We propose an inversion scheme to retrieve characteristics of seismic point sources, which in contrast to common practice, takes into account anisotropy. If anisotropy is neglected durig inversion, the moment tensors retrieved from seismic waves due to sources situated in anisotropic media may be biased. Instead of the moment tensor, the geometry of the source is retrieved directly in our inversion; if necessary, the moment tensor can be then determined from the source geometry. The source geometry is defined by the orientation of the slip vector and the fault normal as well as the strength of the event given by the size of the slip and the area of the fault. This approach allows direct interpretation of the source geometry in terms of shear and tensile faulting. It also makes possible to identify volumetric source changes that occur during rupturing. We apply the described algorithm to one event of the year 2000 earthquake swarm episode in West Bohemia (Central Europe). For inversion we use information of the direct P waves. The structure is approximated by three different models determined from travel-time observations. The models are inhomogeneous isotropic, inhomogeneous anisotropic, and homogeneous anisotropic. For these models we obtain seismic moments MT = 3.2 - 3.8 * 1014 Nm and leftlateral near-vertical oblique normal faulting on a N-S trending rupture surface. The rupture surface is consistent with fault-plane solutions of earlier studies and with the spatial distribution of other events during this swarm. The event seems to be accompanied by a small amount of crack opening. The amount of crack opening is slightly reduced when the inhomogeneous anisotropic model is assumed, but it persists. This result seems to indicate a role of fluids on the triggering of this event.
Seismic source, tensile earthquake, anisotropy, West Bohemia.
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