A new depiction of slow earthquake fault zone based on the quantitative evaluation of anomalous structures around tremor sources

2022 Fiscal Year Final Research Report

• Project/Area Number: 19K04010
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: The University of Tokyo
• Principal Investigator: Toh Akiko 東京大学, 大学院理学系研究科(理学部), 特別研究員 (70587344)
• Project Period (FY): 2019-04-01 – 2023-03-31
• Keywords: 南海トラフ / 地震波散乱 / スロー地震 / 震源と構造の相互作用 / 微動 / 超低周波地震 / 海底広帯域地震計 / 低周波微動

Outline of Final Research Achievements

[1]　Locations and focal mechanisms of shallow, very low frequency earthquakes in the Nankai Trough were estimated using near-field and broadband　ocean bottom seismic records. The obtained distribution showed that it is influenced by a subducting-ridge in this region. Because the subducting-ridge appears to press seismically active area landward, tsunamigenic slip may occur closer to the land compared to the regions without the ridge.
[2] We discovered tremors that exhibit short duration signals when recorded close from the source which we referred to as "short-duration tremors". They suggest that tremors' source process is not always long. We performed numerical simulations on elastic wave propagation and demonstrated that the observations can be qualitatively reproduced by assuming a strongly scattering material surrounding the seismic source. Such a structure could be embedded along the slow-earthquake fault zone and play a key role in their source process.

Free Research Field

地震学

Academic Significance and Societal Importance of the Research Achievements

[1]南海トラフ東部の超低周波地震の分布が、沈み込む海嶺の影響を受けていることを明らかにした。この結果は、この領域における津波発生の空間様式を予測するうえで重要である。
[2]普通地震と微動の波形の違いが、震源過程だけでなく震源周辺構造の違いにも起因することを明らかにした。微動震源周辺には、強い地震波散乱構造が在る可能性を示した。この異常構造がスロー地震断層構造そのものである可能性が高く、今後のスロー地震のメカニズム解明の鍵となると考えられる。これまでの地震学であまり着目されてこなかった「地震の震源とその周辺構造の相互作用」という現象の重要性を示唆する結果を得た。