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A detailed microseismicity study and current stress regime in the Peninsular Ranges of northern Baja California, Mexico; the Ojos Negros region

J. Frez, J. J. Gonzalez, J. G. Acosta, F. A. Nava, I. Mendez, J. Carlos, R. E. Garcia-Arthur and M. Alvarez
A detailed microseismicity study and current stress regime in the Peninsular Ranges of northern Baja California, Mexico; the Ojos Negros region
Bulletin of the Seismological Society of America (October 2000) 90 (5): 1133-1142

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< or =M< or =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 (+ or -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.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 90
Serial Issue: 5
Title: A detailed microseismicity study and current stress regime in the Peninsular Ranges of northern Baja California, Mexico; the Ojos Negros region
Affiliation: CICESE Seismology Department, Ensenada, Mexico
Pages: 1133-1142
Published: 200010
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 37
Accession Number: 2001-006308
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
N31°30'00" - N32°15'00", W116°30'00" - W115°45'00"
Secondary Affiliation: New Mexico Institute of Mining and Technology, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200103
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