The Nature of Deep-Focus Earthquakes

Wadati (1928) first proved convincingly that some earthquakes occur at depths well beneath the Earth's crust. He essentially proposed the nomenclature used today by the International Seismological Centre (ISC), i.e. earthquakes with focal depths exceeding 300 km are deep earthquakes, and those with depths between 70 and 300 km are intermediate earth­ quakes. However, when there is little reason to distinguish between the two groups, both types are called simply deep earthquakes. To avoid confusion, in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes, and those with focal depths between 70 and 300 km as intermediate earthquakes. Deep earthquakes are of interest for at least four reasons. First, they are exceedingly common. Between 1964 and 1986 they constituted no less than 22% of all earthquakes having mb greater than 5 . 0 in the ISC catalog. Second, they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones. One of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups, or Wadati-Benioff zones, apparently delineates the cold down going cores of convection cells in the uppermost mantle. Third, seismologists use body waves of deep earthquakes dispro­ portionately more often than those of shallow earthquakes to inves­ tigate core, mantle, and crustal structure. This is because (a) deep earth­ quakes often possess relatively more impulsive sources, (b) their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station, and (c) surface waves do not contaminate these body phase arrivals on seismograms. Finally, deep earthquakes are