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Source Characteristics of the M_w 6.2 Loreto Earthquake of 12 March 2003 that Occurred in a Transform Fault in the Middle of the Gulf of California, Mexico

¹ Centro de Investigación Científica y de Educación Superior de Ensenada
Departamento de Sismología
Km 107 Carretera Tijuana-Ensenada
Ensenada, Baja California, Mexico CP 22860

We analyzed the Loreto earthquake of 12 March 2003 M_w 6.2, which occurred in the transform fault that joins the Guaymas and Carmen Basins in the middle of the Gulf of California. This event was recorded by a network of autonomous continuously recording broadband seismographs located over the Baja California Peninsula, Sonora, and Sinaloa states of Mexico. The main event was located at 26.615° N and 111.09° W, at a depth of 5.0 ± 2 km. A foreshock of magnitude 4.2 was recorded and located at 26.580° N and 111.011° W. We located 35 aftershocks, 20% of them were located in the rupture area of the mainshock consistent with a nearly vertical right-lateral strike-slip fault with a northwest–southeast trend. Rupture propagation derived from waveform analysis indicated that the rupture propagated from northwest to southeast. Most of the aftershocks occurred southeaster of the main event. From the statistical analysis of 333 aftershocks we calculated a b-value of 0.68.

Focal depth and fault geometry were estimated from body-wave form modeling of P and S waves. Synthetics were calculated using Herrmann’s (1987) reflectivity code with a triangular source time function of 6 sec. The best match between synthetics and recorded P and S waves were obtained with a fault geometry given by a strike of 117° ± 4°, a dip of 79° ± 2°, and a slip of 168° ± 2°. We calculated a seismic moment of 2.68 x 10¹⁸ N m (2.68 x 10²⁵ dyne cm), a source radius of 6.8 ± 0.7 km, and a static stress drop of 3.8 ± 1.2 Mp (38 ± 12 bars). Calculated static stress drops of previous events located in the Gulf of California range from 0.2 to 12.5 Mp (2 to 125 bars).