Relationship between hypocentral distributions and Vp/Vs ratio structures inferred from dense seismic array data: a case study of the 1984 western Nagano Prefecture earthquake, central Japan

Abstract

We conducted a three-dimensional traveltime tomographic reconstruction in and around the source region of the 1984 western Nagano Prefecture earthquake to investigate the generation process for the main shock and associated swarm activity. Up to 220 000 high-resolution traveltime records (2 ms error) were compiled from a dense seismic network. From these records, we performed accurate, high-resolution calculations to estimate hypocentre distributions and three-dimensional velocity structure. Most hypocentres aligned along the same path or within the same plane, rather than in three-dimensional clusters. Hypocentres in the swarm region are located in regions with low Vp/Vs ratios, while few earthquakes occurred in regions with high or normal Vp/Vs ratios. We suggest that differences in the number of small fractures and fluid content between these two regions influenced the seismic activity. Rupture propagation associated with the main shock appears to be confined by relatively higher Vp/Vs surroundings, and a low-velocity region which limits its vertical extent.