Abstract
We have estimated the stress tensor at two earthquake swarm areas in Japan, Nikko-Ashio, Tochigi Prefecture and off Ito, Izu Peninsula, by applying a technique that inverts P-wave polarity data for a large number of events under the following assumptions: 1) Stress field in a small region is uniform, 2) slips occur along pre-existing weak planes randomly distributed in the region, and 3) slip direction is parallel to the direction of the maximum shear stress. We inverted data collected by the 1993 Joint Seismic Observation in the Nikko-Ashio area. The estimated maximum principal stress in this area is nearly horizontal in the NW-SE direction and the minimum principal stress is sub-vertical. Moreover, the state of stress just beneath Nikko-Shirane Volcano is different from that in the surrounding areas. The orientation of the minimum principal stress is sub-horizontal and perpendicular to the strike direction of opening cracks found at the top of Mt. Nikko-Shirane, an active quaternary volcano. This local change is probably related to recent magmatic activity. We also analyzed data from the 1989 Teishi Knoll eruption area. The estimated direction of the minimum principal stress is parallel to that of the maximum extension of the crustal deformation measured during the submarine eruption at Teishi Knoll. Spatial variations in the state of stress off Ito were found to be small.
