The 1973 and 2022 Mw 7.6 Michoacan-Colima earthquakes: implications for recurrent interplate rupture in the Mexico Subduction Zone

Abstract The Michoacan-Colima, Mexico, Mw 7.6 subduction earthquakes of 30 January 1973 and 19 September 2022 occurred within ~ 20 km of each other and have been suggested to be quasi-repeating events due to the similarity in the globally-recorded seismograms. P waves recorded for the two events, however, are distinctly different, indicating clear dissimilarities in the source characteristics of the two interplate ruptures. We apply a multiple time-window finite-fault inversion method to derive source models for the two earthquakes using the available teleseismic P-waveforms. The time-window formulation used in the inversion provides flexibility in the rise time, yielding a more appropriate estimate of the distribution of fault slip than previously obtained for the 1973 earthquake. Rupture models derived for the two events show significantly different distributions of coseismic slip. Slip during the 1973 earthquake covers a 35-km radius with a peak slip of ~ 2 m downdip of the hypocenter. Slip in 2022 occurs in two separate patches with the highest slip (~ 4 m) located near the hypocenter. The two slip zones of the 2022 earthquake straddle the area of high slip observed in 1973, indicating an adjoining pattern of asperity distribution along the plate boundary where interplate slip in 2022 occurs outside of the 1973 asperity area. This behavior has been observed in other recurring large earthquake sequences along the Mexico subduction zone and is consistent with a noncharacteristic asperity model where future high-slip areas occur in regions adjacent to or away from previous asperities. Finite-fault studies conducted to date for recurring large thrust earthquakes in Mexico, including the 1973/2022 sequence analyzed here, do not show a persistence of interplate asperities for consecutive events. Detailed analyses of the recorded seismic waveforms such as the one presented in this study can provide important constraints on the differences in rupture complexity for recurring events with nearly identical globally-recorded waveforms.