SEISMICITY AND STRUCTURE OF THE KAMCHATKA SUBDUCTION ZONE

The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zon e (KSZ) were studied using the catalog of the Kamchatka regional seism ic network, focal mechanism solutions estimated from P wave first moti ons, the formal inversion of long-period waveforms, and centroid momen t tensor solutions. To the south of similar to 55 degrees N, the slab shows an approximately constant dip angle of similar to 55 degrees. To the north of similar to 55 degrees N, the dip of the slab becomes sha llower reaching similar to 35 degrees. The maximum depth of seismicity , D-m, varies from similar to 50 degrees km depth near 50 degrees N to similar to 300 km depth at similar to 55 degrees N. The volcanic fron t is almost linear along the main part of the KSZ whereas it is sharpl y shifted landward to the north of similar to 55 degrees N, The variat ion of D-m, is apparently consistent with the standard empirical relat ion D-m = f(phi), where phi is the thermal parameter of the subducted slab, To the north of similar to 55 degrees N, the slab is offset towa rd the northwest, and it is sharply deformed in a narrow contorted zon e which is similar to 30 km wide (similar to 56 degrees N, similar to 161 degrees E). To the north of this contortion, D-m, decreases to sim ilar to 100 km. The landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the similar to 90-160 km isodepth range of the subducte d slab. The observed value of D-m, in the northern segment significant ly diverges from the global relation D-m = f(phi). We interpret this a s an effective decrease of the thermal thickness of the subducted lith osphere.