Noname manuscript No. (will be inserted by the editor) Fully-coupled simulations of megathrust earthquakes and tsunamis in the Japan Trench, Nankai Trough, and Cascadia Subduction Zone Gabriel C. Lotto · Tamara N. Jeppson · Eric M. Dunham Abstract Subduction zone earthquakes can pro- strate that horizontal seafloor displacement is a duce significant seafloor deformation and devas- major contributor to tsunami generation in all sub- tating tsunamis. Real subduction zones display re- duction zones studied. We document how the non- markable diversity in fault geometry and struc- hydrostatic response of the ocean at short wave- ture, and accordingly exhibit a variety of styles lengths smooths the initial tsunami source relative of earthquake rupture and tsunamigenic behavior. to commonly used approach for setting tsunami We perform fully-coupled earthquake and tsunami initial conditions. Finally, we determine self-consistent simulations for three subduction zones: the Japan tsunami initial conditions by isolating tsunami waves Trench, the Nankai Trough, and the Cascadia Sub- from seismic and acoustic waves at a final sim- duction Zone. We use data from seismic surveys, ulation time and backpropagating them to their drilling expeditions, and laboratory experiments initial state using an adjoint method. We find no to construct detailed 2D models of the subduc- evidence to support claims that horizontal momen- tion zones with realistic geometry, structure, fric- tum transfer from the solid Earth to the ocean is tion, and prestress. Greater prestress and rate-and- important in tsunami generation. state friction parameters that are more velocity- weakening generally lead to enhanced slip, seafloor Keywords tsunami; megathrust earthquake; deformation, and tsunami amplitude. The Japan subduction zone; Japan Trench; Nankai Trough; Trench's small sedimentary prism enhances shal- Cascadia Subduction Zone; tsunami modeling; low slip but has only a small effect on tsunami initial conditions; dynamic rupture height. In Nankai where there is a prominent splay fault, frictional parameters and off-fault material properties both influence the choice of rupture path- 1 Introduction way in complex ways. The splay generates tsunami waves more efficiently than the d´ecollement. Rup- Subduction zones span the surface of the Earth ture in Cascadia is buried beneath the seafloor but and host the world's largest earthquakes and tsunamis. causes a tsunami that is highly complex due to Despite their ubiquity in convergent margins, sub- the rough seafloor bathymetry. Neglecting compli- duction zones are qualitatively and quantitatively ant sediment layers leads to substantially different distinct from one another in several ways that greatly rupture behavior and tsunami height. We demon- influence earthquake rupture and tsunamigenesis. To this point, most models of subduction zone G.C. Lotto · E.M. Dunham earthquakes have been highly idealized, and none Department of Geophysics, Stanford University, Stan- have attempted to couple dynamic rupture to tsunami ford, CA, USA E-mail: [email protected] generation in a realistic setting. In this study, we perform fully-coupled simulations of subduction zone T.N. Jeppson Department of Geology and Geophysics, Texas A and earthquakes and tsunamis in order to gain insight M University, College Station, TX, USA on the following: the influence of geometry, geo- logic structure, friction, and stress on the megath- E.M. Dunham rust rupture process; the role of horizontal and Institute for Computational and Mathematical Engi- vertical seafloor motion in contributing to tsunami neering, Stanford University, Stanford, CA, USA height; and the extent to which the standard tsunami 2 Gabriel C. Lotto et al. modeling procedure makes justifiable assumptions, we observe differences between subduction zones, especially with respect to initial conditions. where fluid pressures in sedimentary prisms are of- We begin by highlighting some of the areas of ten elevated but can fall anywhere in the range greatest difference between subduction zones, mo- between hydrostatic and lithostatic [59]. Fluid ex- tivating our interest in modeling specific detail in pulsion rate can vary depending on whether a mar- addition to general subduction features. Decades gin is accretionary or nonaccretionary, and on the of seismic imaging has revealed major differences overall permeability of a fault zone [74]. in geometry from one subduction zone to the next Given such a diversity in qualitative and quan- (e.g., [13, 42, 57, 61, 62]). Geometrical differences titative subduction zone characteristics, it should include the presence or absence of splay faults, not surprise us to find substantial differences in which have been observed since the 1970s, most no- rupture style and tsunamigenic efficiency. Tsunamis tably in Alaska [69], Costa Rica [87], and Nankai from great megathrust events like the Mw 9.1-9.3 [66]. Virtually all subduction zones feature com- 2004 Sumatra earthquake [91] result in significant pliant prisms of weakly consolidated sediments, loss of life and cause massive damage to property though these vary widely in their landward ex- and infrastructure, but so do those from more- tent between different subduction zones [102] and efficient tsunami earthquakes that release hundreds along-strike within the same convergent margin of times less energy, such as the Mw 7.7 2010 Mentawai (e.g., [62]). Various ocean drilling projects have event [46]. Meanwhile, other major significant sub- demonstrated the extreme elastic compliance of duction zone earthquakes like the Mw 8.6 2005 sedimentary prisms in Cascadia [97], Nankai [71], Nias-Simeulue event [8] generate only small tsunamis Costa Rica [19], Barbados [96], and Tohoku [32, that do limited damage. The non-monotonicity of 61], and while exact values of elastic moduli of sed- the relationship between earthquake magnitudes iments vary from margin to margin, they are typi- and tsunami heights should cause us to ask which cally one or more orders of magnitude smaller than specific properties of subduction zones influence elastic moduli from deeper parts of the subduction tsunami generation, and to what extent they do zone. Lab and drilling results are consistent with so. estimates of rigidity from subduction zone earth- The first major focus of this study is how ge- quake data, which pin on-fault shear moduli be- ometry, friction, stress, and material structure in- tween 1 and 10 GPa at shallow depths [6]. fluence rupture and the ensuing tsunami. We can In addition to prism size and material com- link several subduction zone characteristics to dif- pliance, the thickness of sediments on the incom- ferences in earthquake rupture and tsunamigenic ing crust varies between subduction zones. Sedi- behavior. For instance, fault and seafloor geome- ment thickness, along with crustal age and con- try is a prominent factor in determining tsunami vergence rate, helps to control the temperature on amplitude. A steeply-dipping d´ecollement or splay the fault and thus the updip and downdip limits fault will produce a larger portion of vertical up- of velocity-weakening friction and possibly seismic- lift, which directly corresponds to tsunami height ity [64]. The updip limit of seismicity has generally (e.g., [12, 34]). The angle of a sloping seafloor, in- been associated with the dehydration of stable- cluding the presence of seamounts or any rough sliding smectite clays to velocity-weakening illite bathymetry, also directly contributes to tsunami and chlorite as temperature increases with depth height even if seafloor motion is entirely horizon- [27, 28, 103]. Field and laboratory data indicate tal [48, 93]. Several studies [12, 34, 111] have used that this transformation occurs at temperatures dynamic rupture simulations of branching faults to between 100◦C and 150◦C (e.g., [25,31,59]). These demonstrate that prestress, frictional properties, transitional temperatures occur at depths between and rupture velocity are important in determining 2 and 10 km, depending on the subduction zone rupture pathway, e.g. whether an earthquake rup- [64]. The downdip limit of seismicity may also be tures along a steep splay, a flatter d´ecollement, or determined in part by a velocity-weakening to velocity-both. The results of our Nankai simulations, pre- strengthening transition, relating to increasing tem- sented below, bear out the importance of prestress peratures or mineral transformations [7, 56, 99]. and friction on the choice of rupture pathway. Plate geometry and tectonic forces, which de- In addition to determining rupture pathway, termine the absolute state of stress in a subduction fault friction and prestress (particularly the con- zone, vary around the world. Excess pore pressure tributions of excess pore pressure) help to con- is another major determinant of effective stress trol both rupture speeds and long-term deforma- [74]. As initially saturated seafloor sediments subduct tion [51,74,104]. Our previous work shows that the and pressurize, they expel water through poros- choice of rate-and-state frictional parameter b − a ity reduction and mineral dehydration. Here, too, has a major influence on rupture velocity, total Fully-coupled simulations of megathrust earthquakes and tsunamis 3 slip, and tsunami height for a simple subduction- In addition to exploring the effects of using re- zone-like geometry [50]. That same work demon- alistic frictional and structural parameters, we are strates the effect of compliant prism materials on also motivated