Temporal changes in seismic response of the Tres Vírgenes geothermal field (Mexico) from HVSR: Reservoir-scale variability and depth sensitivity

We analyze more than seven years of continuous ambient seismic noise recorded at the Tres Vírgenes geothermal field (Mexico) to investigate temporal changes in reservoir-scale seismic response using horizontal-to-vertical spectral ratios (HVSR). Daily H/V amplitudes were evaluated at representative frequencies for seven stations with the highest temporal continuity and compared quantitatively with water-injection rates and local seismicity. Network-scale correlations were weak and did not remain robust after accounting for temporal dependence, while objectively defined external perturbations did not, as a group, produce immediate H/V changes larger than those observed during matched non-event periods. Nevertheless, individual episodes exhibited coherent immediate or delayed responses, indicating substantial temporal and spatial heterogeneity across the field. To investigate the physical origin of selected spectral changes, H/V curves from August 2010 and January 2013 at station TV01 were inverted using a geologically constrained one-dimensional model. The preferred model families indicate approximately 21% lower S-wave velocity in the fractured granodioritic unit and approximately 5% lower S-wave velocity in the overlying sandstone unit for January 2013 relative to August 2010, although the inversion remains inherently non-unique. Frequency-dependent sensitivity kernels show that the low-frequency mode near 0.08 Hz is primarily sensitive to deeper crystalline portions of the model, whereas sensitivity progressively shifts toward intermediate and shallow units at higher frequencies. These results show that long-term HVSR monitoring may provide complementary information on temporal changes in fractured geothermal systems when interpreted together with independent operational, seismicity, geological, and geophysical constraints, but the response is episodic, spatially variable, and should not be interpreted as a deterministic proxy for injection, seismicity, or reservoir stress or as a stand-alone operational diagnostic.