New Velocity Structure of the NovyKostel Earthquake-Swarm Region, West Bohemia, Determined by the Isometric Inversion
Abstrakt
The intraplate West Bohemia region is characterized by relatively frequent earthquake swarms mostly concentrated in the vicinity of the NovyKostel village. Records of selected microearthquakes of the 2008 seismic swarm from a small circular area around NovyKostel with a radius of 11 km are processed with focus on availability of precise onset readings from all the selected events at all the selected stations.
These data were used in the joint hypocenter-velocity inversion, employing the so-called isometric method which is especially efficient for this task. The results are two new one-dimensional velocity models, the constrained (smoother) and the unconstrained one, of the upper crust down to the depth of 11 km, and absolute locations of the selected earthquakes in the new constrained model.
The foci delineate two segments of a steeply dipping fault zone. Time residuals at each of the stations display a remarkable azimuthal dependence, for both P and S waves, caused by anisotropy.
No systematic dependence on epicentral distance was found even for stations outside the target area, which means that the new 1D models seem to be reasonable approximations of the real structure in the whole, much larger, West Bohemia region. No time dependence indicating possible changes of vP, vS and their ratio during the swarm was observed.
A set of P-and S-wave station corrections that should be used to correct onset times in order to reach precise absolute locations was derived as an integral part of the new models.