Characteristics of seismicity in the central Kurile region

The distribution of the numbers of representative earthquakes and the seism ic energy released at depths of 0-100, 101-200, and more than 200 km is ana lyzed throughout the island part of the Kurile-Kamchatka are (phi = 43-51 d egrees N, lambda = 142-160 degrees E) and on its central segment between th e Bussol and Kruzenshtern sounds. We used the M greater than or equal to 6. 1 data of 1900 through 1974 from [New Catalog..., 1977] and the M greater t han or equal to 4.0 data from the yearbooks "Earthquakes in the Soviet Unio n" of 1982 through 1991. The goal of the paper is to verify the hypothesis that the central Kurile region is a seismic gap due to prolonged seismic qu iescence here at depths shallower than 100 km (more than 80 years at the bo undaries of the region and more than 220 years inside of it). The distribut ions of numbers of earthquakes over both observation periods in the depth i ntervals mentioned above are found to be similar within the whole are and i ts central area, whereas the seismic energy released in the central area at depths below 200 km is 1.4 to 2.5 orders of magnitude higher than in the w hole are. This result suggests that the central Kurile area cannot be regar ded as an ordinary seismic gap, because the prolonged seismic quiescence in the upper lithosphere (depths shallower than 100-200 km) is due to the hig her energy level of the seismic process at depths greater than 200 km. A po ssible explanation of this seismicity feature is the lower viscosity of the upper lithosphere at depths to 100 km, which prevents a rise in stresses t o a critical level giving rise to catastrophic earthquakes. Therefore, the scheme of seismic prediction and seismic hazard in the central segment of t he Kurile Are should be revised.