Mode of Strain Release Associated with Major Earthquakes in Japan

Abstract

It is now widely accepted that most shallow earthquakes are caused by a sudden release of tectonic stress, and therefore strain, in the form of faulting (56, Chap. 14). The elastic rebound theory explains remarkably well the overall nature of major earthquakes. However, it is now evident that the strain release by actual earthquakes takes place in a variety of ways. For example, some earthquakes arc reported to have been preceded by significant premonitory crustal deformations with or without foreshocks. Most major earthquakes are followed by aftershocks and, in some cases, by creep-like postseismic deformations. This variety obviously results from the difference in the nature of the crust and the stress field in the respective epicentral area. Thus, detailed study on the mode of the strain release for individual earthquakes is important for understanding the physics of earthquakes and for predicting the mode of occurrence of future major earthquakes. The latter problem is of course very important for designing a practical method of earthquake prediction. The crustal deformation represents the low frequency spectrum of the strain release, and seismic waves represent the high frequency spectrum. To understand better the mode of the strain release, it is important to study it over a wide frequency range, from geodetic to seismic. From this point of view, this paper discusses the mode of strain release in several major earthquakes in Japan for which both geodetic and seismic data are available, and we will focus our attention on the relation between the geodetic slip and the seismic slip. Figure 1 shows the locations of the earthquakes discussed in this paper. We do not intend to make an exhaustive review of the literature on the subject that can be referred to (19, 21, 36, 39, 50, 63, 71).