There are many cases in which microseismicities are connected with active faults. Nevertheless, investigating the spatial distribution of microearthquakes more precisely, it is occasionally revealed that some microseismicities have no clear connections with any active fault, but have some correlations to geological structure. On the basis of this observational facts, an analysis was made on the correspondence between the hypocentral distribution and the geological structure of the Hokuriku district.
As a whole tendency, microseismic activities are high in Paleozoic area and they are low in Tertialy and Quaternary volcanic area. Locally, it is recognized that Quaternary volcanic rock areas are specially aseismic and microseismic activities are found around those volcanic areas, and that the granitic areas in Paleozoic area are often aseismic. The sharp spatial contrast of microseismic activities which is not connected with any active fault seems to be resulted from the relative differences of the properties of the neighbouring rocks. The geological structure seems to be one of the control factors of microseismicities.