ALBERT

Beschreibung: We studied the background seismic activity in the Cerro Prieto geothermal field (CPGF). Earthquakes were located below the exploitation area of the CPGF, between 3 and 12 km deep. This activity is located on a horstlike structure below the geothermal field and coincides with the zone of maximum subsidence in the CPGF. Two earthquake swarms occurred, one in November 2000 and the second in February 2001, along the southeast-northwest strike of the Cerro Prieto fault and in the neighborhood of the Cerro Prieto volcano. Magnitudes of the located micro-events range from -0.3 to 3.5. A V (sub P) /V (sub S) ratio of 1.91 for the region below the volcano suggests a water-saturated medium and/or a partial-melt medium. Seventy-six focal mechanisms of individual microearthquakes reveal significant variation in the orientation of the P and T axes. Focal mechanism inversions using the Gephart (1990) code also reveal significant lateral stress variation of the best-fit stresses. Below the CPGF, the orientation of the best-fit maximum, intermediate, and minimum stresses are (plunge, azimuth): sigma (sub 1) = (66 degrees , 72 degrees ), sigma (sub 2) = (6 degrees , 328 degrees ), and sigma (sub 3) = (24 degrees , 235 degrees ). For the November 2000 earthquake swarm, the best-fit principal stress orientations are sigma (sub 1) = (12 degrees , 117), sigma (sub 2) = (11 degrees , 25 degrees ), and sigma (sub 3) = (74 degrees , 253 degrees ), whereas for the February 2001 earthquake swarm they are sigma (sub 1) = (36 degrees , 235 degrees ), sigma (sub 2) = (24 degrees , 127 degrees ), and sigma (sub 3) = (45 degrees , 11 degrees ). The sigma (sub 1) orientations are considerably different from the orientation of the maximum compressive regional stress that lies approximately north-south. On 1 June and 10 September 1999, two earthquakes of M (sub W) 5.2 and 5.3 occurred in the basement at depths of 7.4 and 3.8 km below the CPGF. Waveform modeling resulted in fault-plane geometries given by strike = 236 degrees , dip = 60 degrees , slip = -58 degrees (normal) and strike = 10 degrees , dip = 90 degrees , slip = 159 degrees (right-lateral strike slip), respectively. An observed triangular source time function of 0.7 sec and a double source with a total duration of 1.9 sec were used to calculate the synthetics seismograms for these two events. Static stress drops and seismic moments for the June and September events are Delta sigma = 82.5 MPa, M (sub 0) = 7.65X10 (super 16) N m and Delta sigma = 31.3 MPa and M (sub 0) = 1.27X10 (super 17) N m. These stress drops are typical of continental events rather than of events originating in spreading centers. We concluded from the focal mechanism inversions of the background seismicity and the June and September 1999 events that complex stress heterogeneity exits in the CPGF as a result of the continual thinning of the crust in the Cerro Prieto basin.

Beschreibung: Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude M (sub w) 5.5 event on 26 November 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754 degrees N and 113.708 degrees W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mechanism estimated from first motions (phi = 310 degrees , delta = 83 degrees , lambda = 97 degrees ) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.05-0.1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330 degrees + or -15, dip 85 degrees + or -5, and slip of 165 degrees + or -15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28X10 (super 17) N m (1.28X10 (super 24) dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.