Seismic Anisotropy Observations in the Mexicali Valley, Baja California, México

Abstract

A study of seismic anisotropy was performed using data from earthquakes of the Mexicali Valley. The investigated region encompasses the Cerro Prieto Geothermal Field (CPGF), one of the most important fields in the world. The results showed that at most of our stations the average polarization directions of the fast S waves range from N14°W to N17°E. A N-S polarization direction was obtained for the whole area by averaging the polarization directions from all stations used. In terms of the EDA hypothesis, this average trend agrees with the postulated state of stress for southern California, and with fault plane solutions for events of the Mexicali Valley. Notorious deviations from the N-S global trend were found southeast of the CPGF, with polarization trends between N25°E and N67°E, and in the geothermal field, with polarization directions between N7°W and N14°W. The polarization results for these zones indicated stress conditions that are different from the more regional stress pattern.The delay times that were measured between the fast and slow shear waves reached values of up to 0.6 sec, with a mean value of 0.35 sec. Consistent with our polarization results, the larger delay times (0.2-0.6 sec) were found in the CPGF. Smaller or null values were observed at the periphery of the study area. No temporal trends in the delay times were apparent, as shown by data from the two stations that recorded the larger number of events.Overall, we conclude that the splitting effects of this study result from a shallow anisotropy volume. The splitting results are thus interpreted as caused by the preferred orientation of vertical fluid-filled microcracks aligned in a direction that is parallel to the regional stress field. The stronger splitting effects that were observed in the area of the CPGF were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone, at depths of 1 to 4-5 km from the surface. We thus believe that such marked splitting effects have a direct relation with the reservoir of the CPGF.

Shear‐wave splitting observations at the regions of northern Baja California and southern Basin and Range in Mexico

“…NE70 yields fast axis direction sub parallel to the trend of the plate boundary, which is consistent with the direction reported for the crust by Zúñiga et al [1995]. González and Munguía [2003] report mean delay times of 0.35s for the crust below Mexicali Valley and Zollo and Bernard [1989] a mean value of 0.25s for the crust of the Imperial Valley, suggesting that the crustal anisotropy in this region may contribute to up to 40% of the delay time obtained with teleseismic waves. The delay time of 0.9s at NE70 indicates that the strain in the vicinity of the Imperial‐Cerro Prieto fault system may not extend deeper than ∼100 km.…”

Systematic analysis of crustal anisotropy along the Karadere-Düzce branch of the North Anatolian fault

“…The ∼7 per cent estimated crack density is similar to values measured near the San Andreas fault at Parkfield (Daley & McEvilly 1990) and a normal fault near Oroville, California (Leary et al 1987), while being somewhat higher than the 5 per cent value measured at the Hector Mine rupture zone (Cochran et al 2003). We also note that 7 per cent is lower than inferred crack density at volcanic or geothermal regions, such as the Long Valley caldera in California (Savage et al 1990), southern Hawaii (Munson et al 1995) and the Cerro Prieto geothermal field in the Mexicali valley, Mexico (González & Munguía 2003).…”

Seismic anisotropy in northern and central Gulf of California region, Mexico, from teleseismic receiver functions and new evidence of possible plate capture

“…Thus the characterization of this property provides information about the stress/strain state. Upper crust anisotropy in the GoC region was first characterized in the Mexicali‐Imperial Valley by Zúñiga et al [1995] and González and Munguía [2003], who analyzed the birefringence that affects local S waves. Recently, the installation of broad band stations of the NARS‐Baja and RESBAN arrays have enabled the first studies of the anisotropy of the crust and upper mantle throughout the entire region of GoC using surface waves [ Zhang et al , 2006; Markee and Gaherty , 2006; Zhang et al , 2007] and also SKS waves [ Obrebski et al , 2006; Van Benthem et al , 2006].…”