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

本文收集了日本本州岛中北方北陆和希达地区的P波和S波地震观测资料，以获得精确的震源，特别是震源深度。这些数据是京都大学防灾研究所的微震观测站自1978年至1993年安装遥测系统以来所记录的。在此期间，常规网络定位了25，000多次地震。为了便于分析，收集了所有P波、S波资料以及振幅和总持续时间（F-P），并将其按同一格式排列，再加上临时观测资料，对所有地震进行了重新定位。精确的P波和S波走时或地壳结构是获得准确震源所必需的。因此，也收集和重新分析了由爆炸地震观测得出的走时或地壳结构。在表层P波速度小于6 km/s的地区，厚度约1 km，厚度很薄，下伏层厚约13 km，速度为5.8-6.0km/s。利用该构造对所有地震进行了重新定位，并选取了较准确的地震，研究了震源深度与热结构的关系。同时收集了热低和热梯度资料，并与截止深度进行了比较。因此，沿沿着Atotsugawa断层的事件截止深度较深，为12- 15 km，而在希达山脉下的事件截止深度较浅，约为5- 8 km，那里有一些活火山，热流较高。此外，在希达山脉中最活跃的火山--八岳岳火山下，震源深度仅为3公里。因此，北陆和希达地区地震活动的截止深度与热结构密切相关。

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.