Crustal movements in tectonic areas

Abstract

Horizontal and vertical crustal movements in regions where major earthquakes occur have a cyclic nature which can be correlated with the occurrence of major earthquakes. Recognizable phases in the cycle are: (1) slow continuous movement at a constant rate; (2) more rapid movement commencing from 10–20 years to a few hours or days prior to a major earthquake; (3) sudden movement at the time of the earthquake; and (4) rapid movement following the earthquake for a period of weeks to months. Movement at the time of the earthquake is usually produced by slip on a fault, which can be deduced from dislocation models and field evidence. It is always in the opposite sense to the slow continuous movement, which represents the accumulation of the elastic strain which results in, and is released by, the earthquake. The strain-accumulation field can be modeled as a virtual dislocation opposite to that of the earthquake. Rapid movements following the earthquake usually decelerate rapidly, often logarithmically, after the earthquake, and are often due to aseismic slow slip on the fault. Like aftershocks, these movements are adjustments to the stress state produced by the earthquake. They may be in the same direction as the movements during the earthquake if the earthquake slip was overdamped, or in the reverse sense if it was underdamped. Rapid movements prior to earthquakes are less well studied but may in some cases represent the beginning of fault slip in an aseismic, stable mode. The strain-accumulation field contains all information necessary to determine the maximum size of earthquakes that can be produced at any given time, whereas a sudden increase in rate of movement may give warning of the time of such an earthquake.