Characteristics of the 1985 Michoacan

Source characteristics of the Sept. 19, 1985 Michoacan, Mexico earthquake and its aftershock on Sept. 21 were inferred from broadband and short-period teleseismic GDSN records. We Fourier-transformed the P waves, corrected for instrument response, attenuation, geometrical spreading, and radiation pattern (including the depth phases), and then averaged to obtain the teleseismic source spectrum from 1 to 30 s. The Michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large in terplate subduction even ts. Source spectra for the Sept. 21 aftershock, 1981 Play a Azul, 1979 Petatlan, and 1978 Oaxaca events follow a trend similar to that of the 1985 Michoacan event. This spectral trend may characterize the Mexican subduction zone. A station-by-station least-sq11ares inversion of the Michoacan earthquake records for the source time function yielded three source pulses, which we interpreted as events on the fault plane. The first two are similar in moment, and the third contains only 20% of the moment of the first. Directivity is evident in the timing. At each station, we measured the time differences between the pulses, and performed a least-squares nonlinear estimation of the strike, distance, and time separation between the events to locate them relative to one another. The second event occurred 26 s after the first, and 82 km southeast of it, indicating southeastward rupture along the trench. The third event occui'red 21 s after the second, and about 40 km seaward of it. The two large events are also seen in the near-field strong motions. The mainshock records, spectrum, and time functions contain less high frequency radiation than those of the 1985 Valparaiso, Chile earthquake. Apparently, the Michoacan earthquake ruptured two relatively smooth, strong patches which generated large 30 s waves, but small 1 to 10 s waves. Such behavior contrasts with the Valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy. This difference is consistent with the higher nearfield accelerations recorded for the Valparaiso event.