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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. As in 1985, the accelerogram nearest the epicenter has peak values that are notably smaller than expected, and it seems to display a static offset when integrated. Indeed, accelerations on rock at all distances average below values that were expected based on previous experience in Guerrero. 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 SP 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 …