1998 Hida Earthquake Swarm

1998年8月7日飛騨山脈上高地付近において群発地震が発生した。衛星通信システムの導入によって周辺観測網のデータが得られるようになった最初のイベントであり, 8カ月間に10, 000個以上の震源を求めることができた。活動は, 上高地付近の東西に帯状の地域, 穂高岳から槍ヶ岳にかけての南北の帯状の地域及び野口五郎岳付近の南北に帯状の地域に発生し, これらの活動域を移動, 再帰する現象が見られた。決められたH・2以上の地震約300個のメカニズム解は北西-南東方向に主圧力軸をもつ横ずれ型が卓越し, 群発地震の南北, 東西の並びと調和的である。また隣接する焼岳の火山活動との関連が注目されたが, 火山活動を示す現象は観測されなかった。なお, 今回の活動に関するデータを, 地元岐阜県上宝村へ随時提供して災害対策の基礎資料とした。
An earthquake swarm occurred on August 7, 1998, at the Kamikochi in the Hida Mountain range, central Honshu.The activity continued more than 8 months including a moderate earthquake of M5.4. Some temporary stations were set by use of satellite conununication system near the source area, where seismological observation is very difficult because of steep mountains. Together with the data of routinely operated stations, more than 10, 000 events have been located at the Kamitakara observatory, Kyoto University for 8 months. The activity migrated 20km from Kamikochi towards north along the Hida Mountains and returned to the beginning area again at an average speed of about 1-2km/day. The focal depths are shallow as less than 7km, concentrating at about 3-5km deep. More than 300 focal mechanisms were determined from the directions of initial motions for events with magnitude as small as 2.0, on account of the dense observation network. Focal mechanisms of most events are strike-slip type with maximum pressure axes in northwest-southeast direction, which is consistent with the global stress in this region. The damage of the earthquake was not very severe, but real time information on the seismic activity was very useful for the measures by local government offices.