The remarkable damage area by the 2000 Tottori Seibu earthquake and the bedrock structure at Yumigahama peninsula, Tottori

摘要

The 2000 Tottori Seibu Earthquake (M 7.3) occurred on October 6, 2000. Big damage appeared in the western area of Tottori Prefecture. Specially, much damage was reported in Yumigahama Peninsula area as the collapse of the wooden house. The area that the seismic intensity was high appeared in the most northern part of the Yumigahama Peninsula. Then, it was ascertained by the questionnaire investigation of the seismic intensity in Sakaiminato City. The microtremor observation and the gravity investigation were carried out in this area for the purpose of making a detailed bedrock structure clear. The distribution of horizontal-to-vertical spectral ratios (H/V) with the observation of a microtremor detected systematic areas. And, as for the distribution of the predominant period, the bedrock structure shows a tendency of becoming thick in the central area of the peninsula. A general tendency of Bouguer gravity anomaly of the density 2.4 g/cm~3 is as following. The high anomaly area out the northern area of the peninsula and the low anomaly area around the Nakaumi of belt-like zone are spread out. The analysis of the bedrock structure was estimated by the method of Komazawa. The density of the upper layer is 1.9 g/cm~3 and the lower layer is 2.4 g/cm~3. It becomes rapidly deep near Sakai-Channel, and the deepest area of the bedrock is about 900 m in the central area of the peninsula. A three-dimensional structure analysis (Komazawa, 1995) was shown the same result. Seismic survey carried out in 2002 is still being analyzed. And, it expects that there are many discoveries with regard to the bedrock structure in the western area of Tottori Prefecture.