Transition from interplate slip to double seismic zone along the Kuril‐Kamchatka arc

Abstract

During subduction, the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths. Thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth. To characterize this transition, we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the Kuril‐Kamchatka arc. Precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances. At depths of 30–50 km, our results delineate a transitional thrust zone, characterized by earthquakes showing thrust faulting on moderately dipping nodal planes. The northwesterly dipping nodal plane dips approximately 40°, some 10°–15° steeper than the dip of low‐angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km. For earthquakes along the transitional thrust zone, secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds. In the former case, the transitional thrust zone is interpreted as the deep‐seated portion of the plate interface whose dip increases with depth. The alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the Pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km. The inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region. However, a locked interface does not seem to affect the state of strain in the double seismic zone. The upper, compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km. At this depth, the two layers of seismicity in the double seismic zone are separated by as much as 30–40 km, too large a distance to be accommodated by models of seismogenesis invoking either the gabbro‐eclogite transition or dehydration of subducted oceanic crust.

Double seismic zone and dehydration embrittlement of the subducting slab

“…Although double seismic zones have been also observed in south central Alaska [ Ratchkovsky et al , 1997], Kamchatka‐Kuril [ Veith , 1977; Kao and Chen , 1995], New Britain [ McGuire and Wiens , 1995], and New Zealand [ Eberhart‐Phillips and Reyners , 1997], we do not investigate these areas for the following reasons. The geometries of the double seismic zones observed in south central Alaska and Kamchatka‐Kuril and their subduction histories are similar to those in eastern Aleutians and northeast Japan, respectively.…”

A hypothesis for the seismogenesis of a double seismic zone

“…The precise configuration of double seismic zones is one of the most important constraints on the understanding of the seismogenesis of intermediate-depth earthquakes. Figs 2(a) and (b) show two cross-sections (profiles KS and KP) along the central Kuril-Kamchatka arc where the detailed seismogenic structures between 0 and 200 km were delineated from high-resolution teleseismic studies using body waveform inversion (Kao & Chen 1994, 1995. , where a detailed distribution of seismogenic structures was mapped out using microearthquakes recorded by a densely distributed local seismograph network (e.g.…”

“…In general, the most prominent feature of a double seismic zone is the simultaneous presence of two layers of seismicity Kao & Chen (1995). Only focal mechanisms of earthquakes that occurred in the double seismic zone are shown.…”

“…1. Earthquake hypocentres (small symbols) and mechanisms (large symbols) are adapted fromKao & Chen (1995). Only focal mechanisms of earthquakes that occurred in the double seismic zone are shown.…”

Tsunamis along the Okhotsk Sea Coast Due to Great Central Kurile Earthquakes

“…Solid squares show epicenters of the transitional thrust type shown in Fig. 2a in Kao and Chen (1995). Interval of depth contour is 2000 m. …”