 0 9 - 0 4 - 8 5 1327 5 3 . 0 3 17 .567 101 ,308 20 .5 0 . 0 8 15.1 4 9 . 0 265 4 . 0 3 .2 - -

8517 0 9 - 1 9 - 8 5 1333 10 .12 17 .407 1 0 1 . 5 0 6 4 . 5 1 .15 8 9 . 9 8 7 . 6 3 1 0 4 . 3 - - - -

8523 0 9 - 2 1 - 8 5 0 2 1 0 3 7 . 0 9 1 7 . 3 2 0 100 .968 2 9 . 0 0 . 9 6 81 .6 56 .1 169 4 . 2 - - - -

8 5 2 4 0 9 - 2 1 - 8 5 0 9 0 7 12 .23 17 .161 1 0 1 . 1 3 9 0 . 6 0 . 2 8 - - - - 354 3.1 - - - -

8 5 2 6 0 9 - 2 8 - 8 5 0 3 5 2 5 0 , 2 8 1 7 . 5 3 2 101 .111 9 .8 0 . 2 7 13.1 2 2 . 0 124 4 .5 5.1 5 . 0

8527 0 9 - 3 0 - 8 5 0 9 0 8 11 .03 17 .078 101 ,071 7 . 0 1 .94 34 .3 17.0 314 3 . 2 - - - -

8528 10-03-85 0 6 2 9 5 0 . 9 5 1 7 . 2 8 4 1 0 1 . 1 3 4 23 .1 0 . 0 7 7 . 0 2 . 4 287 4 . 4 4 .3 - -

8529 10 -09 -85 1708 2 7 . 6 6 1 7 . 4 1 6 1 0 0 . 9 1 7 7 . 6 0 ,41 9 .7 3 .9 177 3 .3 - - - -

8534 11-22-85 0 4 0 2 19 .39 1 7 . 1 8 0 1 0 1 . 1 6 0 21 .9 0 . 0 5 - - - - 360 3 .8 - - - -

8535 12-05-85 1542 2 1 . 8 5 17 .853 1 0 1 . 2 8 7 10.4 0 . 7 5 9 . 6 2 1 . 6 198 4 . 4 3 . 7 - -

8 5 3 6 12-21-85 1642 3 4 . 6 7 17 .291 100 .973 14.5 1 .92 86.1 4 8 . 8 155 3 .7 - - - -

8 5 3 7 12-21-85 1702 3 1 . 2 9 17 .303 1 0 1 . 1 7 0 4 . 9 3 . 7 0 8 0 . 4 78 .1 295 3 .4 - - - -

8538 12-22-85 1843 4 4 . 6 3 17 .175 101 .125 14.5 1 .22 9 9 . 0 9 .9 345 3 . 6 - - - -

8 5 3 9 12-24-85 1928 1 7 . 5 6 17 .165 1 0 1 , 1 7 2 10.7 0 .01 9 9 . 0 3 .3 358 3 .3 - - - -

8602 0 1 - 1 2 - 8 6 1651 21 .21 17 .917 101 ,893 4 .1 0 . 4 5 3 .6 2 . 0 222 4 . 7 5.1 - -

8 6 0 6 0 1 - 2 4 - 8 6 0 9 2 6 4 4 . 5 4 1 7 . 3 4 2 9 9 . 9 9 5 6 . 0 0 . 2 0 2 .7 3 .3 251 3 .8 - - - -

8607 0 1 - 2 4 - 8 6 1803 3 2 . 8 4 17 .237 1 0 1 . 1 4 4 20 .3 0 , 1 9 6 . 5 1.0 289 4 . 6 4 .5 - -

8608 0 1 - 2 6 - 8 6 0 0 5 5 5 9 . 6 9 1 7 . 3 7 4 1 0 0 . 9 1 7 12.2 2 . 0 0 6 2 . 8 7 6 . 5 168 4 . 0 4 . 0 - -

8 6 0 9 0 1 - 2 6 - 8 6 0303 14 .42 1 7 . 4 6 9 101 .228 6 . 6 1 ,46 62 .1 8 0 . 4 265 3 .8 4 . 0 - -

8 6 1 2 0 1 - 2 9 - 8 6 2001 2 0 . 5 0 1 7 . 3 5 7 1 0 1 . 4 2 6 .7 0 . 1 9 2 . 2 5 . 0 248 4 . 7 4 . 6 - -

8613 0 2 - 0 1 - 8 6 0331 3 6 . 1 6 1 6 . 9 4 6 1 0 0 . 1 3 9 36 ,1 0 . 3 2 1.8 2 ,0 256 4 . 0 4 .1 - -

8 6 1 4 0 2 - 0 7 - 8 6 2 1 2 6 5 3 . 4 4 17 ,653 101 .455 19,7 0 . 1 5 4 3 . 8 17 .4 223 4 . 7 4 . 9 - -

8 6 1 6 0 2 - 1 8 - 8 6 1359 5 3 . 4 3 17 .009 9 9 . 2 0 9 3 1 , 2 0 . 1 9 3 . 6 3.1 182 4 . 0 - - - -

8 6 1 9 0 3 - 1 8 - 8 6 1114 2 5 . 1 0 17 .614 1 0 1 . 2 4 0 4 , 8 0 . 0 7 13.3 98 .1 193 4 . 5 4 . 6 - -

8 6 2 0 0 3 - 2 4 - 8 6 2 3 3 9 2 2 . 3 6 17 .278 101 .118 16.1 0 . 2 6 - - - - 357 - - 3 ,6 - -

8622 0 4 - 3 0 - 8 6 0 7 0 7 1 8 . 9 l 1 8 , 0 2 4 1 0 3 . 0 5 7 2 0 . 0 0 .81 16 .2 9 9 . 0 220 6 . 4 6 .2 7 . 0

8 6 2 6 0 5 - 2 9 - 8 6 2031 2 1 . 0 3 16 .851 9 8 . 9 3 2 3 5 . 6 0 . 4 4 1.9 4 . 0 157 5 . 0 5 ,2 4 . 2

8627 0 5 - 2 9 - 8 6 2 1 4 2 4 8 . 4 7 16 .803 9 8 . 8 6 6 2 7 . 0 0 . 6 9 14.5 3 3 . 4 173 4 .3 4 , 0 - -

8628 0 6 - 1 1 - 8 6 2 1 3 9 5 2 . 8 3 17 .857 1 0 0 . 3 4 5 50 .3 O. 11 9 .7 2 5 . 9 196 4 . 7 5 ,1 - -

8 6 2 9 0 6 - 1 6 - 8 6 0551 0 5 . 5 5 17 .076 9 9 . 6 2 1 33 .8 0 . 2 2 1.9 3 .0 191 4 .3 4 , 5 - -

8 6 3 0 0 6 - 1 9 - 8 6 0 4 3 9 4 3 . 6 7 18 .168 1 0 1 . 5 7 3 9 .5 2 . 0 4 22 .1 5 3 . 2 129 4 . 8 5 . 2 - -

8 6 3 2 0 6 - 2 7 - 8 6 0 5 1 7 4 1 . 5 6 1 6 . 9 0 9 9 9 , 0 4 7 16.1 0 . 1 2 38 .3 4 4 . 4 212 4 . 0 3 .7 - -

8641 0 9 - 2 1 - 8 6 0743 3 3 . 0 4 17 .085 9 9 . 5 0 6 2 0 . 0 1 .14 15 .2 18 .0 156 3 .2 - - - -

8643 10 -14 -86 2 0 4 7 0 9 . 2 7 16 .920 1 0 0 . 2 9 2 13.3 1.43 9 . 6 10.1 270 3 . 9 - - - -

8 6 4 4 10 -31 -86 1241 2 4 . 9 8 17 .067 9 9 . 7 8 8 3 3 . 4 0 . 1 0 0 . 0 0 . 0 2 7 0 2 . 6 - - - -

8645 11 -04 -86 0 1 5 8 0 6 . 6 5 1 7 . 7 8 9 102 .021 15.2 0 . 1 9 14.5 13.2 300 4 . 8 4 .8 - -

8647 12 -14 -86 0 7 2 8 0 2 . 0 2 17 .373 1 0 0 . 8 1 3 2 0 . 0 0 . 5 8 0 . 0 0 . 0 182 4 .3 - - - -

8648 1 2 - 1 6 - 8 6 1856 4 6 . 3 0 1 7 . 1 8 4 9 9 , 9 4 4 3 7 . 7 0 . 0 6 4 . 2 6 . 2 244 4 .3 - - - -

8 7 0 9 0 3 - 2 6 - 8 7 1838 2 7 . 9 7 1 6 . 8 9 9 100 .061 17 .2 0 . 2 2 1.9 1.1 256 4 . 8 4 ,8 4 .5

8 7 1 0 0 4 - 0 2 - 8 7 1601 5 2 , 4 6 16 .839 9 9 . 6 9 4 18 .0 0 . 2 8 1.7 0 . 6 153 4 . 0 4 .8 - -

8718 0 6 - 0 7 - 8 7 1330 12 .65 1 6 . 5 1 0 9 8 . 9 1 3 17 .9 0 .61 7 .3 2 .2 279 4 . 9 5 .5 4 .8

8719 0 6 - 0 9 - 8 7 1537 0 5 . 4 3 16 .943 9 9 . 8 4 4 29 .7 0 .21 1.5 0 .8 145 4 . 0 4 . 2 - -

8721 0 6 - 2 1 - 8 7 1300 5 0 . 6 7 1 7 . 3 3 6 1 0 0 . 1 0 0 1.4 0 . 4 0 2 .5 9 .5 126 3 .8 - - - -

8723 0 7 - 0 5 - 8 7 1818 4 8 . 5 6 16 .217 9 8 . 8 2 5 8 .0 0 .21 3 .9 1.6 241 4 . 7 4 .5 - -

8737 10 -25 -87 0431 4 9 . 3 4 17 .324 101 .245 16.3 0 . 2 9 2 .0 0 . 6 202 4 . 4 - - - -

8738 10-25-87 2 1 1 9 3 1 . 4 0 17 .451 101 .115 6 . 7 0 . 3 0 5 . 0 5 . 6 195 3 .8 - - - -

8 7 4 2 11 -22 -87 0511 5 3 . 4 6 17 .135 1 0 0 . 0 6 8 2 3 . 6 0 . 2 9 1,2 1.1 83 4 . 2 - - - -

8 7 4 4 12 -03 -87 1206 0 2 . 0 3 17 .455 1 0 1 . 0 9 7 17.9 0 . 1 5 2 .6 1.7 141 3 .9 4 . 0 - -

8801 0 1 - 2 2 - 8 8 2 1 5 2 34 .21 17 .008 101 .063 9 .3 2 ,21 18.2 19.3 275 4 . 2 - - - -

8802 0 1 - 3 0 - 8 8 1157 11.61 17 .293 1 0 0 . 6 8 5 2 6 . 4 1.11 2 . 9 3 .7 181 3 . 6 - - - -

8803 0 2 - 0 5 - 8 8 0 6 0 9 0 0 . 3 4 18 .155 9 8 . 9 1 4 4 2 . 3 0 . 4 0 1 .0 4 . 5 109 4 .3 - - - - 8804 0 2 - 0 8 - 8 8 1351 2 9 . 9 3 17 .494 1 0 1 , 1 5 7 19 .2 0 . 1 9 1.3 0 .5 146 5 . 0 5 .5 5 .7

8806 0 2 - 2 5 - 8 8 2 2 1 2 1 3 . 8 4 1 7 . 2 1 0 9 9 . 8 4 3 5 4 . 0 0 . 5 0 1.9 1.7 117 3 .8 - - - -

8 8 1 6 0 5 - 2 9 - 8 8 0611 4 0 . 7 0 18 .109 1 0 0 . 0 5 0 53 .5 0 . 4 0 1.2 2 . 0 101 4 . 2 4 . 6 - -

8818 0 6 - 1 6 - 8 8 2 2 5 9 1 9 . 4 6 18 .073 9 9 . 9 6 7 59 .8 0 . 1 7 1 .2 1.4 103 4 .1 - - - - 8 8 2 6 0 8 - 1 6 - 8 8 0 4 2 0 5 0 . 2 7 16 .967 9 9 . 8 0 1 21. ' ] 0 . 3 2 0 . 9 0 . 6 103 4 . 6 4 . 2 - -

Seismic Source Parameters from the Guerrero Subduction Zone 1759

Table 1--Continued

Date Origin Lat. Long. Depth rms ERH ERZ Gap Event (m-d-yr) (hr) (sec) (°N) (°W) (kin) (sec) (km) (kin) (0) Mn* mb* Ms t

8831 09-07-88 2017 51.52 17.231 100.255 18.2 0.40 2.0 1.9 80 4.1 - - - - 8833 09-14-88 0607 33.69 17.736 101.474 28.1 0.50 2.6 2.6 170 4.3 4.0 - - 8834 09-14-88 2036 37.53 18.340 102.430 38.2 0.43 1.4 1.1 164 4.7 4.9 - - 8842 11-26-88 1626 01.79 17.391 100.682 37.0 0.56 7.6 7.8 128 3.8 - - - - 8845 12-06-88 1454 41.70 16.887 100.065 11.7 0.47 2.5 1.5 215 4.3 - - - - 8907 03-02-89 1720 01.93 16.945 100.466 9.5 0.49 2.0 1.3 269 3.6 4. t - - 8911 03-09-89 1010 38.27 17.209 99.863 40.0 0.43 1.0 1.9 72 3.7 4.8 - - 8912 03-10-89 0519 51.04 17.446 101.089 17.6 0.43 2.1 1.0 140 5.0 5.1 4.8 8913 03-13-89 0325 23.52 17.002 99.094 27.0 0.41 1.4 4.8 161 3.6 4.8 - - 8915 03-13-89 0408 41.17 16.960 99.048 32.3 0.42 2.2 3.5 123 4.4 4.9 - - 8924 04-25-89 1429 00.36 16.603 99,400 19.0 0.44 1.4 0.6 207 6.5 6.9 6.9 8929 05-02-89 0930 16.72 16.637 99.513 13.4 0.46 1.6 1.1 210 5.1 5.2 4.9 8942 07-06-89 2321 02.36 17.405 101.485 7.6 0.53 3.7 2.0 258 4.9 4.8 - - 8948 08-12-89 1531 49.24 18.126 100.030 56.5 0.56 1.4 3.1 105 4.8 5.0 4.5 8949 08-17-89 0054 03.21 17.118 100.035 25.6 0.38 1.0 1.9 69 4.8 4.7 - - 8950 08-21-89 0933 41.26 17.044 99.487 34.5 0.44 1.3 1.1 67 4.7 4.8 - - 8961 10-08-89 2232 40.57 17.189 100.213 36.0 0.22 0.5 1.0 77 5.0 5.0 4.1 8974 11-09-89 0836 40.85 16.844 99.648 9.9 0.30 0.8 1.0 173 5.1 5.1 - - 9019 05-11-90 2343 49.12 17.046 100.840 11.7 0.28 1.2 1.0 199 5.3 5.3 4.9 9024 05-31-90 0735 26.93 17.106 100.893 15.8 0.37 1.3 0.7 210 5.5 5.8 5.9

*Mn is the coda magnitude calculated by the Instituto de Ingeneria, UNAM, using the SISMEX stations. tm b and Ms are from the USGS Earthquake Data Reports.

Table 2 Events Analyzed in Each Region

Active Regions * Magnitude

Range* Total* Petatlan Acapulco Interior

3 .0 -3 .5 7 (56) 6 (12) 1 (39) 0 (5) 3 . 5 - 4 . 0 19 (81) 9 (30) 9 (42) 0 (3) 4 . 0 - 4 . 5 27 (61) 10 (21) 12 (26) 3 (8) 4 . 5 - 5 . 0 18 (25) 6 (8) 4 (6) 4 (5) 5 .0 -5 .5 7 (8) 3 (3) 4 (4) 0 (0) 5 . 5 - 6 . 0 1 (3) 1 (1) 0 (0) 0 (0) 6 . 0 -6 .5 1 (1) 0 (0) 0 (0) 0 (0) 6 . 5 - 7 . 0 1 (1) 0 (0) 1 (1) 0 (0)

*Magnitude bins based on M~] (see footnote for Table 1). *Total number of events analyzed includes some earthquakes from

outside the active regions, *The first value indicates number of events analyzed; enclosed in

parenthesis is the total number of events recorded for the region.

Data Analysis

To estimate earthquake source parameters from shear- wave amplitude spectra, a model for the other influences on the spectra is required. The recorded spectrum, S( f ) , at a site may be generally described as follows:

S( f ) = I ( f )G(R)P( f )SF( f )A( f ) ,

where I ( f ) is the recording instrument transfer function, G(R) corrects for the attenuation of body waves due to geometrical spreading, P( f ) describes anelastic atten-

10

20

.c" 30

50

60 0

Guerrero: 1985-1989

L 1

I

lO I I ~ I I

20 30 40 50 60 70 Number of events

Figure 3. Depth distribution of all earthquakes recorded by the Guerrero accelerograph array be- tween 1985 and 1989.

80

1760 J .R. Humphrey, Jr. and J. G. Anderson

uation along the ray path, SF(f ) is a frequency-depen- dent site response function accounting for effects on the spectrum due to variations in the seismic structure near the recording site, and A ( f ) is the seismic source spec- trum. The instrument response is flat to acceleration for frequencies less than the comer frequency of the sensor (30 or 50 Hz, depending on the instrument). Body-wave amplitude decay due to geometrical spreading is as- sumed to be proportional to R -1. Since most of the spec- tra are recorded at distances less than 100 kin, this as- sumption seems justifiable (Herrmann, 1985). Separation of attenuation and the site response from the source spec- trum is ambiguous. In view of this ambiguity, this anal- ysis assumes that A( f ) has the shape of the Brune (1970) total stress-drop model. Deviations from this shape at frequencies higher than the source comer frequency are modeled as attenuation effects using the empirical pa- rameter, K, of Anderson and Hough (1984). Variation in the spectral shape not accounted for by the attenuation parameters is interpreted as site effects. Implementation of this approach has been described in detail in Anderson and Humphrey (1991) and Humphrey and Anderson (1992) and is outlined below.

Spectral Analysis

The S-wave amplitude spectra were calculated from the Fourier transform of the horizontal components at each station for each event recorded by the array. The time series were windowed from the start of the shear pulse to a point at which 95% of the shear energy was contained in the window. Each component was baseline corrected by removing the mean. Earthquake source pa- rameters were estimated from each component by fitting the observed spectra with a model spectrum. The model spectrum, M(f) , is parameterized as a smooth approx- imation of the observed acceleration spectrum, S(f) , as follows:

0.85 Mo ( 2 4 ) 2 - ~ s (1) M ( f ) - 41rp/33R 1 + ( f i f e ) 2 e

In equation (1), Mo is the seismic moment, f is the source corner frequency, K is the spectral decay parameter, R is the source to station distance, p is the density, and/3 is the shear-wave velocity. Thus, M(f ) incorporates all of the effects anticipated above for S(f) , except for the site effect SF(f) . The constant, 0.85, accounts for the average shear-wave radiation pattern, amplification due to the free surface, and partitioning of energy into the two horizontal components. For events in Guerrero, we used values of p = 2.83 g / cm 3 and/3 = 3.61 km/sec for the density and shear-wave velocity, based on a seis- mic refraction and gravity experiment by Valdes et al. (1986) southeast of Acapulco. Valdes et al. developed a detailed velocity and density model for a cross section

of the subduction zone. The parameters used are appro- priate for the upper crust immediately above the dipping slab.

The computational method described by Anderson and Humphrey (1991) objectively determines Mo,fc, and K to minimize the difference between the logarithms of M(f ) and the observed acceleration spectrum, S(f) . It is not possible to set up a linear algorithm to obtain all three simultaneously. Therefore, a trial value for the cor- ner frequency, fc', is selected, M0 and K are chosen con- tingent on f ' , and the procedure is repeated until a min- imum in the misfit is obtained.

Site effects, SF(f) , cause variability in the estimates of source parameters between stations, even though the accelerograph stations are almost all located on hard rock (Fig. 4). The work of Castro et al. (1990) quantified an average SF(f) at several of the Guerrero stations. Hum- phrey and Anderson (1992) extended the Castro et al. research and estimated SF(f) as the average residual over all available earthquakes from equation (1), after cor- rection for an average value of K. The spectral decay parameter, K, was parameterized as K(r, S) = Ko(S) + i (r) (Anderson, 1991), where K(r, S) is the observable parameter and is a function of epicentral distance, r, and the site effect at station S. The near-site effects are de- scribed in the term Ko(S), and the change in K with dis- tance is described as ~(r). Though the approach is dif- ferent, the average site residuals determined by Humphrey and Anderson (1992) are consistent with those deter- mined by Castro et al. (1990). After correcting the spec- tra for SF(f) the S-wave spectra are fit again to the model in equation (1), resulting in a best estimate for moment and comer frequency for that seismogram.

The estimates of seismic moment and comer fre- quency are calculated from the log averages of the val- ues determined from three or more site-corrected spec- tra. The final moment magnitude is calculated from the average estimate from each station (Anderson and Hum- phrey, 1991). The mean values calculated are trimmed means; i.e., data that fall outside a 2 or range about the mean are discarded and the mean recalculated. Source parameters for some earthquakes are deemed acceptable with only one or two station estimates if the site response functions for the stations are relatively flat from 0.3 to 30.0 Hz (Fig. 4). The site response is relatively flat at Atoyac, Coyuca, Filo de Caballo, Los Magueyes, Oco- tillo, San Marcos, Suchil, Tonalapa, and Zihuatanejo. Such source parameter estimates are less accurate than others. However, most of these events occurred in the early period of array operations and they represent the only data from the aftershocks of the Michoacan earth- quakes.

Determination of Event Source Parameters

Spectra from 82 events are used in this study; 80 which occurred between 1985 and the end of 1989 and

Seismic Source Parameters from the Guerrero Subduction Zone 1761

two addi t ional events f rom 1990. The lat ter two (9019 and 9024; see Table 1) are included because they are

wel l - recorded thrust events located on the nor thwestern side of the Guerrero gap. Of the 82 earthquakes, 39 events in the magni tude range 3 to 6 are recorded on at least three and sometimes on as many as 14 stations. The source parameters for the remaining 43 events are obtained from one or two stations with flat site response.

F rom these der ived mean es t imates o f the momen t and the c o m e r f requency, the source radius , re, is cal- culated in the manner o f Brune (1970, 1971):

2 . 3 4 / 3 rc - - - , (2)

2~rfc

and the stress drop, Ao-, f rom

7M0 Ao- - (3)

16r~"

The average slip across the fault p lane is de te rmined as fol lows:

Mo O - (4)

A/z'

where A is the fault area assuming a c i rcular source and /z is the shear modulus .

The results are summar ized in Table 3. In column 1, the events are referred to by the GAA cata log number that is l is ted in Table 1. The number of spectra used in de termining the mean values is l is ted in column 2. The

1.0

.8

.6

.4

.2

.0

- - . 2

- - . 4

- - . 6

- - . 8

- 1 . 0

A

Xaltianguls Paraiso La U ~ Petatlan Las Mesas

t 1

.0 .0 1 I

1.0 2.0

1.0

.8

.6

.4

.2

.0

- - . 2

- - . 4

- - . 6

- - . 8

-1 .0

B

.0 2.0

Ocotlto Papanoa Las VicJas Teocalco Lo C o m m u n i d a d Cerro de Piedra

I I I I

.0 1.0

v (D

Cn O

_.J

1.0

.8

.6

.4

.2

.0

--.2

--.4

--.6

--.8

-I .0

Coyuca Tono lapa Son Marcos Suchil Zihuatenojo Atoyac Los Magueyes Ocotitlo Filo de Caballo

I I I I I

.0 .0 1.0

Log Frequency, Hertz 2.0

.4

.2

.0

- . 2

- . 4

- - . 6

- - . 8

-1 .0 -1 .0

Acapu l co El Bolcon LO Venta Atoyac La Union Cayoco Caleta de Campos

I I I I I

.0 1.0 2.0 Log Frequency, Hertz

Figure 4. Site functions for Guerrero stations, from Humphrey and Anderson (1992). (A) Stations with amplifications in a narrow frequency band. (B) Stations with amplification over a broad bandwidth. (C) Stations with flat site response from 0.3 to 30 Hz. (D) Stations with a more complex response. Stations with near-fiat site response, in group (C), are the only ones used for single-station estimates of stress drop, and at least one of the stations in other stress-drop de- terminations must be from this group.

1762 J .R. Humphrey, Jr. and J. G. Anderson

Table 3 Source Parameters from the Guerrero Subduction Zone

Log M0 Log fc Log Act Radius t Slip ~ Event N* (dyne-cm) (Hz) (bars) Mw (m) (cm) FM §

8506 1 21 .81 0 . 8 0 2 . 4 6 3 .87 2 1 3 . 1 12 .23 - -

8507 1 2 3 . 3 0 0 . 3 0 2 . 4 6 4 . 8 7 6 7 2 . 2 3 8 . 1 3 - -

8 5 1 0 1 2 2 . 1 3 0 . 8 5 2 .93 4 . 0 9 1 8 9 . 9 3 2 . 1 5 - -

8511 1 2 2 . 4 0 0 . 5 5 2 .31 4 . 2 7 3 7 8 . 7 15 .26 - -

8512 1 2 1 . 3 0 0 . 8 5 2 .11 3 . 5 4 189 .9 4 . 8 2 - -

8513 1 2 1 . 8 4 0 . 5 0 1 .59 3 . 8 9 4 2 5 . 5 3 . 2 6 - -

8 5 1 4 1 2 2 . 2 2 0 .65 2 . 4 2 4 .15 3 0 0 . 8 1 5 . 7 4 - -

8517 1 2 2 . 7 5 0 .15 1 .46 4 . 5 0 9 5 3 . 5 5 . 3 6 - -

8523 1 2 2 . 7 5 0 . 1 5 1.45 4 . 5 0 9 5 3 . 5 5 .35 - -

8 5 2 4 1 2 1 . 0 3 1 ,10 2 . 5 9 3 .35 106 .8 8 . 1 6 - -

8526 2 2 3 . 9 0 + 0 . 3 0 - 0 . 2 3 ± 0 . 0 4 1 .48 --- 0 . 1 9 5 . 2 7 -+ 0 , 2 0 2 2 6 3 . 5 13 .42 SS

8527 1 2 1 . 4 6 0 . 8 5 2 .27 3 .64 189 .9 6 . 9 8 - -

8528 1 2 3 . 0 4 0 . 3 5 2 .35 4 . 6 9 6 0 0 . 2 2 6 . 4 1 - -

8529 1 2 1 . 2 9 0 . 6 0 1 .34 3 .53 3 3 7 . 8 1 .47 - -

8 5 3 4 1 2 2 . 2 3 0 . 7 0 2 . 5 9 4 . 1 6 2 6 8 . 4 2 0 . 5 5 - -

8535 2 2 2 . 8 2 -+ 0 . 0 4 0 .33 ± 0 .11 2 .05 ± 0 . 2 7 4 . 5 5 --- 0 . 0 3 636 .1 14 .23 - -

8 5 3 6 1 2 1 . 5 8 0 . 6 5 1 .78 3 . 7 2 3 0 0 . 8 3 . 6 0 - -

8537 1 2 0 . 7 8 1.05 2 .18 3 .18 119 .8 3 . 6 0 - -

8538 1 2 1 . 5 9 1 .00 2 . 8 4 3 . 7 2 134 .4 18.41 - -

8539 1 2 1 . 2 4 1 .05 2 . 6 4 3 .49 119 .8 10 .38 - -

8602 1 2 3 . 2 8 0 . 2 0 2 .13 4 . 8 5 8 5 0 . 9 2 2 . 7 7 - -

8606 2 2 2 , 3 3 ± 0 . 2 6 0 . 6 0 ± 0 . 0 7 2 .38 --- 0 . 4 7 4 . 2 2 ± 0 . 1 7 3 3 7 . 5 16 .14 - -

8607 2 2 2 . 9 9 ± 0 . 1 4 0 . 0 2 ± 0 . 0 3 1 .32 -+ 0 . 0 3 4 . 6 6 ± 0 . 0 9 1 2 7 0 . 4 5 . 2 8 - -

8608 1 2 2 . 6 8 - 0 . 0 5 0 . 7 9 4 . 4 6 1510 .6 1,83 - -

8609 1 2 2 . 9 4 - 0 . 1 5 0 .75 4 . 6 3 1 8 9 3 . 6 2 .11 - -

8612 2 2 3 . 3 2 ± 0 . 2 9 0 . 1 8 ± 0 . 2 5 2 . 1 0 ± 0 . 4 5 4 . 8 8 ± 0 . 2 0 8 9 8 . 3 2 2 . 1 2 - -

8613 3 22 .51 ± 0 .11 0 . 5 0 ± 0 . 0 9 2 .27 ± 0 . 1 8 4 . 3 4 ± 0 . 0 8 4 2 5 . 1 15 .56 - -

8 6 1 4 2 2 3 , 2 9 --+ 0 .11 - 0 . 0 7 -+ 0 .11 1 .32 ± 0 .21 4 . 8 6 ± 0 . 0 7 1595 .6 6 .61 - -

8616 4 2 2 . 5 4 ± 0 .21 0 . 5 0 ± 0 . 1 2 2 . 3 0 ± 0 . 1 7 4 . 3 6 ± 0 . 1 4 4 2 5 . 2 16 .74 - -

8 6 t 9 1 2 3 . 3 6 0 . 2 5 2 . 3 7 4 .91 7 5 5 . 3 3 4 . 7 7 - -

8 6 2 0 1 2 1 . 9 9 0 . 7 5 2 . 5 0 4 . 0 0 2 3 9 . 2 14 .84 - -

8622 2 2 5 , 3 9 ± 0 . 0 4 - 0 . 4 8 ± 0 . 0 3 2 . 2 2 -+ 0 . 0 4 6 . 2 6 ± 0 .03 4 0 1 7 . 3 130 .39 T

8 6 2 6 5 2 3 , 7 9 ± 0 .21 0 . 1 7 -+ 0 . 1 2 2 .55 ± 0 . 2 6 5 . 1 9 ± 0 . 1 4 9 0 8 . 8 6 3 . 7 9 T

8627 1 2 2 . 3 9 0 . 8 0 3 . 0 4 4 . 2 6 213 .1 4 6 . 1 7 - -

8628 2 2 3 . 8 0 ± 0 . 3 0 - 0 . 0 3 ± 0 . 0 4 1.98 ± 0 . 2 0 5 . 2 0 ± 0 . 2 0 1425.1 2 6 . 6 1 - -

8 6 2 9 6 2 2 , 7 4 ± 0 . 3 0 0 . 5 7 ± 0 .21 2 . 7 0 ± 0 . 5 4 4 . 4 9 ± 0 . 2 0 3 6 4 . 9 3 5 . 6 9 - -

8 6 3 0 3 2 3 . 7 8 -+ 0 . 1 6 0 . 0 2 ± 0 . 0 3 2 .08 + 0 . 0 9 5 . 1 8 -+ 0 .11 1294 .6 3 0 . 7 5 - -

8632 1 2 1 . 8 5 0 . 8 0 2 .51 3 . 9 0 213 ,1 13 .54 - -

8641 1 2 1 . 0 8 1.15 2 .78 3 .38 95 .1 1 1 . 3 4 - -

8643 1 2 2 . 3 6 0 . 5 0 2. l 1 4 . 2 4 4 2 5 . 5 10 .88 - -

8644 1 2 0 . 5 7 1 .45 3 .17 3 .05 4 7 . 7 14 .07 - -

8645 2 2 3 . 6 0 ± 0 . 3 7 0 , 0 5 ± 0 . 1 4 2 . 0 0 --- 0 . 0 6 5 . 0 6 -+ 0 . 2 4 1 2 0 0 . 2 2 3 . 6 9 - -

8647 2 2 3 . 2 2 ± 0 . 3 5 0 . 0 5 ± 0 . 1 4 1 .62 ± 0 . 0 8 4 .81 ± 0 . 2 3 1200 .2 9 .95 - -

8648 3 2 2 . 5 9 ± 0 .31 0 . 5 0 ± 0 . 1 3 2 . 3 4 ± 0 .11 4 . 3 9 ± 0 .21 4 2 5 . 1 18 .48 - -

8709 6 23 .11 ± 0 . 1 4 0 .31 ± 0 . 1 0 2 . 2 9 ± 0 .21 4 . 7 4 ± 0 . I0 6 6 0 . 9 2 5 . 5 4 - -

8 7 1 0 5 22 .41 -+ 0 . 2 4 0 . 6 3 ± 0 . 2 3 2 . 5 6 ± 0 . 4 4 4 . 2 8 ~- 0 . 1 6 3 1 5 . 2 2 2 . 5 7 - -

8718 9 2 3 . 7 2 ± 0 . 1 3 0 . 2 4 -+ 0 .11 2 .71 ± 0 .31 5 .15 -+ 0 . 0 9 7 6 5 . 9 7 7 . 0 0 T

8719 9 22 .41 --- 0 .21 0 . 6 6 -+ 0 .21 2 .65 ± 0 . 4 8 4 . 2 8 ± 0 . 1 4 2 9 3 . 4 2 6 . 0 3 - -

8721 3 2 1 . 9 9 -+ 0 . 1 0 0 . 5 2 --- 0 . 1 5 1 .80 ± 0 . 3 7 3 . 9 9 --- 0 . 0 7 4 0 8 . 9 5 . 0 6 - -

8723 3 2 3 . 0 4 --+ 0 . 1 0 0 . 3 8 -+ 0 . 0 8 2 .45 ± 0 . 1 3 4 . 6 9 -+ 0 . 0 6 5 5 5 . 8 3 0 . 6 8 - -

8737 3 2 3 . 3 3 ± 0 . 1 0 0 . 0 2 ± 0 .11 1.63 -+ 0 . 2 6 4 . 8 9 ± 0 . 0 6 1296 .6 10 .93 - -

8738 1 2 1 . 8 6 0 . 5 5 1 .76 3 . 9 0 3 7 8 . 7 4 .31 - -

8742 6 2 2 . 5 6 -+ 0 . 2 9 0 . 4 3 ± 0 . 3 2 2 . 1 2 --- 0 . 7 5 4 . 3 8 ± 0 . 1 9 4 9 5 . 7 12 .86 - -

8744 3 2 2 . 2 4 ± 0 . 1 2 0 . 4 5 -+ 0 . 1 8 1 .85 -+ 0 . 5 6 4 . 1 6 ± 0 . 0 8 4 7 6 . 7 6 . 6 0 - -

8801 2 2 3 . 1 9 ± 0 . 0 3 - 0 . 2 5 ± 0 . 0 7 0 . 6 9 ± 0 . 2 4 4 . 7 9 ± 0 . 0 2 2 3 9 5 , 5 2 .31 - -

8 8 0 2 1 2 2 . 4 8 0 . 0 0 0 .73 4 . 3 2 1344 .4 1.43 - -

8803 2 2 2 . 7 4 ± 0 . 2 7 - 0 . 0 3 +- 0 .11 0 .91 ± 0 . 0 6 4 . 4 9 -+ 0 . 1 8 1429 .3 2 .31 - - 8 8 0 4 13 2 4 . 1 5 +- 0 . 1 7 0 . 1 0 ± 0 . 0 8 2 . 7 2 ± 0 .21 5 . 4 4 -4- 0 . 1 2 1059 .3 1 0 9 . 5 6 T

8806 3 2 1 . 8 4 -+ 0 . 2 0 0 . 8 7 ± 0 . 1 3 2 . 7 0 -+ 0 . 4 8 3 . 9 0 -+ 0 , 1 3 182 .8 18 .07 - -

8 8 1 6 4 2 2 . 8 6 ± 0 . 3 0 0 . 2 4 -+ 0 . 1 5 1.83 ± 0 . 1 7 4 . 5 8 + 0 . 2 0 7 7 8 . 8 10 .42 - -

8818 1 2 2 . 3 9 0 . 4 0 1 .85 4 . 2 6 5 3 5 . 6 7 . 4 2 - -

8 8 2 6 7 2 2 . 8 6 ± 0 . 3 2 0 . 5 6 -+ 0 .11 2 . 7 9 - + 0 . 2 6 4 . 5 8 ± 0 . 2 2 3 7 2 . 7 4 5 . 2 8 - -

Seismic Source Parameters from the Guerrero Subduction Zone 1763

T a b l e 3--Continued

Log M0 Log fc Log Ao- Radius '~ Slip :~ Event N* (dyne-cm) (Hz) (bars) Mw (m) (cm) FM §

8831 5 22.11 4- 0.08 0.83 4- 0.09 2.85 4- 0.26 4.07 4- 0.05 198.9 27.95 - - 8833 3 22.78 --- 0.16 0.47 - 0.25 2.43 + 0.64 4.52 4- 0.11 459.7 24.65 - - 8834 2 23.70 - 0.16 -0 .07 4- 0.11 1.73 4- 0.15 5.13 4- 0.11 1595.6 16.99 N 8842 3 21.97 - 0.10 0.77 4- 0.29 2.53 4- 0.94 3.98 + 0.07 230.1 15.35 - - 8845 3 22.33 4- 0.38 0.47 _4- 0.16 1.99 -+ 0.11 4.22 4- 0.25 459.0 8.86 - - 8907 3 22.52 4- 0.07 0.33 + 0.03 1.78 4- 0.08 4.35 +_ 0.05 623.3 7.38 - - 8911 8 22.95 4- 0.23 0.42 + 0.15 2.46 4- 0.35 4.64 4- 0.15 512.5 29.46 - - 8912 8 23.83 -+- 0.09 0.09 4- 0.09 2.37 +- 0.21 5.22 4- 0.06 1084.0 49.91 - - 8913 4 23.09 - 0.24 0.43 4- 0.18 2.63 -+ 0.34 4.73 --- 0.16 504.8 42.06 - - 8915 5 23.18 +-- 0.20 0.29 4- 0.16 2.31 4- 0.37 4.79 --- 0.13 689.2 27.81 - - 8924 11 25.94 + 0.17 -0 .75 --- 0.09 1.95 +0 .15 6.63 4- 0.11 7581.8 131.99 T 8929 11 23.95 4- 0.16 -0.01 4- 0.08 2.19 4- 0.13 5.30 4- 0,10 1360.0 41.52 - - 8942 5 23.42 4- 0.26 0.12 4- 0.26 2.03 +- 0.58 4.95 4- 0.17 1019.7 21.76 - - 8948 6 24.00 4- 0.41 -0.15 4- 0.20 1.80 +-- 0.27 5.33 -+ 0.27 1903.0 23.65 - - 8949 9 22.85 4- 0.27 0.52 4- 0.16 2.67 - 0.43 4.57 4- 0,18 403.8 37.54 - - 8950 4 22.84 - 0.30 0.56 4- 0.28 2.78 4- 0.62 4.56 4- 0.20 368.5 44.05 - - 8961 14 23.41 -+ 0.14 0.27 -+ 0.15 2.47 4- 0.39 4.94 4- 0.09 725.6 41.76 - - 8974 6 23.41 4- 0.40 0.13 4- 0.16 2.06 -+- 0.29 4.94 --- 0.27 991.2 22.63 - - 9019 11 24.14 -+ 0.18 -0 ,17 + 0.09 1.89 4- 0.18 5.42 _+ 0.12 1986.1 29.89 T 9024 14 24.75 4- 0.13 -0.28 4- 0.07 2.16 --- 0.17 5.83 4- 0.09 2572.1 73.14 T

*N refers to the number of stations used in determining the average source parameters. *Radius calculated from averagef~ using equation (2), *Slip calculated from average Mo using equation (4). ~Focal mechanism from Earthquake Data Reports. T = thrust; SS = strike slip; N = normal.

Table 4 Comparison of Guerrero Moments with Harvard CMT Data

Harvard CMT Ratio of Shear Guerrero Moment Depth Moment Depth Moduli

Event (x 1023 dyne-cm) (km) (x t023 dyne-cm) (km) (/~CMT//~GAA)

8526 7.8 9.8 13.0 41.2 1.84 8626 5.1 35.6 7.6 57.2 1.84 8718 5.1 22.9 11.0 46.7 1.84 8804 14.4 19.2 74.0 47.8 1.84 8912 4.3 20.3 14.0 46.8 1.84 8929 7.9 13.4 19.0 47.9 1.84 8948 6.6 23.8 13.0 76.1 1.83 8961 2.5 36.0 4.7 35.0 1.38 9019 15.1 11.7 25.0 15.0 1.20 9024 39.8 15.8 75.0 26.0 1.84

f inal en t ry in T a b l e 3 is a no ta t ion to i den t i fy the foca l

m e c h a n i s m f r o m the E a r t h q u a k e D a t a R e p o r t p u b l i s h e d

by the U . S . G e o l o g i c a l Su rvey .

Discussion

Seismic Moments

The scaling of seismic moment from the low-fre- quency a s y m p t o t e o f the s h e a r - w a v e spec t r a d e p e n d s on

the m o d e l u s e d fo r ma te r i a l p r o p e r t i e s in the s o u rce re-

gion. As d i scussed earlier, the cons tants used in this s tudy

are f r o m a m o d e l for the c rus t sou theas t o f A c a p u l c o .

T h e va lues fo r dens i t y and s h e a r ve loc i t y y ie ld a shea r

m o d u l u s o f 3 .7 x 1011 d y n / c m 2. In this s ec t ion , w e ex-

a m i n e the r e l a t ion b e t w e e n e s t ima te s o f s e i s m i c m o m e n t

f r o m the GAA r e c o r d s and t h o s e f r o m o b s e r v a t i o n s o f

t e l e s e i s m i c b o d y w a v e s ,

Tab le 4 l is ts the t en G u e r r e r o e a r t h q u a k e s t2or w h i c h

the re are e s t i m a t e s o f the cen t ro i d m o m e n t t e n s o r (CMT)

f r o m l o n g - p e r i o d b o d y w a v e s . T h e s e da ta are f r o m the

H a r v a r d CMT ca t a log [us ing the m e t h o d o f D z i e w o n s k i

and W o o d h o u s e (1983)] . T h e m o m e n t s in the H a r v a r d

CMT ca ta log are s y s t e m a t i c a l l y g rea t e r t han those re-

p o r t ed he re . T a b l e 4 a l so s h o w s that the d e p t h s o f the

ev en t s u s e d to ca l cu l a t ed the CMT are o f t en t w o or th ree

t i m e s d e e p e r than t h o s e d e t e r m i n e d us ing s h o r t - p e r i o d

1764 J.R. Humphrey, Jr. and J. G. Anderson

25.0,

2 4 . 8 l . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . ~ . . . . . . . . . T , o ~ /

24 .4

2 4 . 2

° 2 4 . 0

Cn O --J 2.3.8

23.6

25 .4

23.2

B929

r 1 8

,;; ...... it ;8;; ...........

. . . ~ . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

: i

2 3 . 0 ~ 2 3 . 2 2 3 . 6 24 .0 2 4 . 4

Log M o, CMT

Mexican stations and phase arrivals from the Guerrero accelerographs. This contributes to a discrepancy in the scalar seismic moment since, at this greater depth, the crustal model used for the CMT solution has greater ve- 26 locity and rigidity than those used in our moment esti- mates. For each event, Table 4 shows the ratio ( / . £ C M T ] / 2 5

/ZCAA) of shear modulus as used for the CMT solution, taken from the Preliminary Reference Earth Model. E (PREM; Dziewonski and Anderson, 1981) to the shear ~ 24 modulus used in our moment determination. It is ex- ~=>" pected that the Harvard CMT estimate of seismic mo- ~g 23 ment will exceed our estimates by this ratio.

Figure 5 compares the CMT estimate of M0, reduced :~ u by the ratio /XCMT/~GAA, and the GAA estimates of M0. "~ 22

In general, there is good agreement between the two de- terminations, within the standard error of the GAA mo- ~ 21 ments. The only exception is event 8804, an M s = 5.7 earthquake in the northwest part of the Guerrero gap. The cause of the discrepancy (a factor of 3) is not known, 2o but the GAA moment is well determined from 11 sta- tions. Because of the agreement for the other nine earth- 19 quakes where teleseismic moments are available, we conclude'that our estimates of the seismic moment are accurate.

24 .8

Stress Drops

The relation between seismic moment and corner frequency is shown in Figure 6, along with lines of con- stant stress drop from 10 to 1000 bars. The majority of the 82 earthquakes analyzed have stress drops greater

Figure 5. A comparison of seismic mo- ments estimated from the Guerrero acceler- ograph stations with adjusted moments from the Harvard CMT catalog. Harvard mo- ments are adjusted to compensate for the difference in shear modulus between the PREM earth model (Dziewonski and An- derson, 1981) at the depth used to deter- mine the CMT, and crustal model used in this study at the depth we located the earth- quakes. Error bars represent one standard deviation about the mean for Guerrero es- timates. Numbers beside each symbol are the GAA catalog numbers in Table 1.

_ _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

\ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

i ~

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- 1 . 0 - . 5 .0 .5 1.0 1.5 2.0

Log Corner Frequency, Hz

Figure 6. Relationship between seismic mo- ment and corner frequency for 82 events recorded by the Guerrero accelerograph network. Lines with constant stress drops are based on equations (2) and (3).

Seismic Source Parameters from the Guerrero Subduction Zone 1765

than 100 bars, and the logarithmically averaged stress drops is 150 bars (Fig. 6; Table 3). Singh et al. (1990) show that peak acceleration for most observations in Guerrero is best predicted by rms stress drop of over 100 bars. Our stress-drop estimates are greater than those ob- tained by Castro et al. (1990) for common events. We attribute this to two factors. Castro et al. constrained moments to equal the teleseismic values, which as shown here are consistently greater than estimates from local stations. The intersection of these greater moments with accelerogram spectra occurred at much lower frequen- cies than the comer frequencies picked here, leading to lower stress drops. Second, since we are using a spectral shape with a single comer frequency and fitting it to the high frequencies, our approach tends toward the higher rms stress drops, as found by Singh et al. (1989a, b; 1990). Among the larger events (M0 > 3 × 1022 dyn-

cm), about half of the events have stress drops over 100 bars and half have smaller stress drops. Among the smaller events (M0 < 3 × 10 z2 dyn-cm), there is a distinctly smaller proportion of events with stress drops below 100 bars.

Figure 7 shows stress drops (with their uncertainties) as a function of distance from the Minster and Jordan (1978) pole of rotation for the Cocos Plate relative to the North American Plate. The graph groups stress drops of all events that are located at the same distance along the trench axis. This shows that most of the lower stress- drop events occur at distances corresponding to the Pe- tatlan region. This prominent characteristic of the spatial distribution of stress drops is also shown in Figure 8. Since 150 bars is near the average stress drop, Figure 8 displays the epicenters in terms of high stress-drop (greater than 150 bars), intermediate stress-drop (50 to 150 bars),

L"• 3.0 {3 .o v 2.5 o_ o

"~ 2.0

~D L 1.5 E~ jO 1.0

4 . 0 i

3 . 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

............................................... : ................ i . . . . . . . . . . . . . . . :~ ............... i .........

!iIi i i j_ i i :: !

5 , :: : i [ : : .o I i l I I

2300 2400 2500 2600 2700 2800

D is tance f r o m pole ( k m )

Figure 7. Stress drop as a function of distance from the pole of rotation of the Co- cos Plate relative to the North American Plate for the 82 events studied. Pole of rotation is at 29.8 ° N, 121.28 ° W from Minster and Jordan (1978). Error bars represent one standard deviation about the mean. Sym- bols with no error bars are single-station es- timates.

19.5

19.0

18.5

o 18.0

L~ 17.5 o _d

17.0

16,5

16,0 - 1 0 3

G u e r r e r o S t r e s s D r o p s : 1 9 8 5 - 1 9 8 9

MICHOACAN / ÷ ( /

,,-,2--- wzx,co / ~:'x 1 ~"~ MORELOS ~

. ) -C --' "~ _)~ ,,~..~ PUEBI.A

- " ~ " ~ A 9~- "~ GUERRERO

|

- 1 0 2 - 1 0 1 - 1 0 0 - 9 9 - 9 8

L o n g i t u d e (aE)

Figure 8. Map of Guerrero region showing the spatial distribution of stress drops. Circles signify Z~r _-< 50 bars, aster- isks signify 50 =< Act < 150 bars, and tri- angles signify Act > 150 bars.

1766 J .R. Humphrey, Jr. and J. G. Anderson

and low stress-drop (less than 50 bars) events. The figure shows significant spatial variation among events in the coastal zone. Earthquakes in the Petatlan active region, west of the central part of the Guerrero gap; show a mix of high, intermediate, and low stress drops. From Table 2, it can be seen that these data represent 61% of the M 4 and greater events recorded for this region. Earth- quakes in the Acapulco active region, east of the central Guerrero gap, have higher than average stress drops. The data for this region represent 58% of the M 4 and greater events recorded for this region. The central portion of the Guerrero seismic gap has not been active during this study.

Earthquakes in the interior region (i.e., more than about 75 km from the coast in the inland zone of normal faulting) generally show intermediate stress drops. Ap- proximately 54% of the moderate events recorded in this region have been analyzed. These earthquakes are lo- cated greater than 40-km deep, and following Suarez et al. (1990), they are most probably related to intraplate deformation of the subducting slab. Focal mechanisms for events in this region usually suggest normal faulting (LeFevre and McNally, 1985). One event in this region (8834, see Table 3) appears in the Harvard catalog as a normal faulting event. Shallow crustal earthquakes also can be expected in this region, although none are present in the Guerrero catalog; when these occur, their stress drops and focal mechanisms might be different from the deep events studied here.

Source Parameter Scaling

The relation between stress drop and seismic mo- ment for the earthquakes listed in Table 3 is shown in Figure 9. Moment and stress drop are essentially uncor- related for these earthquakes, taken as a whole. Figure 10 shows the same relationship for successively more

limited subsets of events that are recorded on four-or- more, six-or-more, and eight-or-more stations. In these restricted subsets of data, however, there is a tendency for stress drop to decrease as seismic moment increases. Regressions through the subsets, also shown in Figure 10, are quite similar, and extrapolate to a reasonable es- timate for the static stress drop during the 1985 M 8. ! Michoacan earthquake (Anderson et al., 1986).

The interpretation of Figures 9 and 10 depends crit- ically on whether the stress-drop estimates from three or fewer station measurements are reliable. A systematic source of error for the stress drop would be a consistent underestimate of the corner frequency fc, which would cause r~ to be overestimated (equation 2) and stress drop to be underestimated (equation 3). Since many of the low stress-drop events are located in the Petatlan region, this implies that a limited number of commonly triggered stations are critical. These stations are Zihuatanejo, Suchil, Papanoa, and Petatlan. The corner frequencies that these stations yield for high stress-drop events are not system- atically different from other stations, and their records exhibit the full range of stress drops inferred for this re- gion. Therefore, we conclude that this is not an expla- nation for the observed low stress drops and that they are a real feature of the seismicity northwest of the seis- mic gap.

Figures 9 and 10 suggest two alternative models for the behavior of stress drop along the Guerrero subduc- tion zone. One alternative is that stress drop tends to decrease as the seismic moment increases. The extrap- olation of stress drop from the small earthquake spectral fits to the M 8. l Michoacan event in Figure 10 suggests that the regressions may represent a real aspect of the data; i.e., stress drop decreasing with increasing mo- ment.

On the other hand, it should be noted that a low

104

? -13 v ~L o ¢m

o

10 3

10 2

101

I 0 ° 20

0

0

0 o o 8

o O o o 8 0 ~ o o o o- oo o o

0 C ~ -~O- o 0

o 8 ° o % o f 3 ° 0 0 0 0 0 0 0 0 ° O

o oo o o

o o o

0 O 0 0 0

R = - 0 . 1 7

I

21 22 23 24- Log Moment ( d y n e - c m )

I

25 26 Figure 9. Scatter plot of A~r as a func- tion of M0 for all 82 events studied R is the correlation coefficient.

Seismic Source Parameters from the Guerrero Subduction Zone 1767

stress-drop earthquake will tend to cause lower peak ac- celerations, and will tend to trigger fewer accelero- graphs. Thus, the procedure of selecting events that trig- ger only large numbers of stations, to assure redundancy and reliability in the estimates, biases the earthquake se- lection toward events with high stress drops. Consider- ing this and the difficulty in rejecting the low stress-drop events based on known sources of errors, we conclude that Figure 9 gives a more realistic representation of the overall stress-drop characteristics as a function of mo- ment along the Michoacan and Guerrero segments of the Mexican subduction zone than the subsets in Figure 10. The more complete data in Figure 9 suggest that the stress drop is statistically independent of seismic moment in this magnitude range. Based on this figure, the stress drop in M 8 earthquakes in this region might fall into a rather broad range. Conceivably the stress drop for large events could be limited by a decreasing upper bound as moment increases, but that upper bound would be very poorly resolved by the limited numbers of events avail- able at large magnitudes.

Implications for Hazard Assessment

There are several plausible explanations for the dif- ferences in stress drop along the coast of Guerrero. One seemingly straightforward explanation for this difference is in terms of the supposed state of tectonic stress in the region. This explanation is emphatically not unique, be- cause the relationship between stress and stress drop is ambiguous. The last major earthquakes in the vicinity of the Petatlan active region were in 1979 (Ms = 7.6) and

1985 (Ms = 7.5). These events are expected to have been large enough to relieve significant quantities of accu- mulated seismic strain. However, some parts of the re- gion might still be highly stressed as part of the Guerrero gap. Since the earthquakes occurred relatively recently, it is reasonable that the stress drops in this region show a mix of high and low values. On the other hand, in the Acapulco active region, the last major earthquake was in 1957. Thus, it is more than 30 yr since the last major strain-relieving event. Since 30 yr is a significant frac- tion of the average repeat time in the Guerrero subduc- don zone, it is plausible that the region is more highly stressed, causing the consistently high stress drops that are observed.

Another hypothesis to explain the spatial distribution of stress drops is that there is a relationship between stress drop and focal mechanism. Some support for this idea could come from considering that the earthquakes in the interior have lower than average stress drops and prob- ably normal mechanisms. Suarez et al. (1990) have shown that some of the events near the coast also have normal mechanisms. The regional variation in stress drop could then be caused by a different mix of normal and thrust earthquakes. We do not yet have adequate control over focal mechanisms to resolve this issue. There is also the possible distinction, within the Petatlan and Acapulco active regions, that some of the earthquakes are on the main subduction thrust and others are within the crust above the main thrust, or within the subducted slab be- low the main thrust; differences in stress drop among these three categories are possible. The location preci-

13

o tm

10 3

10 2 o 0 0 0 0

R=-0.46

101 I I I I I

22 25 24 25 26 27

103

102 0 0 0 0 °

R=-0.52

10 ~ ~ I I I 1

22 23 24 25 26 27

Z~

I

28 29

I

28 29

103

10 2

101 22

R = - 0 . 6 8

I I I I f I

25 24 25 26 27 28 29 Log Moment (dyne-cm)

Figure 10. Scatter plot of Ao- as a func- tion of M0 for subsets of the 82 events stud- ied. (A) Events recorded on four or more stations. (B) Events recorded on six or more stations. (C) Events recorded on eight or more stations. R is the correlation coeffi- cient.

1768 J .R. Humphrey, Jr. and J. G. Anderson

sion is not sufficient to resolve this with confidence, but with the present data there is no pattern of stress drop related to focal depth. We also could not find any con- vincing relationship between stress drop and magnitude, year of the earthquake, or distance from the trench be- yond what was discussed above. A final hypothesis is that the pattern has nothing to do with any of the factors discussed above, but is rather a consequence of differ- ences in the character of the subduction zone, which might persist for very long time periods ( - 1 0 5 to 10 6 yr), de- termined by the time required to subduct an inhomoge- neity in the ocean floor, or the persistence of an inho- mogeneity in the upper plate of the thrust zone.

Conclusions

This study examined seismic moments, stress drops, and their correlation for a region of coastal Mexico. Seismic moments from this study are similar to those obtained from moment tensor inversions of long-period body waves that have been corrected with the shear modulus from more accurate depth estimates. The Brune stress-drop parameter shows an interesting pattern of variation within and along the subduction zone. The in- terior region of the Guerrero subduction zone is char- acterized by lower than average stress drops and deeper focal depths in a region shown by LeFevre and McNally (1985) to be dominated by extensional stress in the lower plate. The coastal region of the Guerrero subduction zone may be divided into an active zone with both high and low stress-drop events and an active zone with predom- inantly high stress-drop events, bounded by several zones of very low seismicity. Although the most obvious ex- planation could be that stress drop is correlated with the temporal position of the region in the cycle of stress ac- cumulation and release, several alternative explanations cannot yet be ruled out. These include correlation with focal mechanism, depth, distance relative to the trench, and location along the coast and above, on, or below the main subduction thrust.

Acknowledgments

Raul Castro provided many helpful and stimulating discussions dur- ing the course of this study. We thank John Vidale for his many com- ments and suggestions that improved this manuscript. This study was funded by National Science Foundation Grants Numbers EAR 88-18641 and BCS 88-08357.

References

Anderson, J. G. (1991). A preliminary descriptive model for the dis- tance-dependence of the spectral decay parameter in southern California, Bull. Seism. Soc. Am. 81, 2186-2193.

Anderson, J. G. and S. E. Hough (1984). A model for the shape of the Fourier amplitude spectrum of acceleration at high frequen- cies, Bull. Seism. Soc. Am. 74, 1969-1993.

Anderson, J. G. and J. R. Humphrey, Jr. (1991). A least-squares method for objective determination of earthquake source param- eters, Seism. Res. Lett. 62, 201-209.

Anderson, J. G., P. Bodin, J. Brune, J. Prince, S. Singh, R. Quaas, M. Onate, and E. Mena (1986). Strong ground motion and source mechanism of the Mexico earthquake of Sept. 19, 1985, Science 233, 1043-1049.

Anderson, J. G., R. Quaas, D. Almora, J. M. Velasco, E. Guevara, L. E. dePavia, A. Gutierrez, and R. Vasquez (1987a). Guerrero, Mexico accelerograph array: summary of data collected in 1985, Report GAA-2, Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico.

Anderson, J. G., R. Quaas, D. Almora, J. M. Velasco, E. Guevara, L. E. dePavia, A. Gutierrez, and R. Vasquez (1987b). Guerrero, Mexico accelerograph array: summary of data collected in 1986, Report GAA-3, Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico.

Anderson, J. G., R. Quaas, D. Almora, J. M. Velasco, E. Guevara, L. E. dePavia, A. Gutierrez, and R. Vasquez (1988). Guerrero, Mexico accelerograph array: summary of data collected in Jan- uary to June, 1987, Report GAA-5, Seismological Laboratory, Mackay School of Mines, University of Nevada-Reno, Reno, Nevada.

Anderson, J. G., R. Quaas, R. Castro, S. K. Singh, J. M. Velasco, D. Almora, E. Mena, R. Vasquez, G. Castro, C. Perez, B. Lo- pez, and R. Mejia (1989a). Accelerograms from the Guerrero, Mexico, strong motion array for the April 25, 1989 earthquake (Ms = 6.9): a preliminary report, Report GAA-6, Seismological Laboratory, University of Nevada-Reno, Reno, Nevada and In- stituto de Ingenieria, Universidad Nacional Autonoma de Mex- ico, Mexico City.

Anderson, J. G., S. K. Singh, J. M. Espindola, and J. Yamamoto (1989b). Seismic strain release in the Mexican subduction thrust, Phys. Earth Planet. Interiors 58, 307-322.

Anderson, J. G., R. Quaas, R. Vasquez, D. Almora, J. R. Hum- phrey, J. M. Velasco, R. Castro, P. Perez, C. Perez, B. Lopez, and R. Mejia (1990a). Guerrero, Mexico accelerograph array: summary of data collected in July to December, 1987, Report GAA-8, Seismological Laboratory, Mackay School of Mines, University of Nevada-Reno, Reno, Nevada.

Anderson, J. G., R. Quaas, R. Vasquez, D. Almora, J. R. Hum- phrey, J. M. Velasco, R. Castro, P. Perez, S. K. Singh, E. Nava, C. Perez, B. Lopez, R. Mejia, and G. Castro (1990b). Guerrero, Mexico accelerograph array: summary of data col- lected in 1988, Report GAA-9, Seismological Laboratory, Mackay School of Mines, University of Nevada-Reno, Reno, Nevada.

Anderson, J. G., R. Quaas, S. K. Singh, and J. R. Humphrey (1991a). Guerrero accelerograph array: six years of operation, in Proc. of the 4th International Conf. on Seismic Zonation, 25-29 Au- gust 1991, Vol. II, Earthquake Engineering Research Institute, Oakland, California, 435-442.

Anderson, J. G., R. Quaas, J. R. Humphrey, R. Vasquez, D. A1- mora, J. M. Velasco, R. Castro, C. Perez, and G. Castro (1991b). Guerrero, Mexico accelerograph array: summary of data col- lected in 1989, Report GAA-11, Seismological Laboratory, Uni ~ versity of Nevada-Reno, Reno, Nevada and Instituto de Ingen- ieria, Universidad Nacional Autonoma de Mexico, Mexico City.

Anderson, J. G., R. Quaas, R. Vasquez, D. Almora, Y. Lei, J. M. Velasco, G. Castro, C. Perez, and J. R. Humphrey (1991c). Guerrero, Mexico accelerograph array: summary of data col- lected in 1990, Report GAA-12, Seismological Laboratory, Uni- versity of Nevada-Reno, Reno, Nevada and Instituto de Ingen- ieria, Universidad Nacional Autonoma de Mexico, Mexico City.

Anderson, J. G., J. Brune, J. Prince, R. Quaas, S. K. Singh, D. Almora, P. Bodin, M. Onate, R. Vazquez, and J. M. Velasco (1994). The Guerrero accelerograph network, Geofis. Int. 33, 341-371.

Seismic Source Parameters f r o m the Guerrero Subduct ion Zone 1769

Brune, J. N. (1970). Tectonic stress and the spectra of seismic shear waves from earthquakes, J. Geophys. Res. 75, 4997-5009.

Brune, J. N. (1971). Correction, J. Geophys. Res. 76, 5002. 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, Bull. Seism. Soc. Am. 80, 1481-1503.

Chael E. P. and G. S. Stewart (1982). Recent large earthquakes along the Middle American trench and their implications for the sub- duction process, J. Geophys. Res. 87, 329-338.

Dean, B. W. and C. L. Drake (1978). Focal mechanism solutions and tectonics of the Middle America arc, J. Geol. 86, 111-128.

de Cserna, Z. (1989). An outline of the geology of Mexico, in The Geology of North America--An Overview, A. W. Bally and A. R. Palmer (Editors), Vol. A, Geological Society of America, Boulder, Colorado, 233-264.

Dziewonski, A. M. and J. H. Woodhouse (1983). An experiment in systematic study of global seismicity: centroid-moment tensor solutions for 201 moderate and large earthquakes of 1981, J. Geophys. Res. 88, 3247-3271.

Dziewonski, A. M. and D. L. Anderson (1981). Preliminary refer- ence earth model (PREM), Phys. Earth Planet. Interiors 25, 297- 356.

Herrmann, R. B. (1985). An extension of random vibration theory estimates of strong ground motion to large distances, Bull. Seism. Soc. Am. 75, 1447-1453~

Humphrey, J. R., Jr. and J. G. Anderson (1992). Shear wave atten- uation and site response in Guerrero, Mexico, Bull. Seism. Soc. Am. 81, 1622-1645.

Karig, D. E., R. K. Cardwell, G. F. Moore, and D. G. Moore (1978). Late Cenozoic subduction and continental margin truncation along the northern Middle America trench, Geol. Soc. Am. Bull. 89, 265-276.

Kelleher, J., L. Sykes, and J. Oliver (1973). Possible criteria for pre- dicting earthquake locations and their application to major plate boundaries of the Pacific and the Caribbean, J. Geophys. Res. 78, 2547-2585.

Klein, F. W. (1979). Hypocenter location program HYPOINVERSE: a users guide to versions 1, 2, 3, and 4, U.S. Geol. Surv. Open- File Rept. 78-694, 113 pp.

Lefevre, L. V. and K. C. McNally (1985). Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone, J. Geophys. Res. 90, 4495-4510.

McNally, K. C. and J. B. Mi~aster (1981). Nonuniform seismic slip rates along the Middle America trench, J. Geophys. Res. 86, 4949-4959.

McNally, K. C., J. R. Gonzalez-Ruiz, and C. Stolte (1986). Seis- mogenesis of the 1985 great (Ms = 8.1) Michoacan, Mexico earthquake, Geophys. Res. Lett. 13, 585-588.

Minster, J. B. and T. H. Jordan (1978). Present day plate motions, J. Geophys. Res. 83, 5331-5354.

Nishenko, S. P. and S. K. Singh (1987). Conditional probabilities for the recurrence of large and great interplate earthquakes along the Mexican subduction zone, Bull. Seism. Soc. Am. 77, 2095-2114.

Pindell, J. and J. F. Dewey (1982). Permo-Triassic reconstruction of western Pangea and the evolution of the Gulf of Mexico/Car- ribbean region, Tectonics 1, 179-211.

Priestley, K. F. and T. G. Masters (1986). Source mechanism of the September 19, 1985 Michoacan earthquake and its implications, Geophys. Res. Lett. 13, 601-604.

Quaas, R., J. G. Anderson, and D. Almora (1987). The Guerrero accelerograph array to record strong earthquakes, in Memorias, VII Congreso Nacional de lngenieria Sismica, 19 al 21 de no- viembre de 1987, Queretaro, Qro., Mexico, B-40-B-54 (in Spanish).

Quaas, R., D. Almora, J. M. Velasco, R. Vazquez, P. Perez A., M. Ordaz, J. G. Anderson, S. K. Singh, E. Mena, G. Castro, C. Perez, and E. Nava (1988). Accelerograms from the Guerrero array for the Mexico earthquake of February 8, 1988 (Ms = 5.8): a preliminary report, Report GAA-4, Instituto de Ingenieria, Universidad Nacional Autonoma de Mexico, Mexico City, Mex- ico.

Quaas, R., J. Anderson, S. K. Singh, J. M. Velasco, D. Almora, R. Vazquez, G. Castro, and P. Perez (1990). Accelerograms from the Guerrero, Mexico strong motion array for May 11 (Ms = 4.9) and May 31 (Ms = 5.8), 1990 earthquakes, Report GAA- 10, Seismological Laboratory, University of Nevada-Reno, Reno, Nevada and Instituto de Ingenieria, Universidad Nacional Au- tonoma de Mexico, Mexico City, Mexico.

Rebollar, C. J., L. Munguia, A. Reyes, A. Uribe, and O. Jimenez (1991). Estimates of shallow attenuation and apparent stresses from aftershocks of the Oaxaca earthquake of 1978, Bull. Seism. Soc. Am. 81, 99-108.

Singh, S. K., L. Astiz, and J. Havskov (1981). Seismic gaps and recurrence periods of large earthquakes along the Mexican sub- duction zone: a reexamination, Bull. Seism. Soc. Am. 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 sub- duction zone, Bull. Seism. Soc. Am. 79, 1697-1717.

Singh, S. K., M. Ordaz, E. Mena, R. Quaas, M. Rodriguez, M. Ottaviani, J. G. Anderson, and D. Almora (1989b). Maximum acceleration expected in the epicentral region of earthquakes in the Mexican subduction zone, in Memorias, VIII Congreso Na- cional de lngenieria Sismica y VII Congreso Nacional de ln- genieria Estructural, Acapulco, Gro., 1989, Vol. I, A-125-A- 145 (in Spanish).

Singh, S. K., E. Mena, J. G. Anderson, R. Quaas, and J. Lermo (1990). Source spectra and RMS acceleration of Mexican sub- duction zone earthquakes, Pure Appl. Geophys. 133, 447-474.

Suarez, G., T. Monfret, G. Wittlinger, and C. David (1990). Ge- ometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico, Nature 345, 336-338.

UNAM Seismology Group (1986). The September 1985 Michoacan earthquakes: aftershock distribution and history of rupture, Geo- phys. Res. Lett. 13, 573-576.

Valdes, C. M., W. D. Mooney, S. K. Singh, R. P. M~yer, C. Lom- nitz, J.H. Luetert, C. E. Helsley, B. T. R. Lewis, and M. Mena (1986). Crustal structure of Oaxaca, Mexico, from seismic re- fraction experiments, Bull. Seism. Soc. Am. 76, 547-563.

Seismological Laboratory and Department of Geological Sciences Mackay School of Mines University of Nevada-Reno Reno, Nevada 89557-0047

Manuscript received 18 August 1992.