Global strain accumulation and release as revealed by great earthquakes

Abstract

The strain-rebound characteristic of the sequence of all great shallow earthquakes of magnitudes 8.0 and greater which have occurred since 1904 exhibits a saw-tooth shape with very nearly linear segments. The serration amplitudes and periods have been decreasing linearly with time since the beginning of the sequence. If a proposed interpretation is correct, the characteristic indicates the following conclusions: (1) Earthquakes in this magnitude range are not independent events, but are related in some form of world-wide stress system. (2) From 1908 (and presumably 1904) to 1950 the rate of total secular strain generation in the crustal layer in which these earthquakes originate was remarkably constant. (3) The strain was released in five active periods of decreasing lengths separated by quiescent intervals of very small or no activity during which crustal strain accumulated at a constant rate. (4) During the active periods the strain release proceeded at approximately twice the rate of secular strain generation. The strain-rebound characteristic of the sequence of all great earthquakes of intermediate depth (h = 70-300 km) exhibits no resemblance to that of the shallow sequence. The strain-rebound characteristic of the sequence of all great deep earthquakes (h ≥ 300 km) can be represented by the equation S = a + b log t indicating that activity at this depth has been falling off continuously since 1904. The different behavior of the three sequences is considered evidence for the existence of three layers in the earth's crust having different relative movements in the tectonic sense. Heretofore evidence for layering in the earth's crust has been derived from studies of the propagation of seismic waves and is thus limited to short-time mechanical properties (less than 5 minutes) only. The world strain-rebound characteristics presented herein provide the first evidence for layering on the basis of secular tectonic mechanical properties. From this point of view, the crust is made up of three distinct layers each with its own proper movement, the shallow 0-70 km, intermediate 70-300 km, and deep 300-680 km approximately.

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