Spatio-temporal distribution of the b-value in the volcanic complex and geothermal field of Tres Vírgenes; Baja California Sur, Mexico

Analysis of 1275 seismic events recorded at least at 4 seismic stations during 2009–2013, enabled to conduct the analysis of the b-value from the Gutenberg-Richter relation in time and space in the Tres Vírgenes Volcanic and Geothermal Complex (TVVGC). A mean value of 1.64 (b = 1.64 ± 0.08) was calculated for all the TVVC, which is alike to estimations obtained at other geothermal zones and volcanic regions. The spatial distribution can be characterized by four zones with distinctive ranges of b-values. Zone A (b = 2.07 ± 0.2) corresponds to the central part of the TVVC. Zone B (b = 0.63 ± 0.04) comprises El Aguajito caldera, and El Cimarrón Fault. Zone C (b = 2.33 ± 0.3) is centered between the La Virgen Volcano, and La Virgen Fault. The zone west of La Virgen volcano, delimited by the Mezquital, and Bonfil Faults, constitutes zone D (b = 0.946 ± 0.1). A large regional and local tectonic heterogeneity is associated with zones A and C. Zone A comprises the geothermal production operation zone. Zones B and D can be correlated with regional tectonics. Larger seismicity levels are observed in zones A and C (a-values of 6.52 and 7.8 respectively) with respect to zones B (a = 2.81) and D (a = 3.05). The minimum magnitude of completeness (Mc) was 2.3 and 2.6 for zones A and C, and 1.2, and 1.5 for zones B and D. At depths of 3.5 and 7.0 km, values of b = 1.47 ± 0.1 and 2.24 ± 0.2 were obtained. For depths less than 2.5 km, and larger than to 7.1 km, we estimated b = 0.93 ± 0.1 and 0.789 ± 0.05. Similarly, the largest values of the a-parameter (i.e., 4.72 and 7.66), are associated with depths between 3.5 and 7.0 km. The lowest values, 3.23 and 3.25, correspond to shallow depths of less than 2.5 km, or larger than 7.1 km. The largest values observed between 3.5 and 7.0 km might indicate that permeability is low at the Mesozoic granitic basement, and intense seismic activity (seismic swarms) is associated with tectonic and geothermal processes. Cylindrical to vertical oblate volumes are associated with four high b-value anomalies which correlate with deep reaching NW-SE regional faults (i.e., El Mezquital, La Virgen-El Campamento, and El Azufre-Reforma). These anomalies stretch along several depth intervals. A recently reported distribution of Vp, and Vp/Vs beneath the volcanic complex further indicates that the combination of tectonic and geothermal process giving rise to two of such anomalies might be due to the presence at depth of partially melted magma. In particular, beneath the northwestern portion of the La Virgen volcano, and north of the geothermal field. This interpretation implies that magma emplacement was controlled, in particular by deep reaching NW-SE regional faults, which were emplaced at different times along the NE-to-SW directed magmatic evolution of this volcanic complex. Another issue is that there are several independent heat sources beneath the TVVC.