Comparison of absolute and relative moment tensor solutions for the January 1997 West Bohemia earthquake swarm

Moment tensor solutions for 70 clustered events of the 1997 West Bohemia mi croearthquake swarm, as calculated by two different methods, are given. The first method is a single-event, absolute moment tensor inversion which inv erts body-wave peak amplitudes using synthetic Green functions. The second method is a multiple-event, relative method for which Green functions are " reduced" to 2 geometrical angles of rays at the sources. Both methods yield similar moment tensors, which can be divided into at least two or three di fferent classes of focal solutions, indicating that, during the swarm activ ity, different planes of weakness were active. The major source component o f most events is a double couple. However, the deviations from the double-c ouple mechanisms seem to be systematic for some classes of solutions. Error analysis was based on transforming the estimate of the standard deviation of amplitudes extracting from the seismograms into confidence regions of th e absolute moment tensor. They show that the non-DC components are signific ant at a fairly high confidence level.