Moment Tensor Inversion for Microearthquakes and Isolated Volcanic Tremors near Nikko-Shirane Volcano, Northeastern Japan

Abstract

The Nikko area, the northwestern part of Tochigi prefecture, is one of the most seismically active regions in the northeastern Japan. In recent years, seismic activity has become higher and isolated volcanic tremors have begun to occur beneath the Nikko-Shirane volcano.
To investigate the cause of these changes, we determined moment tensors of microearthquakes in and around the Nikko area by using broad-band seismic waveforms observed near the hypocenters. In order to stabilize the solutions, we utilized long-period band-passed waveforms to suppress the effect of short wave-length crustal inhomogeneity and to emphasize the near-field term.
The smallest event whose moment tensor could be determined is as small as M_w 1.3. We also tried to estimate the moment tensors of isolated volcanic tremors just beneath Nikko-Shirane volcano. The result from the analysis of the beginning parts of the tremors shows little amount of non-doublecouple components, which implies that these tremors contain ordinary double-couple sources at least in their beginnings. The stress held estimated from the moment tensors is considerably different from the tectonic stress field in this region. The extremely low-velocity high-attenuation body beneath Nikko-Shirane volcano is presumably the cause of the stress disturbance. The pattern of the disturbance indicates that the pressure inside the low-velocity body have a little increased recently.