A Seismological Study of the Michoacán-Colima, Mexico, Earthquake of 19 September 2022 (Mw7.6)

Michoacán-Colima earthquake of 19 September 2022 (Ms 7.6, Mw 7.6) ruptured the NW end of the Cocos-North American plate interface, causing severe damage to many towns and cities in the states of Michoacán and Colima. The damage was further exacerbated by a major aftershock (Mw 6.7) on 22 September. The mainshock initiated below the coast at a hypocentral distance of 22 km from the seismic station of Maruata (MMIG) where peak ground acceleration and velocity, PGA and PGV, of ~ 1 g and 28 cm/s were recorded. The epicenter of the major aftershock was located ~ 30 km SE of the mainshock. Finite fault modeling of the mainshock by the U.S. Geological Survey reveals a rupture propagation along the strike towards the NW and yields a static stress drop, Δσs, of 3.7 MPa. Our estimated radiated energy, ER, is 3.44x1015J, so that ER /M0 is 1.27 × 10−5 similar to other large Mexican thrust earthquakes whose rupture areas do not extend to the trench. Aftershocks of the 2022 mainshock overlap that of the Colima earthquake of 30 January 1973 (Mw7.6). Galitzin seismograms of the two earthquakes at DeBilt (DBN), The Netherlands, are reasonably similar so that they may be classified as quasi-repeated events. On the other hand, the DBN seismogram of the earthquake of 15 April 1941 (MS 7.7), whose location is poorly known but occurred in the same region, differs greatly from those of the 1973 and 2022 earthquakes, suggesting a different source area for the 1941 event. An analysis of the extensive regional recordings exhibits the effect of the directivity on the ground motion and on the ratio of ground motion during the mainshock to the major aftershock. The directivity explains the observed azimuthal dependence of PGA and PGV ratios, spectral ratios, and PGA and response spectra at 2s, Sa (T = 2 s). Because of the directivity, PGA, PGV, and Sa (T = 2 s) in the Valley of Mexico during the mainshock and the major aftershock were about the same in spite of the magnitude difference of 0.9. At CU (the reference, hard site in Mexico City), PGA and PGV during both events were ~ 6 cm/s2 and 2 cm/s, respectively, lower than expected for the mainshock and higher than expected for the aftershock.