Rheological control on earthquake source kinematics and dynamics at the Hengill geothermal field

We analyze seismic source parameters of the induced earthquakes at the Hengill (Iceland) between 2018 and 202 to investigate rupture processes in a complex volcanic–geothermal setting. Our analysis reveals a source scaling relation that deviates from the commonly assumed M0 ∝ fc^-3. By combining stress tensor orientation, lithostatic and hydrostatic pressure, and frictional strength estimates, we quantify how much of the available effective stress is released by each earthquake. The results show a consistent depth transition: shallow earthquakes (< 3–4 km) rupture in a fluid–weakened regime, whereas deeper events (> 5 km) are increasingly controlled by ambient tectonic stress. This, combined with low Savage-Wood efficiency indicating significant dynamic overshoot, suggests that a large portion of the available strain energy is dissipated aseismically. To explain these observations, we propose a model where ductile, rate-dependent fracture energy suppresses earthquake cascades. This model successfully predicts the observed null/positive correlation between b-value and stress drop, a signature of a high-dissipation regime where increased stress drops suppress rupture propagation rather than promoting it.