講演情報
[S08-17]動力学モデルを用いた津波地震の断層破壊に与える付加体の影響の検討
*津田 健一1、芝崎 文一郎2、Jean-Pual Ampuero3 (1. 清水建設㈱技術研究所 安全安心技術センター、2. 建築研究所、3. コード・ダジール大)
A “tsunami earthquake” was originally defined as “an earthquake that directly causes a regional and/or tele-seismic tsunami that is greater in amplitude than would be expected from its seismic moment magnitude” (e.g., Polet and Kanamori, 2022). Tsunami earthquakes occur in the shallow part of subduction megathrust interfaces, which is often in contact with the accretionary wedge (Qiu and Barbot, 2022). The low S-wave velocity of accretionary wedge materials is a possible explanation for the slow rupture velocity (Vr) of tsunami earthquakes. In addition, the ratio Er/Mo is known to be smaller for slow ruptures, based on rupture models and past earthquake analyses (e.g., Venkataraman & Kanamori, 2004). Furthermore, at lower Vr, the longer T leads to a lower corner frequency, which decreases the excitation of high-frequency seismic motions. The presence of an accretionary wedge overlying the tsunami earthquake source decreases the rupture velocity, but this is not its only possible effect. The low rigidity of the accretionary wedge can also induce large slip (Sallarès et al., 2021) and high slip velocities. Furthermore, seismic waves may be trapped and amplified within the accretionary wedge, while they may be attenuated outside of the wedge. However, these effects have not yet been investigated quantitatively because of lack of records observed outside the accretionary wedge for the past tsunami earthquakes.
In this study, we develop a 3D dynamic rupture model that includes the accretionary wedge and reproduces the typical features of tsunami earthquake, such as the slow rupture and large shallow slip, of the 2010 Mentawai tsunami earthquake (Mw 7.8). We validate the model by comparing the simulated ground motions with the available observed records and ground motion models. We further found that seismic wave energy was trapped within the accretionary wedge, reducing the energy released outward based on the analysis of empirical ground motion model (Shi and Midorikawa, 1999). This wave trapping effect could be one of the important factors contributing to the weak surface ground motion reported in historical tsunami earthquakes on land, such as the 1896 Meiji-Sanriku Tsunami earthquake. Owing to the general presence of accretionary wedges in the source area of tsunami earthquakes, our findings should be applicable to other past and future tsunami earthquakes.
In this study, we develop a 3D dynamic rupture model that includes the accretionary wedge and reproduces the typical features of tsunami earthquake, such as the slow rupture and large shallow slip, of the 2010 Mentawai tsunami earthquake (Mw 7.8). We validate the model by comparing the simulated ground motions with the available observed records and ground motion models. We further found that seismic wave energy was trapped within the accretionary wedge, reducing the energy released outward based on the analysis of empirical ground motion model (Shi and Midorikawa, 1999). This wave trapping effect could be one of the important factors contributing to the weak surface ground motion reported in historical tsunami earthquakes on land, such as the 1896 Meiji-Sanriku Tsunami earthquake. Owing to the general presence of accretionary wedges in the source area of tsunami earthquakes, our findings should be applicable to other past and future tsunami earthquakes.