Inference of fracturing zones and degrees of fluid content in the Las Tres Virgenes volcanic complex based on an analysis of seismic anisotropy

Abstract

Anisotropy strength in the Tres Vírgenes Volcanic Complex, Baja California Sur, Mexico, is analyzed employing 558 seismic events collected from 2009 to 2013. It was possible to delineate zones and volumes with the highest fracture densities, which are mainly located between the El Viejo and El Azufre volcanoes and around the La Reforma–El Azufre fault system, near some other mapped faults in the area (e.g., El Azufre, El Partido, El Volcán, El Viejo 1, and El Viejo 2 faults); likewise toward the La Virgen volcano and around the La Virgen-El Campamento and El Volcán faults. Individual delay times reached values of up to 0.16 s and an anisotropy percentage of up to 10.3%, with a pervasive anisotropy observed from at least a hypocentral distance of 3.5 km. High fracturing levels are observed from a depth of 7.0 km. Differences between splitting delays and the dominant frequency peaks obtained from the fast S phases allowed considering fracture systems with different degrees of fluid contents. Fractures with minor fluid contents were assumed for delay times higher than 0.03 s with lower dominant frequency peaks (< 1.0 Hz). Higher concentrations of fluid inclusions were assumed for splitting delays higher than 0.03 s but with larger dominant frequency peaks (> 1.0 Hz). Fractures systems chemically sealed or impermeable sealing caps were assumed for low splitting delays (< 0.02 s) with low dominant frequencies (< 1.0 Hz). These different fracture systems seem to be observed at least from 5- to 6-km depth intervals. Likewise, an analysis of the fast polarization directions with respect to different depth ranges (spanning from 3.0 to 8.0 km) has allowed observations of a strong NW–SE regional fracture system accompanied by minor NE–SW fracture systems. However, noteworthy variations from NW–SE to NE–SW, N–S, and E–W in fast polarization directions in rose diagrams have been preferentially observed for those seismic events deeper than 4–5 and 5–6 km in some areas, which could be indicating the location of magmatic bodies that probably caused the reorientation on fracture systems by changes in the local stress field. These magmatic bodies might be supported by a decrease in the dominant frequency peaks (lower than 1.0 Hz), percentage of anisotropy (from 0.1 to 2.5%), and S-wave velocities (from 1.0 to 2.7 km/s), which seem to be located from the 4.0-km depth but more concentrated from the 5–6-km depth interval.