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

[SCG40-07] P- and S-wave velocity imaging from active and passive seismic data for seamount subduction at Hyuga-Nada

*Rie Nakata¹, Kimihiro Mochizuki¹, Norimitsu Nakata^4,5, Masataka Kinoshita¹, Yuri Hashimoto³, Yohei Hamada², Yasuyuki Nakamura², Seiichi Miura², Ryuta Arai² (1.Earthquake Research Institute, University of Tokyo, 2.JAMSTEC, 3.Kochi University, 4.Lawrence Berkeley National Laboratory, 5.MIT)

Keywords:slow earthquakes, seismic imaging, active source, noise

Subducting seamounts are becoming a critically important controlling factor for megathrust fault slip behaviors through modulating structural, rheological and geomechanical properties. Determining these properties on, over and in the area surrounding seamounts provide essential insights into the effects of such a topographic relief. At Hyuga-Nada, the subducting Kyushu-Palau Ridge (KPR), a remnant of the Izu-Bonin Arc, separates the Ryukyu trench and the Nankai Trough. A rich distribution of tremors and very low frequency earthquakes observed around a currently subducting seamount suggests a strong of structural and pressure modulations due to the rough topography. Estimating both P-wave and S-wave velocities provides not only low velocity zones but also anomalous Vp/Vs (i.e., Poisson ratio) zones, constraining lithologies and fluid migrations further than a single parameter inversion. We use a dense Ocean Bottom Seismograph survey performed in 2020 to estimate both P-wave and S-wave velocity structures. The short-period OBSs recorded continuously over a month. Extracting waves excited by air-gun shots allows us to estimate P-wave velocities by traveltime tomography and full waveform inversion. Low P-wave velocity zones are indicated in the vicinity of topographic highs of the subducting seamount. In turn, ambient noise data provide S-wave velocity distribution by using interferometry techniques. We extract multiple modes of Scholte waves propagating between 0.5 to 1.0 km/s and then estimate the velocity model down to 500 m from the seabed. By integrating P- and S-wave velocities from the OBS survey with previously obtained seismic reflection images, we infer fluid properties, such as pore pressure distributions, and their links to the structural features such as faults and structures. We compare the results with the slow-earthquake distribution and discuss how anomalous physical properties may be associated with the generation of slow earthquakes.