Crust-Mantle Transition and the Distribution of Earthquakes Beneath Klyuchevskoy Volcanic Group, Kamchatka

An extremely vigorous Klyuchevskoy Volcanic Group (KVG) in central Kamchatka is a region of high seismic activity that is broadly distributed throughout the crust, and may even extend into the upper mantle. This seismicity has been monitored by a regional network of varying configuration over the past 50 years, and an extensive catalog has been built up. The catalog contains a certain number of earthquakes with deep (over 30 km) hypocenters located beneath the volcanic front, but clearly separated from the main Benioff-Wadati zone. An outstanding problem in understanding this unusual seismic activity is the complexity of the crustal structure. Two key facts are repeatedly found by a variety of geophysical investigations: that the uppermost mantle is very slow; and that the crust-mantle transition is hard to define, with multiple candidates identified at depths ranging from 30 to 50 km. Data from sites of KVG monitoring network (short period and broadband sensors) have been recorded in digital form since early 2000s. We use these data together with records from a recently operated portable broadband network around the Bezymyanny volcano to investigate vertical distrubution of seismic properties beneath the region, with an emphasis on the depth region of the crust-mantle boundary (20 - 60 km). We apply a multitaper spectral correlation receiver function analysis to both broadband and short-period records of teleseismic earthquakes from sites within and around the KVG. We show that short-period sensors used in the regional seismic network yield results comparable with those from true broadband sensors. We construct directional and epicentral distance gathers of P-S converted waves, and identify phases most likely originating from the crust-mantle transition at each of the analyzed sites. We develop estimates of crustal thickness and use spectral content of the target phases to characterize the crust-mantle boundary beneath KVG. We examine the distribution of earthquakes beneath the KVG with reported hypocentral depths over 20 km, scrutinize the quality of depth determination and consider the possibility of lateral mislocation. Events whose hypocentral depths and locations are deemed reliable are examined in the context of the newly constrained velocity structure of the region.