A DOUBLE-PLANED SEISMIC ZONE IN KAMCHATKA FROM LOCAL AND TELESEISMIC DATA

The fine structure of a double-planed deep seismic zone is studied over a wide area of the Kamchatka peninsula. This prominent feature of deep seismic zone configuration is ascertained through the analysis of microearthquake hypocenters from the local seismic network of the Institute of Volcanology of Kamchatka and 22 focal mechanism solutions from the formal inversion of long-period P and SH waves for events with m(b) greater-than-or-equal-to 5.5. Additionally, 11 focal mechanism solutions estimated from the first motion of P-waves and 12 centroid moment tensor solutions of Harvard University are used. The maximum depth of the double seismic zone is 170-180 km. The two planes of seismicity are separated by 40 km at a depth of 50 km, and by 10-15 km at 180 km depth. The focal mechanism solutions of shallow earthquakes show an abrupt change from the thrust events to down-dip compressional events at approximately 60 km depth at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip tensional axis form the lower plane of the double-planed deep seismic zone. Several earthquakes with down-dip tensional axis are discovered in a narrow area of the upper seismic zone at the depth of about 50 km. The double seismic zone is revealed clearly in the area between approximately 52-degrees-N to approximately 54-degrees-N and probably extends up to approximately 56-degrees-N.