Direct comparison of stress drop from waveform and geomechanical stress from in-situ observation at the Basel EGS site

Fluid injection for developing an Enhanced Geothermal System was conducted in Basel, Switzerland, in December 2006, and thousands of induced microearthquakes were observed. Our study enables the first direct comparison between the resolved geomechanical stress parameters on the faults,using borehole logging in-situ stress data, and the stress drop of induced earthquakes derived from seismic waveforms. A significant portion of the resulting stress drops was less than 0.1 MPa, a value lower than those typically observed in natural tectonic earthquakes. High stress drop events (exceeding 1 MPa) showed no significant spatio-temporal clustering, farther from the injection point. This spatial and temporal distribution suggests that the initial pore pressure increase locally reduced the strength of faults near the well, leading to predominantly low-stress-drop events. Subsequently, higher-stress-drop events were triggered in more distant regions. A comparison between the shear stress and stress drop revealed a positive correlation, establishing a mechanically plausible link. There was a negative correlation between the stress drop and changes in pore pressure, implying that higher pore water pressure affects normal stress and makes it easier for the fault to slip. This was consistent with the low stress drop observed. The stress drop estimates were typically lower for cluster events and higher for unclustered events. Isolated high stress drop events following the shut-in operation suggest pore pressure distribution changed relating to the shut-in process. These findings definitively link fault stress and pore pressure to the stress drop of induced earthquakes, providing crucial insights for mitigating seismic risk in geothermal development.