Tectonic setting and source parameters of the September 19, 1985 Michoacan, Mexico earthquake

Abstract

Analysis of body waves and long-period surface waves from the September 1985 earthquake in coastal Michoacan, Mexico shows that the event was an interplate subduction event with a low dip angle fault plane (δ=9°) striking parallel to the Mid-America trench (ϕ=288°) and a small component of left lateral motion (λ=72°) with a point source depth of 17 km, and a seismic moment in excess of 1 × 10^(28) dyn cm. The earthquake was a multiple event, with a second source of identical moment, fault geometry, and depth occurring approximately 26 s after the first. Directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first. This suggests that the earthquake first broke the northern portion of the Michoacan gap, propagated with low moment release through the rupture zone of the 1981 Playa Azul earthquake, and then broke the remaining asperity in the southern section of the gap. The seismic moment determined from Rayleigh and Love waves is between 1.0 - 1.7 × 10^(28) dyn cm (M_W = 7.9 - 8.1), the largest moment determined to date for a Mexico subduction earthquake. Comparison of seismograms at Pasadena with records of other large Mexico events shows that the Michoacan earthquake is basically the same size as the 1932 Jalisco, Mexico earthquake, and clearly larger than other significant events in Mexico since 1932. The seismic moment and the time since the last large earth-quake in Michoacan (in 1911) fit an empirical relation between moment and recurrence time found for the Guerrero-Oaxaca region of the subduction zone. The large aftershock on September 21 (M_s=7.5) has the same geometry as the mainshock, a somewhat larger source depth (22 km), a simple time function, and a seismic moment between 2.9 - 4.7 × 10^(27) dyn cm (M_w = 7.6 - 7.7).