Detailed velocity ratio mapping during the aftershock sequence as a tool to monitor the fluid activity within the fault plane

0464264 - GFÚ 2017 RIV NL eng J - Journal Article
Bachura, M. - Fischer, Tomáš
Detailed velocity ratio mapping during the aftershock sequence as a tool to monitor the fluid activity within the fault plane.
Earth and Planetary Science Letters. Roč. 453, November (2016), s. 215-222. ISSN 0012-821X. E-ISSN 1385-013X
R&D Projects: GA ČR GAP210/12/2336; GA MŠMT LM2010008
Institutional support: RVO:67985530
Keywords : velocity ratio * earthquake swarms * West Bohemia/Vogtland
Subject RIV: DC - Siesmology, Volcanology, Earth Structure
Impact factor: 4.409, year: 2016

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.
Permanent Link: http://hdl.handle.net/11104/0263217