Tsunami excitation in the outer wedge of global subduction zones
Introduction
The recycling of oceanic plates at subduction zones is a key part of plate tectonics, enabling many geochemical cycles that make our planet inhabitable. The subduction of oceanic material occurs at a shallow angle in the lithosphere and is resisted by frictional forces. The wide area offered by subduction megathrusts releases more than 90% of global seismicity and powers Earth's largest earthquakes and tsunamis (Fig. 1a and Table S1). The Rim of Fire, which extends around the Pacific Rim to the Java-Sumatran arc in Southeast Asia, produced many of the largest earthquakes and tsunamis of the past century, including the 1960 moment magnitude (Mw) 9.5 Valdivia, Chile and the 1964 Mw 9.2 Alaska giant earthquakes. Seismically triggered tsunami waves cause great loss of life and property and have been described as the scourge of the Pacific [Lockridge, 1985].
After several decades of relative quiescence, giant earthquakes reoccurred in the 21st century with the 2004 Mw 9.2 Sumatra-Andaman earthquake on the Sunda megathrust [Chlieh et al., 2007; Ammon et al., 2005; Lay et al., 2005], the 2010 Mw 8.7 Maule earthquake at the Chilean subduction zone [Moreno et al., 2010; Weiss et al., 2019], and the 2011 Mw 9.1 Tohoku-Oki megaquake along the Japan Trench [Lay and Kanamori, 2011; Wei et al., 2012; Simons et al., 2011]. More great and giant earthquakes are expected at many remaining seismic gaps of global subduction zones [McCann et al., 1979; Heaton and Kanamori, 1984; Thatcher, 1989] or at their boundary [Herman and Furlong, 2021] and they will be almost certainly be accompanied by trans-oceanic tsunami waves. Understanding the relationship between seismicity and tsunami-genesis is a major challenge in tectonophysics [Satake and Tanioka, 1999; Polet and Kanamori, 2000; Geersen, 2019]. This is exemplified by the conundrum of tsunami earthquakes that occur close to the trench and generate tsunami waves comparable to that of larger-magnitude earthquakes [Kanamori, 1972; Kanamori and Kikuchi, 1993; Kanamori et al., 2010]. Tsunami earthquakes have occurred in many sections of the Pacific Rim (Fig. 1a and Table S1) and represent a major category of subduction seismicity. Current models of subduction seismicity suggest a depth-dependent rupture behavior controlled by chemical and geothermal gradients and structural boundaries [e.g., Hyndman et al., 1997; Song and Simons, 2003; Lay et al., 2012; Obara and Kato, 2016; Gao and Wang, 2017; Bilek and Lay, 2018; Shi et al., 2020; Barbot, 2020]. The seismogenic zone underlies the inner wedge and the arc or continental crust, bounded by the 100 and 350°C isotherms of unstable friction for quartz-rich gouge [Blanpied et al., 1991, Blanpied et al., 1995; Scholz, 1998]. Long-term and short-term slow-slip events, whether or not associated with low-frequency earthquakes and tremors, occur in stable-weakening regions promoted by regional metamorphism, high temperature, and the presence of fluids [Bürgmann, 2018; Behr and Bürgmann, 2021; Condit et al., 2020; Kirkpatrick et al., 2021]. The brittle-ductile transition occurs below the cold nose, underneath the volcanic arc when it exists [Wada and Wang, 2009; Qiu et al., 2018; Weiss et al., 2019; Barbot, 2020; Luo and Wang, 2021]. The seismogenic and tsunamigenic potentials of the megathrust underneath the shallow accretionary prism is less well understood. Observations of trench-breaking slip during the 2004 Sumatra-Andaman [e.g., Singh et al., 2008; Dean et al., 2010; Hüpers et al., 2017] and the 2011 Tohoku-Oki [Fujiwara et al., 2011; Ito et al., 2011; Kido et al., 2011; Kodaira et al., 2012] earthquakes, of shallow slow-slip events [Wallace et al., 2016; Todd et al., 2018; Saffer and Wallace, 2015; Araki et al., 2017] and low-frequency earthquakes [Ito and Obara, 2006; Obana and Kodaira, 2009; Nakano et al., 2018], and of tsunami earthquakes, provide direct evidence for unstable fault slip near the trench. The relative scarcity of shallow megathrust seismicity may be explained by distributed deformation in the thrust-and-fold belt of the outer wedge [Li et al., 2015; Sathiakumar et al., 2020; Shi et al., 2020] or by the complex evolution of the frictional properties of the overlying sediment, mostly clay [Brantut et al., 2008; Ujiie and Tsutsumi, 2010; Tsutsumi et al., 2011; Ikari and Kopf, 2017; Aretusini et al., 2021]. Several mechanisms have been proposed to explain large shallow slip on the décollement during trench-breaking ruptures, including thermal pressurization [Sibson, 1977; Wibberley and Shimamoto, 2005; Ishikawa et al., 2008; Noda and Lapusta, 2013], hydraulic lubrication [Brodsky and Kanamori, 2001; Di Toro et al., 2006], frictional heating [Hirono et al., 2008; Tanaka et al., 2006], and flash weakening [Beeler et al., 2008; Rice, 2006], but an explanation for the elevated tsunami potential of shallow ruptures is still elusive.
Various source mechanisms can be invoked to explain extreme tsunami wave heights. Efficient seafloor uplift can be explained by inelastic deformation within the outer wedge [Ma and Hirakawa, 2013; Kozdon and Dunham, 2014; Lotto et al., 2017; Ma and Nie, 2019], enhanced deformation due to low rigidity of the frontal wedge [Bilek and Lay, 1999; Polet and Kanamori, 2000; Sallares and Ranero, 2019], submarine slump failure [Kanamori and Kikuchi, 1993; Ward, 2001], hanging wall plasticity [Seno, 2000; Tanioka and Seno, 2001; Hill et al., 2012], structural control from seafloor roughness at sediment-starved trenches [Polet and Kanamori, 2000; Geersen, 2019], or exceptionally large shallow megathrust slip [Satake and Tanioka, 1999; Satake et al., 2017; Lay et al., 2011b]. Despite this progress, a simple explanatory framework for the generation of earthquakes and exceptionally large tsunamis at the shallow portion of subduction zones is still missing.
To shed new light on the problem, we draw insights from a global catalog of run-up of seismically triggered tsunamis comprising a wide range of tectonic settings spanning different plate age, geometry, convergence rates, and sediment fluxes (Fig. 1b). The distribution of maximum and median tsunami run-up as a function of earthquake magnitude can be grouped into three earthquakes categories based solely on the location and extent of the rupture: First, the blind ruptures that do not extend to the trench, e.g., the 2005 Mw 8.6 Nias-Simeulue and the 2014 Mw 8.1 Iquique events, and that generate only moderate or minor tsunami waves; second, the trench-breaking ruptures of great and giant earthquakes that generate substantial tsunamis; and third, the tsunami earthquakes that are confined to the frontal portion of the accretionary prism and break to the trench, which generate similar-sized tsunamis to that of category-two events. These results highlight that tsunami-genesis is not only controlled by earthquake size, but also by the proximity of earthquake ruptures to the trench and the structural fabric of the hanging wall.
Seismic imaging of active margins over the past decades has increased our understanding of the structure and mechanical properties of accretionary wedges (Fig. 2), illuminating at least three mechanically distinct segments [von Huene et al., 2009; Wang et al., 2006; Wang and Hu, 2006; Kimura et al., 2007; Wang et al., 2010; Watt and Brothers, 2020]. The actively deforming outer wedge is a thrust-and-fold belt, characterized by the presence of numerous parallel or conjugate faults overlain by folds, that sits under steep seafloor topographic slopes [Mandal et al., 1997; Seno, 2000; Wang et al., 2006; Singh et al., 2008; Kopp et al., 2009; Kamei et al., 2012; Zhu et al., 2013]. The outer wedge includes a frontal prism near the trench, which is often the most intensely deforming segment. The inner wedge encompasses a thicker pile of sediment under mild seafloor slopes and undergoes little internal deformation contemporaneously. The basal layer of the inner wedge includes more lithified sediment due to higher temperature and pressure. The third offshore segment of subduction margins lies under the continental or arc shelf or under forearc basins and represents the frontal keel of the crust from the overriding plate. The inner and outer wedges are separated by a dynamic backstop while a static backstop bounds the inner wedge and the continental or arc crust [Watt and Brothers, 2020]. In this study, we argue that tsunami-genesis is largely controlled by the overlap of seismic ruptures with the outer wedge of the accretionary prism (Fig. 2e). The high-angle splay faults of the outer wedge provide an efficient mechanism to transfer sub-horizontal slip on the megathrust to seafloor uplift and tsunami excitation. The down-dip width of the outer wedge controls the number of splay faults involved during rupture, individual fault slip, and the resulting coseismic seafloor uplift.
To investigate the structural control of tsunami excitation, we review multiple geophysical, seismo-geodetic, and high-resolution bathymetric datasets. We delineate the inner and outer wedges of accretionary prisms by interpretation of seismic horizons in vertical cross-sections and by mapping surface faults and folds. In the following section, we analyze the morphology surrounding the rupture areas of historical and more recent tsunami earthquakes. We find that the rupture width, moment magnitude, and tsunami run-up of tsunami earthquakes are positively correlated with the width of the outer wedge. This relationship allows us to discuss tsunami hazards at other subduction zones based on morphological data. In a subsequent section, we calibrate an empirical model of maximum tsunami run-up of trench-breaking ruptures based on the width of the outer wedge and the earthquake moment magnitude using data from historical great and giant earthquakes. We conclude by making predictions of maximum tsunami run-up for hypothesized trench-breaking ruptures at global subduction zones based on the morphology of the outer wedge. Our model indicates the highest tsunami run-up for tsunami earthquakes at the Western Makran (Iran), Western Aleutian, Lesser Antilles, Hikurangi (New Zealand), and Cascadia subduction zones, although there is notable trench-parallel variability. Detailed seismic imaging of the outer wedge at megathrust seismic gaps will be key to better mitigate the tsunami risks induced by subduction seismicity.
Section snippets
Relationship between tsunami earthquakes and the outer wedge
We start by describing the structural setting of well-documented tsunami earthquakes to delineate their relationship with the outer wedge. We assemble high-resolution bathymetric data and multi-channel seismic profiles and use slip distribution models to constrain the rupture location. The bathymetric data is from high-resolution multi-beam surveys or coarser datasets [Ryan et al., 2009]. For each case, we consider several bathymetric and topographic profiles going through the maximum coseismic
Structural control on tsunami excitation
A structural analysis of the berthing ground of tsunami earthquakes (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11 summarized in Fig. S1) illuminates their relationship with the outer wedge of accretionary prisms at subduction zones. We present a quantitative summary of the structural observations in Table S2 and Fig. 12. The location of tsunami earthquake ruptures is generally subject to epistemic uncertainties associated with the limited resolution of
Empirical model of maximum tsunami run-up for trench-breaking ruptures
The analysis of tsunami run-up from tsunami earthquakes illuminates a structural control of tsunami excitation that may operate for all types of trench-breaking ruptures. Activation of distinct structures in the outer wedge have also been observed to excite disproportional tsunami waves in great and giant earthquakes. For instance, mega-splays were involved in the rupture of the 1944 Mw 8.0 Tonankai [Park et al., 2002; Moore et al., 2007], 2004 Mw 9.2 Sumatra-Andaman [Singh et al., 2008;
Implications for tsunami hazard at remaining seismic gaps
As the correlation with outer-wedge width provides useful estimates of maximum runup for tsunamis triggered by tsunami earthquakes and trench-breaking ruptures within a wide range of moment magnitudes, we survey the morphology of global subduction zones to shed light on tsunami hazards at remaining seismic gaps. We conduct structural analyses of the Hikurangi, Makran, Lesser Antilles (Fig. S3), Sumatran (Fig. S4 and S5), Nankai (Fig. S7), Ryukyu (Fig. S6), Aleutian (Fig. S9), Cascadia,
Discussion
Several forms of structural control of tsunami excitation have been tested [Satake and Tanioka, 1999], including the incoming seafloor roughness, the sediment vertical thickness in the outer wedge and frontal prism, and the accretionary versus erosive nature of sedimentary prisms [Geersen, 2019]. However, the vertical thickness of the outer wedge may not be a systematic controlling factor for tsunami excitation [Geersen, 2019]. The efficiency of seafloor uplift depends on the presence of
Conclusions
Tsunami earthquakes occur in the complex structural setting of the outer wedge of accretionary prisms at subduction zones. The presence of active deformation structures off the plate interface allows rupture propagation on high-angle faults that displace the seafloor more effectively than slip on the shallow dipping décollement. Active faults of the outer wedge include forward-, backward-, and bi-vergent thrusts permitting shortening and accretion as well as normal faults that accompany
Data availability
The structural data presented in Fig. 13 is available in digital format at the DOI https://doi.org/10.5281/zenodo.5854751. The topography is from Ryan et al. [2009]. The finite slip distributions are from Hayes [2017]. Our analyzed data sets in this study are given in the supplementary document.
Author contributions
QQ and SB designed the study, gathered the data, and wrote the manuscript.
Declaration of Competing Interest
The authors declare no competing financial or non-financial interests.
Acknowledgments
QQ and SB are supported by the funding from the National Science Foundation, under award number EAR-1848192. QQ is also supported by grants from the National Natural Science Foundation of China (42076059, 41890813, 41976066, 41976064), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering, Guangdong Laboratory (Guangzhou) (GML2019ZD0205), Chinese Academy of Sciences (Y4SL021001, QYZDY-SSW-DQC005, 133244KYSB20180029, ISEE2021PY03, 131551KYSB20200021), The
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