Fine fracture structures in the geothermal region of Hakone volcano, revealed by well-resolved earthquake hypocenters and focal mechanisms

We investigated precise hypocentral distribution and the mechanisms of small earthquakes in the geothermal region of Hakone volcano, central Japan, where swarm activity has been frequently observed. Earthquake swarms were remarkably prevalent in 2001 and 2006, and were accompanied by crustal deformation. We first determined initial hypocenters, using station corrections and one-dimensional velocity structures as estimated by the joint hypocenter determination method. Then, we applied the double-difference method to relocate the initial hypocenters using the differential arrival time obtained by both manual picking and waveform cross-correlation analysis. Subsequently, we determined the focal mechanisms from the absolute P- and SH-wave amplitudes and P-wave polarities. From the relocated hypocenter distribution, we found that most swarm earthquakes are distributed on thin plane-like zones with width/length of 100 m to 1 km. We found that these plane-like hypocentral distributions range from the E–W to N–S strikes and are consistent with one of the nodal planes of the focal mechanism. It is likely that most swarm earthquakes occurred on pre-existing fracture planes, shaping the plane-like hypocenter distributions. Previous studies suggested that a highly permeable fracture system has developed in the fault interaction area, and these highly permeable fractures affect the occurrence of swarm earthquakes and surface geothermal activity. Since Hakone volcano is located in the interaction area of the active Tanna and Hirayama faults, it is suggested that the fracture planes revealed by the relocated hypocenter distribution have been developed by the activity of the two faults. The fracture planes in the caldera might be channels for the hydrothermal water of hot springs in the caldera of Hakone volcano which are rich in sodium chloride and considered to be composed of fluid heated by a deep-seated magma. The strike of the fracture plane on which the swarm earthquakes of the 2001 activity occurred is inconsistent with the orientation of open crack models estimated from geodetic data. It is therefore possible that the hydrothermal fluid aseismically intruded the open cracks during the 2001 activity.