SOURCE CHARACTERISTICS OF SHALLOW EARTHQUAKES IN THE NORTHERN PART OF SANRIKU-OKI REGION, JAPAN

Six shallow earthquakes, which occurred in the Sanriku-Oki region during the period from 1965 to 1970, are studied by using long-period P-and S-waves. Focal mechanism solutions are determined from the S-wave polarization angles. Fault planes, which are distinguished from the auxiliary planes by means of the hypocenter distribution of aftershocks, are slightly dipping toward the island arc. A linear correlation is found between the dip angle of fault plane and the distance of epicenter from the trench axis; the dip angle increases with increasing distance. The motion directions of the foot wall blocks relative to the hanging wall sides are in the range from N58°W to N72°W and are consistent with the expectation from the plate tectonics theory, the motion direction of the Pacific plate relative to the Eurasia plate. The focal depths re-determined by the use of depth phases such as pP and sP reveal that the hypocenters of the six earthquakes lie almost on a plane slightly dipping toward the island arc. The spectral analysis of P-waves is made to elucidate the dynamic source process. To do this, the effects of multiple reflection in the crust near the source region as well as beneath the station are taken into account. A unilateral bidirectional model of faulting appropriate to each event is determined by comparing the trough frequencies in the observed P-wave spectra with those expected theoretically. The fracture is found to have propagated in the motion direction of the foot wall block with a velocity between 2.0 and 3.1km/sec.ArticleJournal of Physics of the Earth. 26(3):275-297 (1978)journal articl