Questions and answers about “seismic gaps”

Abstract

At a time when the field of earthquake prediction is still very much in its infancy, and also a time when recent earthquake disasters have stimulated considerable awareness, the U.S. Geological Survey's Office of Earthquake Studies has reported what it considers significant progress in identifying areas of high seismic potential, i.e., they are deemed likely to produce large earthquakes in the next few decades. Recently the USGS prepared the following questions and answers about these so‐called ‘seismic gaps.’What is a seismic gap? According to the theory of plate tectonics, the earth's outer shell, including the crust and upper mantle, is composed of a number of large semirigid plates that are in constant slow motion. Most of the world's earthquakes occur where the plates slide against each other along vertical ‘transform faults,’ or where one slides over another along a dipping ‘subduction zone.’ The term ‘seismic gap’ refers to regions along these active plate boundaries where (1) large earthquakes have occurred in the past, but not within the past 30 years; and (2) nearby parts of plate boundaries either have experienced strain‐releasing major earthquakes within the past 30 years or have no history of past great earthquakes (suggesting that strain has been released by aseismic creep or by small to moderate earthquakes). A seismic gap might be considered as a quiet zone where strain, that must eventually be released, has been building up in an area that has a history of releasing such strain through large earthquakes.