Active faults in the deformation zone off Noto Peninsula, Japan, revealed by high- resolution seismic profiles

Recently, a lot of earthquakes occur in Japan. The deformation zone which many faults and folds have concentrated exists on the Japan Sea side of Japan. The 2007 Noto Hanto Earthquake (MJMA 6.9) and 2007 Chuetsu-oki Earthquake (MJMA 6.8) were caused by activity of parts of faults in this deformation zone. The Noto Hanto Earthquake occurred on 25 March, 2007 under the northwestern coast of Noto Peninsula, Ishikawa Prefecture, Japan. This earthquake is located in Quaternary deformation zone that is continued from northern margin of Noto Peninsula to southeast direction (Okamura, 2007a). National Institute of Advanced Industrial Science and Technology (AIST) carried out high-resolution seismic survey using Boomer and 12 channels short streamer cable in the northern part off Noto Peninsula, in order to clarify distribution and activities of active faults in the deformation zone. A twelve channels short streamer cable with 2.5 meter channel spacing developed by AIST and private corporation is designed to get high resolution seismic profiles in shallow sea area. The multi-channel system is possible to equip on a small fishing boat, because the data acquisition system is based on PC and the length of the cable is short and easy to handle. Moreover, because the channel spacing is short, this cable is very effective for a high- resolution seismic profiling survey in the shallow sea, and seismic data obtained by multi-channel cable can be improved by velocity analysis and CDP stack. In the northern part off Noto Peninsula, seismic profiles depicting geologic structure up to 100 meters deep under sea floor were obtained. The most remarkable reflection surface recognized in the seismic profiles is erosion surface at the Last Glacial Maximum (LGM). In the western part, sediments about 30 meters (40 msec) thick cover the erosional surface that is distributed under the shelf shallower than 100m in depth and the sediments thin toward offshore and east. Flexures like deformation in the sediments continue in the ENE- WSW direction along the faults shown by Okamura (2007b). In addition, other flexures in Holocene that had not been known up to now were found along the coastline of northwestern side of Noto Peninsula in shallower sea area. The deformation has been growing by displacements of an underlying active fault but does not cut the reflections. The vertical offset of the flexure on the LGM erosion surface is larger than those on the reflections in the sediments covering the erosinal surface and the offset decrease upward. The sea bottom topography deformed by tectonic activities is maintained in offshore area. These tectonic activity periods are uncertain because sediments are thin, however, it is presumed that there were several activities in the past. These observations indicate that the fault slipped several times during the last 20,000 years. Okamura, 2007a, AFRC News, No66, pp1-3 (in Japanese) Okamura, 2007b, Geological Map West of Noto Peninsula. Marine Geology Map Series, no. 61, GSJ, AIST.