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.

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Yufu-Tsurumi volcano group lies at the northeastern part of Kyushu island, Japan. Based on tephrochronological studies, Yufu-Tsurumi volcano group started their activities more than 35,000 years ago. In the latest eruptions of Yufu-dake, pyroclastic flows and ash falls due to collapse of lava domes emplaced, and one of their ash falls, the Yufu-dake 1 Ash (Yf-1), is well-known as one of the tephra late Quaternary around eastern Central Kyushu. Beppu Bay is located about 5 km on the eastern part of Yufu-Tsurumi volcano group. The Site-P core was obtained with a piston core sampler from central part of Beppu Bay. It has two intercalated thin volcanic ash layers at depth of 3-5 and 5-1 horizon. The layer at depth 3-5 horizon is characterized by the existence of greenish brown or brown micro pumice-type volcanic glass shards with high refractive indices of 1.558-1.563. The 5-1 horizon is chacterized by the occurrence of colorless volcanic glass shards with minerals of low refractive indeices of 1.493-1.503. The one of their volcanic ash layers (the 5-1 horizon) in the core was correlated with the Yufu-dake 1 Ash, based on their morphology and refractive index of glass shards. The eruption age of the Yufu-dake 1 Ash was examined by AMS (accelerator mass spectrometry) -¹⁴C dating method using shells on two levels which are intercalated with volcanic ash layers at depth of 5-1 horizon and was estimated to be about 2,500 year B. P. As a result of this study, the avarage frequency of eruption at Yufu-Tsurumi volcano group is estimated as every 1,000-1,500 years intervals.

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