LAUSR

Abstract We analyze the seismicity that occurred in the Tres Virgenes Volcanic Complex, TVVC, (Baja California Sur, northwestern Mexico) from 2009 to 2013, whose depths range from 2 to 6 km and are located in granitic rocks of the Peninsular Range Batholite that constitutes the substratum of the geothermal field. A shear wave splitting analysis of 558 micro-earthquakes was carried out. Based on the birefringence study (fast shear wave polarization and delay time parameters) of the seismic records, we detected irregular spatial and temporal changes. Our results show NW-SE, N-S, and NE-SW polarization directions as well as mean delay times ranging from 0.015 s to 0.058 s for all analyzed seismic stations, with some individual values reaching 0.16 s. The relation of both parameters with 1) location of epicenters in different sectors, 2) faults affecting the TVVC, and 3) location of recording seismic stations indicates that the anisotropy is strongly controlled by the fault systems affecting the TVVC. We observe that the birefringence parameters are due to anisotropic systems, which vary azimuthally. The first system comprises NW-SE fast polarizations that extend at least 5 to 6 km deep. Superimposed to this anisotropic system there is a local and shallower second system (up to 5 km deep), with NW-SE, N-S, and NE-SW fast polarizations. This anisotropy is associated with local faults with similar orientations that affect the TVVC (i.e., El Viejo, El Cimarron faults). The first system is of regional character associated with NW-SE faults (i.e., El Mezquital, El Campamento, La Virgen, Reforma, and El Azufre faults) related to Gulf of California opening (i.e., related to an NW-SE transpressive system). The geothermal field is characterized by NW-SE polarizations correlated with local faults following this trend (i.e., El Viejo Fault). Immediately to the north of the TVVC, there are N-S to NE-SW faults (i.e., El Cimarron Fault). We infer that the two groups of faults controlling the observed polarizations intersect at depths between 5 and 6 km giving rise to fracture networks that allow the flow of geothermal fluids towards the surface. Below a depth of 6 km, that corresponds to the brittle-ductile transition (i.e., 350°C), would be located the heat source of the geothermal system (magmatic reservoir, or a dike system). Seismicity between 1 to 2 km depths is scarce, corresponding to the depth of the actual geothermal reservoir.