High-resolution gravity survey of the south extension of the Yanagase fault on the northeastern coast of Lake Biwa

Authors: Masahiko Makino, Yasuaki Murata, Masao Komazawa and Shigeo Okuma

Abstract: High-resolution gravity survey was conducted on the northeastern coast of Lake Biwa to reveal concealed structures of the south extension of the Yanagase fault, an NNW-trending active fault in the Kohoku region (northern coastal area of Lake Biwa). The survey was carried out at 592 stations located in the northeastern coastal area, using LaCoste & Romberg gravimeters (D-205 and G-911) and GPS receivers (Trimble 4000ssi and Magellan Promark). We compiled those data together with 90 existing gravity stations and made a Bouguer gravity anomaly map with Bouguer
density 2.66 g/cm3. We also made a residual gravity anomaly map by removing a regional trend reflecting the structure more than a few kilometers deep in order to detect shallow structures. Several NNW- to NW-trending linear structures shown by a steep gradient of gravity are recognized on the residual gravity anomaly map. The arrangement of these
linear structures suggests that the Yanagase fault diverges southward into several faults concealed beneath the plain. The basement structure around Mt. Toragozeyama in Torahime Town may have a certain relation to the distribution of damage caused by the 1909 Anegawa earthquake (M 6.8).
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