JMA reported that seismicity increased at Hachijo-jima beginning on 13 August around 1600. By 1700 that day 16 earthquakes were felt, with a maximum magnitude of 3. The hypocenters were located ~2 km off of the island's W shore at depths of 5-15 km, depending on assumed seismic-wave velocity models. During the afternoon of the 15th, the earthquakes migrated to the N, without a clear change in depth. By the 16th, volcanic tremor-like waveforms were also recorded. By the evening of the 16th seismicity began to decrease. JMA stated that the overall increase in seismicity might not lead directly to an eruption, though seismicity may continue at a similar level. According to the Geographical Survey Institute, GPS measurements revealed in the center of the island there was 5-6 cm of movement to the E and 10 cm of uplift until about 1500.

Geological Summary. Hachijojima, in the central Izu Islands about 300 km S of Tokyo, consists of two small Quaternary dominantly basaltic stratovolcanoes forming an elongated NW-SE-trending island. The eroded Pleistocene-to-Holocene Higashiyama volcano occupies the SE end of the 14-km-long island, and the symmetrical Holocene Nishiyama volcano the NW end. Parasitic cones occur on the SE flank of Nishiyama. The small volcanic island of Kojima lies several kilometers W of Hachijojima. Growth of Higashiyama began several tens of thousands of years ago, and included the formation of two small calderas. The initial submarine and early subaerial eruptions of Nishiyama took place from 10,000 to 8000 years before present (BP). Its latest major activity, from the early Holocene until about 4,000 BP, was restricted to flank eruptions. Eruptions recorded since the 15th century have been restricted to the summit of Nishiyama and a submarine vent of unknown location.

Source: Japan Meteorological Agency (JMA) via the Volcano Research Center