Studies on the Bouguer anomaly of active faults along the western margin of the Kanto plain

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Gravity observations were made at about 1,500 stations with a LaCoste and Romberg gravimeter over an area of about 1,700 km_2 in the western part of the Tokyo metropolisphere for the purpose of clarifying fault structure there. Our conclusion is as follows:(1) The Kodama fault (newly discovered by the present survey), the Kushibiki fault and the Kasukawa fault stand continuously in a single fault-line. The 1931 Nishi-Saitama earthquake may have been caused by a joint motion of these faults.(2) From the topographical point of view the Tachikawa fault is considered to be a vertical fault subsiding southwestward the present gravity survey concludes, however, that this fault may possibly be a left-lateral strike-slip fault including vertical displacements.