We conducted three-dimensional travel time tomography in and around the source region of the 1984 Western Nagano Prefecture Earthquake to investigate the generation process of the mainshock and swarm activity near the source. As many as about 250,000 travel time data of good quality (1 ms error) from a dense network were compiled and we obtained hypocentral distribution and three-dimensional P wave velocity structure with high accuracy and resolution. Most of the estimated hypocenters were aligned in lines or planes, not in the form of masses. We found the hypocenter distribution corresponding to the mainshock fault plane, and it was sandwiched by high-velocity regions on both horizontal sides. A low-velocity region spreading horizontally at the bottom of this hypocenter alignment was also seen. In the northeastern side of the mainshock fault, where there is swarm activity, we detected several other alignments of hypocenters, most of which may be located at the boundaries of geological strata. A low-velocity region was also found at the bottom of some of these alignments. This low-velocity region may be due to fluids from below the seismogenic zone. The generation of both the mainshock and the swarm activity might be related to fluid intrusions from the lower side of the hypocenter alignments.


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