Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms

Abstract

The occurrence and specific properties of earthquake swarms in geothermal areas are usually attributed to a highly fractured rock and/or heterogeneous stress within the rock mass being triggered by magmatic or hydrothermal fluid intrusion. The increase of fluid pressure destabilizes fractures and causes their opening and subsequent shear‐tensile rupture. The spreading and evolution of the seismic activity are controlled by fluid flow due to diffusion in a permeable rock (fluid‐diffusion model) and/or by redistribution of Coulomb stress (intrusion model). These models, however, are not valid universally. We provide evidence that none of these models is consistent with observations of swarm earthquakes in West Bohemia, Czech Republic. Full seismic moment tensors of microearthquakes in the 2008 swarm in West Bohemia indicate that fracturing at the starting phase of the swarm was not associated with fault openings caused by pressurized fluids but rather with fault compactions. This can physically be explained by a fault‐weakening model, when the essential role in the swarm triggering is attributed to degradation of fault strength due to long‐lasting chemical and hydrothermal fluid‐rock interactions in the focal zone. Since the rock is exposed to circulating hydrothermal, CO2‐saturated fluids, the walls of fractures are weakened by dissolving and altering various minerals. The porosity of the fault gauge increases, and the fault weakens. If fault strength lowers to a critical value, the seismicity is triggered. The fractures are compacted during failure, the fault strength recovers, and a new cycle begins.

Magmatic or Not Magmatic? The 2015–2016 Seismic Swarm at the Long-Dormant Jailolo Volcano, West Halmahera, Indonesia

“…However, large normal faulting events are absent in the catalogs of focal mechanisms of past seismicity in the area (Cardwell et al, 1980;Ekström et al, 2012), although with our waveform similarity study we could find some small-magnitude events (i.e., M l < 4) before 2015 that are compatible with a normal faulting style. Finally, earthquakes triggered by a high pore-pressure front are expected to have a minor or negligible non-DC component in their MT solutions (Vavryčuk and Hrubcová, 2017). Therefore, within the porepressure diffusion model it is difficult to explain the occurrence of the large (>40%) non-DC component we observe in all our MT solutions.…”

“…Usually the source mechanics of earthquakes triggered by a pore-pressure increase are well-described by DC mechanisms compatible with the regional tectonic stress regime (Simpson et al, 1988;Valoroso et al, 2009;Stabile et al, 2014). Non-DC components are generally very small or negligible (Vavryčuk and Hrubcová, 2017).…”

Detection of Stress Anomaly Produced by Interaction of Compressive Fault Steps in the West Bohemia Swarm Region, Czech Republic

“…Fluid‐rich geothermal regions are usually characterized by high heat flow with elevated temperatures and with mixed brittle‐ductile failures manifested by the occurrence of a swarm‐like seismicity (Ben‐Zion & Lyakhovsky, ; Zaliapin & Ben‐Zion, ). Accordingly, a long‐term erosion of faults in West Bohemia by fluids caused probably their weakness with absence of strong asperities and produced repeating occurrence of earthquake swarms as documented by Vavryčuk and Hrubcová () by analysis of non‐DC components of seismic moment tensors. However, an interaction of the fault tips altered physical conditions in the focal zone.…”

“…The Young's modulus and the Poisson's ratio needed in the modeling are 8 × 10 4 MPa and 0.22. The Poisson's ratio of 0.22 corresponds to a rather low v P / v S ratio of 1.67 observed in the area and typical for geothermal regions (Vavryčuk & Hrubcová, ).…”

“…Focal mechanisms of the 2008 swarm earthquakes were calculated by Vavryčuk and Hrubcová () using the moment tensor inversion of amplitudes of direct P waves. The selected 483 swarm earthquakes with magnitudes ranging from 1.0 to 3.8 recorded at the WEBNET seismic network were located with high precision by Bouchaala et al () using the HypoDD method.…”