Abstract

The NW-trending San Miguel fault system is one of the most important seismogenic systems in northern Baja California, and the Ojos Negros region, comprising the Ojos Negros valley and bordering areas, is one of its most active regions. Within this region are found most of the mapped faults of the system: Ojos Negros, Tres Hermanos, most of San Miguel, and portions of the Vallecitos fault, which makes this a very important region from the points of view of intraplate tectonics and regional seismic hazard.

A detailed microseismicity (0.2 ≤ M ≤ 4.0) survey of the Ojos Negros region, carried out in 1997 (one month, 13 Reftek stations recording at 200 samples/sec, plus two permanent RESNOM stations and other less sensitive instruments), yielded important results about the fault activity and the stress regime in the region. Our results are based on 278 hypocenters and 50 focal mechanisms selected from almost 2500 earthquakes recorded at a minimum of four stations. The selected database is comprised of good quality local events, for which the hypocentral depth can be reliably estimated. Locations and focal mechanisms were obtained using an improved velocity model (Sierra97) for this part of the Peninsular Ranges.

The hypocenters tend to cluster in space and time, with cluster interepicenter separations of the order of the location error (±1 km). The Ojos Negros valley (as defined by its sedimentary soil) is roughly covered by epicenters. The Tres Hermanos fault shows no significant seismicity, and the few earthquakes near (although not very close to) its southern third seem to be associated with seismicity that extends into the valley. Seismicity associated with the Ojos Negros fault consists almost exclusively of one large cluster. The San Miguel fault, the most active fault in the area, has epicenters within a 6–8 km wide band centered along its mapped trace.

Most focal mechanisms are strike-slip with a minor normal component, while others are dominantly normal. The resulting pattern for the valley indicates a regional extensional regime with the average T axis in the ENE-WSW direction, and P axes distributed along an N-S strip with a slight inclination and concentrated near the poles.

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