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Vavryčuk, V., Hrubcová, P. - 2017 - Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms

Vavryčuk, V., Hrubcová, P. - 2017 - Seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms

Václav Vavryčuk and Pavla Hrubcová, researchers of the Institute of Geophysics of the Czech Academy of Science, investigated seismological evidence of fault weakening due to erosion by fluids from observations of intraplate earthquake swarms.

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 is controlled by fluid flow due to diffusion in a permeable rock (fluid-diffusion model) and/or by the redistribution of Coulomb stress (intrusion model). These models, however, are not valid universally. We provide evidence that none of these models is inconsistent with observations of swarm earthquakes swarms in West Bohemia, Czech Republic. Full seismic moment tensors of micro-earthquakes in the 1997 and 2008 swarms 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 ‘fluid-erosion fault weakening model’, when the essential role in the swarm triggering is attributed to 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 the 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.

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