Anomalous Crustal Deformation in the Northeastern Izu Peninsula and Its Tectonic Significance. Tension Crack Model.

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Abstract
Since the autumn of 1978, anomalous crustal deformation, such as crustal extension and crustal uplift, and earthquake swarm activity have continued in the northeastern Izu Peninsula and its seas off the east coast (northwestern Sagami Bay). Repeated precise geodetic surveys have revealed the horizontal deformation as if the crust of the northwestern Sagami Bay has rifted. The horizontal deformation leads to the conclusions that the NE-SW extensional stress field is predominant and a tension crack (fissure), such as strike of N120°E, length of 15km, and dislocation of 130cm, has formed in the crust of the northwestern Sagami Bay. On July 13, 1989, a submarine eruption occurred at the northwestern extension of the tension crack and a monogenetic volcano, the Teisi Knoll, was born. It is concluded from these events that the present-day stress state in the northwestern Sagami Bay and the northeastern Izu Peninsula is the NE-SW tension and that the monogenetic volcano is born in the NW-SE-trending tension crack (fissure zone) under the tensional stress field. It is supposed that the NE-SW tensional stress is caused by the bending of the Philippine Sea plate due to its subduction.

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