Worldwide Distribution of Earthquakes

Abstract

A COMPILATION by Barazangi and Dorman¹ of the worldwide epicentres of earthquakes reported by the US Coast and Geodetic Survey for the years 1961 to 1967 strikingly demonstrates the limited extent of seismically active areas. Almost all earthquakes occur in very narrow bands along oceanic ridges and transform faults, along continental transcurrent faults, or in broader zones below and behind island arcs. The depth distribution of earthquakes is also very limited. Almost all seismic energy is released in the crust². Intermediate and deep focus earthquakes occur almost without exception in narrow planar zones which dip at angles of about 45° below island arcs^2,3. Apart from minor seismicity associated with volcanic activity, seismic activity decreases to a very low level at quite shallow depths in all other regions. In California, for example, no earthquakes have been observed below about 15–20 km. This remarkably non-uniform geographical distribution implies that the occurrence of earthquakes requires very special conditions. It is the purpose of this communication to suggest that a necessary condition is the presence of relatively acidic rock.