A Source Study of the Mw 7.0 Acapulco, Mexico, Earthquake of 8 September 2021

Abstract

AbstractThe Acapulco earthquake of 2021 broke a segment of the southeast Guerrero seismic gap along the Mexican subduction thrust. The rupture initiated offshore Acapulco (16.770° N, 99.942° W) and propagated down-dip toward northeast. This source directivity is confirmed from both (1) an analysis of local and regional recordings as a function of azimuth and (2) kinematic inversion of near-source, band-pass filtered (0.025–0.5 Hz) displacement seismograms and Global Positioning System static coseismic displacement vectors. The inversion reveals little slip near the hypocenter (<0.5 m) and significant slip distributed over an area of ∼184  km2, with the large slip patches in the northeast part of the fault. The estimated average slip and static stress drop are 260 cm and 18.6 MPa, respectively. Moment rate function reported by National Earthquake Information Center–U.S. Geological Survey from finite-fault modeling is simple, and it resembles other Mexican subduction earthquakes in the 7.0 ≤ M ≤ 7.5 range. Moment rate spectrum is well fit by the Brune ω−2 source model. Radiated seismic energy from teleseismic P waves is 7.5×1014  J, and ER/M0 is 2.1×10−5. Radiated energy enhancement factor—a measure of source complexity—is small, 5.8, similar to other Mexican subduction thrust earthquakes. Seismograms at DeBilt of the 2021 and the 11 May 1962 Acapulco earthquakes show an extraordinary similarity, seldom seen at M 7.0 level. The 2021 earthquake seems a repeat of the 1962 earthquake. The slip deficit since 1962 corresponding to a plate convergence rate of 6.2 cm/yr and perfect coupling is 366 cm. Thus, the seismic slip of 260 cm during that 2021 earthquake suggests a coupling ratio of 0.7, greater than 0.3 and 0.5 reported from geodetic measurements. Large moment release in the southeast seismic gap appears to have a periodicity of ~60 yr. Because 60 yr have elapsed since the last sequence earthquakes (1957 MS 7.5; 1962 MS 7.0 and 6.8), a renewal of large earthquakes in the region may be expected.