Tensile Fault Steps Indicated by Positive Non-Double-Couple Components of Seismic Moment Tensors in West Bohemia Swarm Earthquakes

Session: Earthquake Source Parameters: Theory, Observations and Interpretations
Type: Oral
Date: 4/30/2020
Time: 11:45 AM
Room: 120 + 130
Description: 

The West Bohemia region is a geothermal area with a permanent seismic activity in the form of earthquake swarms. This activity is related to Tertiary and Quaternary volcanism manifested by numerous CO2 emanations and mineral springs. The earthquake swarms in West Bohemia last typically from several days to several months and consist of thousands of microearthquakes with strongest events rarely exceeding ML of 4. The earthquakes are mostly characterized by the left-lateral strike-slip focal mechanism associated with the main fault in the focal zone. However, some earthquakes display also other focal mechanisms indicating the presence of differently oriented fault segments, which can mutually interact and cause local stress anomalies.

Analyzing about 1500 microearthquakes, which occurred during the 2018 swarm, we show that non-double-couple (non-DC) components of seismic moment tensors vary along the fault. Most frequently, the isotropic component (ISO) of the moment tensors is typically negative and indicates compaction of fault gouge during the rupture process. However, we detected an area in the focal zone, which displays a positive ISO indicating rather extensive motions and fault opening. A high-resolution imaging of the fault zone and a detailed stress analysis reveal that this anomalous area is produced by interaction of tensile fault steps. A local stress anomaly under tensile regime is confirmed by numerical modelling of the Coulomb stress. The results point to a possibility of utilizing the non-DC components of moment tensors as a sensitive indicator of stress regime in the focal zone.

Presenting Author: Vaclav Vavrycuk

Authors