Paleoseismological analysis on the basis of precise sea bottom topography and sonic prospecting along the normal fault in the Beppu-Haneyama Fault Zone in Kyushu, Japan

Abstract:

The subaqueous topography of bays or lakes along the large active faults are influenced by displacement on fault and strong motion related sediments such as land slide, turbidite etc. We carried out precise topographic survey using multi-beam sonic survey, and seismic reflection survey to about 40m deep sediments in Beppu Bay, which is a pull apart basin with normal faults related to right lateral movements of Median Tectonic Line in southwest Japan. In west central Kyushu, long active fault zone named as Beppu – Haneyama Fault zone runs with E-W direction normal fault zone. The southwest boundary of Beppu Bay is a part of Beppu-Haneyama Fault zone and normal fault of pull apart basin.

 The multi beam sonic data show the characteristic altitude distribution (topography) of steep inclining slope from shore side to the deepest part with 70m below sea level along the coast, and also submarine slidings occurred at off Beppu and off Oita. Within those areas, several blocks of more than 100m has preserved shape and developed to sliding direction. From the viewpoint of sliding topography, sliding movements are thought sector collapse during short interval, and main cause is thought the movement of directly below active fault and related strong seismic motion.

The sonic prospecting data show several reflection horizons indicating volcanic ashes and sand seams. Around two submarine sliding deposit areas, continuation of clear reflections are sparse influenced by event sedimentation and thick coarse sediments.

88 m sediment cores from 7 sites (core length: 8m to 20m long per site) from deepest part and submarine sliding area in late July this year (2015) will make clear that construction age of these topography and construction mechanism from lithological characteristics, and comparison to historical record including large earthquake occurred in 1596.