[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-09] Postseismic deformation of the 2011 Tohoku earthquake: Insights from GNSS, GNSS-A, and repeating earthquake observations

★Invited Papers

*Fumiaki Tomita¹, Tianhaozhe Sun^2,3, Takeshi Iinuma⁴, Motoyuki Kido¹, Yusaku Ohta⁵, Naoki Uchida^5,6, Kelin Wang^2,3 (1.International Research Institute of Disaster Science, Tohoku University, 2.Pacific Geoscience Centre, Geological Survey of Canada, 3.School of Earth and Ocean Sciences, University of Victoria, 4.Japan Agency for Marine-Earth Science and Technology, 5.Graduate School of Science, Tohoku University, 6.Earthquake Research Institute, University of Tokyo)

Keywords:Postseismic deformation, 2011 Tohoku-oki earthquake, Repeating earthquake, Seafloor geodesy

Postseismic deformation following a giant interplate earthquake is primarily controlled by viscoelastic relaxation (VR), afterslip (AS), and fault locking (FL). A comprehensive understanding of these postseismic behaviors is crucial for discussing coseismic rupture process and underground physical properties (e.g., frictional properties along a plate interface and rheological structures) because postseismsic deformation is driven by coseismic stress perturbations (e.g., Miyazaki et al., 2004, GRL; Wang et al., 2012, Nat.; Tomita et al., 2020, EPS).
For the 2011 Tohoku earthquake, onshore GNSS and offshore GNSS-Acoustic (GNSS-A) observations have revealed the spatiotemporal variation in the postseismic deformation (e.g., Watanabe et al., 2021, EPS; Fujiwara et al., 2022, EPS); however, extensive seafloor crustal deformation along the entire Japan Trench has only been clarified up to 2016 (e.g., Honsho et al., 2019, JGR).
In this study, we employed the latest GNSS-A data collected by Tohoku University, JAMSTEC (e.g., Tomita et al., 2024, AGU), and Japan Coast Guard (e.g., Yokota et al., 2018, Sci. Data), and onshore GNSS data (F5 solutions; Takamatsu et al., 2023, EPS), to elucidate spatiotemporal characteristics of postseismic deformation of this earthquake over 10 years. Additionally, repeating earthquake (RE) data (updated from Uchida and Matsuzawa, 2013, EPSL) over this period were also incorporated to directly constrain the contribution of AS.
The offshore geodetic results show ongoing landward motion since immediately after the massive event in the offshore central region, where large coseismic rupture occurred (mainly, off Miyagi). This suggests that VR and FL are dominant in this region. Meanwhile, they also show trenchward motion in the offshore regions to the north and south (off northern Iwate and Fukushima) near the trench that persisted until 2016, suggesting short-lived shallow ASs. The onshore geodetic result show continuous trenchward motion, especially in the central and northern regions, reflecting VR and long-lived deep AS. The difference in the duration of the shallow and deep ASs inferred from the geodetic data is consistent with the RE activity.
We then compared the observational results with our updated finite-element viscoelastic model that combines VR, AS, and FL (Sun et al., 2024, AGU). The model successfully reproduces the overall patterns of the observed postseismic deformation well. In particular, the decaying AS better explains the temporal variation in the geodetic data. Moreover, the combination of the VR and FL models can fully explain ongoing (i.e., after ~2019) offshore deformation pattern; however, the VR model inherently includes the contribution of deep AS as stress-driven AS. The observed and modelling results indicate high, moderate, and low interplate couplings in the 2011 Tohoku rupture area and the 1968 Tokachi rupture area, and off Fukushima–Ibaraki, respectively. Our findings demonstrating the spatiotemporal variation in the postseismic slip/coupling conditions may indicate the spatial differences in frictional properties on the plate interface and stress transfer following the massive cosesimic rupture.