Development of identifying unclear active faults and quantitative evaluation method for fault activity around Quaternary volcanoes

2022 Fiscal Year Final Research Report

• Project/Area Number: 20K04082
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17030:Human geosciences-related
• Research Institution: Shinshu University
• Principal Investigator: Takeshita Yoshihiro 信州大学, 学術研究院教育学系, 准教授 (00578271)
• Co-Investigator(Kenkyū-buntansha): 廣内 大助 信州大学, 学術研究院教育学系, 教授 (50424916)
• Project Period (FY): 2020-04-01 – 2023-03-31
• Keywords: 活断層 / 活動履歴 / 変位量 / 第四紀火山 / 活火山 / 変動地形 / 先行谷 / テフラ分析

Outline of Final Research Achievements

We conducted a topographic and geological survey around a fault outcrop discovered in Shinano-machi town, northern Nagano Prefecture, Japan. Detailed investigation of the fault outcrop indicates that the fault has been active at least three times in the last 30,000 years. Furthermore, the total displacement of three faulting events was approximately 5 m, and the displacement per event was found to be 1.5 to 2.0 m. In addition, continuous tectonic geomorphology was observed around the fault outcrop, and antecedent valley was found to be useful in the search for unknown active faults. Based on these tectonic geomorphologies, the fault can be traced in a northeast-southwest direction for at least 6 km. Trench and drilling survey were conducted on the hanging and footwaall sides of the fault. As a result, the vertical displacement rate of this fault was found to be about 0.25 mm/yr.

Free Research Field

第四紀地質学

Academic Significance and Societal Importance of the Research Achievements

2008年の岩手・宮城内陸地震は，活火山である栗駒山周辺の活断層が認識されていなかった地域で発生した．このことは，若い第四紀火山周辺では大規模地震が繰り返し発生しても明瞭な変動地形が残されない可能性を示しており，活断層の認定に関して問題を提起した。
本研究では若い第四紀火山である黒姫山の東側において、断層露頭および先行谷などの変動地形に基づき新たな活断層を検出し，活動履歴や変動量を明らかにすることができた。以上の成果はこれまで活断層が認定できていなかった地域、特に若い第四紀火山周辺において、活断層の認定やその活動性に関する研究に貢献するだけでなく地震防災に関する基礎資料としても重要である。