On Deep Structure, Properties of the Upper Mantle, and Volcanism of the Kuril-Kamchatka Island Arc According to Seismic Data

The results of detailed seismic investigations during the period 1961 to 1964 are described. Accurate data of focus location for Kamchatka and the Commander Islands are cited. The majority of earthquakes are located in the Pacific focal zone and the others are found in such remarkable tectonic regions as the east Kamchatka ranges, the continental slope of the Commander Islands, etc. The focal zone seismic activity decreases with increasing depth. The seismic activity at a depth of 250 km is 100 times less than the activity at a depth of 0–20 km. Kamchatka earthquake locations in relation to the Kuril-Kamchatka Island arc and deep water trench are approximately the same as those of the south Kuril Island earthquakes. The Kamchatka active volcano belt coincides with the region of earthquakes having focal depths of 100–200 km, especially between 125 and 175 km. S-wave screening in the magma chambers under the volcanoes is observed. The Avacha volcanic cluster magma chamber at a depth of 20–80 km has the form of a column, 25 km in diameter. P-wave velocity in the upper mantle under the Pacific Ocean and between the Aleutian trench and the Kuril-Kamchatka trench is about 8.2 km/sec, and under Kamchatka 7.7 km/sec. Local velocity decreases to basaltic range (Vp = 72 km/sec) in the upper mantle at a depth near 70 km under the east Kamchatka active volcano belt