1:30 PM - 1:45 PM
[S09-3-01] Seismic valve as a driving mechanism of the 2014 aftershock sequences in West Bohemia
The West Bohemia/Vogtland region is specific for its collocation of earthquake swarm activity and degassing of CO2 of mantle origin in a relatively small area. We present an analysis of three mainshock-aftershock sequences that occurred in May-August 2014 and was associated by fast increase of CO2 flow in a mofette, which reached six-fold of the original level during the subsequent 150 days. The following slow decay suggests that the seismic activity could have been driven by the fault-valve mechanism.
Our analysis of the spatiotemporal characteristics and focal mechanisms of the aftershock sequences shows that the three mainshocks occurred with unfavorably oriented mechanism in a step-over region of the fault plane with increased coulomb stress due to previous activity. The ETAS modeling shows that an additional aseismic source with an exponentially decaying strength was needed to trigger a large fraction of the aftershocks. Corresponding pore pressure simulations with an exponentially decreasing flow rate show a good agreement with the observed spatial migration of the aftershocks. We propose a scenario that the mainshock opened fluid pathways into the fault plane participating in the aftershock triggering. The migrating fluid pulse is then supposed to reach the surface and be observed as a long-term increase of CO2 flow rate in the mofette few days later.
We simulated gas flow in a two dimensional model of Earth crust composed of a sealing layer located in the hypocentre depth, which is penetrated with earthquake fault and releases fluid flow from a low-permeable lower crust to vertical high-permeable channel pointing to the surface. We get excellent and robust fit of the simulated flow to the data using a realistic vertical channel diffusivity showing that even this simple model is capable of explaining the observations including the short travel time of the flow pulse from 8 km depth to the surface, the following flow increase and later long-term decrease.
Our analysis of the spatiotemporal characteristics and focal mechanisms of the aftershock sequences shows that the three mainshocks occurred with unfavorably oriented mechanism in a step-over region of the fault plane with increased coulomb stress due to previous activity. The ETAS modeling shows that an additional aseismic source with an exponentially decaying strength was needed to trigger a large fraction of the aftershocks. Corresponding pore pressure simulations with an exponentially decreasing flow rate show a good agreement with the observed spatial migration of the aftershocks. We propose a scenario that the mainshock opened fluid pathways into the fault plane participating in the aftershock triggering. The migrating fluid pulse is then supposed to reach the surface and be observed as a long-term increase of CO2 flow rate in the mofette few days later.
We simulated gas flow in a two dimensional model of Earth crust composed of a sealing layer located in the hypocentre depth, which is penetrated with earthquake fault and releases fluid flow from a low-permeable lower crust to vertical high-permeable channel pointing to the surface. We get excellent and robust fit of the simulated flow to the data using a realistic vertical channel diffusivity showing that even this simple model is capable of explaining the observations including the short travel time of the flow pulse from 8 km depth to the surface, the following flow increase and later long-term decrease.