Source mechanism of the September 19, 1985 Michoacan Earthquake and its implications

Abstract

Moment tensor inversion of low frequency teleseismic data for the September 19, 1985 Michoacan, Mexico earthquake gave an average strike 298°, slip 268°, and seismic moment 10.3×10 27 dyne‐cm. Low frequency body waves clearly show the complex nature of this event. Cross‐correlation of the September 19 P‐wave with that of the smaller, and colocated October 25, 1981 Playa Azul earthquake, show that the Michoacan earthquake consisted of two subevents separated in time by 25 s, and located approximately 30 km northwest and 40 km southeast respectively, of the Playa Azul epicenter. Body wave inversion of the first subevent gave strike 298°, slip 266°, and seismic moment 3.0×10 27 dyne‐cm. The second subevent had a comparable seismic moment. The cumulative moment for large earthquakes occurring along the Michoacan‐northwest Guerrero coast indicate that seismic displacement along this segment of the Mexican subduction zone is similar to that occurring on other portions, indicating that the "former" Michoacan gap was not anomalously aseismic.