Seismic Gap Earthquakes.pdf

The Michoacan seismic gap in Mexico experienced a series of large earthquakes from 1981 to 1986, including the 1981 Playa Azul earthquake (Ms = 7.3), the 1985 Michoacan earthquake (Ms = 8.1), and two large aftershocks in 1985 (Ms = 7.5) and 1986 (Ms = 7.0). The rupture pattern of the Michoacan gap during this period can be characterized by a sequential failure of five distinct asperities.

The 1981 Playa Azul earthquake had a seismic moment of 7.2 x 10^27 dyne-cm and a depth of 27 km. The 1985 Michoacan earthquake had a seismic moment of 1.1 x 10^28 dyne-cm and a depth of 27.9 km. The aftershock on April 30, 1986, had a seismic moment of 2.0-3.1 x 10^26 dyne-cm and a depth of 21 km.

The seismic moment and 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. The distribution of asperities in the Michoacan gap is considered characteristic, but the temporal sequence exhibited by the 1981-1986 sequence is unclear.

The Middle America trench has been the site of numerous large thrust earthquakes that rupture discrete segments 100-200 km long, with an average recurrence interval of 33 ± 8 years. The Michoacan gap is part of this subduction zone, and its seismic activity is influenced by the subduction of the Cocos plate under the North American plate.

Historical seismicity in the region shows that large earthquakes have occurred along most of the Middle America trench in the last hundred years, with some segments experiencing longer recurrence intervals. The Guerrero gap, located south of the 1979 Petatlan rupture zone, is another major gap in the Mexican subduction zone that has not experienced a major event since the turn of the century.