Cutoff depth of seismicity and thermal structure in the Hokuriku and Hida regions.

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P and S wave data of seismological observations have been collected to obtain precise hypocenters, especcially focal depths for the earthquakes in the Hokuriku and Hida regions, northern central Honshu, Japan. The data have been recorded by the microearthquake observatories of the Disaster Prevention Research Insatitute, Kyoto University during the period from 1978 to 1993, since the telemetered system was installed at the observatories. Durihg the period more than 25,000 earthquakes were located by the routine networks. All P and S wave data as well as amplitude and total duration times (F-P) were collected and aranged in a same format for convinience of analyzes.In this study, all earthquakes were relocated using the data, adding those obtained by temporary observations. Precise travel times of P and S waves, or crustal structures are necessary to obtain accurate hypocenters. Therefore, travel times or crustral structures derived from explosion seismic observations were also collected and reanalyzed. In the regions at the surface layr with P-wave velocity less than 6 km/s is about 1km and very thin, underlained by 5.8-6.0km/s layr with a thickness of about 13km. By using this structure all earthquakes were relocated and accurate ones were selected to study the relationship between focal depths and thermal structure. Data of heat low and thermal gragient were also collected to compare them with the cutoff depths. In consequence, the cutoff depths of events along the Atotsugawa fault are rather deep as 12-15km, while those under the Hida mountains, whehe some active volcanoes exist and heat flow is high, are shallow as about 5-8km. Besides under the Yake-dake volcano, which is the most active volcano in the Hida mountains, focal depths are as shallow as 3km. Thus, Cutoff depth of seismicity in the Hokuriku and Hida regions are closely related to the thermal structure.