[S-CG40] Science of slow-to-fast earthquakes

[SCG40-09] Structural controls on the spatial extent of tectonic tremors in Kumano-nada

★Invited Papers

*Takeshi Akuhara¹, Kazuya Shiraishi², Takeshi Tsuji³, Yusuke Yamashita⁴, Yoshihiro Ito⁴, Atikul Haque Farazi⁵, Ohyanagi Shukei⁵, Hiroko Sugioka⁶, Eiichiro Araki², Takashi Tonegawa², Ryosuke Azuma⁷, Ryota Hino⁷, Kimihiro Mochizuki¹, Shunsuke Takemura¹, Tomoaki Yamada¹, Masanao Shinohara¹ (1.Earthquake Research Institute, University of Tokyo, 2.JAMSTEC, 3.School of Engineering, The University of Tokyo, 4.Disaster Prevention Research Institute, Kyoto University, 5.Graduate school of science, Kyoto University, 6.Graduate School of Science, Kobe University, 7.Graduate School of Science, Tohoku University)

Keywords:Subduction zone, Tectonic tremor

The generation mechanisms of slow earthquakes have been extensively discussed in association with structural factors, such as high pore fluid pressure zones, subducted sea mounts, smaller-scale topographic irregularities, and material heterogeneities. Many studies have identified such structural factors as being spatially correlated to the first-order feature of slow earthquake distributions, such as centroid. However, what controls the spatial extent of slow earthquakes, that is, what hinders the slip propagation is less clear, primarily due to significant uncertainties in accurately locating these events. This study addresses this gap by comparing precisely located shallow tectonic tremors to structural features identified in bathymetry and seismic reflection profiles. Utilizing a novel Bayesian technique and densely deployed ocean-bottom seismometers, we achieve horizontal location errors of less than 1 km for the majority of events. This level of precision offers unprecedented spatial resolution sufficient for elucidating the spatial extent of slow earthquakes.
Our results reveal a good correlation between the tremor distribution and fold-and-thrust features observed in the bathymetry, with tremors predominantly located beneath the imbricate thrust zone and absent beneath the frontal thrust zone. Seismic reflection profiles elucidate that the upper limit of the tremor distribution aligns with the portion where decollement branches to frontal thrusts, suggesting that the abrupt change in the fault geometry plays a role in impeding slip propagation. Furthermore, in the eastern area outside the tremor zone, slump scars overprint the bathymetrical feature of fold-and-thrust structures, highlighting the dominant tectonic influence from the subducted ridge. In this area, the accretionary prism shows only gentle deformation, suggesting that the stress shadow created by the ridge subduction could inhibit the development of high-pore fluid pressure conditions necessary for slow earthquake generation.
These findings not only shed light on the interplay between subsurface structure and slow earthquakes but also offer novel insights into assessing hazards associated with future megathrust earthquakes. Extending this research framework to other regions could enhance our understanding of megathrust slip behaviors.