Local Earthquake Tomography at the Los Humeros Geothermal Field in Mexico

Abstract

Between September 2017 and September 2018, a seismic network consisting of 23 broadband and 20 short period stations, was deployed to monitor and study the currently exploited Los Humeros geothermal field. Los Humeros is a volcanic complex located at the eastern edge of the Trans Mexican Volcanic Belt (TMVB) forming the northern boundary of the Serdán-Oriental basin. It hosts one of Mexico’s main geothermal fields, which is operated by the Federal Electric Comission (CFE, by its Spanish acronym). The shallow subsurface has been studied extensively, but knowledge of the geothermal system at depths greater than ~2.4 km is still rather sparse. For this reason, in the framework of the European H2020 project GEMex, several geophysical, geological, and geochemical surveys have been carried out to better understand the structure and behavior of the geothermal reservoir, and to investigate future development areas. In this work, we analysed the continuous seismic records and applied a recursive STA-LTA algorithm to detect local micro-seismic events mainly associated to exploitation activities. Manual review and picking led to the location of around 500 local events, mainly clustered around injection wells. Focal depths range for the main part between 1 and 4 km, which is the known reservoir interval. The extracted seismic catalogue was used to derive a 1D velocity model. Then, we employed this model as a priori information to obtain a 3D velocity model by jointly inverting for earthquake locations and lateral velocity variations. We will present the derived P-wave velocity model and Vp/Vs models to highlight new information on the geothermal field and possible variations due to changes in fluid composition, rock porosity and temperature.