Detailed velocity ratio mapping during the aftershock sequence as a tool to monitor the fluid activity within the fault plane
Abstract
The rheological properties of Earth materials are expressed by their seismic velocities and V-P/V-S ratio, which is easily obtained by the Wadati method. Its double-difference version based on cross-correlated waveforms enables focusing on very local structures and allows tracking, monitoring and analysing the fluid activity along faults.
We applied the method to three 2014 mainshock-aftershock sequences in the West Bohemia/Vogtland (Czech Republic) earthquake swarm area and found pronounced V-P/V-S variations in time and space for different clusters of events located on a steeply dipping fault zone at depths ranging from 7 to 11 km. Each cluster reflects the spatial distribution of earthquakes along the fault plane but also the temporal evolution of the activity.
Low values of V-P/V-S ratio down to 1.59 +/- 0.02 were identified in the deeper part of the fault zone whereas higher values up to 1.73 +/- 0.01 were estimated for clusters located on a shallower segment of the fault. Temporally the low V-P/V-S values are associated with the early aftershocks, while the higher V-P/V-S ratios are related only to later aftershocks.
We interpret this behaviour as a result of saturation of the focal zone by compressible fluids: in the beginning the mainshock and early aftershocks driven by over-pressured fluids increased the porosity due to opening the fluid pathways. This process was associated with a decrease of the velocity ratio.
In later stages the pressure and porosity decreased and the velocity ratio recovered to levels of 1.73, typical for a Poissonian medium and Earth's crust.