Strong Ground Motion during the 8 September 2021 (Mw7.0) Acapulco Earthquake: Rupture Directivity and its Effect on Simulated Motions in Mexico City from Postulated Mw 7.5 – 8.0 Earthquakes

The 8 September 2021, Mw7.0, earthquake occurred in the SE Guerrero seismic gap along the Mexican subduction zone, just below Acapulco. The earthquake was strongly felt in Acapulco with peak ground acceleration (PGA) exceeding 0.6 g at some sites. An analysis of seismograms and accelerograms reveals a strong azimuthal variation of the seismic intensities. Unusually large intensities are observed in the NE direction with respect to the regional ground motion prediction equation (GMPE). This is consistent with the reported rupture directivity towards NE inferred from the slip distribution on the fault obtained from the inversion of local strong motion data. Using recordings of the 2021 earthquake and five other Guerrero thrust events (5.8 ≤ Mw ≤ 7.3) at CU as empirical Green´s functions (EGFs), we simulate ground motions for postulated Mw 7.5, 7.7, and 8.0 earthquakes in the Guerrero seismic gap. The simulated motions strongly depend on the source directivity of the EGF. The largest and smallest synthesized motions occur when the 8 May and 10 May 2014 events are used as the EGFs, respectively. Directivity towards and away from CU during the 8 May and 10 May earthquakes, respectively, is well-documented in a previous study. Three of the six EGFs yield synthesized motions from a postulated Mw 8.0 earthquake that exceed, two EGFs produce motions which are smaller, and one EGF gives similar motions to those observed at CU during the devastating1985 Michoacan earthquake. Under adverse directivity conditions, as observed during the 8 May 2014 earthquake, even an Mw 7.5 event in Guerrero seismic gap may produce motions similar to that recorded in1985. This may have been the case during the 28 July 1957 (Ms7.5) Guerrero earthquake, the third most damaging event in the history of Mexico City.