[U-13] 2023年2月トルコ・東アナトリア断層帯の地震

[U13-06] The sharp turn: backward rupture branching during the 2023 Mw 7.8 Turkey earthquake

*Xiaotian Ding¹、Shiqing Xu¹、Yuqing Xie²、Martijn van den Ende²、Jan Premus²、Jean-Paul Ampuero² (1.Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen, China.、2.Université Côte d’Azur, IRD, CNRS, Observatoire de la Côte d’Azur, Geoazur, Nice, France)

キーワード：Turkey earthquake, Earthquake triggering, Backward rupture branching, Cascade process

Multiple lines of evidence indicate that the Mw 7.8 Turkey earthquake started on a splay fault, then branched bilaterally onto the nearby East Anatolian Fault (EAF). Unlike the favorite multi-segment rupture scenarios with rupture jumping to the forward direction, this rupture pattern includes one feature deemed implausible: rupture jumping to the SW segment of the EAF through a sharp corner (< 90 degrees between the two faults)—known as backward branching. To understand this backward branching, we perform 2.5D dynamic rupture simulations with the software package SEM2DPACK (Ampuero, 2012; Weng and Ampuero, 2019, 2020). We consider a large set of possible scenarios and obtain the following results. (1) Various rupture modes including subshear and supershear on the splay fault can trigger bilateral ruptures on the EAF, which themselves can be either subshear, supershear, or a mixture of the two. (2) While at earlier times the SW segment of the EAF may be transiently loaded towards failure by the splay fault rupture, later that segment will be under a stress shadow once the splay fault rupture terminates near the junction. (3) In many cases, successful backward branching onto the SW segment of the EAF is aided by the forward branching onto the NE segment, which itself is favorably triggered by the splay fault rupture. These findings suggest that implausible backward branching may be realized by a forward-then-backward cascade process over three fault segments, which has important implications for understanding the mechanisms of multi-segment rupture and the complexity of rupture processes, as well as for a more accurate assessment of earthquake hazards.