Rupture process of four medium-sized earthquakes that occurred in the Gulf of California

摘要

Four medium-sized earthquakes (M w Global CMT project 5.5, 5.6, 5.9, 6.3; hereinafter named Topolobampo, Angel de la Guarda, San Lorenzo, and Loreto earthquakes, respectively) located in the Gulf of California Extensional Province were studied to obtain their kinematic rupture processes. A network of broadband seismic stations located around the Gulf of California recorded the events (Network of Autonomously Recording Seismographs–Baja and Red Sísmica de Banda Ancha). Inversion of the seismic moment tensor and body waveform modeling were used to obtain the fault geometry and slip distribution on the fault plane, respectively. From these analyses, we obtained source depths of the order of 5.5 ± 0.5 km. We found also that the source rupture processes of the Topolobampo and Angel de la Guarda events have simple moment rate functions and source time durations of 5.0 ± 1.2 and 4.2 ± 1.2 s, respectively. The Topolobampo event was a right-lateral strike-slip event, and Angel de la Guarda was a normal event. The San Lorenzo and Loreto shocks show a rather complex rupture, with source time durations of 7.5 ± 1.2 and 9.0 ± 1.2 s, respectively. For these earthquakes, we tested the resolution of numerical results, performing an extra inversion with smoother waveforms. The new inversions do not show the separated patches of slip, as in the first analysis, but the slip distribution has an elongated shape not characteristic of simple events. We cannot therefore conclude rupture propagation for the San Lorenzo event, although the extent of the patch for the Loreto earthquake agrees with aftershock locations. Estimates of source time durations for these earthquakes are at the upper limit of the values found for earthquakes elsewhere. Directions of P axes are in the same order of magnitude as the maximum horizontal stress obtained for the so-called Gulf of California stress province from borehole elongations, focal plane solutions, and fault slip data.