Abstract

An earthquake of magnitude 6.6 occurred on 4 January 2006, at 28.081° N and 112.381° W along a transform fault that joins the San Pedro Martir and the Guaymas basins in the Gulf of California extensional province. We located 17 foreshocks and 38 aftershocks. The foreshocks occurred on a fault perpendicular to the transform fault, where the main event occurred. The aftershocks were located along a fault length of approximately 18 km with a northwest–southeast trend. The average Brune static stress drop of the San Pedro Martir event was 8 MPa. From a time‐domain moment tensor inversion, we obtained the fault geometry given by strike of 129°±1°, dip of 86°±4°, and rake of 168°±12°, which was constrained to have a nonisotropic component and a source depth of 6±2  km. We used the inversion code from Yagi et al. (1999) to invert near field and teleseismic P waves to obtain the spatial slip distribution over the fault. The event had a single source and a moment rate function (MRF) displaying a triangular shape with a duration of 12 s. The rupture propagated toward the northwest from the hypocenter over a rupture area of 28×12  km2 with a maximum slip displacement of 2.3 m and a seismic moment of 8.79×1018  N·m. The directivity confirmed that the rupture propagated from the southeast to the northwest. Few aftershocks were located in the rupture area obtained from the inversion. Most aftershocks occurred toward the southeast of the epicenter. All these source analyses were performed to have a well‐calibrated excitation term for future regional modeling of ground‐motion parameters. The magnitudes of the foreshocks preceding this peculiar earthquake were higher than those of the aftershocks. Our results show that earthquakes with magnitudes of five or higher present a simple and self‐scaling law using a constant stress parameter, but for earthquakes with magnitudes lower than five, the high frequencies are depleted, and the earthquake can be replicated by a low‐stress parameter of 0.28 MPa. We also observed that the aftershocks and foreshocks differ in their frequency content. Although the foreshocks follow Brune’s omega‐squared source term, the aftershocks have larger contents of high frequencies.

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