Tectonic landform surveys in the northwestern area of Sagami Bay.

Abstract

In the past, earthquakes registering a magnitude of about 7 on the Meteorological Agency Scale have occurred repeatedly in the Odawara area. The earthquakes have often caused injury and damage to property. The next earthquake will do likewise. The repeating cycle of earthquakes is considered to be about 70 years and it’s feared that the next earthquake will occur within the next 10. The purpose of this survey and study is to contribute to the earthquake prediction program. The Hydrographic Department of Japan performed three kinds of investigation in the sea area. First, we surveyed the ocean-floor topography using the narrow-multi beam swath survey system for shallow seas (Hydrochart II). Second, we got further detailed topographic data with a side scan sonar. Third, we observed the bottom geology with a ROV (Remotely Operated Vehicle) . Thus we have narrowed the survey area down. The survey areas given priority were the seaward extention of the Kozu-Matsuda Fault Line, and the area near Manazuru Knoll. The Kozu-Matsuda Fault is believed by most seismologists and geologists to be one of the most active and serious faults. Around Manazuru Knoll, it is suspected that there is Nishi-Sagami Dislocation from south to north, which provokes earthquakes registering a magnitude of about 7. After these investigations, the following two results were obtained : 1. The movement of the crust is still active around the seaward extention of the Kozu-Matsuda Fault Line. 2 . The crust around the Manazuru Knoll is bent downwards having a north-south axis. It may be caused by the subduction at the Nishi-Sagami Dislocation.