Stress estimate for the highest mountain system in Japan

The Hida range, the largest mountain system in Japan, is a contemporary uplifting feature (uplifting rate∼1–5 mm/yr). No large earthquakes nor active faults have been located in this mountain range but small earthquakes occur rather frequently, often as swarms, in its axial portion. Their focal mechamisms are found to be of strike slip type with the P axes in the NW‐SE to WNW‐ESE direction. This direction is perpendicular to the trend of the mountain chain and coincides with the orientation of the tectonic compression now causing intraplate earthquakes in most regions of the Japanese islands. It also coincides with the convergence direction of the Pacific plate and the Eurasian plate. The Bouguer anomaly profile across the Hida range suggests an upper crustal root beneath it. A buoyancy load associated with this upper crustal root is calculated from the Bouguer anomaly profile, which is found to be not large enough to sustain the surface topographic load isostatically. If any tectonic forces were removed from the mountain system, mountains would subside through inelastic deformation. A formula is presented to calculate, using the known topographic load and buoyancy load, the stress differences arising at the virtual instant of removal of tectonic forces. The Newtonian fluids of uniform viscosity are assumed. The calculated stress difference is horizontally tensile, about 290 bars, at the axial portion of the mountain range. This horizontal tension must be overcome tectonically to ensure the mountain uplifting and the earthquake occurrence. We suggest that the Hida range is now uplifting by horizontal compression of plate tectonic origin with a compressive stress at least of the order of 300 bars. During uplifting, the mountain system seems to maintain its upper crustal configuration as the one in a fairly close state of minimum stress difference.