Seismotectonics of the Hida region, central Honshu, Japan

Abstract

Seismotectonic features of the northern Hida region, central Honshu, Japan have been investigated in some detail, mainly on the basis of the last 6 years of observations of seismicity and focal mechanism of a large number of earthquakes, with reference to the geological and tectonic setting and evidence of a large earthquake in the past. It was found that high seismicity is concentrated with a remarkably clear lineation, but with a relatively low activity in the central section, along the Atotsugawa fault extending for about 70 km. This is one of major Quaternary faults in this region. The high seismicity with spatially nonuniform distribution may be related to postseismic stress concentration after the 1858 Hida earthquake (M = 7.0), and to heterogeneous fault strength. Seismicity is also high with belt-like extension beneath the Hida mountain range which is the highest mountain system in the Japan Islands. The depth distribution of seismicity is clearly bounded at 15 km below the Atotsugawa fault and 8 km beneath the Hida mountains. The local variations can be interpreted as being due to the difference in the brittle-ductile transition depth which suggests higher temperature beneath the mountains, involving active volcanoes. The focal mechanism solutions indicate that the maximum compressive stress is oriented in an ESE-WNW direction, which is more consistent with the direction of motion of the Pacific plate relative to the Eurasian plate rather than the suggested relative motion between the North America and Eurasian plates. The magnitude of the shear stress working in this region is estimated to be less than 700 bar.