Stress accumulation and release since 1882 in Ometepec, Guerrero, Mexico: Implications for failure mechanisms and risk assessments of a seismic gap

Abstract

We found evidence that four episodes of large shallow (h < 30 km) interplate earthquakes ruptured approximately the same segment of the Middle America subduction zone in Ometepec, Guerrero, Mexico, during the last century: in 1890 (M = 7.2), 1937 (MS = 7.5), 1950 (MS = 7.3), and 1982 (MS = 6.9 and 7.0). This segment is about 70 km long, which is near the limit of resolution to define seismic gap dimensions. We synthesized the spatial, temporal, and mechanistic patterns of regional relocated seismicity (mb ≥ 4.0) to investigate the mechanics of rupture within the same seismic gap for the relatively large number of times (four) offered by the Ometepec case history. Despite the similarities in source depths and geometries of the shallow main shocks, we found marked differences in their magnitudes, recurrence intervals, and rupture modes (e.g., degree of source complexity and event multiplicity). The episodic amounts of energy release do not appear to scale with the observed elapsed times between the shallow main shocks (47.1, 13.0, and 31.5 years in chronological order) according to time‐ or slip‐predictable models of earthquake recurrence. We interpret these findings in the context of a variable rupture mode of the Ometepec region. This variable rupture mode suggests that probabilistic forecasts of future large earthquakes in the region, which are based solely on the historic record of main shocks, can be uncertain by at least ±57% of the average local repeat time of 30.5 years and 1/2 unit of magnitude. Specific predictions of future strong ground motions are obviated because earthquakes within the gap do not exhibit a “characteristic” behavior. We also found that all four episodes of interplate main shocks were preceded by large downdip normal fault earthquakes at intermediate depths (M ≥ 6.5, h > 60 km). The time intervals between extensional and subsequent compressional events range from 1.6 to 9.9 years with an average of 6.3±3.7 years. This apparent pattern which initiates at depth was further documented for the final 2/3 of the last seismic cycle. From the spatiotemporal distribution of seismicity during this final period, we identified three general stages of stress release. Stage I, from 1964 to January 1980, was characterized by scattered seismicity. Stage II, from February 1980 to June 6, 1982, showed a significant increase in downdip seismic activity while the shallow zone became quiescent. (There was also a period of apparent seismic quiescence from mid‐1975 to early 1978). Most seismic energy in stage II was released on October 24, 1980, by a downdip normal fault earthquake at intermediate depth (MS = 7.0, h = 65 km). Stage III began on June 7, 1982, with the occurrence of two large interplate thrust fault earthquakes at shallow depths (MS = 6.9, h = 18 km, and MS = 7.0, h = 19 and 23 km). The documentation of stage III through June 1986 shows subsidence of previous downdip activity and an aftershock zone which is spatially coincident with the shallow quiet zone identified in stage II. The spatiotemporal characteristics of the 1982 main shock‐after shock sequence suggests that rupture propagated to the northwest toward the site of the 1957 (MS = 7.5) Acapulco‐San Marcos earthquake. This propagation is of interest because the latter site is presently considered to possess high seismic potential. We suggest that downdip normal fault seismic activity at intermediate depths reflects a symptomatic state of stress associated with some basic mechanism of plate interaction prior to interplate slippage in a large earthquake. This mechanism may have an important bearing on the generation and possible intermediate‐term forecasts of large shallow interplate earthquakes along the subduction zone in Mexico.