Marine seismological observations in the Central Kurile segment before catastrophic earthquakes on November, 2006 (M = 8.3) and January, 2007 (M = 8.1)

Abstract

The results of detailed seismological observations with bottom seismographs in the Central Kurile segment in August-September, 2006 are discussed. The system of six bottom seismographs was placed on the island slope of the Kurile deep-sea trench southeast of Urup Island and southwest of the Bussol Strait. Over 230 earthquakes with MLH = 0.5–5.5 were registered in the area with a radius of 150 km around the center of the observation system at depths up to 300 km during 16 days. Records of 80 earthquakes with hypocenters in the earth crust (h = 0–30 km) beneath the island slope of the Kurile deep-sea trench were first obtained by bottom seismographs. These data are inconsistent with previous concepts of aseismicity of this zone. The discovery of the unique morphological structure of the Benioff zone beneath the central Kurile Arc represents the most important result of detailed seismological observations. The zone consists of an inner seismoactive subzone, which is located beneath the island slope of the arc at depths of 15–210 km, being characterized by an angle of incline of 50° under the latter and crosses the ocean bottom approximately 80 km away from the trench axis, and outer low-activity subzone. The latter is traceable beyond the trench almost parallel to the inner zone beginning from a depth of 50 km below the sea bottom up to a depth of approximately 300 km. Due to the slightly lower incline (∼45°) of the outer subzone, both subzones gradually converge downward. The integral thickness of the Benioff zone varies from 150 km in its upper part to 125 km at depths of 210–260 km. The medium sandwiched between these subzones is practically aseismic. The reality of this defined structure is confirmed by the distribution of aftershocks of the earthquake that occurred on November 15, 2006 (M = 8.3). These seismic events served as foreshocks for the subsequent strong earthquake of January 13, 2007 (M = 8.1) with the hypocenter located beyond the trench under the ocean bottom. Such a structure of this zone within the central Kurile Arc segment is unique, having no analogues either in the flanks of the Kurile-Kamchatka Arc or other arcs. The results of detailed seismological observations obtained two months before the first of the catastrophic Central Kurile earthquakes appeared to be typical for the period of foreshocks (the lower seismic activity of the Simushir block, which hosted the hypocenter of the earthquake that occurred on November 15, 2006, particularly at depths of 0–50 km, the gentler incline of the recurrence plot, and other features).