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

Abstract

AbstractObservations of the 2008–2014 seismic activity in West Bohemia, Czech Republic, provide evidence of interaction of compressive fault steps that created local stress anomaly and triggered a seismic sequence with exceptional properties. The West Bohemia is a geothermal area, characterized by persistent fluid‐driven seismicity in the form of earthquake swarms. The focal zone is formed by two weak and fluid‐eroded parallel strike‐slip faults with a step of about 200 m. The fault segments were activated successively by the 2008 and 2011 swarms with magnitudes of the strongest events of 3.8 and 3.7, respectively. In 2014, a fracture linking both segments was formed or activated by a mainshock‐aftershock sequence. The aftershock decay was very fast, and the focal mechanism of the strongest event with magnitude of 4.2 was inconsistent with the regional background stress. The stress inversion of 957 focal mechanisms revealed a stress anomaly characterized by interchanging the σ2 and σ3 principal stress axes in the area of fault interaction. The modeling of the Coulomb stress change confirmed that the stress anomaly could completely disturb the regional background stress and produce the rotation of the principal stress axes retrieved from focal mechanisms. The faults activated or newly formed within the compressive stress anomaly were of high strength, which caused the anomalous mainshock‐aftershock character of the 2014 activity and the rapid aftershock decay. Linking the two previously active isolated faults during the 2014 activity increased the expected moment magnitude Mw of a possible strongest earthquake from 5.0 to 5.4.