Seismicity and a Focal Mechanism of Low-frequency Earthquakes Occurring in the Western Part of Tochigi Prefecture, Japan

Abstract

Low-frequency earthquakes lying near the Moho discontinuity have been, recently, observed just beneath the volcanic front. In this paper, we analyzed seismograms recorded in the western part of Tochigi prefecture from September 1992 to January 1993 and from July 1994 to January 1995, in order to reveal the characteristic and seismicity of low-frequency earthquakes in this region. The seismograms of the low-frequency earthquake on November 17, 1992, are also analyzed by using a waveform inversion method to get a focal mechanism of low-frequency earthquake. Major results are as follows. 1) 269 low-frequency earthquakes were recorded for 15 months in the western part of Tochigi prefecture, and their activity is similar to an earthquake swarm. 2) The depths of low-frequency earthquakes during September, 1992-January, 1993 range from 39 km to 30 km, but most of the events occurred about 30 km deep. On the other hand, all low-frequency earthquakes which occurred during July, 1994-January, 1995 lay at 27 km deep. 3) There are two groups of low-frequency earthquake family composed by earthquakes having waveforms similar to each other. The hypocenters of earthquake family migrated about 1 km from west to east during August, 1994 to January, 1995. 4) The low-frequency earthquake of November 17, 1992 has a focal mechanism of strike-slip type with a maximum compression axis directed west-northwest to east-southeast which is consistent with a regional tectonic stress. The source models composed by a single force, an open crack, or CLDV can not explain the observed seismic waveforms of this low-frequency earthquake.