HR: 0800h
AN: S41B-0992    [Abstracts]
TI: Microseismicity Studies in Northern Baja California: General Results.
AU: * Frez, J
EM: jofrez@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Acosta, J
EM: chang@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Gonzalez, J
EM: javier@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Nava, F
EM: fnava@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AU: Suarez, F
EM: fsuarez@cicese.mx
AF: CICESE, POB 434843, San Diego, CA 92143- United States
AB: Between 1997 and 2003, we installed local seismological networks in northern Baja California with digital, three-component, Reftek instruments, and with 100-125 Hz sampling. Each local network had from 15 to 40 stations over an area approximately of 50 x 50 km². Surveys have been carried out for the Mexicali seismic zone and the Ojos Negros region (1997), the San Miguel fault system (1998), the Pacific coast between Tijuana and Ensenada (1999), the Agua Blanca and Vallecito fault systems (2001), the Sierra Ju rez fault system (2002), and other smaller areas (2001 and 2003). These detailed microseismicity surveys are complemented with seismograms and arrival times from regional networks (RESNOM and SCSN). Selected locations presented here have errors (formal errors from HYPO71) less than 1 km. Phase reading errors are estimated at less than or about 0.03 s. Most of the activity is located between mapped fault traces, along alignments which do not follow the fault traces, and where tectonic alignments intersect. The results suggests an orthogonal pattern at various scales. Depth distributions generally have two maxima, one secondary maximum, at about 5 km; the other, located at 12-17 km. The Agua Blanca fault is essentially inactive for earthquakes with M_L > 1.7. Most focal mechanisms are strike-slip with a minor normal component; the others are dominantly normal; the resulting pattern indicates a regional extensional regime for all the regions with an average NS azimuth for the P-axes. Fracture directions, obtained from directivity measurements, show orthogonal directions, one of which approximately coincides with the azimuth of mapped fault traces. These results indicate that the Pacific-North American interplate motion is not being entirely accommodated by the NW trending faults, but rather is creating a complex system of conjugate faults.
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: Fall Meeting 2005