The Hot Springs Research Institute (HSRI) has developed seismic observation network in Hakone volcano since 1968. In the first decade of the observation by the HSRI, hypocenters were determined by using S-P times and the Omori equation and were confined in the Owakudani geo-thermal area. This is remarkable, because earthquakes are known to occur in an extended area in the Hakone caldera recently. In this paper, we try to clarify whether the past hypocenters are real. For the purpose we introduce a fitness value to estimate the most probable location of swarm activity based on the S-P times. First, we calculate synthetic S-P times, from every grid point that covers the Hakone caldera with a spacing of 500m for all observation sites. Then, Root Mean Square (RMS) between observed S-P times for each earthquake in a certain swarm activity and the synthetic S-P times is calculated for all the grid points. The fitness value given to grid points is defined as the summation of the inverse RMS for all earthquakes belonging to the swarm activity concerned. It is considered that the grid points with large fitness values exhibit the most probable area of the swarm activity. From the analysis we found that earthquakes in the period of 1970s occurred in an extended region from Mt. Kintoki to Motohakone, not confined to the Owakudani area. Some of the swarms seem to have occurred near Lake Ashi. In conclusion, we think that hypocentral distribution of swarm earthquakes in the past is not much different from that in recent years and swarm activities in the Hakone caldera have occurred in a similar way at almost the same region since 1970s.

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Zircon fission-track dating was applied to determine the age of the Hiyoriyama Cryptodome in the Noboribetsu Geothermal Field, Kuttara Volcano, southwestern Hokkaido, Japan. We dated two rock samples (HY-10 and HY-11) collected from the wall of an explosion crater at the summit of the cryptodome. A total of 20.5kg of dacite (HY-10) and 25.5kg of dacite (HY-11) was crushed, and 1004 (HY-10) and 1008 (HY-11) zircon grains were used to determine the relatively young fission-track ages. Dating was performed using the external detector method, and the ages were calculated from the densities of spontaneous and induced tracks in the whole zircon grains. The obtained ages are 15±4ka (HY-10) and 14±4ka (HY-11), which are the same within error. The dating results suggest that the cryptodome formed at ca. 15ka.

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