Moment‐tensor spectra of the 19 Sept 85 and 21 Sept 85 Michoacan, Mexico, earthquakes

IDA records of the 19 Sept 85 Michoacan earthquake and its large aftershock of 21 Sept 85 have been used to estimate source‐mechanism spectra and total‐moment spectra MT(ω) in 1‐mHz bands over the frequency interval 1‐11 mHz. Source mechanisms, obtained by the phase‐equalization method of Riedesel and Jordan, show no significant frequency dependence and no significant non‐double‐couple components. The best average double‐couple solution has a strike, dip and rake of 289° ± 4°, 19° ± 15° and 76° ± 5° for the main event, and 293° ± 3°, 24° ± 12° and 73° ± 4° for the aftershock. Total‐moment spectra, obtained by the power‐spectral method of Silver and Jordan, are parametrized by a total integrated moment MT0 and a characteristic time τc. For the main event, we obtain MT0 = (10.7 ± 2.0) × 1020 N−m and τc = 49 ± 7 s; the corresponding parameters for the aftersnock are MT0 = (2.6 ± 0.6) × 1020 N−m and τc = 30 ± 11 s. Both events are relatively slow; in particular, the aftershock has a larger characteristic time than either the fast‐rupturing 29 Nov 78 Oaxaca or the 14 Mar 79 Petalan earthquakes, although the static moments are about the same. Our results support the model of the main shock as a multiple‐rupture event with moment release occuring as long as 100 s after the initial break.