Abstract

On 3 August 2009 an earthquake of magnitude Mw 6.9 occurred near Canal de Ballenas, in the north-central region of the Gulf of California, Mexico. The focal mechanism of the main event, reported in the Global Centroid Moment Tensor (CMT) catalog, is right lateral strike-slip with a strike of 216° and a dip of 78°. The initial location reported by the National Seismological Service of Mexico [Servicio Sismólogico Nacional (SSN)] and the Array Network Facility (ANF) suggested that the epicenter was on the North American plate near the Tiburón fault, which is considered inactive. This earthquake was preceded by a magnitude mb 5.5 event that occurred about 5 min before. In the next 40 min after the main event two aftershocks with magnitudes mb 4.9 and Mw 6.2 occurred, and on 5 August a third aftershock of Mw 5.7 was located in the Canal de Ballenas region. The events of August 2009 were recorded by the regional stations of the broadband network Red Sismólogica de Banda Ancha (RESBAN) that Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) operates and by stations of the SSN also located in the region of the Gulf of California. We used body-wave arrivals to determine precise epicentral locations and to estimate the rupture area of this important sequence of earthquakes. The resulting hypocentral coordinates indicate that the main event of this sequence occurred along the Canal de Ballenas transform fault, with a rupture length of 50 km. Based on the aftershock distribution, we estimate that the main event had a rupture area of approximately 600 km2, an average slip of 1.3 m, and a stress drop of 2.2 MPa.

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