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SEISMOLOGICAL LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125
Abstract
We investigated the source characteristics of large earthquakes which occurred in the Michoacan, Mexico, seismic gap during the period from 1981 to 1986 in relation to historical seismicity in the region. The rupture pattern of the Michoacan gap during this period can be characterized by a sequential failure of five distinct asperities. Before 1981, the Michoacan gap had not experienced a large earthquake since 1911 when an MS = 7.8 earthquake occurred. The recent sequence started in October 1981 with the Playa Azul earthquake which broke the central part of the gap. Body-wave modeling indicates that the Playa Azul earthquake is 27 km deep with a seismic moment of 7.2 x 1027 dyne-cm. It is slightly deeper than the recent Michoacan earthquakes, and its stress drop is higher, suggesting a higher stress level at depths in the Michoacan gap. The seismic moment of the 19 September 1985 (Mw = 8.1) earthquake was released in two distinct events, with the rupture starting in the northern portion of the seismic gap and propagating to the southeast with low moment release through the area already broken by the 1981 Playa Azul earthquake. The rupture propagated further southeast with an Mw = 7.5 event on 21 September 1985. Another aftershock occurred on 30 April 1986 to the northwest of the 19 September main shock. Body-wave modeling indicates that this event has a simple source 10 sec long at 21 km depth, and fault parameters consistent with subduction of the Cocos plate (
= 280°, 
= 12°, and 
= 70°) and M0 = 2.0 to 3.1 x 1026 dyne-cm (Mw = 6.8 to 6.9). Although this distribution of asperities is considered characteristic of the Michoacan gap, whether the temporal sequence exhibited by the 1981 to 1986 sequence is also characteristic of this gap or not is unclear. It is probable that, depending on the state of stress in each asperity, the entire gap may fail in either a single large event with a complex time history or a sequence of moderate to large events spread over a few years. The seismic moment and the time since the last earthquake in Michoacan (in 1911) fit an empirical relation between moment and recurrence time found for the Guerrero-Oaxaca region of the Mexico subduction zone.
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