RESEARCH LETTER Dynamic Wedge Failure and Along‐Arc Variations 10.1029/2019GL083148 of Tsunamigenesis in the Japan Trench Margin Key Points: Shuo Ma1 and Shiying Nie1 • Inelastic wedge deformation can induce large seafloor uplift 1Department of Geological Sciences, San Diego State University, San Diego, CA, USA efficiently landward from trench and diminish near‐trench slip on the fault • Along‐arc variation of sediment Abstract Elastic dislocation models require large near‐trench slip to explain large tsunamigenesis, thickness in the Japan Trench which is probably best exemplified in the 2011 M9 Tohoku earthquake. However, it is puzzling that the margin can lead to important largest Tohoku tsunami heights occurred about 100 km north of the largest slip zone, where bathymetric along‐arc variations of tsunamigenesis surveys indicate no large slip at the trench or submarine landslides. Here we show that coseismic • The large tsunami heights along the yielding of plentiful sediments in the northern Japan Trench margin can induce large inelastic uplift Sanriku coast in the 2011 Tohoku landward from the trench and diminish slip near the trench. The scarcity of sediments in the south leads earthquake can be due to inelastic wedge deformation to nearly elastic response with large slip at the trench and mostly horizontal seafloor displacement. Thus, the variations of sediments along the Japan Trench and sediment yielding can explain the puzzling Supporting Information: variations of tsunamigenesis and near‐trench slip in this earthquake. Inelastic wedge deformation can • Supporting Information S1 be an important mechanism of tsunamigenesis in accretionary and other sediment‐filled plate margins. • Movie S1 Plain Language Summary The general consensus about the devastating 2011 Tohoku tsunami is that it was due to large slip at the trench. This can be somewhat misleading because the largest tsunami Correspondence to: heights occurred about 100 km north of where the largest trench slip occurred. Using elastic dislocation S. Ma, [email protected] theory, inversions of tsunami data require large trench slip ~30 m in the north (north of 39°N). However, the bathymetry data before and after the earthquake suggest no large trench slip or submarine landslides in the north. Here we present a mechanism that involves the yielding of sediments in the overriding wedge. Citation: Ma, S., & Nie, S. (2019). Dynamic wedge Our inelastic wedge deformation model can explain the puzzling observations of large tsunami heights failure and along‐arc variations of along the Sanriku coast in the north with no or small trench slip and small tsunami in the south with tsunamigenesis in the Japan Trench large trench slip. The amount of sediments in a subduction margin can be very important in evaluating margin. Geophysical Research Letters, 46. https://doi.org/10.1029/ tsunami hazard. 2019GL083148 Received 3 APR 2019 Accepted 25 JUL 2019 1. Introduction Accepted article online 30 JUL 2019 One puzzle about the tsunami generated by the 2011 M9.0 Tohoku earthquake is that the largest tsunami heights occurred in the Sanriku coast, which is about 100 km north of where the largest slip occurred at the trench (Mori et al., 2011). The tsunami runup along the rugged Sanriku coast, up to nearly 40 m, is about 2–3 times larger than the south, causing immense devastation. Most slip models inverted from seismic and geodetic data resolve slip 50 m or larger at the trench updip from the hypocenter (Lay, 2018; Sun et al., 2017, and references therein), supported by bathymetry data before and after the earthquake (Fujiwara et al., 2011). These slip models, based on seismic and/or geodetic data alone, however, resolve little slip north of 39°N (Lay, 2018), which cannot explain the large tsunami heights along the Sanriku coast, even when high‐resolution near‐shore bathymetry and topography are used in the runup calculations (e.g., Tappin et al., 2014; Yamazaki et al., 2018). In order to explain the large tsunami heights in the north, near‐trench slip as large as ~30 m is required north of 39°N from inversions of tsunami data (e.g., Satake et al., 2013; Yamazaki et al., 2018). However, two pro- files of bathymetry data before and after the earthquake north of 39°N indicate no large slip or submarine landslides at the trench (Fujiwara et al., 2017). These findings conflict with the existing tsunami‐based inverse slip models and also a submarine landslide model (Tappin et al., 2014). Without large trench slip or submarine landslides, what caused the devastating tsunami along the Sanriku coast? Also, what gave rise to the variations of near‐trench slip from south to north? An important characteristic of the Japan Trench margin is that the amount of the sediments increases sys- ©2019. American Geophysical Union. tematically from south to north (see Figure 17 of Tsuru et al., 2002, and supporting information Figure S1). All Rights Reserved. South of 38°N, sediments subduct along the plate interface in thin sedimentary channels. However, MA AND NIE 1 Geophysical Research Letters 10.1029/2019GL083148 sediments north of 38°N increase dramatically to form large sedimentary wedges up to 30–40 km wide and 10–15 km deep in the overriding plate. These large sedimentary wedges directly face the Sanriku coast to the west. Sediments, due to their low shear strength, can fail easily under dynamic stress change during an earthquake. In this work, we will show that it is the inelastic deformation of wedge sediments (wedge fail- ure) that may have caused the devastating tsunami along the Sanriku coast, not large near‐trench slip as required by elastic dislocation models. The tsunami heights along the Sanriku coast for both the 2011 Tohoku earthquake and the 1896 Sanriku tsunami earthquake (Kanamori, 1972) were very similar (e.g., Lay, 2018; Satake et al., 2013) suggesting that both devastating tsunami were likely due to the failure of wedge sediments. In an elastic dislocation model, large slip at the trench on a shallow dipping fault causes mostly horizontal seafloor displacement. When the seafloor is not flat, the horizontal displacement of seafloor slope also leads to certain uplift of ocean, thus contributing to tsunami generation (Tanioka & Satake, 1996). This contribution, however, can be inefficient because seafloor slope landward from the trench is generally small. In the Japan Trench, the seafloor slope landward from the trench is about 6° and 10° along the bathymetry profile(s) in the south (Fujiwara et al., 2011) and north (Fujiwara et al., 2017), respectively. Large earthquake‐generated tsunami generally require more efficient seafloor uplift. Seno (2000) and Tanioka and Seno (2001) first proposed the concept of sediment failure in contributing to seafloor uplift. Ma (2012) and Ma and Hirakawa (2013) modeled inelastic wedge deformation in two‐ dimensional dynamic rupture models. They showed that for a nearly critical wedge (e.g., Dahlen, 1984; Wang & Hu, 2006), coseismic wedge failure is more efficient in generating seafloor uplift than slip on a shal- low dipping fault. With significant inelastic wedge deformation, large uplift can occur landward from the trench with little slip at the trench. Elastic dislocation models with buried slip can also generate peak sea- floor uplift landward from the trench, but the uplift is much smaller than that from inelastic wedge deforma- tion given the same seismic moment. The higher efficiency of generating uplift from inelastic deformation (irrespective of wedge criticality) can be understood by considering frictional sliding on conjugate Coulomb microfractures with steeper dips. Inelastic deformation is also a large energy sink (Ma & Hirakawa, 2013), which can lead to the slow rupture velocity, deficiency of high‐frequency radiation, and low moment‐scaled radiated energy, anomalously observed for shallow subduction zone earthquakes (e.g., Lay et al., 2012; Lay & Bilek, 2007). In the following, we will show that by extending these inelastic wedge deformation models to three dimensions (3‐D), the variation of sediments and hence the variation of mate- rial strength in the Japan Trench margin can explain the puzzling along‐arc variations of tsunamigenesis and near‐trench slip observed in the 2011 Tohoku earthquake. 2. 3‐D Dynamic Rupture Model With Inelastic Wedge Deformation We consider a 10°‐dipping subduction plate interface underlying a homogeneous sedimentary wedge with surface slope 5° and backstop thickness 15 km (Figure 1a). The fault is 57.735 km along dip and 240 km along 3 strike. The material properties in the wedge are ρ = 2,500 kg/m , VP = 3,464 m/s, and VS = 2,000 m/s, where ρ is density, VP is P wave speed, and VS is S wave speed. The material properties outside the wedge are every- ρ ′ 3 ′ ¼ ; = ′ ¼ ; = where = 2,500 kg/m , V P 6 000 m s, and V S 3 464 m s. This simple crustal structure includes a compliant sedimentary wedge (shear modulus is 10 GPa vs. 30 GPa outside the wedge) but ignores other heterogeneities that are important in affecting wave propagation. The velocity heterogeneities may have only second‐order effects on inelastic deformation, which will be incorporated in a future study. A compliant sedimentary wedge can interact constructively with the free surface to significantly enhance the slip at trench and hanging wall ground motion (e.g., Lotto et al., 2017; Ma & Beroza, 2008). Wedge compliance can also lead to slow rupture velocity (Lotto et al., 2017). The fault in our model is confined to the upper 15 km; therefore, this is not a Tohoku earthquake model but a model aiming to capture important rupture characteristics in the upper 10–15 km of the Tohoku earthquake that may be general to many other subduction zone earthquakes (e.g., Lay et al., 2012). Our fault dimensions and geometry are similar to the model of Tanioka and Seno (2001) for the 1896 Sanriku earthquake.