Inelastic Wedge Failure and Along-Strike Variations of Tsunami Generation in the Shallow Subduction Zone

Shuo Ma, & Shiying Nie

Published August 14, 2018, SCEC Contribution #8521, 2018 SCEC Annual Meeting Poster #280

One remaining and important puzzle about the 2011 Tohoku tsunami is that the largest tsunami heights were observed along the Sanriku coast (between 39.5 and 40.25°N), ~100 km north of where the largest slip (>50 m) occurred in the trench, with tsunami more than a factor of two larger in the north than the south (Mori et al., 2011). Most seismic and geodetic slip models have no or little slip to the north of 39°N and can not explain the large tsunami heights in Sanriku. Tsunami inversions suggest that near-trench slip needs extend further to the north of 39°N (e.g., Satake et al., 2013; Yamazaki et al., 2017). However, bathymetry surveys to the north of 39°N before and after the 2011 Tohoku earthquake indicate no large slip at the trench (Fujiwara et al., 2017). Here we show that inelastic wedge failure can provide a good explanation to this puzzle. In the Japan subduction zone the amount of sediments in the overriding wedge increases from south to north (Tsuru et al., 2002), which suggests that coseismic wedge failure can occur more easily in the north. We construct a simple 3D shallow subduction earthquake model allowing wedge failure, with a closeness-to-failure (CF) parameter larger in the north motivated by the above observation. Our simulations show that large slip at the trench occurs in the south due to the wedge far from failure (small CF), which causes large horizontal seafloor displacement and small uplift due to shallow fault dip. However, in the north, due to large presence of sediments (large CF) the inelastic deformation in the wedge diminishes the fault slip at the trench, but causes uplift efficiently landward from the trench, similar to our previous 2D results (Ma, 2012; Ma and Hirakawa, 2013). The uplift in the north can be well more than twice larger than in the south. This along-strike seafloor uplift pattern is consistent with the tsunami height observations of Mori et al. (2011) and bathymetry surveys of Fujiwara et al. (2017). The resulting tsunami from large inelastic uplift in the north is expected to be shorter-wavelength and more dispersive than that from the slip on the fault, also consistent with tsunami observations along the Sanriku coast. Thus we propose that the largest tsunami heights during the 2011 Tohoku earthquake are likely due to inelastic wedge failure to the north of 39°N, not large slip at the trench.

Key Words
earthquake rupture dynamics, tsunami, subduction zone

Citation
Ma, S., & Nie, S. (2018, 08). Inelastic Wedge Failure and Along-Strike Variations of Tsunami Generation in the Shallow Subduction Zone. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)