[Abstract]

Hakone volcano is situated at the northern tip of the Izu-Mariana volcanic arc in eastern Japan, and area that is both tectonically and volcanically active. Fumarolic activity is observed at post-caldera cone volcanoes within the caldera, and the northern extension of the Kita-Izu fault, the source of a M7.3 earthquake in 1930 (Kita-Izu earthquake), traverses the southern part of the caldera. Although there is no historical record of eruptive activity, many intense earthquake swarms have been reported since 1786 within the caldera. In this study, literature on earthquake swarms in 1917, 1920, 1933-35, 1943, 1944, 1953, 1959-60 are re-examined to reveal detailed development of the activity, seismic intensity and the epicentral region of these events. Two epicentral regions are recognized; the central cones region (1), and the southern part of the caldera (2). Earthquake swarms in (1) are often accompanied by rumblings and the main shock is not distinct; successive earthquakes are felt almost continuously during the peak of activity. On the other hand, earthquake swarms in (2) are rarely accompanied by rumblings and have obvious sequence of foreshocks, a mainshock and aftershocks. The largest earthquakes in the swarms in (2) are larger than those in (1). The two epicentral regions are both on the northern extension of the Kita-Izu fault system. Differences in the style of earthquake swarm activity in regions (1) and (2) may be due to differences of the geology and the source depth. Correspondence between fumarolic activity in the solfataras at central cone volcanoes and seismic activity was not observed except for the 1933-35 swarm. Most of the earthquake swarms at Hakone volcano are therefore probably tectonic earthquakes on the Kita-Izu fault system rather than being related to hydrothermal or magmatic activity within the caldera. Earthquake swarms at Hakone appear to have been rare before 1917, and except for 1786, no historical records exist even though one of the most important highways in Japanese history passed across the volcano. An interpretation that attributes the earthquake swarms since 1917 to foreshocks and aftershocks of the 1930 Kita-Izu earthquake would broadly explain the frequency of earthquake swarms at Hakone volcano since the early twentieth century.

Key words:

Hakone Volcano, earthquake swarm, historical record, Kita-Izu fault systen, caldera unrest

e-mail:

mannen@onken.odawara.kanagawa.j