Mechanism of large earthquakes along the eastern margin of the Japan sea

Aftershock distribution of the 1983 Japan Sea earthquake as determined from helicopter-dispatched OBS observation.

“…The mechanism solutions have been obtained for the most recent three of the above-mentioned earthquakes; the 1940 Off-Shakotan earthquake by FUKAO and FURUMOTO (1975), the 1964 Off-Oga earthquake by ICHIKAWA (1971), and the 1964 Niigata earthquake by ABE (1975). Their results show that the mechanisms of these earthquakes are almost pure dip-slip type, with pressure axes directing east-west.…”

Seismological evidence indicating rupture along an eastward dipping fault plane for the 1964 Niigata, Japan earthquake.

“…Plates are identified as; EU, Eurasian; NA, North American; P, Pacific; and Ph, Philippine. Included on this figure are the source mechanisms for the 1964 event (ABE, 1975), the 1983 Akita-Oki event (SHIMAZAKI and MORI, 1983), and the 1971 Sakhalin and1940 Shakotan events (FUKAO andFURUMOTO, 1975).…”

Aftershock distribution of the 1983 Nihonkai-chubu (Japan Sea) earthquake determined from relocated hypocenters.

“…The hypothesis, however, has not yet been fully substantiated by seismological data. Although the focal mechanisms of larger earthquakes in the seismic zone are known to be predominantly the reverse-type (ICHIKAWA, 1971;FUKAO and FURUMOTO, 1975), it has been difficult to determine their fault planes for lack of near-field geodetic data and precise aftershock distributions. Even with the 1964 Niigata earthquake of moderately large size (MJMA 7.5), the dipping direction of the fault plane is still uncertain although much discussion has so far been made HAMADA, 1983;MORI and BOYD, 1985).…”