Activity of low frequency (LF) earthquakes in the depth range from 25 to 50 km was investigated in central Japan, by using data obtained by the seismic network of National Research Institute for Earth Science and Disaster Prevention. A total of 15 LF events were found in five regions, southwest Fukushima Pref., north Tochigi Pref., west Tochigi Pref,. central Gunma Pref., and Izu-Oshima, from 1980 to 1992. All of the LF earthquakes are located along the volcanic front in the Kanto district. Predominant frequencies of the LF earthquakes, magnitudes of which range from 1.4 to 2.7, are 1 to 5 Hz, significantly lower than those of typical microearthquakes of the same magnitude range. The seismograms are dominated by clear P and S phases, S amplitudes being larger than P amplitudes at most of the stations. P waves of the LF earthquakes are often contaminated by high frequency waves at close stations. In most cases, more than two earthquakes successively occurred in a short time. Spectra of the LF earthquakes in southwest Fukushima, west Tochigi and Izu-Oshima exhibit a single strong peak at almost same frequency for P and S waves, while those in the other regions show more complicated structure, presumably reflecting complicated spectra at the sources. Although source process of the LF earthquakes are not modeled in this study, large amplitude ratios of P and S phases, which are estimated to be 5.6 in average, suggest a contribution of shear mode rather than tensile mode at the sources.

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Magma plumbing system beneath Hachijo-Higashiyama volcano was discussed based on whole-rock and mineral chemistry and their petrologic characters. The volcanic history of Higashiyama volcano during recent ca. 30ka was divided into four stages ; the Pre-Sueyoshi (ca. 30-22 ka), Sueyoshi (22 ka), Nakanogo (20-10 ka), and Mitsune(10-4ka) stages. The Pre-Sueyoshi and Nakanogo stages are characterized by basaltic pyroclastics and lavas from central and parasitic vents. Considerable amount of dacitic pumice fall and flow were discharged within short period of the Sueyoshi stage. The evacuation of magma resulted in formation of small (1.4 km×0.9 km) caldera. In Mitsune stage, erupted magma was restricted to andesitic to dacitic composition. Another characteristic point of this stage is that the summit crater was completely inactive. All eruptions took place on the flank of Higashiyama. These stages are closely related to both chemical and magma plumbing system evolution. From the viewpoint of whole-rock bulk and isotope chemistry, comagmatic differentiation of Higashiyama volcano was suggested. Magma mixing, however, played a supplemental role on the evolution of magma. The dacitic magma discharged in the Sueyoshi stage was resided in a zoned reservoir of several km³ in volume. This reservoir was independent of both Pre-Sueyoshi and Nakanogo stage magma plumbing system, which was revealed by whole rock and mineral chemistry. Magma plumbing system beneath Higashiyama volcano in recent ca. 30 ka was replaced every few to ten thousand years.

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