[S-SS08] Crustal Structure

[SSS08-P04] Adjoint tomography of an accretionary wedge and shallow slow-slip regions in the North Island of New Zealand

*Shun Adachi¹, Bryant Chow², Yoshihiro Kaneko¹, Yasunori Sawaki¹, Kimihiro Mochizuki³, Yusuke Yamashita⁴, Tomoaki Yamada⁴, Martha K Savage⁵, Emily Warren-Smith⁶, Katie Jacobs⁶ (1.Graduate School of Science, Kyoto University, 2.University of Alaska Fairbanks, 3.Earthquake Research Institute, University of Tokyo, 4.Disaster Prevention Research Institute, Kyoto University, 5.Victoria University of Wellington, 6.GNS Science)

The Hikurangi subduction zone in the North Island of New Zealand hosts repeating slow-slip events, a thick accretionary wedge, subducted seamounts, and a fully-locked plate interface that is capable of generating megathrust earthquakes. Recently, Chow et al. (2022a,b) undertook the first application of earthquake-based adjoint tomography of the Hikurangi subduction zone and imaged two sections of high-velocity anomaly below the East Coast of the North Island, which have been interpreted as previously unidentified, deeply subducted seamounts. The presence of these seamounts is supported by independent evidence including seafloor bathymetry data and the presence of nearby geophysical anomalies. They are also linked with spatial variations in slip behavior observed along the Hikurangi subduction margin. In this tomographic study, we extend the domain of Chow et al. (2022a,b) 400 km to the northeast to include the 2017-2018 IODP (International Ocean Discovery Program) drill sites (Barnes et al., 2020), the source region of the 2021 M7.3 East Cape earthquake (Okuwaki et al., 2021), offshore seamounts identified by active source seismics (Bell et al., 2010) and a thick low-velocity wedge in the northern margin (Kaneko et al., 2019). Using 60 geographically well-distributed events recorded by 101 permanent or temporary broadband seismic stations, including 9 OBSs (Ocean Bottom Seismographs), we perform iterative model updates using spectral element and adjoint simulations to fit waveforms with periods ranging from 6 - 30 s. We present ongoing efforts towards imaging and understanding the accretionary wedge of the Hikurangi subduction zone and its link to spatial variations in megathrust slip behavior.