第5回Slow-to-Fast地震学カフェ（連携セミナー）を開催しました
前日まで開催されていたInternational Joint Workshop on Slow-to-Fast Earthquakes 2024に出席するために来日されたKelin Wang博士に、「The strength of subduction megathrusts and implications to earthquake processes」という題目で、特別セミナーを実施していただきました。沈み込み帯における断層強度とすべり挙動に関する最新の研究をご紹介いただきました。これは、測地学、地震学、断層レオロジー、鉱物学など学際的な知識を統合したもので、広範な研究分野から集っている当研究領域の研究者にとって、とても刺激的な内容でした。当日は、当研究領域の内外から、セミナー室と、オンラインそれぞれに20人程度の参加者が集まりました。
| 日時 | 2024年9月20日（金）16:00-17:30 |
| 会場 | 東京大学地震研究所1号館セミナー室 |
| 世話人 | 総括班/A02班（北佐枝子）、B01班（伊東優治） |
| 内容 | Kelin Wang博士（Geological Survey of Canada, University of Victoria）によるセミナー 「The strength of subduction megathrusts and implications to earthquake processes」 |
| 概要 | The strength of subduction megathrusts and how it varies during slip are important to earthquake physics. Actual megathrusts feature multi-scale heterogeneity, but fundamental understanding can be gained by focusing on the big picture and representative end members. This seminar provides a big-picture sketch of what has been learned over the past three decades. An ideal creeping fault has only one strength, but an ideal stick-slip fault has three strengths: the true strength, apparent strength, and dynamic strength. At a regional scale, there are only two useful methods to determine megathrust strength: the study of force balance in which gravity provides a reference and the study of thermal fields in which frictional heat dissipation reflects fault strength. Global applications of these methods yield effective friction coefficients of megathrusts in the range 0.03 – 0.13, much lower than the value of about 0.4 predicted by Byerlee’s law with hydrostatic fluid pressure. Despite this remarkable weakness, stress drop in megathrust earthquakes generally is only 10 – 30% of the absolute fault strength. Seismogenesis depends not on the absolute strength but how the fault weakens or strengthens during slip. Thirty years ago, updip and downdip limits of the megathrust seismogenic zone were proposed on the basis of thermal controls on weakening or strengthening behaviour. Today, new understanding of fault mechanics and fault gouge petrology and rheology requires the development of a new conceptual framework. I will explain why and how the previously proposed limits should be changed to “soft barriers” to seismic slip. |