DYNAMIC RUPTURE PROCESSES OF INLAND EARTHQUAKES IN JAPAN WEAK AND STRONG ASPERITIES

Reconstruction of dynamic rupture process of four large earthquakes in Japan shows two types of high stress drop region on the fault planes, i.e., high stress drop part with low strength excess, and that with high strength excess, where the strength excess is the difference between strength and applied initial stress. Here, we call the first type of the above high stress drop part "weak asperity" or "asperity with low strength" and the second one "strong asperity" or "asperity with high strength". In the present paper, the dynamic rupture processes of four earthquakes in Japan, i.e., the 1969 Gifu earthquake, the 1974 Izu-Hanto-Oki earthquake, the 1980 Izu-Hanto-Toho-Oki earthquake, and the 1984 Western Nagano earthquake, are studied by using the method proposed in Miyatake (1992). The method revealed heterogeneous distributions of the stress drop and the strength excess over the fault planes for these events. In rupture processes of two events occurred in the Izu region (the 1974 Izu-Hanto-Oki earthquake and the 1980 Izu-Hanto-Toho-Oki earthquake), the southeast Japan, weak asperities and a barrier were predominant. The strength excess and the stress drop distributions seem to be complementary to each other. From this point of view, two events in the Izu have a slightly simple feature, although the 1980 Izu-Hanto-Toho, Japan, earthquake seems to have a complex rupture propagation. On the other hand, other two events occurred in the central Japan are more heterogeneous and complex. Both types of asperities and barriers appeared. Some parts of high stress drop regions for these events seem to overlap with the areas of high strength excess, but other parts do not. These features of heterogeneities might be attributed to its tectonics and the heterogeneity of the structure and applied stress.