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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北海道南西部クッタラ火山日和山潜在ドームのフィッショントラック年代測定を行った.測定は日和山潜在ドームの山頂爆裂火口から採取した2個のディサイト試料(試料名HY-10,20.5kgおよびHY-11,25.5kg)から分離した1004個および1008個のジルコンについて行い,測定法はジルコンの外部面を用いる外部ディテクター法(ED2)を用いた.その結果,HY-10から15±4ka,HY-11から14±4kaの年代値が得られた.これらの年代値は誤差の範囲内で一致する.日和山潜在ドームの形成年代は,約15000年前であると考えられる.

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