A detailed microseismicity study and current stress regime in the peninsular ranges of northern Baja California, Mexico: The Ojos Negros Region

The NW-trending San Miguel fault system is one of the most important seismo genic systems in northern Baja California, and the Ojos Negros region, comp rising the Ojos Negros valley and bordering areas, is one of its most activ e regions. Within this region are found most of the mapped faults of the sy stem: Ojos Negros, Tres Hermanos, most of San Miguel, and portions of the V allecitos fault, which makes this a very important region from the points o f view of intraplate tectonics and regional seismic hazard. A detailed microseismicity (0.2 less than or equal to M less than or equal to 4.0) survey of the Ojos Negros region, carried out in 1997 (one month, 1 3 Reftek stations recording at 200 samples/sec, plus two permanent RESNOM s tations and other less sensitive instruments), yielded important results ab out 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 usi ng an improved velocity model (Sierra97) for this part of the Peninsular Ra nges. The hypocenters tend to cluster in space and time, with cluster interepicen ter separations of the order of the location error (il km). The Ojos Negros valley las defined by its sedimentary soil) is roughly covered by epicente rs. The Tres Hermanos fault shows no significant seismicity, and the few ea rthquakes near (although not very close to) its southern third seem to be a ssociated with seismicity that extends into the valley. Seismicity associat ed with the Ojos Negros fault consists almost exclusively of one large clus ter. The San Miguel fault, the most active fault in the area, has epicenter s 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 indicate s a regional extensional regime with the average T axis in the ENE-WSW dire ction, and P axes distributed along an N-S strip with a slight inclination and concentrated near the poles.