Space-time variations in the parameters of split S waves excited by local earthquakes in southern Kamchatka

An analysis of the distribution of split S-wave parameters due to small local earthquakes occurring down to a depth of 200 km beneath the PET IRIS station (in the town of Petropavlovsk-Kamchatskii) for the period 1993–2002 revealed an inhomogeneous distribution of anisotropic properties and time-dependent changes in the state of stress and strain in the sinking slab. Three time intervals have been identified from the behavior of the split-wave parameters: 1993–1995, 1996–1998, and 1999–2002. The most orderly orientation of fast-wave polarization azimuths at all depths and lower stresses in the earth was observed in 1996–1998. Regions of relatively stable and unstable parameter behavior consistent with increased and decreased P and S velocities have been identified. The central block down to a depth of 120 km and the lower part of the Benioff zone are classified as stable regions dominated by E-SE fast-wave azimuths. Unstable behavior of fast-wave azimuths was noticed for depths of 60 to 90 km and around the top of the Benioff zone. The highest values of S-wave delay time and regions where the delay times were migrating mostly occur at contacts between more rigid and weakened blocks.