Tectonic implications of subcrustal, normal faulting earthquakes in the western Shikoku region, Japan

Abstract

Tectonic implications of subcrustal, normal faulting earthquakes in the western Shikoku region have been investigated in sorme detail, mainly based on the faulting mechanism of the Bungo channel earthquake of August 6, 1968 (M=6.6, h=45km) and its aftershocks. A synthetic study of the focal mechanism solution, spatial distribution of aftershocks, seismic waves observed in the near and far fields, and vertical tectonic movements, indicates that the main shock was caused by normal faulting with some left-lateral motion of a western block downwards relative to the eastern side, along a steep westerly-dipping fault plane with dimension of 18-20km, which was formed by the rupture initiating at a depth of 45km and spreading upwards and northeastwards. The fault parameters estimated by a comparison between the observed and synthetic seismograms are: seismic moment 2.0-2.2×1026dyne·cm, average fault displacement 0.8-1.0m and stress drop 33-40bars. These parameters explain the measured elevation changes. The focal mechanism of a series of the 1968 earthquakes, a foreshock, main shock and aftershocks, as well as many other earthquakes around the region, is consistently of the normal faulting type, in contrast to low-angled reverse faulting in Hyuganada earthquakes. This result suggests that all must have been generated under the same tensional stresses working horizontally in an E-W direction. Possible causes of the tensional stresses are discussed in relation to the descending Philippine Sea plate. Some tectonic models such as tearing, simple bending of the plate along the hinge line, gravitational drag, and thermal stress due to cooling, are tentatively examined.