Earthquakes occurring at depths below about 60 km are called deep earthquakes. They are frequently classified into two groups, intermediate-depth and deep (in the restricted sense) earthquakes, depending on their focal depths. The boundary between the intermediate and deep foci is usually set at 300 km. The deep seismic activity extends to about 700 km in the mantle. The deeper parts of the Earth are apparently completely aseismic.
Wadati in 1928 first showed clear evidence for the existence of deep earthquakes in and near Japan. Subsequently, it became clear that deep earthquakes occurred in several different areas. The worldwide distribution of deep earthquakes is apparent in
Fig. 1. which reveals that the deep seismicity is restricted to island-arcs and island-arc-like structure around the circum-Pacific belt and in the Alpide belt. Since their discovery, deep earthquakes have furnished important information about the interior of the Earth and mantle dynamics.
This is a preview of subscription content, log in to check access.
References
Hasegawa, A., N. Umino, A. Takagi, and Z. Suzuki, 1979, Double-planed deep seismic zone and anomalous structure in the upper mantle beneath northeastern Honshu (Japan), Tectonophysics 57, 1–6.Google Scholar
Isacks, B. L., and P. Molnar, 1971, Distribution of stresses in the descending lithosphere from a global survey of focal-mechanism solutions of mantle earthquakes, Rev. Geophys. Space Phys. 9, 103–174.Google Scholar