Search for the source fault of the Genroku Earthquake by the integrated seismic exploration along the on- and off-shore zone of the southern Boso peninsula, Japan

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The integrated seismic exploration was carried out in 2005 along the on- and off-shore zone of the southern Boso peninsula, Japan, in order to estimate the source fault of the Genroku Earthquake of 1703. 804 receiver points were set up onshore, and 1005 air-gun shots were done offshore. The seismic line reached more than 50 km long from Shirahama to Katsuura. As the line might not run perpendicular to the estimated trend of the upper surface of the Philippine Sea plate (PHS), we developed a new technique for processing instead of the conventional reflection method. The principle of the technique, called OAS (Optimum Azimuth Search) method, is to search the optimum azimuth for stacking by trial and error. By this technique, the prominent image of a big knoll emerges beneath the southern part of the Boso Peninsula. The combination of this result with reprocessed seismic profiles of the previous explorations (SK-78, Daidaitoku-Boso2002, NIED96 etc.) on and off the southern part of the Boso peninsula provides the 3 D image of a knoll with its base of about 30 km in diameter and its height of about 2 km relative to its surroundings. The knoll was probably formed originally as a volcanic sea mount, considering that volcanic sea mounts occur in common on the PHS between the Izu-Mariana trench and the Izu-Bonin arc. It is estimated that the knoll has been playing a role of an asperity on the subducting PHS. This provides a reasonable hypothesis that the northern slope of the knoll corresponds to the source fault of the Genroku Earthquake.