3D Anisotropic Vs Model of the Los Humeros Volcanic Complex, México, Using Ambient Noise Tomography

Abstract:

The Los Humeros Volcanic Complex (LHVC) is located in the Trans-Mexican Volcanic Belt, and holds one of the main geothermal fields in Mexico (T ~ 400^oC, 90MW). It is composed by two nested collapses: Los Humeros and Los Potreros. Two geothermal reservoirs are present: a shallow liquid-dominated (1.2 to 1.7 km depth) and a deep mixed liquid-steam (1.9 to 2.7 km depth).

Several geological, geochemical and geophysical studies have been done to characterize in detail the hydro-geothermal system and the main structural pattern. However, the features of the caldera deeper than 3km and the position of the heat sources are still poorly constrained. As part of the international consortium between Mexico and Europe (GEMex), in 2017 we deployed a seismic network (25 broad-band and 20 short-period seismometers) for more than a year, in order to image the deep structure of the LHVC.

In this study we constructed a 3D anisotropic Vs model using group and phase velocities of the Rayleigh and Love waves reconstructed applying the seismic ambient noise tomography technique. A rigorous pre-processing and data quality-control (time-synchronization, sensor’s orientation) allowed us to obtain reliable and highly resolved models. The 2D group and phase velocity maps were obtained from 0.5 to 9 s, using variable cell-size grids. Depth inversion was carried out using surf96 code.

Our models imaged fairly well the geothermal reservoir and the related shallow structures, which are in strong agreement with the findings retrieved applying other geophysical methodologies (e.g. magnetotelluric, passive travel-time tomography, gravimetric, etc.). We also imaged two deep low-velocity bodies interconnected between them by thin vertically elongated anomalies: 1) at 3-7 km of depth, and 2) from 11 km depth downward. These regions match with two melting storage zones proposed recently by petrological studies and forming a complex magmatic plumbing system beneath the LHVC.

This work is performed in the framework of the Mexican European consortium GEMex (Cooperation in Geothermal energy research Europe-Mexico, PT5.2 N: 267084 funded by CONACyT-SENER: S0019, 2015-04, and Horizon 2020, grant agreement No. 727550). We thank the CFE for the access and support during the station installation in the field.