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The Focal Distribution of Earthquakes in Shikoku and Its Surrounding Regions

1985; Seismological Society of Japan; Volume: 38; Issue: 1 Linguagem: Inglês

10.4294/zisin1948.38.1_93

2186-599X

Kennosuke Okano, Shozo Kimura, Tadashi KONOMI, Masao Nakamura,

Earthquake and Disaster Impact Studies

From examining the spatial and depth distributions of earthquakes in Shikoku and its surrounding regions, some interesting inferences are drawn, as described in the following. The extremely low activity of crustal earthquakes shallower than about 10km depth in the northern Kii channel between Shikoku and the Kii peninsula is supposed to be attributed to hydropressure in the uppermost part of the crust, resulting from permeation into rocks with water. This feature is observed from place to place in Japan. The Median Tectonic Line is a boundary between the two regions of different distributions of crustal earthquakes.Intermediate-depth earthquakes beneath Kyushu and mantle earthquakes beneath Shikoku are generally considered to be intraplate earthquakes in the Philippine Sea Plate generated by its subduction under the Eurasian Plate. The subducting direction of the plate is nearly E-W for Kyushu and NW-SE for Shikoku. It is natural to expect, therefore, that fissures produce between the two plates and thus earthquakes scarcely occur in the fissure region. However, such phenomena are not noticed from the distribution of microearthquakes. Crustal and mantle earthquakes beneath Shikoku occur in zones dipping down and up from north to south, respectively, and then the two seismic zones appear to make contact each other at the south of about 20km from the Muroto promontory.