# Constraining the Effect of Bathymetric High Subduction in an Accretionary Wedge: Evidence From the Tokai Area of the Nankai Trough, Japan

Andrzej Górszczyk, Rafael Almeida, Romain Brossier, Ludovic Métivier, Stéphane Operto

► To cite this version:

Andrzej Górszczyk, Rafael Almeida, Romain Brossier, Ludovic Métivier, Stéphane Operto. Constraining the Effect of Bathymetric High Subduction in an Accretionary Wedge: Evidence From the Tokai Area of the Nankai Trough, Japan. Geochemistry, Geophysics, Geosystems, 2025, 26 (3), 10.1029/2024GC011754. hal-05054154

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Distributed under a Creative Commons Attribution 4.0 International License
# Geochemistry, Geophysics, Geosystems®

## RESEARCH ARTICLE

10.1029/2024GC011754

**Key Points:**

* FWI based pseudo-reflectivity models reveal detailed geological structure of the Tokai segment

* Imaged subducting highs are tilted fault blocks in oceanic crust rather than volcanic ridges

* Their subduction causes fault reactivation, imbricate fan and duplex formation, and basin inversion

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

A. Górszczyk,
agorszczyk@igf.edu.pl

**Citation:**

Górszczyk, A., Almeida, R., Brossier, R., Métivier, L., & Operto, S. (2025). Constraining the effect of bathymetric high subduction in an accretionary wedge: Evidence from the Tokai area of the Nankai Trough, Japan. *Geochemistry, Geosystems*, 26, e2024GC011754. https://doi.org/10.1029/2024GC011754

Received 11 JUL 2024
Accepted 4 MAR 2025

**Author Contributions:**

**Conceptualization:** Andrzej Górszczyk, Rafael Almeida, Stéphane Operto

**Data curation:** Andrzej Górszczyk

**Formal analysis:** Andrzej Górszczyk, Rafael Almeida

**Investigation:** Andrzej Górszczyk, Rafael Almeida, Stéphane Operto

**Methodology:** Andrzej Górszczyk, Romain Brossier, Ludovic Métivier, Stéphane Operto

**Project administration:** Andrzej Górszczyk, Romain Brossier, Ludovic Métivier, Stéphane Operto

© 2025 Institute of Geophysics, Polish Academy of Sciences and The Author(s). Geochemistry, Geophysics, Geosystems published by Wiley Periodicals LLC on behalf of American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

## Constraining the Effect of Bathymetric High Subduction in an Accretionary Wedge: Evidence From the Tokai Area of the Nankai Trough, Japan

Andrzej Górszczyk¹,² ID, Rafael Almeida³ ID, Romain Brossier², Ludovic Métivier²,⁴ ID, and Stéphane Operto⁵

¹Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland, ²University Grenoble Alpes, ISTerre, Grenoble, France, ³San Diego State University, San Diego, CA, USA, ⁴CNRS, University Grenoble Alpes, LJK, Grenoble, France, ⁵Université Cote d'Azur, CNRS, OCA, Valbonne, France

<figure><img src="image_2.png" /></figure>

**Abstract** The Tokai area in the eastern Nankai Trough is characterized by the subduction of a rough plate interface caused by bathymetric highs related to the adjacent Izu-Bonin arc and fault fabrics in the oceanic crust. However, the effect of this process on the structural development of its accretionary wedge and megathrust seismicity is still poorly understood. To get better insight into Tokai area we apply full-waveform inversion to legacy wide-angle seismic data exploiting available signal frequency up to 15 Hz. From this model we extract pseudo-reflectivity attributes, which are complementary to the section obtained with Kirchoff pre-stack depth migration of streamer data. The high-resolution images reveal detailed geological characterization of the subducting oceanic crust, the geometry of the megathrust and splay faults, and the structure of the wedge. The latter is characterized by imbricate fans, out-of-sequence thrusts, and duplexes that significantly contribute to its thickening. We compare these features to published structural models and propose that the increased structural complexity in the Tokai segment is due to fault reactivation triggered by the subduction of the bathymetric highs. We suggest that this fault reactivation also controls the shallow seismogenic behavior of this region. We show that the evolution of the forearc basin is related to the ongoing underplating process and out-of-sequence thrusting within the wedge. We compare our seismic imaging results with bathymetric, magnetic, and gravity data, revealing no evidence of large ridges beneath our profile, which previous studies have proposed as a cause of the persistent Tokai seismic gap.

**Plain Language Summary** The Tokai segment is located at the eastern Nankai Trough, where the Philippine Sea Plate is sliding underneath Japan. This segment is distinct from other parts of the Nankai Trough, showing a more irregular seafloor bathymetry and narrower wedge of deformed sediments. This is thought to be the effect of seamounts and ridges plowing into the wedge during subduction. Here we use novel approaches to process legacy seismic data to create new images, and interpret in detail the faults and deformation caused by this process. Based on the type and intensity of deformation we divide the wedge into three sections. We show how the adding of material to the wedge from below, a process called underplating, is an important process here. This, combined with faults moving in an unexpected order (out-of-sequence), contributes significantly to the uplift and thickening of the wedge. We place observational constraints on the variation of the shape of the plate interface, how the new faults develop in the wedge, and how sediments are accumulated and deformed below and above the wedge. Finally, we do not identify a large-scale subducted ridge beneath our profile, which previous studies had suggested might govern earthquake occurrence in this region.

## 1. Introduction

Megathrusts at convergent margins are the sites of the most devastating and largest earthquakes ever recorded. The factors that determine the nucleation, propagation and size of these earthquakes are still widely debated (Hsu et al., 2013; Hubbard et al., 2016; Lay et al., 2012; Olsen et al., 2020; L. Ye et al., 2018). The great number of studies published in recent years about the effect of seamount subduction on megathrust earthquake nucleation, and the effects of associated deformation on inter-seismic coupling of the megathrust, emphasize the increasing interest in processes related to the subduction of different types of bathymetric highs. It has long been thought that subduction of rugged seafloor bathymetry causes widespread deformation in the accretionary wedge (e.g., A. Mogi & Nishizawa, 1980; Lallemand et al., 1986; Von Huene & Lallemand, 1990). However, the nature of the deformation has been hitherto studied using mostly numerical (Morgan & Bangs, 2017; Ruh et al., 2016; Sun
**Resources:** Andrzej Górszczyk,
Romain Brossier, Ludovic Métivier

**Software:** Romain Brossier,
Ludovic Métivier

**Supervision:** Andrzej Górszczyk

**Validation:** Andrzej Górszczyk,
Stéphane Operto

**Visualization:** Andrzej Górszczyk

**Writing – original draft:**
Andrzej Górszczyk, Rafael Almeida

**Writing – review & editing:**
Andrzej Górszczyk, Rafael Almeida,
Romain Brossier, Ludovic Métivier,
Stéphane Operto

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Map of the Nankai Trough divided into A, B, C, D fault segments (Ando, 1975). Red dashed lines mark slip area of the twentieth century large earthquakes. Yellow stars mark the shallow tremors (Vannucchi et al., 2022). Blue dashed line mark the Tokai Gap area. Yellow arrow marks the convergence of the Philippine Sea Plate with respect to Japan (DeMets et al., 2010). White arrows mark the volcanic chains at the edge of Izu-Bonin Arc. Inset shows the zoom on the bathymetry area of Kumano Basin and Enshu Basin. (b) Tokai area - black dashed square in panel (a) - with the location of the major faults and the TKY-21 OBS-seismic profile. The red relief image of bathymetry is adopted from Yamashita et al. (2017).</figcaption></figure>

et al., 2020; C. Wang et al., 2021) and analogue models (Dominguez et al., 1998; Rijsingen et al., 2019). One of the reasons for this is that most accretionary wedges are commonly located offshore, making them inaccessible to field studies, as well as commonly used geodetic methods (e.g., Almeida et al., 2018). However, these settings are ideal for large-scale active-source seismic experiments that can probe the lithosphere down to the upper mantle to determine the structure of the underlying subduction zone. Despite the difficulties of obtaining coherent seismic images in complexly deformed regions, the redundant seismic wavefields generated by these experiments illuminate various deep structural elements therein and allow for the geometry of the megathrust and other faults of the accretionary wedge to be determined through the use of cutting edge data processing and visualization methods.

The Nankai Trough (NT), offshore Japan (Figure 1a), has been a site of intensive geophysical investigations due to its high seismic hazard. There, the Shikoku Basin of the Philippine Sea Plate subducts beneath Japan, on the Eurasian Plate, toward the northwest at a rate of ~43 mm/yr (DeMets et al., 2010), forming a sediment-dominated accretionary wedge (Tsuji et al., 2015). Numerous marine geophysical surveys have been carried out in the region, however, these existing case studies have generally focused on a single type of observation that is processed using a single method. Given that the accuracy and detail of reconstructed subsurface models strongly rely not only on the quality of the recorded data but also on the robustness of the applied processing techniques, such approaches limit the reliability of the final models and therefore the accuracy of subsequent geologic interpretations.

In this study we showcase an approach where we build our geologic interpretation of the easternmost segment of the NT (i.e., the Tokai Segment - TS) based on complementary archival ocean-bottom seismometer (OBS) data and multi-channel seismic (MCS) data processed with state-of-the-art techniques (Górszczyk et al., 2017, 2021; Gorszczyk et al., 2019) utilizing a full-waveform inversion (FWI, Virieux and Operto (2009), Tromp (2019)) framework. Here we derived the high-resolution velocity model from the OBS data with broader-frequency bands (up to 15 Hz), which allowed the extraction of pseudo-reflectivity profiles that strongly correlate and complement the reflectivity section derived from the pre-stack depth migration of MCS data. The successful combination of the two seismic data sets allowed us to construct a geologic model with unprecedented detail for this complex region that is better resolved and supported by different independent data and imaging approaches.

Strong lateral velocity variations within the accretionary wedge divide it into a shallow and slower outer wedge, a middle wedge with large-scale steeply dipping velocity perturbations, interpreted as thrust sheets, and an inner wedge that acts as a backstop on the landward side of the wedge. We carry out wavefront and ray tracing modeling to demonstrate the relation between the wide-angle seismic wavefields and the underlying geological features of our structural interpretation. We combine the high-resolution attributes of the velocity model with bathymetry
data to track the branching of the megathrust along the major faults that cut through the wedge up to the seabed. The new images allow us to identify a few bathymetric highs in the subducting oceanic crust, which we interpret as tilted fault blocks. We describe in detail the complex deformation caused by the subduction of bathymetric highs, and compare these results with predictions from analogue and numerical modeling. We show that the evolution of the forearc basin is related to the ongoing underplating process and out-of-sequence thrusting within the wedge. We suggest that fault reactivation controls the shallow seismogenic behavior of TS. Finally, we use potential field and bathymetry data to test the existence and characterization of the subducting ridges that have been previously proposed in the TS (e.g., Le Pichon et al., 1996).

## 2. The Tokai Segment

The TS is the easternmost segment of the NT (segment D in Figure 1a) that historically has ruptured individually or combined with others during megathrust earthquakes (Ando, 1975). Rupture distribution of the most recent megathrust earthquakes (i.e., Kanto (1923), Tonankai (1944), and Nankai (1946); see the regions marked with red-dashed lines in Figure 1a), and the fact that TS last ruptured seismically in 1854, has led to the rationale that it is a seismic gap with a large seismic hazard (K. Mogi, 2004). Therefore, there is significant concern that a large earthquake involving the TS will occur this century.

Though the overall structure of the wedge in the TS is similar to the other, well-characterized segments of the NT (e.g., N. Bangs et al., 2009) some important features differentiate it. The TS is characterized by a narrower and steeper accretionary wedge with more complex bathymetry than the rest of the NT (i.e., smooth bathymetry of the Kumano Basin vs. ridged bathymetry of the Enshu Basin; see the inset in Figure 1a). The eastern boundary of the TS is marked by the Izu-Bonin Arc, formed by the subduction of the Pacific Plate underneath the Philippine Sea Plate. The western edge of the TS is defined by an abrupt seaward step in the deformation front of the Kumano Basin accretionary wedge that roughly coincides with the location of the Tenryu Canyon (Figure 1b). Measurements of structural features on the walls of this canyon indicate that there is a change in the orientation of folds and abundant subsidiary strike-slip faulting there, suggesting that the boundary is defined by large-scale transcurrent faulting (Hayman et al., 2011).

South of the TS accretionary wedge, in the Shikoku Basin, 15–20 Ma crust is subducting with an approximate NW-SE spreading fabric (Okino et al., 1999). There are also series of W-NW striking sub-parallel volcanic ridges on the western flank of the Izu-Bonin Arc (white arrows in Figure 1a). The largest and northernmost of these is called the Zenisu Ridge, which has been over-printed by southward verging intraplate thrusting that accommodates a portion of the Eurasian-Philippines convergence (Mazzotti et al., 2002). Several authors have suggested that the structure and seismogenic behavior of the TS is differentiated from the rest of the NT by the cyclic subduction of these volcanic ridges. Based on modeled gravity and magnetic data they proposed the presence of two ridges (so-called Paleo-Zenisu Ridges) currently located under the outer wedge and the backstop of the TS (Kodaira et al., 2003; Le Pichon et al., 1996; Park et al., 2004).

Until recently, velocity model reconstructions of the TS have been carried out using mainly wide-angle ray-based traveltime tomography techniques (e.g., Dessa et al., 2004; Kodaira et al., 2004; Nakanishi et al., 1998), which allowed to infer only its first-order velocity structure. Moreover, MCS data acquisition carried out with short streamers (~4 km) provided data that allowed the study of only its shallow (upper few kilometers) structure and stratigraphy (e.g., Martin et al., 2004; Mazzotti et al., 2002). Prior studies have classified the wedge into a weakly deformed unit (incoming sediments), a moderately deformed unit (outer wedge), a heavily deformed unit (middle wedge), and a backstop (inner wedge; Le Pichon et al. (1996), Henry et al. (2004), Gorszczyk et al. (2019)). Though their structural interpretations were hindered by limited resolution at depth, Martin et al. (2004), Dessa et al. (2004), and Henry et al. (2004) proposed that the outer and middle wedges are separated by a large out of sequence thrust locally called the Tokai Thrust, which is laterally equivalent to the “megasplay fault” observed to the west in the Tonankai segment of the NT (Park et al., 2002). The boundary between the middle wedge and the inner wedge is locally referred to as the Kodaiba Thrust, which has been interpreted as a fault with a considerable strike-slip component (see Figure 1b for the location of the main faults; Martin et al. (2004), Takahashi et al. (2002), Le Pichon et al. (1994)).
## 3. Methods and Data

### 3.1. High-Resolution Crustal-Scale Seismic Imaging

The cornerstone of the geological interpretations we present here are the acoustic FWI results obtained from OBS data (JAMSTEC, 2001b), and the reflectivity profile obtained by Kirchoff Prestack Depth Migration (K-PSDM) of short streamer (4.5 km) MCS data (JAMSTEC, 2001a) acquired by IFREMER and JAMSTEC along profile TKY-21 in 2001 (Figure 1). Here we use the time-domain FWI approach, and we push the inversion to 15 Hz. We started our inversion from the velocity model obtained by Górszczyk et al. (2021) where the maximum inverted frequency was 3.5 Hz (Figure S3 in Supporting Information S1) and for consistency of workflow we perform our FWI in time-domain. Inverting a frequency band up to 15 Hz allowed for recovery of changes in seismic velocity at exceptionally high-resolution. The final FWI velocity model is presented in Figure 2a. The imaging results are consistent with previous studies using various approaches including Laplace-Fourier FWI (Brossier et al., 2009; Shin & Cha, 2008) and time-domain FWI of the OBS data, as well as slope tomography (Lambaré, 2008; Tavakoli et al., 2017) and K-PSDM of the MCS data (processing details can be found in Górszczyk et al. (2017, 2021), Gorszczyk et al. (2019)). Despite the acoustic, isotropic approximation used to derive this high-resolution model, the range and distribution of obtained absolute velocity values can be used for first-order quantitative interpretation of lithology and comparison with well-log data from the deep oceanic drilling to support their interpretation.

The model in Figure 2a has a resolution that is incomparable to the classic traveltime tomographic methods used to study the TS previously (e.g., Bassett et al., 2022; Dessa et al., 2004; Kodaira et al., 2003). However, since the amplitude spectra of the Earth is dominated by its low-wavenumbers it can be difficult to visually detect the small-scale structural information encrypted in the high-wavenumber components of this spectra. To better emphasize the velocity perturbations at small and intermediate spatial scale, we remove the low-wavenumber velocity components from the FWI model to allow for a better visualization of the geometry of the underlying geological units. Below, we briefly review the four approaches used to extract the fine-scale velocity attributes of the FWI model, which would have been impossible to obtain from smooth traveltime velocity models.

In our first method, the low-wavenumber part of the model (Figure S1 in Supporting Information S1) is obtained by approximating vertical velocity profiles with 5th order polynomial curves that are subsequently subtracted from the FWI model. The resulting detrended model (Figure 2b) highlights fast and slow velocity regions with saturated red and blue colors respectively. This helps to emphasize the shape of the intermediate-scale geological units defined by similar velocities. The second method used applies a Gaussian smoothing with $\sigma$ equal to 5, where $\sigma$ is the characteristic of the Gaussian operator controlling the scale of the velocity details removed during smoothing. In Figure 2c we show the velocity perturbations obtained after the smoothed model (Figure S2 in Supporting Information S1) was subtracted from the original FWI model. Note that the scale of the resulting perturbations is smaller than in case of detrended model in Figure 2b, because the smoothing operator was selected to highlight the geometry of interfaces in the accretionary wedge. Our next approaches are akin to edge (sharp contrast) detection methods from the field of image processing that use operators approximating spatial derivatives. The third approach is based on calculating the spatial derivative of the velocity field - hereafter referred to as a gradient model, and highlights sharp velocity contrasts and areas of rapidly increasing/decreasing velocity. We derive the gradient model (Figure 2d) as a sum of horizontal and vertical spatial derivatives of velocity. The fourth and final approach is to calculate the morphological gradient of the FWI model (Gonzalez et al., 2020) by treating it as an image (matrix of pixels). We use a 2D operator of a given size $n \times n$ (where n is a number of pixels) to recover the maximum changes of velocity within it (Figure 2e). The resulting image indicates the maximum velocity changes in the close neighborhood of a given point. The four obtained high-wavenumber attribute models (Figures 2c-2e) can be interpreted as pseudo-depth-migrated images, and we collectively refer to them as pseudo-reflectivity profiles.

The velocity model recovered with FWI of OBS data and the pseudo-reflectivity profiles, are complementary to the K-PSDM derived from the MCS data (Figure 2f) albeit at lower resolution. This is because the velocity model obtained by the FWI is strictly determined by the local wavelength that is spanned at a given position of the model during the wave propagation. This wavelength, in turn is constrained by the frequency content of the inverted data and the angular illumination of the target (Fichtner & Trampert, 2011). In other words, due to the limited frequency band of the data (<15 Hz) and wide scattering angles of the OBS data at crustal depths we cannot expect the reconstruction of sharp interfaces - especially at the deeper parts of the model where the velocities are higher,
<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) FWI velocity model derived from ocean-bottom seismometer data with smooth contours marking areas of similar velocity range; (b) Detrended model obtained by removing from model in panel (a) the 5th order polynomial trend of vertical velocity profiles; (c) Pseudo-reflectivity model obtained by removing from model in panel (a) a background velocity model obtained with Gaussian smoothing; (d) Pseudo-reflectivity gradient model derived as a sum of horizontal and vertical spatial derivatives of model in panels (a, e) Pseudo-reflectivity model of maximum changes in velocity derived as a morphological gradient of model in panels (a, f) K-PSDM profile derived from the multi-channel seismic data and migrated using velocity model in panel (a).</figcaption></figure>

and therefore the local wavelength increases. Moreover, the regularization applied to reduce noise and artifacts will partially smooth the velocities between different units - for example, crust and upper-mantle. This is why, for example, rather than seeing the Moho as a singular reflector in our pseudo-reflectivity profiles (Figures 2c–2e),
we observe its low-passed representation in the form of a continuous anomaly (marked by light-red color) around 15 km of model depth. However, due to the short streamer, complex geology, 2D acquisition etc. there is no evidence at all of this interface in the K-PSDM profile. While we observe a correlation between the K-PSDM section and velocity attributes down to approximately 7–8 km depth, deeper structures are primarily visible in the velocity attributes only. All images compiled together provide fine-scale complementary information that allows us to carry out a more refined characterization of the geology of the TS. Therefore, we consider our approach as a template that can lead to improved geological interpretations in other subduction zones as well.

## 3.2. Waveform and Ray Forward Modeling

Analysis of the wave and ray propagation within the FWI velocity model helps to better understand which geological features have a significant impact on the anatomy of the seismic data. We choose representative examples of OBS gathers from different locations along the seismic profile and perform time-domain forward wavefield and ray modeling through the velocity model using the source-receiver configuration of the real data. To visualize the evolution of the wavefield across the model, we extract the wavefront at 1 s intervals during propagation. The resulting snapshots are stacked and superimposed on the velocity model together with the raypaths (reflected and first-arrival) to illustrate the penetration of rays within the model (a tool commonly used in traveltime tomography studies). Visualizing the wavefield evolution as it propagates through the model is a valuable bridge from the data domain (i.e., the seismogram extracted at the receiver position) to the model domain. It allows to identify locations where the waves are reflected or channeled/trapped, and hence to assess questions such as what interface in the model is associated with a given arrival in the seismic data. Moreover, it helps to understand how the resolution of the velocity model is related to the data, through the difference in wavenumber coverage between the shallow and deep parts of the model and different wavefield propagation regimes.

## 3.3. Potential Field and Bathymetric Data

To aid the interpretation of the 2D imaging results and position it within the 3D context of the margin, we use magnetic, bathymetry and gravity data. We extract the magnetic data from the Magnetic Anomaly Map of East and Southeast Asia (AIST, 2021). In our study area, the source data for this consists of marine magnetic surveys compiled into a 2 min grid. These data were reduced-to-pole (RTP) to shift the magnetic anomaly over the source using routines from Blakely (1995) with a magnetic inclination and declination of 47° and 6° respectively. The bathymetry used for interpretation (Figures 1b, 3a, and 6a) is adopted from Yamashita et al. (2017) who present the bathymetry of the Nankai Trough in a Red Relief Image Map format designed to highlight the details in the relief of a given surface. Finally, we also compare our results with free-air gravity data (Sandwell & Smith, 2009) and the residual bathymetry data that is obtained by subtracting the average bathymetry of the NT from the bathymetry of the profile of interest (Bassett & Watts, 2015).

# 4. Structure of the Tokai Segment From Seismic Data

## 4.1. Accretionary Wedge

To highlight the overall structure of the TS accretionary wedge we overlay the velocity model with smoothed contour lines (Figure 2a). The absolute velocity in the FWI model ranges from 1,500 m/s to >8,000 m/s. The extent of the sedimentary cover is marked with light-red color above the 3,000 m/s contour and thickens toward in the frontal part of the wedge and the trench area. Within this sedimentary cover we observe continuous features, both sub-horizontal and tilted, marked by increased and decreased saturation of red color that correspond to the layering of the strata with different velocities. There is also general trend of increasing velocity in the accretionary wedge starting from the trench (light-red) through the central wedge (saturated red; contour 4,000 m/s) to the backstop (yellow and light-green; contour 5,000 m/s). The abrupt change from red color to saturated green color throughout the lower part of the model (roughly marked by contour 6,000 m/s) delineates the top of the subducting oceanic crust, while the less sharp transition from green to blue (around the 7,900 m/s contour) indicates the Moho. Based on these overall characteristics, the accretionary wedge of the TS can be divided into four subunits with different intensity of deformation. The differences in the underlying wedge structure are also reflected by the different domains of the bathymetry along the profile shown in Figure 3a. We also use the bathymetry to infer the 3D geometry of structures that form scarps on the seabed (Figures 3b–3d) by tracking them along strike.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Bathymetry relief image around the TKY-21 profile; (b) K-PSDM profile and the structural interpretation. Yellow and green solid lines mark faults and approximate strata orientation within the wedge. Red solid lines mark the strata of the basins B1 and B2. Cyan and yellow dotted lines mark the strata of the forearc basin FB. Green dotted lines mark basin fans BF. Thin and thick black dashed lines mark the decollement and top of the oceanic crust respectively. Black solid line marks the Moho. Red dashed lines mark the normal faults in the oceanic crust. Blue dashed lines mark axial planes of fault-related folds within the wedge. FT - frontal thrust; FT' - developing frontal thrusts; AT - abandoned thrust; TT - Tokai Thrust; KT - Kodaiba Thrust; EF - Enshu Fault; B1, B2 - wedgetop basins; FB - forearc basin; BF - basin fans; TC - Tenryu Canyon; BU - basal unconformity; DX1, DX2, DX3 - duplexes; D - decollement; BH1, BH2, BH3, BH4 - bathymetric highs; NF - normal faults. (c, d) Interpretation from (b) superimposed on detrended and gradient model respectively (Figures 2b and 2d).</figcaption></figure>

4.1.1. Incoming Sediments

First we delineate the incoming sediments (blue-shaded area in Figures 4a–4c and 4p). This sub-section corre-
sponds to the strata of the Suruga Trench, which has not yet been incorporated into the accretionary wedge and has
a gentle northward dip. The youngest layers within the Suruga Trough are mostly flat and correspond to deep-
water, trench-fill deposits, such as turbidites, that are filling the depression between the active frontal thrust of
the wedge (FT in Figures 4a–4c) and the Zenisu Ridge. Within the trench there is a buried southward-verging
anticline (marked by green lines at 95 km of Figure 3b) associated to an abandoned frontal-most thrust of the
accretionary wedge (FT' in Figures 4a–4c) that overlies a thickened stratigraphic interval of decollement
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a–o) Zoom panels from Figure 3 with interpretation. Left, central and right columns show panels extracted from the K-PSDM profile, gradient model and detrended model respectively. (p) K-PSDM profile with crude interpretation and rectangles corresponding to the position of zoom panels presented in panels (a–o). Blue, magenta, yellow and white shaded zones correspond to incoming sediments, outer, middle and inner wedge respectively.</figcaption></figure>

mudstones characterized by chaotic reflectivity (i.e., a mushwad, (Thomas, 2018)). Because there is no increase of magnetic anomaly associated with this thickened section, we do not consider that the underlying igneous oceanic crust is involved.
The crest and forelimb of the anticline are cut by a small north-verging thrust fault with an associated small wedgetop basin with internal unconformities. At the seafloor there is an uplift of ~40 m in height and ~3 km in width, just above the culmination of this structure that can be faintly observed in the bathymetry relief image (Figure 3a). This fault may be a long-lived gravity-driven slump, as it originates in the strata flanking the Zenisu Ridge, and has a downslope motion. There is a small thrust cutting the “backlimb” syncline of the slide which may have formed as an out-of-syncline thrust to accommodate the folding, or as the continuation of FT’, but now offset by the slide. To the south of the Suruga Trench the profile shows the northern end of the Zenisu Ridge.

### 4.1.2. Outer Wedge

Toward the north-west we outline the outer wedge (magenta-shaded area in Figures 4d–4f and 4p). It is limited to the south by the frontal thrust FT that causes the initial bathymetric expression of the wedge, and is defined by a shallowing in bathymetry from ~3.7 km depth to ~2.4 km depth. The decollement (thin dashed black line D in Figure 4) at the base of the FT is deformed as it passes over a series of bathymetric highs (BH1, BH2, BH3 in Figures 4a–4i). The FT, which ramps-up the northern flank of the bathymetric high BH1, is blind and creates a relatively simple fault-bend-fold. Beneath this thrust, at ~6.2 km below sea level, there is a small thrust that is no longer active (AT in Figures 4a–4c). This fault ramps-up a short distance from the decollement, which at this location has been uplifted and then subsided in the wake of BH1 (these is discussed later in the context of the oceanic crust).

The next structure to the north is an imbricate fan that creates a local bathymetric high at a depth of ~2.3 km (~80 km of distance; Figures 4d–4f). Sharp velocity variations in Figures 4e and 4f highlight very well the main interfaces within this unit (also note how they manifest in other velocity attributes in Figure 2). The internal geometry of this structure is complex. There is evidence of older faults being back-rotated to steeper dips by younger ones, and out-of-sequence thrusts cutting through older thrust sheets (dashed yellow lines in Figures 4d–4f). Some faults reach the seafloor and create a break in slope indicating that they are active. The resulting structural culminations created a wedgetop basins (B1 and B2 in Figures 4d–4f) that record details of the deformation history through internal unconformities and growth strata. In both cases the wedgetop basins have slower velocities than the underlying thrust sheets (indicated by blue color in Figure 4f), and the older strata within the basins are tilted. The reflections are well imaged in this part of the accretionary wedge and it is apparent that all the thrusts sole into a fault that forms the roof thrust of an underlying duplex at ~70 km of distance (DX1 in Figures 4d–4f). This feature is observed in both reflectivity and velocity gradient profiles (Figures 4d and 4e) and has clearly lower velocities than surrounding structures (Figure 4f). The uplift related to the formation of DX1 may be the cause of the pronounced northward tilting of basin B2 observed in its deeper strata. DX1 is underlain by the decollement (black dashed line D in Figure 4e), under which the reflections are tenuous and disturbed/ discontinuous (Figure 4d) suggesting significant deformation and out-of-plane wavefield propagation that hampers correct depth migration in 2D. However, the velocity variations in Figures 4e and 4f show zones of increased velocity at 75 km of distance and 8 km of depth that could correspond to a second bathymetric high (BH2 in Figures 4d–4f), either a small subducted seamount or another tilted fault block.

### 4.1.3. Middle Wedge

The middle wedge (yellow-shaded area in Figures 4g–4i and 4p) is defined by a sharp change in bathymetry at ~65 km of distance, where the water depth starts decreasing gradually until it reaches a depth of ~0.8 km. The transition between the well-imaged thrust sheets of the outer wedge, and thicker, less sharply imaged thrust sheets on the middle wedge is marked by the Tokai Thrust (TT in Figures 4g–4i). This fault occupies the same structural position as the mega-splay fault described at other locations along the NT, which is generally considered to be an active out-of-sequence thrust (Park et al., 2004). From the Tokai Thrust northward, the wedge is capped by the basal unconformity of the forearc basin (magenta line BU in Figures 4g–4i and 4p). In the middle wedge, the reflectivity becomes less clear with depth (Figure 4g), but the pseudoreflectivity profiles (Figures 4h and 4i) allow us to infer the general geometry of its internal stratigraphy. The shallow reflections show the location of axial planes of fault-related folds (blue dashed lines in Figures 4g–4i) and stratal cutoffs that are consistent with a decollement and ramp structure similar to that observed at present in the outer wedge. In the detrended model, there is a well-imaged large thrust sheet (Figure 4i), characterized by faint interfaces in the K-PSDM profile that is overriding a duplex formed at the transition between the middle and outer wedge (DX2 in Figures 4g–4i and 4p). The interfaces within DX2 are not well focused in the K-PSDM profile. However, our interpretation of a duplex
structure is supported by a southward-dipping reflector located ~55–60 km of distance and ~7 km of depth in Figure 4g (also Figure 4p). This reflector marks the southern flank and culmination of the structure - note the change in reflectivity characteristics on either side of it. In Figure 4h, we observe landward-dipping interfaces in the gradient model, which correspond to the northern flank of the duplex. Additionally, there is an abrupt velocity change in the detrended model at the southern end of DX2 (indicated by a blue anomaly at 60 km of distance and 8 km of depth in Figure 4i), suggesting a lithological change at this location. The duplex uplifts and folds the overlying strata and thrusts, indicating that its movement initiated after these features were formed. We do not consider this feature to be a seamount because the velocities there are significantly lower than those of the oceanic crust, and instead are consistent with those of sedimentary strata at the base of the wedge (Figure 2a). Instead we observe a bathymetric high with increasing velocities (BH3 in Figures 4g–4i) slightly to the south and below the duplex DX2.

### 4.1.4. Inner Wedge

The inner wedge (white-shaded area in Figures 4j–4l and 4p) has much less coherent reflectivity, and what is present in the deeper parts of the profile has lower wavenumbers than the outer or middle wedge. However, the pseudo-reflectivity profiles have coherent features that were used to support the interpretation. The thrust that corresponds to the boundary between the inner and middle wedge is generally referred to as the Kodaiba Thrust (KT in Figures 4g–4i), and as the Tokai Thrust, it separates areas of different reflectivity and structural styles. Le Pichon et al. (1996) interpreted strike-slip faulting parallel to the trench along this structure, however, we find no evidence of this in our seismic profile and conclude that any strike-slip faulting there is probably local and related to the complex strain fields that can form around subducting nearby seamounts (see Figure 6b, K. Wang and Bilek (2011)). The internal structure of the inner wedge is complex, and dominated by thrust sheets that appear to be larger than those in the outer and middle wedge. These thrust sheets probably have large amounts of internal deformation, which limits their reflectivity. The geometry and truncation of these sheets suggests a protracted history of reactivation. A clear indication of this is the offset of the BU by a thrust at 40 km (Figures 4j–4l) and the folding of the forearc strata by the KT at 50 km (Figures 4g–4i).

At the base of the inner wedge there is an interval of underplated material that is faulted into thrust sheets ~2 km thick (DX3 in Figures 4m–4o). The underplated thrust sheets, are defined in the K-PSDM profile by a band of reflectors that roughly follow velocity changes at 14 km depth. The decollement steps down stratigraphically down-dip of a possible bathymetric high that is marked by locally increased velocities above the top of the ocean crust (BH4? in Figures 4n–4o). The step-down is a ramp in the decollement, which leads to the transfer of an interval of strata that is below the basal decollement to the base of the inner wedge. This is probably an ongoing process as the spatial scale of the folding of the thrust sheets is consistent with the broad uplift of the sediments of the overlying forearc basin between 25 and 45 km of distance (FB in Figures 4j–4l).

### 4.2. Subducting Plate

The oceanic crust is characterized by abrupt changes in seismic velocities that make both its top and undulating base (the Moho; thick black line in Figure 3b) readily identifiable in the pseudoreflectivity profiles. The seismic velocities observed for the crust are varying laterally and range between ~ 6,000 m/s and ~7,300 m/s, which is consistent with velocities measured elsewhere (Christeson et al., 2019; Fowler, 2005; Grevemeyer et al., 2018). The subducting plate is characterized by a series of paleo-bathymetric highs with reliefs of up to ~1 km that can be observed underneath the accretionary wedge (BH3 - 57 km; BH2 - 70 km; BH1 - 80 km of distance in Figure 3). These could correspond to seamounts that would be analogous to those observed on the modern seafloor on the western flank of the Izu-Bonin arc in this region, though in this case they would be much smaller than the one imaged by Martin (2003) in a parallel seismic profile located ~14 km south-west from our line. More likely, these features correspond to tilted fault blocks produced by normal faults that cut the oceanic crust (NF red dashed lines in Figure 3b). Extracted velocity attributes models (e.g., Figures 3c and 3d) show a faint fabric in the oceanic crust that may correspond to these normal faults. Locally, the uppermost crust has some perturbed reflectivity (e.g., Figure 4d) that likely corresponds to the sedimentary fill of these normal fault-bounded basins. The sediments deposited on the oceanic basement are mostly incorporated into the accretionary wedge, but the bottom-most package (~1 km thick) is subducted. This sedimentary interval, however, thickens and thins deeper in the section (Figure 3) suggesting that it is being sheared and deformed as it moves relative to the irregular top of the oceanic basement.
### 4.3. Megathrust Fault

The basal decollement (or megathrust; thin dashed black line D in Figure 4p) is marked by a discontinuous reflection beneath the accretionary wedge, that is located above the bathymetric highs on the oceanic crust. It is also well-imaged in the pseudo-reflectivity profiles beneath the outer wedge (e.g., Figure 4e). Starting from a distance of 54 km and depth ~9 km (Figures 4g and 4p), where TT is branching from the megathrust, there is a change in the character of the decollement in the K-PSDM profile and we start to observe a band of northward-dipping pronounced reflectors ~1–2 km thick, consistent with previous observations for Cascadia (Nedimović, Hyndman, et al., 2003; Nedimović, Mazzotti, & Hyndman, 2003) and for Alaska (Bécel et al., 2017; Kuehn et al., 2024). This area corresponds to a low-velocity zone (see Figure 4i), which could be due to either a thick damage zone associated to the megathrust, sediments on the subducting plate, or the development of a subduction channel. At the northern end of this low-velocity zone (marked by BH4 in Figures 4m–4o) the velocity values match those of the top of the oceanic crust (see the pull-up of the contour line marking 6,000 m/s in Figure 2a at 38 km distance) and the reflections there (just above BH4 in Figure 4m) become more focused and stronger than within the low-velocity zone toward south, suggesting a change of the megathrust proprieties. This site coincides with an interpreted duplex DX3 of underplated material being accreted to the base of the inner wedge.

### 4.4. Forearc Basin

The forearc basin (FB in Figures 4j–4l) is well imaged in the K-PSDM profile. The top of the basin (i.e., the seafloor) is folded into a broad anticline between the Kodaiba and Enshu faults (see bathymetry uplift between 20 and 50 km in Figure 4p). As the strata of the forearc basin was deposited, there was continued deformation evidenced by several internal unconformities (Figure 4j). Within the basin, three unconformities are identified (from oldest to youngest: magenta, cyan and yellow). The base of the basin is marked by a basal unconformity (magenta line BU in Figure 4p) where reflective strata overlies the less reflective, higher velocity inner wedge. Over the inner wedge the BU surface truncates thrust sheets and has an irregular geometry on which the basin was formed (see Figures 4j–4l). The BU deepens abruptly at ~42 km of distance, where a thrust of the inner wedge cuts it and overrides the oldest strata interval of the forearc in a geometry virtually identical to that documented by Boston et al. (2016) in the Kumano Basin segment of the NT, suggesting that these structures may correspond to the same event affecting a large segment of the NT. The package between the BU and the next unconformity (cyan dotted line in Figures 4j–4l) onlaps the BU velocities (best observed at the northern end of the profile as a zone of deep blue color in the detrended model; Figure 4k), and thins over the crest of the anticline related to the thrust that cuts the BU, indicating that deposition is coeval with this phase of thrusting. The cyan unconformity follows in the basin and is marked by onlap of the overlying strata that show it was folded into a broad syncline with a hinge at ~40 km distance. Between 30 and 35 km distance the cyan unconformity is eroded by strata with lenticular geometry, similar to that found in basin floor fans (BF, green dotted lines in Figures 4j–4l; Saller et al. (2004)). This interval is formed by amalgamated lenses of up to ~0.5 km in thickness, and ~5 km in length for a total thickness of ~1 km. The base of the BF interval truncates the underlying stratigraphy between 25 and 38 km (Figure 4j) suggesting that the deposits are filling a paleocanyon related to sediment input from the Tenryu River (TC in Figure 4p and its outcrop in bathymetry in Figure 3a). The paleocanyon fill is covered by the youngest strata of the package above the cyan unconformity indicating that it formed within this interval. Finally, there is a youngest unconformity (yellow dotted line in Figures 4j–4l) that shows onlap with the same syncline as the cyan unconformity, showing that the fold growth was a protracted process. At the northern edge of the profile the yellow unconformity deepens and forms the basement of a half-graben formed by a normal fault, the Enshu Fault (EF in Figure 3b). The basin fill is younger than the yellow unconformity and is ~1.5 km thick (Figure 3b). The active channel of the Tenryu Canyon is located adjacent to the footwall of the normal fault, which in conjunction with evidence of abandoned channels further south (Figure 1b, just south of red star marking MITI well), suggests that it is migrating toward the basin axis, probably due to northward tilt of the basin caused by ongoing slip on the Enshu Fault. At the Kodaiba Thrust, all the unconformities are folded over a basement culmination (Figures 4g–4i). South of this, the strata over the middle wedge between 55 and 63 km of distance downlap onto BU (see cyan and yellow dotted arrows in Figures 4g–4i).

### 4.5. Waveform and Ray Penetration of the Tokai Segment

Before carrying out a detailed geological discussion, we first compare our results with wide-angle data waveform and ray-tracing modeling. Ray-paths and wavefront snapshots illustrate which parts of the velocity structure are
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Ray and wavefield modeling. (a, c) Examples of common ocean-bottom seismometer (OBS) gathers. Dashed lines correspond to predicted arrivals associated with reflected ray-paths (shown in panels (b, d) with matching colors). Solid lines correspond to predicted first arrivals associated with diving and refracted waves (ray paths shown in panels (b, d) with matching colors; red = *Pg* arrivals, yellow = *Pn* arrivals). (b, d) Sampling of the model space with wavepaths (wavefront snapshots extracted at 1 s intervals) and raypaths for the OBS gathers in panels (a, c), superimposed on the detrended velocity model.</figcaption></figure>

illuminated by specific seismic waves and, therefore, constrained by particular segments of the OBS data,
therefore allowing us to correlate specific arrivals in the OBS gathers to interpreted structures in the profile.

We show two examples of modeling through our FWI velocity model (Figure 5). In Figure 5a we show the results for an OBS located in the NW part of the profile. Four reflections associated with velocity contrasts of various magnitudes are marked with dashed lines. These reflections correspond to the base of the forearc strata, faults within the inner wedge and the roof thrust of duplex of underplated material (DX3) (black lines in Figure 5b). Weak first arrivals (solid lines) marked in green and light-blue lines in Figure 5a correspond to the waves trapped by the large-scale velocity alternations located between 20 and 40 km that correspond to faults within the middle wedge (Figure 5b) that cause the wavefront to split into separate arrivals. The dark-blue line in Figure 5a marks a small arrival of similar nature that is trapped by the megathrust until 42 km of offset. Further, red and yellow lines
in Figure 5a mark the *Pg* and *Pn* arrivals. Although they are penetrating the deepest regions of the model, subtle distortions of the arrivals show that they are still sensitive to the velocity changes in the frontal accretionary wedge and trench.

The second OBS gather (Figure 5c) corresponds to a receiver located in the central part of the profile. We observe four different reflectors within the wedge (black lines in Figure 5d) that correspond to the base of the forearc, and faults within the middle wedge, particularly the Tokai Thrust, which separates the outer and middle wedges. The weak first-arrivals marked by the magenta solid line (–30 km to –10 km in Figure 5c) correspond to rays that are guided by the faults within the inner wedge that took a slightly different path during the first few seconds of propagation. Similarly to the previous OBS example, we observe variations of the first arrivals marked by light-blue and blue lines in Figure 5c that are associated with the wavefronts and rays sampling thrusts located toward the outer wedge. Again we observe the same behavior of *Pg* and *Pn* arrivals (red and yellow lines in Figure 5c), that are affected by the faults and thrusts located in the shallow part of the model. In both cases the low-velocity zone above the megathrust (marked by blue) shows almost no first-arrival ray coverage, which might give the impression that it is not constrained by data. However, the wavefront snapshots show several secondary arrivals illuminating this region, which are fitted during the waveform inversion process yielding the observed results.

Overall, these models show that throughout the accretionary wedge, the base of the forearc basin and the changes in velocity associated with main faults, such as the megathrust or Tokai and Kodaiba thrusts, constitute the main velocity discontinuities that serve to channel the seismic waves. The existence of the low-velocity damage zones around the long-lived faults produces variations in the propagating wavefront that are reflected by subtle changes in the recorded waveforms. These waveforms are examples of trapped waves in a fault zone (Li & Leary, 1990), and while an accretionary wedge has abundant damage zones due to the many faults located there, this type of modeling can allow us to distinguish which ones have the largest effect on the propagating waves and thus the largest changes in velocities, which in turn correspond to greater damage zone and slip. We consider that these results show that the high-resolution structure of our model is genuinely data-driven, and that the agreement between these features and our structural interpretation reinforces our confidence in the latter.

# 5. Discussion

Our detailed geologic interpretation allows us to compare and contrast the deformation of the TS with that interpreted from studies carried out on other segments of the NT, as well as other subduction zones and analogue modeling. In the following sections we will highlight the most salient aspects of the geology of the TS that can allow us to constrain its dynamic evolution.

## 5.1. Structural Style and Shape of the Tokai Segment Accretionary Wedge

The structural style in the area of Enshu Basin is noticeably different from that observed by other studies in the Kumano Basin area, to the southwest of our profile (Figure 6a). The outer accretionary wedge of Kumano Basin is characterized by a relatively simple set of forward-breaking thrusts, with intervening piggy-back basins (e.g., N. Bangs et al., 2009; Moore et al., 2007). Our profile instead shows imbricate stacks with evidence of younger thrusts cutting through these stacks (e.g., yellow dashed faults in Figure 4d). We interpret these differences to be fundamentally related to the repeated subduction of bathymetric highs in the TS, most likely the seamount chains that form oblique to the Izu-Bonin arc (white arrows in Figure 1a), and thus we consider it an ideal location to better understand the effect of this process on the overlying accretionary wedge. While we only have seismic data in two dimensions, (which imposes limitations when imaging complex geological settings such as this one) additional insights along strike can be gleaned from non-seismic data. Here we use bathymetry, gravity and magnetic data (Figures 6a–6c) and combine them with the velocity perturbation profile presented in Figure 6d.

The shape of the TS accretionary wedge can be described using the bathymetry and residual bathymetry (the difference between local bathymetry and average bathymetry of subduction zone; Bassett and Watts (2015)) along the profile (green and dashed-green curve in Figure 6c). The accretionary wedge in the TS is narrower and steeper than other segments of the NT (see inset in Figure 1a), due to the widespread out-of-sequence thrusting within the wedge. The residual bathymetry shows several highs of various wavelengths (indicated by green horizontal arrows in Figure 6c) that correspond to the outer, middle and inner wedges. The anomalies in the outer wedge correspond to the individual thrust-related structural culminations, while in the middle wedge, the scale of anomalies could be related to either the Tokai Thrust or the duplex DX2. However, we consider it is the latter
<figure><img src="image_8.png" /><figcaption>**Figure 6.** (a) Bathymetry red relief image of the eastern NT area. Yellow stars mark the shallow tremors (Vannucchi et al., 2022). Yellow dashed line marks the boundary between the Kumano and Enshu basins. Yellow arrow marks the convergence between the Philippine Sea Plate and Eurasian Plate. Black line marks the location of the TKY-21 profile; (b) Magnetic anomaly map superimposed on the Tokai bathymetry relief map with the main faults and the location of MITI well and the TKY-21 profile; (c) Bathymetry and potential field data extracted along the TKY-21 profile, red arrows indicate the location of the magnetic anomalies in the reduced-to-pole data associated with bathymetric highs, green horizontal arrows indicate the extent of the signal in the residual bathymetry of each section of the wedge (EF: Enshu Fault, IW: inner wedge, MW: middle wedge, OW: outer wedge) (d) Morphological gradient of the FWI model (Figure 2d) with the baseline interpretation including location of bathymetric highs.</figcaption></figure>

because it is folding the Tokai Thrust, suggesting the thrust is not presently active. In the inner wedge the uplift is broader than the thrusts there, but similar to duplex DX3, and is folding the youngest sediments of the forearc, and thus we consider it to reflect the uplift related to ongoing underplating. The residual bathymetry over the Enshu Fault at the northern end of the profile (20 km distance in Figures 6c and 6d) is lower than the rest of the uplifted basin, consistent with the normal fault being active and down-dropping the seafloor.

## 5.2. Ridge and Seamount Subduction

The potential field data combined with our results allows us to test the hypothesis that there are ridges on the scale of the modern Zenisu Ridge located beneath the TS (i.e., the paleo-Zenisu Ridges; Le Pichon et al. (1996), Kodaira et al. (2003)). The apparent seismic gap in the TS has been attributed to the effect of such features on the overlying megathrusts and accretionary wedge. However, the gravity anomaly profile (blue curve in Figure 6c) correlates with the bathymetry profile (green curve in Figure 6c) suggesting that there are no large, high-density bodies underneath the accretionary wedge along our seismic line. The magnetic field also broadly correlates with the bathymetry, but after reduction-to-pole (RTP, dashed red line in Figure 6c) we clearly observe three anomalies under the outer wedge (around 60 km, 70 and 80 km of distance) and one under the inner wedge (around 40 km of
distance) that correlate with the bathymetric highs interpreted in the seismic images (see the top of the oceanic crust marked by the black dashed line in Figure 6d). These magnetic anomalies support the interpretations that the bathymetric highs are originated in the oceanic crust while the duplexes are not. When comparing these anomalies with that generated by the Zenisu Ridge we can see that their amplitudes are far smaller, even when considering that the bathymetric highs are buried and thus their signal is muted. Furthermore, no large ridge is observed in the seismic data along our profile. An abrupt thickening of the oceanic crust would be expected beneath such a feature, but instead, we observe an undulating Moho that produces only a gentle thickening of the subducting crust in localized areas (e.g., at 55–65 km and 75–85 km of model distance in Figure 2e) that is not associated with changes in plate interface topography. All this suggests that there are no elongated and continuous ridges extending along strike beneath the current accretionary wedge. At present we can observe ~80 km of subducted oceanic plate, which at current convergence rates corresponds to only ~2 Myr of subduction, and therefore we cannot preclude that a large-scale ridge may have been subducted in the past, prior to 2 Ma.

To test whether other large features are being subducted at a location not sampled in our profile, we superimpose the magnetic anomaly data on the seabed bathymetry (Figure 6b). Although there are no large-scale magnetic anomalies along the seismic profile, there are clear anomalies on either side of it (dashed circles in Figure 6b), that correlate with recesses of the frontal thrust (areas where the position of the frontal thrust of the TS is more landward). An anomaly of similar nature can be observed at the mouth of the Tenryu Canyon. We consider these anomalies to represent subducting seamounts, which is consistent with Martin (2003) who imaged such feature to the southwest of our profile. The undulating shape of the Moho along our profile might be related to the proximity of these seamounts, as their footprint is probably greater toward deeper parts of the oceanic crust.

The boundary between the deformed Enshu Basin and the undeformed Kumano Basin appears at first glance to be marked by the Tenryu Canyon (Figure 6a). However, a more careful inspection reveals a series of elongated uplifts further west of the canyon that we interpret as folds that cause a large bathymetric culmination. The edges of these features (yellow dashed line in Figure 6a) align with the edge of the recessed frontal thrust of the TS along the direction of motion of the Philippine Sea Plate (yellow arrow in Figure 6a) suggesting that the observed change in overall structure and width of the wedge has been caused by bathymetric highs carried by the subducting plate.

### 5.3. Nature of Imaged Bathymetric Highs

If there are no seamounts or ridges subducting along the interpreted seismic profile then what is the origin of the imaged bathymetric highs? Their magnetic signature (red arrows in Figure 6c) suggests they represent relief of the oceanic crust, but smaller than seamounts. We interpret them as tilted fault blocks that correspond to either the structural fabric inherited from the spreading center that formed the Shikoku Basin (Figure 1a) or to faults formed at the outer high of the subduction zone. The magnetic anomalies in the Shikoku Basin just south of the TS are approximately north-south (Okino et al., 1999), and are consistent with normal faults of the same orientation. Seismic profiles acquired west of the TS show tilted blocks with a throw of ~1 s two-way traveltime (Kimura et al., 2021), roughly consistent with the scale of the bathymetric highs imaged on our profile. The geometry of the normal faults should allow to distinguish their origin because faults formed in the outer rise should strike more or less parallel to the subduction zone, and almost orthogonal to the Shikoku fabric. However, we only have one profile and therefore cannot constrain the strike of these features. However, the bathymetry and seismic data in the Suruga Trough do not show evidence of young normal faulting, which would be expected if new faults were formed at the outer rise. Furthermore, the presence of the compressional Zenisu Ridge just north of the outer rise may inhibit the development of new faults. Even if there are no new faults being formed, the old ones may have undergone reactivation as they passed through the outer rise. Kimura et al. (2021) showed that the extensional fabric faults were reactivated during their translation toward the trench, prior to them entering the outer rise, however we cannot place constraints on this with our data set.

Deciphering whether the passing of the smaller scale bathymetric highs results in thrust reactivation in the outer wedge is difficult, but the piggy-back basin fill can provide some information. The basins are very gently tilted northward with no apparent folding of the youngest strata. If the basin fill is mostly Pliocene (>2 Ma) in age, then the lack of significant deformation of the youngest strata would suggest that the smaller bathymetric highs imaged do not contribute significantly to fault reactivation, which would lead to piggy-back basin tilting and folding. If the basin fill is Quaternary, as is the case with similar basins located ~100 km to the southwest (Kington, 2012),
then at least a part of the observed deformation in them would be the result of the wedge interacting with the smaller bathymetric highs. Even in the latter case, it is still likely that a greater amount of the thrust reactivation occurs during the subduction of larger features such as seamounts or ridges. The seamounts subducted just north and south of our profile (Figure 6b) also may have contributed to this deformation as they likely have a stress shadow that extends beyond their footprint.

## 5.4. Comparison of Models of Seamount Subduction

Even though there are no seamounts subducting at present in the studied profile, this process has almost certainly occurred in the past. Furthermore, as described above, it is likely that most, if not all, of the thrust reactivation in the wedge occurs during that process. Because of this, the unprecedented detail in which we image the effect of the passing bathymetric highs of several scales on the accretionary wedge allows us to compare it to different scenarios for seamount subduction that have been proposed based on conceptual, numerical and analogue modeling. These include seamount decapitation (Cloos, 1992), flexural accommodation of the accretionary wedge (Scholz & Small, 1997), widespread fracturing and faulting of the accretionary wedge by the seamount (Dominguez et al., 1998; Ruh et al., 2016; K. Wang & Bilek, 2011) and increased slip and steepening of faults within the wedge (Morgan & Bangs, 2017).

Our results show no evidence of seamounts incorporated into the accretionary wedge, as expected in the seamount decapitation model of Cloos (1992), and while there is uplift of the accretionary wedge at different wavelengths, we interpret these effects as permanent due to slip on the faults within the wedge, rather than transient effects that occur just above the seamount as predicted by Scholz and Small (1997). While there is intense deformation of the wedge due to this process, it seems to focus on through-going faults and not on a dense network of small faults that form locally around each passing seamount as proposed by Dominguez et al. (1998) and K. Wang and Bilek (2011). Though we recognize that the structures proposed in these studies may be below the spatial resolution of the data presented here, we suggest that the occurrence of such a pervasive fracturing would likely affect the seismic velocity of the rocks and lead to the overprinting of velocity changes due to the thrusting and damaging of coherent sheets, contrary to what is observed; the wavefield modeling shows that the low-velocity zones that channel the seismic waves are generally localized and correspond to large thrust faults, not diffuse areas within the wedge. Our results, however, are in agreement with the numerical results of Morgan and Bangs (2017) that show a complex deformation pattern in the outer wedge. This is consistent with the reactivation of faults as the seamounts pass under them, though we suggest that this process is not confined to the outer wedge only, and instead occurs throughout the entire wedge as evidenced by the development of duplexes at several scales along the wedge, the warping of the seafloor and the overlying forearc basin, as well as by the out-of-sequence activity of the thrusts in the middle and inner wedge. Models predicting widespread fracturing of the wedge do so because they do not take into account reactivation of pre-existing faults. For example, the numerical models of Ruh et al. (2016) generally predict widespread fracturing, but when they allow for velocity weakening behavior in the wedge, their results are similar to those of Morgan and Bangs (2017) forming discrete shear zones throughout the wedge that are later reactivated.

## 5.5. Warping of the Megathrust or Decollement

Beneath the outer wedge the megathrust is warped over the observed bathymetric highs in the downgoing plate, in a manner that is virtually identical to that reported by Barnes et al. (2010) and Bell et al. (2010) in the Hikurangi subduction zone—although in those studies the seismic data is presented in the time domain and thus the effect of velocity pull-ups exaggerates both the seamount shape and the decollement warping. Barker et al. (2018) carried out a pre-stack depth migration of similar data along the Hikurangi subduction zone margin, removing the pull-up effect, and still observed the decollement deforming over the seamount, as we do in the NT. The effect of these gentle undulations of the decollement on the detailed structure of the rocks adjacent to it is not well imaged here, but detailed studies of decollement geometry elsewhere (Edwards et al., 2018; Kirkpatrick et al., 2020) have demonstrated how roughness of a smaller scale (100's of m) lead to complex deformation patterns around the decollement. Kirkpatrick et al. (2020) shows how normal faults in the oceanic crust can cut the megathrust leading to fault roughness, but here we show that even where normal faults are not directly cutting the megathrust, they still can cause warping of the decollement due to basement topography across them. Given the prevalence of megathrust roughness at different scales in both accretionary and erosional wedges, we suggest that this should be
considered alongside other important factors such as changes in frictional properties when modeling shallow
megathrust earthquake ruptures.

**5.6. Subduction Channel and Shallow Tremor**

The subduction channel, as originally proposed, postulates that a significant portion of subduction-driven deformation is taken up by shearing of the poorly consolidated sediments underthrusted beneath the accretionary wedge (Cloos & Shreve, 1988). The observation of the sediments thinning and thickening underneath the accretionary wedge is consistent with them behaving as a subduction channel. The scale of the variations in shear zone thickness are also consistent with those observed along strike in other parts of the NT (Fahrudin & Tsuji, 2022).

In the megathrust underlying the Kumano Basin, shallow tremor is thought to be caused by fluid cycling and contrasting rheologies within the block-in-matrix deformed subduction channel (see yellow stars in Figure 1a; Ranero et al. (2008), Vannucchi et al. (2022)). The eastern limit of this shallow tremor coincides with the boundary of the TS at the transition from the Kumano to the Enshu basins (Figure 6a), suggesting that there is a fundamental difference in the shallow megathrust there. We propose that the difference in shallow seismogenic behavior within the megathrust subduction channel is due to the extent of fault reactivation within the accretionary wedge in the TS. These structures may provide fluid conduits that affect pore-pressure cycling in the megathrust. This would be consistent with reported observations of cold seeps around large faults throughout the TS (Ashi et al., 2002), interpreted as forming due to the faults serving as high permeability avenues for fluid flow.

N. L. Bangs et al. (2004) used 3D MCS data of the central NT to show a decrease in amplitudes along the megathrust in the landward direction starting ~25 km from the trench, interpreting this as due to the depletion of excess fluid sources. Similarly, Ribodetti et al. (2011) analyzed the seismic response of the megathrust along the subduction channel offshore Ecuador as a function of fluid pressure and porosity and proposed that the lack of a strong seismic response is an indicator for lack of over-pressured fluids. This might be the case for our results, which show a low-amplitude, but coherent decollement interface in the K-PSDM and pseudo-reflectivity profiles extending approximately 20 km from the frontal thrust FT, to the location of BH3 (Figures 4d and 4g). Further landward from BH3, the decollement interface becomes less coherent (suggesting problems with precise migration of the data) but the amplitude of the reflections is higher (Figure 4g), which together with low-velocity zone in this area (Figure 4g) suggests a change in properties of the subduction channel there.

**5.7. Growth of Frontal Thrusts**

In our profile, we image a fault just south of the active frontal thrust FT that propagated up from the decollement and was abandoned having gone through only part of the strata of the Suruga Trough (see AT in Figure 4a). We also interpret two unconformities in this area marked by green lines in Figures 4a–4c. The deeper unconformity is evidenced by the onlap of strata onto AT marked by red arrows. This unconformity marks the top of the stratigraphic section at the moment the fault AT became active that was being folded as the fault grew in a bottom-up manner. We compare this observation to the numerical models of frontal thrust growth from Weiss et al. (2018) that suggest that top-down or bottom-up growth of the frontal thrust is a function of the pore pressure along the decollement. They find that their models result in bottom-up growth of faults when the pore pressure along the decollement reaches ~0.8 times the total pressure, suggesting that this might be the case in the trench area of TS. Though this is only one example, it provides geological observations that can place constraints on basal pore pressure, a parameter that is critical for assessing earthquake rupture dynamics. This also suggests that the current low-pressure scenario for the megathrust may be a transient condition.

The second unconformity in the shallowest part of the section (upper green line in Figure 4a) formed when the frontal-most thrust (FT' in Figures 4a–4c) stepped south ~10 km abruptly after the deposition of the conformable strata between the two unconformities. The location of the step forward of the thrust FT' coincides with the location of a bathymetric high in the oceanic crust and may have been controlled by this feature. The movement on FT' that created the anticlinal culmination above it, was short lived as indicated by the undeformed strata onlapping the second unconformity (horizontal red arrows just below the seabed in Figure 4a). This is in contrast, with the youngest strata deposited in basins B1 and B2 over the outer wedge (Figure 4d), which are gently tilted northward, suggesting that shortening has stepped back and is accommodated in the thrusts within the outer
wedge. Unfortunately the lack of age control for this shallow stratigraphy precludes us from estimating the timing of these changes in frontal fault activation.

## 5.8. Underplating

Our results suggest local accumulation of subducting material at the base of the accretionary prism. Although the seismic signature of these structures is difficult to retrieve from MCS data due to factors such as the use of a short streamer during acquisition, 3D geometrical complexities, and the lack of a strong and continuous impedance contrast, the combination of the MCS data with various velocity attributes led us to interpret them as possible duplex systems on two distinct scales. The first involves the creation of two duplexes: one underneath the outer wedge (DX1) and another at the transition between the outer and middle wedge (DX2, see Figures 4d–4i). Though we cannot clearly image the internal structure of these features, their presence is inferred based on the location of flat-ramp transitions in several thrust sheets that overlay the duplex and suggest the existence of a roof thrust. Associated low-velocity zones may indicate the presence of highly fractured rocks, or accreted sediments that still preserve high-porosity. In both cases, the duplexes have developed completely above the basal decollement. A low-velocity zone in a similar structural position has been observed in the central NT (Park et al., 2010) and has also been interpreted as a duplex. Analogue models of seamount subduction have shown that duplexes may form ahead of the leading edge of a subducting seamount, and eventually be left behind (C. Wang et al., 2021) suggesting that this mechanism may be responsible for the development of these features in the TS.

The second, broader underplating process occurs beneath the inner wedge where thrusts sheets (~2 km thick) are being transferred from the downgoing plate to the accretionary wedge (DX3 in Figures 4m–4o). This large-scale duplexing is similar to that described by Meneghini et al. (2009) for outcrops in the Apennines, where the accreted thrust sheets have been estimated to be ~2 km thick. Similarly, an analogous process may be occurring under Kodiak Island and observed both in outcrop (Sample & Fisher, 1986), and seismic studies (S. Ye et al., 1997). Duplex DX3 is formed by a stepdown of the basal decollement at a depth of ~13 km, which is collocated with the local maxima in the rate of shear stress accumulation related to the downdip edge of the seismogenic zone (Saito & Noda, 2022). The correlation between this large scale underplating and the seismic transition suggest that they are genetically related as appears to be the case for the Main Himalayan Thrust continental megathrust under central Nepal (Hubbard et al., 2016) where a large scale mid-crustal ramp coincides with the downdip edge of the seismogenic zone. Furthermore, numerical models of underplating by Ruh (2020) show the development of features analogous to those observed here - such as broad warping of the surface of the accretionary wedge and the linkage of the roof thrusts of the duplexes with faults overlying them to reach the surface, as we interpret to occur with both the Tokai and Kodaiba thrusts.

## 5.9. Mechanisms of Adjustment of the Critical Taper of the Accretionary Wedge

The adjustment of critical tapers is generally framed in terms of slip of out-of-sequence thrusts to increase the taper, and frontal accretion or hinterland extension to reduce it. Our results show that the middle wedge of the TS has undergone thickening, and thus an increase of the taper, by duplexing and large scale underplating, in addition to continued slip on pre-existing faults not in the frontal position (Figure 7). This transition is evidenced by the change in structural style that occurs above the large thrust sheet overriding DX2 (Figures 4g–4i). The folded faults and thrust sheets there are similar in scale and style to those in the outer wedge (see the dashed blue lines that represent axial planes of some of the better preserved fault bend folds in Figure 4g), as expected, since they would have occupied a proto-outer wedge position at the time of their formation (Figure 7a). The sole thrust of these faults (black dotted line in Figure 7), which at present forms the roof thrust of the underlying large thrust sheet, would have been the basal decollement of the proto-outer wedge. This decollement was subsequently uplifted and folded during the process of duplex formation and wedge thickening, with the active decollement (red dotted line in Figure 7c) stepping down to its current position. This process could have occurred to maintain the taper of the wedge as the current outer wedge was accreted, highlighting the link between underplating and frontal accretion processes. The development of the duplex at the transition between the middle and inner wedge (DX3) was likely part of the same process. This is in contrast to the wedge thickening caused by the development of the megasplay fault in the Tonankai segment of the NT, which is observed to sole into the basal decollement of the wedge, and to have a protracted slip history, forming shortly after the outer wedge sediments that it cuts were accreted (Moore et al., 2007).
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Evolution of the Tokai accretionary wedge between the formation of the middle wedge (MW) and outer wedge (OW). (a) MW (shaded in yellow) is formed at the frontal part of the wedge bounded by the Kodaiba Thrust (KT) and Tokai Thrust (TT); (b) Accretion of the OW (shaded in green) and passage of bathymetric highs lead to the thickening of the wedge through duplex (DX2,DX3) formation at the MW/OW transition and under the inner wedge (IW). The uplift related to DX2 (blue arrow) and out-of-sequence slip on the TT leads to the accumulation sediment in the forearc basin (FB, dark green lines with arrows showing stratal terminations); (c) Continued shortening leads to the leading edge of DX3 to break forward and a thick thrust sheet is emplaced over the roof thrust of DX2, uplifting the forearc strata above it. Black dotted and blue solid lines mark uplifted basal decollement and slope basins respectively. D - decollement; OC - oceanic crust; M - Moho; IS - incoming sediments.</figcaption></figure>

5.10. Evolution of the Forearc Basin

Finally, our results allow us to constrain the evolution of the forearc basin of the TS. In Figure 8a we present two intersecting K-PSDM profiles from Martin (2003). Profile SFJ084 crosses the MITI Nankai Trough well (drilled by the Ministry of International Trade and Industry of Japan in 1999; red star in Figure 6b), and is perpendicular to profile MCS SFJ003 - the profile we present in this study. We use the geological interpretation of Martin (2003) to extend the stratigraphy encountered in the MITI borehole to our profile (Figure 8a). The drill hole reached a major unconformity between forearc basin sediments and Eocene-age accreted sediments at 3 km depth (extending as BU in Figure 8b). According to well-log data, the basin fill overlying that unconformity is formed by three main stratigraphic units separated by hiatus. The basal unit is Early to Early Middle Miocene (older than 15 Ma, be- tween the magenta and cyan unconformities), the middle unit is Pliocene (2-4 Ma; between the cyan and yellow unconformities), and the upper unit is Quaternary (between the yellow unconformity and the seafloor). Note that in Figure 8a the interpreted horizons of Martin (2003) in line SFJ003 are discontinued at ~46 km due to the lack of coherent interfaces within the area of the middle wedge. However, this gap is filled in this study by the pseu- doreflectivity profiles (see e.g. detrended model with our interpretation in Figure 8b).

Martin et al. (2004) estimated that the overall folding of the Enshu Basin occurred ~1 Ma, however, we consider that though this may be the case for the youngest phase of deformation that warps the seafloor, the internal complexities in the Enshu Basin fill points toward a more protracted history of uplift and subsidence. This broad young folding is of a similar spatial extent of both the thrust that cuts the BU around ~40 km and the duplex DX3 (Figures 4j-4l). However, the onlap onto the BU above the thrust indicates that the motion of this fault is pre- Middle Miocene, and the thrust itself does not offset any strata younger than Miocene. This leads us to inter- pret the young broad folding as due to the ongoing underplating at DX3. The younger sequences are locally

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Extrapolation of the MITI well stratigraphy to our profile. (a) K-PSDM profiles and their interpretation from Martin (2003) (Figures 4.5 and 4.7 therein). See Figure 6b for location of the line SFJ084. Profile SFJ003 is the multi-channel seismic profile collocated with the TKY-21 ocean-bottom seismometer line. (b) The same as panel (a) but the SFJ003 profile is exchanged with detrended FWI model and our interpretation.</figcaption></figure>
onlapping onto the cyan and yellow unconformities indicating folding from the pre-Pliocene into the Quaternary. This may be caused by slip on the Kodaiba Thrust (Figures 4g-4i). Even though this fault does not reach the seafloor, the fold above it has tilted all but the most recent sediments, indicating that the structure has remained active throughout the Quaternary. The basin floor fan (BF, green dotted lines in Figures 4j-4l) deposits filling the paleocanyon are Pliocene in age. The incision related to the paleocanyon eroded the Miocene strata above BU almost completely, but Pliocene strata overlap the deposits just below the yellow unconformity.

The downlap terminations observed in the southern end of the forearc basin (cyan and yellow dotted arrows in Figures 4g-4i) likely originated as onlap terminations against a bathymetric high that impounded the sediments of the basin (as is still observed in the Tonankai segment of the NT, Boston et al. (2016)). The initiation of deposition of this strata marks the timing of the bathymetric uplift, probably as a result of the formation of DX2, or slip on the Tokai Thrust. Later the strata of the basin were uplifted relative to the bathymetric high resulting in the downlapping stratal terminations observed presently. This change from onlap to downlap was likely due to the formation of DX3 or more broadly slip on other thrusts within the inner wedge (Figures 7b and 7c). This is consistent with a dynamic wedge that undergoes intermittent phases of out-of-sequence thrusting and duplex formation that folds the overlying forearc strata, creating transient depocenters.

Finally, the age of the Enshu Fault can be inferred from its associated half-graben, as the strata within it is onlapping onto the base of the Quaternary stratigraphic package (see yellow arrows in Figures 4j-4i). This implies that the Enshu Fault has been active throughout the Quaternary - and considering the thinning of this package to the south away from the fault - this half-graben represents the main depocenter (up to 1.5 km thick) during this time. Normal faults in a similar position within an overall compressive wedge have been observed elsewhere (e.g., in Alaska, Bécel et al., 2017), but their origin and significance remains poorly understood.

## 6. Conclusion

We presented detailed structural interpretations of reflectivity and pseudo-reflectivity seismic profiles from the TS of the NT and discussed the evolution of the wedge in this area. The high-frequency inversion of the OBS data produced a velocity model that allowed us to extract pseudo-reflectivity attributes that have a resolution comparable to the K-PSDM profile obtained from the MCS data. This allowed for direct comparison of the structures reconstructed using two independent data sets and imaging approaches. Our interpretation was further supported by gravity, magnetic, and bathymetric data, as well as seismic modeling of wide-angle data.

We showed that different segments of the accretionary wedge of the TS are characterized by distinct structural styles. These segments are separated by large, out-of-sequence thrusts, the Tokai and Kodaiba thrusts. The wedge is characterized by complex structures such as imbricate fans, reactivated thrust faults, as well as unconformities and folding within piggy-back basins recording their protracted deformation. This contrasts with the well-studied Tonankai segment in the central NT characterized by a forward-breaking thrust sequence.

Shortening in the TS wedge is accommodated through a combination of slip of existing faults and development of duplexes at several structural positions at the base of wedge. These have contributed to the thickening of the wedge - possibly in response to frontal accretion of material. The underplating observed in TS is comparable to that proposed to other subduction zones based on geophysical and outcrop studies.

Though there is clear evidence of subducted seamounts around our seismic profile, we do not observe large-scale ridges - that is, Paleo-Zenisu Ridge - beneath our profile at present.

Instead, we observe series of smaller scale ~1 km bathymetric highs that we interpret as tilted fault blocks that are part of the extensional fabric inherited from the Shikoku Basin spreading center. These features cause warping of the decollement of the wedge and deformation of the subduction channel. Even though there are no seamounts at present along our profile, we consider it likely that the subduction of larger features in the past is responsible for the observed structural style.

We compare our observations to the predictions from different proposed models of deformation associated to seamount subduction, frontal thrust nucleation, and forearc basin development. Our interpreted structure is consistent with the models of seamount subduction that predict fault reactivation within the wedge as seamounts, or other bathymetric highs, impinge on it. We suggest that this fault reactivation also controls the shallow seismogenic behavior of TS by controlling fluid cycling along the wedge.
We show that the process of frontal thrust growth is complex with advance and abandonment of frontal thrusts and fault growth consistent with elevated pore pressures on the decollement.

Finally, supported by the data form the MITI well, we show that the forearc basin in the TS has a complex internal stratigraphy that records several episodes of out-of-sequence thrusting in the inner and middle wedge.

## Data Availability Statement

The access to the TKY-21 OBS (JAMSTEC, 2001b; https://www.jamstec.go.jp/obsmcs_db/e/survey/data_year.html?cruise=KY01-06) and TK-2 MCS (JAMSTEC, 2001a; https://www.jamstec.go.jp/obsmcs_db/e/survey/data_year.html?cruise=KR01-08) field data sets presented in this study can be negotiated with JAMSTEC agency and is accessible through their database (http://www.jamstec.go.jp/obsmcs_db/e/index.html).

## Acknowledgments

This study was partially funded by: (a) the Polish National Science Center (Grant 2019/33/B/ST10/01014); (b) the SEISCPE consortium (http://seiscpe2.osug.fr), sponsored by AKERBP, CGG, CHEVRON, EXXON MOBIL, GEOLINKS, JGI, SHELL, SINOPEC, SISPROBE and TOTAL. The study was granted access to the HPC PL-Grid Infrastructure (grant id: plg3dwind4). The FWI velocity model was derived using visco-acoustic full-waveform modeling and inversion solver TOYxDAC TIME developed within the framework of SEISCPE consortium. We would like to thank Editor Claudio Faccenna, reviewer Mladen Nedimović, and five anonymous reviewers for their constructive comments, which helped us improve this article.

## References

Almeida, R., Lindsey, E. O., Bradley, K., Hubbard, J., Mallick, R., & Hill, E. M. (2018). Can the updip limit of frictional locking on megathrusts be detected geodynamically? Quantifying the effect of stress shadows on near-trench coupling. *Geophysical Research Letters*, *45*(10), 4754–4763.
https://doi.org/10.1029/2018gl077785

Ando, M. (1975). Possibility of a major earthquake in the Tokai district, Japan and its pre-estimated seismotectonic effects. *Tectonophysics*, *27*(1-2), 69–85.
https://doi.org/10.1016/0040-1951(75)90011-6

Ashi, J., Tokuyama, H., & Taira, A. (2002). Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough. *Marine Geology*, *187*(1-2), 177–191.
https://doi.org/10.1016/s0025-3227(02)00265-7

Bangs, N., Moore, G., Gulick, S., Pangborn, E., Tobin, H., Kuramoto, S., & Taira, A. (2009). Broad, weak regions of the Nankai megathrust and implications for shallow coseismic slip. *Earth and Planetary Science Letters*, *284*(1-2), 44–49.
https://doi.org/10.1016/j.epsl.2009.04.026

Bangs, N. L., Shipley, T. H., Gulick, S. P., Moore, G. F., Kuromoto, S., & Nakamura, Y. (2004). Evolution of the Nankai Trough decollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging. *Geology*, *32*(4), 273.
https://doi.org/10.1130/g20211.2

Barker, D. H. N., Henrys, S., Tontini, F. C., Barnes, P. M., Bassett, D., Todd, E., & Wallace, L. (2018). Geophysical constraints on the relationship between seamount subduction, slow slip, and tremor at the north Hikurangi subduction zone, New Zealand. *Geophysical Research Letters*, *45*(23).
https://doi.org/10.1029/2018gl080259

Barnes, P. M., Lamarche, G., Bialas, J., Henrys, S., Pecher, I., Netzeband, G. L., et al. (2010). Tectonic and geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand. *Marine Geology*, *272*(1-4), 26–48.
https://doi.org/10.1016/j.margeo.2009.03.012

Bassett, D., Arnulf, A., Kodaira, S., Nakanishi, A., Harding, A., & Moore, G. (2022). Crustal structure of the Nankai subduction zone revealed by two decades of onshore-offshore and ocean-bottom seismic data: Implications for the dimensions and slip behavior of the seismogenic zone. *Journal of Geophysical Research: Solid Earth*, *127*(10).
https://doi.org/10.1029/2022jb024992

Bassett, D., & Watts, A. B. (2015). Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief. *Geochemistry, Geophysics, Geosystems*, *16*(5), 1508–1540.
https://doi.org/10.1002/2014gc005684

Bécel, A., Shillington, D. J., Delescluse, M., Nedimović, M. R., Abers, G. A., Saffer, D. M., et al. (2017). Tsunamigenic structures in a creeping section of the Alaska subduction zone. *Nature Geoscience*, *10*(8), 609–613.
https://doi.org/10.1038/ngeo2990

Bell, R., Sutherland, R., Barker, D. H. N., Henrys, S., Bannister, S., Wallace, L., & Beavan, J. (2010). Seismic reflection character of the Hikurangi subduction interface, New Zealand, in the region of repeated Gisborne slow slip events. *Geophysical Journal International*, *180*(1), 34–48.
https://doi.org/10.1111/j.1365-246x.2009.04401.x

Blakely, R. J. (1995). *Potential theory in gravity and magnetic applications*. Cambridge University Press.
https://doi.org/10.1017/cbo9780511549816

Boston, B., Moore, G. F., Jurado, M. J., & Sone, H. (2016). Deformation of the Nankai Trough inner accretionary prism: The role of inherited structures. *Geochemistry, Geophysics, Geosystems*, *17*(2), 485–500.
https://doi.org/10.1002/2015gc006185

Brossier, R., Operto, S., & Virieux, J. (2009). Seismic imaging of complex onshore structures by 2D elastic frequency-domain full-waveform inversion. *Geophysics*, *74*(6), WCC105–WCC118.
https://doi.org/10.1190/1.3215771

Christeson, G. L., Goff, J. A., & Reece, R. S. (2019). Synthesis of oceanic crustal structure from two-dimensional seismic profiles. *Reviews of Geophysics*, *57*(2), 504–529.
https://doi.org/10.1029/2019rg000641

Cloos, M. (1992). Thrust-type subduction-zone earthquakes and seamount asperities: A physical model for seismic rupture. *Geology*, *20*(7), 601.
https://doi.org/10.1130/0091-7613(1992)020(0601:tttsza)2.3:co;2

Cloos, M., & Shreve, R. L. (1988). Subduction-channel model of prism accretion, melange formation, sediment subduction, and subduction erosion at convergent plate margins: 1. Background and description. *Pure and Applied Geophysics PAGEOPH*, *128*(3–4), 455–500.
https://doi.org/10.1007/bf00874548

DeMets, C., Gordon, R. G., & Argus, D. F. (2010). Geologically current plate motions. *Geophysical Journal International*, *181*(1), 1–80.
https://doi.org/10.1111/j.1365-246x.2009.04491.x

Dessa, J. X., Operto, S., Kodaira, S., Nakanishi, A., Pascal, G., Uhira, K., & Kaneda, Y. (2004). Deep seismic imaging of the eastern Nankai Trough (Japan) from multifold ocean bottom seismometer data by combined traveltime tomography and prestack depth migration. *Journal of Geophysical Research*, *109*(B02111).
https://doi.org/10.1029/2003JB002689

Dominguez, S., Lallemand, S., Malavieille, J., & von Huene, R. (1998). Upper plate deformation associated with seamount subduction. *Tectonophysics*, *293*(3-4), 207–224.
https://doi.org/10.1016/s0040-1951(98)00086-9

Edwards, J. H., Kluesner, J. W., Silver, E. A., Brodsky, E. E., Brothers, D. S., Bangs, N. L., et al. (2018). Corrugated megathrust revealed offshore from Costa Rica. *Nature Geoscience*, *11*(3), 197–202.
https://doi.org/10.1038/s41561-018-0061-4

Fahrudin, C. C., & Tsuji, T. (2022). Influence of shear zone thickness and strike-slip faulting on tectonic tremor in the Nankai Trough, southwest Japan. *Tectonophysics*, *838*, 229519.
https://doi.org/10.1016/j.tecto.2022.229519

Fichtner, A., & Trampert, J. (2011). Resolution analysis in full waveform inversion. *Geophysical Journal International*, *187*(3), 1604–1624.
https://doi.org/10.1111/j.1365-246x.2011.05218.x
Fowler, C. (2005). *The solid earth: An introduction to global geophysics*. Cambridge University Press.

Gonzalez, R., Woods, R., & Eddins, S. (2020). *Digital image processing using MATLAB*. Gatesmark Publishing.

Górszczyk, A., Brossier, R., & Métivier, L. (2021). Graph-space optimal transport concept for time-domain full-waveform inversion of ocean-bottom seismometer data: Nankai trough velocity structure reconstructed from a 1D model. *Journal of Geophysical Research: Solid Earth*, 126(5), e2020JB021504. https://doi.org/10.1029/2020JB021504

Górszczyk, A., Operto, S., & Malinowski, M. (2017). Toward a robust workflow for deep crustal imaging by FWI of OBS data: The eastern Nankai Trough revisited. *Journal of Geophysical Research: Solid Earth*, 122(6), 4601–4630. https://doi.org/10.1002/2016jb013891

Gorszczyk, A., Operto, S., Schenini, L., & Yamada, Y. (2019). Crustal-scale depth imaging via joint FWI of OBS data and PSDM of MCS data: A case study from the eastern Nankai Trough. *Solid Earth*, 10(3), 765–784. https://doi.org/10.5194/se-10-765-2019

Gretemeyer, I., Ranero, C. R., & Ivandic, M. (2018). Structure of oceanic crust and serpentinization at subduction trenches. *Geosphere*, 14(2), 395–418. https://doi.org/10.1130/ges01537.1

Hayman, N. W., Burmeister, K. C., Kawamura, K., Anma, R., & Yamada, Y. (2011). Submarine outcrop evidence for transpressional deformation within the Nankai accretionary prism, Tenryu Canyon, Japan. In *Accretionary prisms and convergent margin tectonics in the northwest pacific basin* (pp. 197–214). Springer. https://doi.org/10.1007/978-90-481-8885-7_9

Henry, P., Operto, S., Lallemand, S., Dessa, J.-X., Martin, V., Noble, M., et al. (2004). Deformation processes and earthquakes in Nankai. In *2004 geohazards workshop abstract book*.

Hsu, S.-K., Yeh, Y.-C., Sibuet, J.-C., Doo, W.-B., & Tsai, C.-H. (2013). A mega-splay fault system and tsunami hazard in the southern Ryukyu subduction zone. *Earth and Planetary Science Letters*, 362, 99–107. https://doi.org/10.1016/j.epsl.2012.11.053

Hubbard, J., Almeida, R., Foster, A., Sapkota, S. N., Bürgi, P., & Tapponnier, P. (2016). Structural segmentation controlled the 2015 mw7.8 gorkha earthquake rupture in Nepal. *Geology*, 44(8), 639–642. https://doi.org/10.1130/g38077.1

JAMSTEC. (2001a). R/V KAIREI KR01-08 Cruise Data. JAMSTEC. https://doi.org/10.17596/0000964

JAMSTEC. (2001b). R/V KAIYO KY01-06 Leg1 Cruise Data. JAMSTEC. https://doi.org/10.17596/0000155

Kimura, G., Nakamura, Y., Shiraishi, K., Fujie, G., Kodaira, S., Yamaguchi, A., et al. (2021). Rejuvenated extension of the Philippine Sea plate and its effect on subduction dynamics in the Nankai Trough. *Island Arc*, 30(1). https://doi.org/10.1111/iar.12402

Kington, J. (2012). *The structure and kinematics of the Nankai Trough accretionary prism, Japan* (PhD Dissertation). University of Wisconsin-Madison.

Kirkpatrick, J. D., Edwards, J. H., Verdecchia, A., Kluesner, J. W., Harrington, R. M., & Silver, E. A. (2020). Subduction megathrust heterogeneity characterized from 3D seismic data. *Nature Geoscience*, 13(5), 369–374. https://doi.org/10.1038/s41561-020-0562-9

Kodaira, S., Lidaka, T., Kato, A., Park, J.-O., Iwasaki, T., & Kaneda, Y. (2004). High pore fluid pressure may cause silent slip in the Nankai Trough. *Science*, 304(5675), 1295–1298. https://doi.org/10.1126/science.1096535

Kodaira, S., Nakanishi, A., Park, J. O., Ito, A., Tsuru, T., & Kaneda, Y. (2003). Cyclic ridge subduction at an inter-plate locked zone off Central Japan. *Geophysical Research Letters*, 30(6), 1339. https://doi.org/10.1029/2002gl016595

Kuehn, H., Nedimovic, M. R., Shillington, D. J., & Becel, A. (2024). *Variations in reflection signature and slip behavior of the subduction interface offshore Alaska Peninsula from 152° i to 161° iW*. Wiley. https://doi.org/10.1002/9781394195947.ch15

Lallemand, S., Chamot-Rooke, N., & Aubouin, J. (1986). Sur la cause du décrochement senestre entre les fosses du Japon et des Kouriles: Subduction-collision d'un ancien volcan sous-marin. *Comptes Rendus de L'Académie des Sciences, Paris, Série II*, 303(16), 1443–1448. Retrieved from https://hal.archives-ouvertes.fr/hal-03272803

Lambaré, G. (2008). Stereotomography. *Geophysics*, 73(5), VE25–VE34. https://doi.org/10.1190/1.2952039

Lay, T., Kanamori, H., Ammon, C. J., Koper, K. D., Hutko, A. R., Ye, L., et al. (2012). Depth-varying rupture properties of subduction zone megathrust faults. *Journal of Geophysical Research*, 117(B4). https://doi.org/10.1029/2011jb009133

Le Pichon, X., Lallemand, S., Fournier, M., Cadet, J., & Kobayashi, K. (1994). Shear partitioning in the eastern Nankai Trough: Evidence from submersible dives. *Earth and Planetary Science Letters*, 128(3–4), 77–83. https://doi.org/10.1016/0012-821x(94)90136-8

Le Pichon, X., Lallemand, S., Tokuyama, H., Thoué, F., Huchon, P., & Henry, P. (1996). Structure and evolution of the backstop in the eastern Nankai Trough area (Japan): Implications for the soon-to-come Tokai earthquake. *Island Arc*, 5, 440–454.

Li, Y.-G., & Leary, P. C. (1990). Fault zone trapped seismic waves. *Bulletin of the Seismological Society of America*, 80(5), 1245–1271. https://doi.org/10.1785/bssa0800051245

Martin, V. (2003). *Structure et tectonique du prisme d'accrétion de Nankai dans la zone de Tokai par imagerie sismique en trois dimensions* (Unpublished doctoral dissertation). Université de Paris Sud.

Martin, V., Henry, P., Nouzé, H., Noble, M., Ashi, J., & Pascal, G. (2004). Erosion and sedimentation as processes controlling the BSR-derived heat flow on the Eastern Nankai margin. *Earth and Planetary Science Letters*, 222(1), 131–144. https://doi.org/10.1016/j.epsl.2004.02.020

Mazzotti, S., Lallemand, S., Henry, P., Pichon, X. L., Tokuyama, H., & Takahashi, N. (2002). Intraplate shortening and underthrusting of a large basement ridge in the eastern Nankai subduction zone. *Marine Geology*, 187(1–2), 63–88. https://doi.org/10.1016/s0025-3227(02)00245-1

Meneghini, F., Marroni, M., Moore, J., Pandolfi, L., & Rowe, C. (2009). The processes of underthrusting and underplating in the geologic record: Structural diversity between the Franciscan complex (California), the Kodiak complex (Alaska) and the internal Ligurian units (Italy). *Geological Journal*, 44(2), 126–152. https://doi.org/10.1002/gj.1144

Mogi, A., & Nishizawa, K. (1980). Breakdown of a seamount on the slope of the Japan trench. *Proceedings of the Japan Academy, Series B*, 56(5), 257–259. https://doi.org/10.2183/pjab.56.257

Mogi, K. (2004). Two grave issues concerning the expected Tokai Earthquake. *Earth Planets and Space*, 56(8), li–lxvi. https://doi.org/10.1186/bf03353074

Moore, G. F., Bangs, N. L., Taira, A., Kuramoto, S., Pangborn, E., & Tobin, H. J. (2007). Three-dimensional splay fault geometry and implications for tsunami generation. *Science*, 318(5853), 1128–1131. https://doi.org/10.1126/science.1147195

Morgan, J. K., & Bangs, N. L. (2017). Recognizing seamount-forearc collisions at accretionary margins: Insights from discrete numerical simulations. *Geology*, 45(7), 635–638. https://doi.org/10.1130/g38923.1

Nakanishi, A., Shiobara, H., Hino, R., Kodaira, S., Kanazawa, T., & Shinamura, H. (1998). Detailed subduction structure across the eastern Nankai Trough obtained from ocean bottom seismograph profiles. *Journal of Geophysical Research*, 103(B11), 27151–27168. https://doi.org/10.1029/98jb02344

Nedimović, M. R., Hyndman, R. D., Ramachandran, K., & Spence, G. D. (2003). Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface. *Nature*, 424(6947), 416–420. https://doi.org/10.1038/nature01840

Nedimović, M. R., Mazzotti, S., & Hyndman, R. D. (2003). Three-dimensional structure from feathered two-dimensional marine seismic reflection data: The eastern Nankai Trough. *Journal of Geophysical Research*, 108(B10). https://doi.org/10.1029/2002jb001959

Okino, K., Ohara, Y., Kasuga, S., & Kato, Y. (1999). The Philippine Sea: New survey results reveal the structure and the history of the marginal basins. *Geophysical Research Letters*, 26(15), 2287–2290. https://doi.org/10.1029/1999g1900537
Olsen, K. M., Bangs, N. L., Tréhu, A. M., Han, S., Arnulf, A., & Contreras-Reyes, E. (2020). Thick, strong sediment subduction along south- central Chile and its role in great earthquakes. *Earth and Planetary Science Letters*, **538**, 116195. https://doi.org/10.1016/j.epsl.2020.116195

Park, J. O., Fujie, G., Wijerathne, L., Hori, T., Kodaira, S., Fukao, Y., et al. (2010). A low-velocity zone with weak reflectivity along the Nankai subduction zone. *Geology*, **38**(3), 283–286. https://doi.org/10.1130/g30205.1

Park, J. O., Moore, G. F., Tsuru, T., Kodaira, S., & Kaneda, Y. (2004). A subducted oceanic ridge influencing the Nankai megathrust earthquake rupture. *Earth and Planetary Science Letters*, **217**(1–2), 77–84. https://doi.org/10.1016/s0012-821x(03)00553-3

Park, J.-O., Tsuru, T., Kodaira, S., Cummins, P. R., & Kaneda, Y. (2002). Splay fault branching along the Nankai subduction zone. *Science*, **297**(5584), 1157–1160. https://doi.org/10.1126/science.1074111

Ranero, C. R., Grevemeyer, I., Sahling, H., Barckhausen, U., Hensen, C., Wallmann, K., et al. (2008). Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis. *Geochemistry, Geophysics, Geosystems*, **9**(3), 10.1029/2007gc001679

Ribodetti, A., Operto, S., Agudelo, W., Collot, J.-Y., & Virieux, J. (2011). Joint ray+born least-squares migration and simulated annealing optimization for high-resolution target-oriented quantitative seismic imaging. *Geophysics*, **76**(2), R23–R42. https://doi.org/10.1190/1.3554330

Rijsingen, E., Funiciello, F., Corbi, F., & Lallemand, S. (2019). Rough subducting seafloor reduces interseismic coupling and mega-earthquake occurrence: Insights from analogue models. *Geophysical Research Letters*, **46**(6), 3124–3132. https://doi.org/10.1029/2018gl081272

Ruh, J. B. (2020). Numerical modeling of tectonic underplating in accretionary wedge systems. *Geosphere*, **16**(6), 1385–1407. https://doi.org/10.1130/ges02273.1

Ruh, J. B., Sallarès, V., Ranero, C. R., & Gerya, T. (2016). Crustal deformation dynamics and stress evolution during seamount subduction: High-resolution 3-D numerical modeling. *Journal of Geophysical Research: Solid Earth*, **121**(9), 6880–6902. https://doi.org/10.1002/2016jb013250

Saito, T., & Noda, A. (2022). Mechanically coupled areas on the plate interface in the Nankai Trough, Japan and a possible seismic and aseismic rupture scenario for megathrust earthquakes. *Journal of Geophysical Research: Solid Earth*, **127**(8). https://doi.org/10.1029/2022jb023992

Saller, A. H., Noah, J. T., Ruzuar, A. P., & Schneider, R. (2004). Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems. *AAPG Bulletin*, **88**(1), 21–46. https://doi.org/10.1306/09030303003

Sample, J. C., & Fisher, D. M. (1986). Duplex accretion and underplating in an ancient accretionary complex, Kodiak Islands, Alaska. *Geology*, **14**(2), 160. https://doi.org/10.1130/0091-7613(1986)14(160:daauia)2.0.co;2

Sandwell, D. T., & Smith, W. H. F. (2009). Global marine gravity from retracked Geosat and ERS-1 altimetry: Ridge segmentation versus spreading rate. *Journal of Geophysical Research*, **114**(B1). https://doi.org/10.1029/2008jb006008

Scholz, C. H., & Small, C. (1997). The effect of seamount subduction on seismic coupling. *Geology*, **25**(6), 487. https://doi.org/10.1130/0091-7613(1997)025(0487:teosso)2.3.co;2

Shin, C., & Cha, Y. H. (2008). Waveform inversion in the Laplace domain. *Geophysical Journal International*, **173**(3), 922–931. https://doi.org/10.1111/j.1365-246x.2008.03768.x

Sun, T., Saffer, D., & Ellis, S. (2020). Mechanical and hydrological effects of seamount subduction on megathrust stress and slip. *Nature Geoscience*, **13**(3), 249–255. https://doi.org/10.1038/s41561-020-0542-0

Takahashi, N., Amano, H., Hirata, K., Kinoshita, H., Lallemand, S., Tokuyama, H., et al. (2002). Faults configuration around the eastern Nankai Trough deduced by multichannel seismic profiling. *Marine Geology*, **187**(1–2), 31–46. https://doi.org/10.1016/s0025-3227(02)00243-8

Tavakoli, F. B., Operto, S., Ribodetti, A., & Virieux, J. (2017). Slope tomography based on Eikonal solvers and the adjoint-state method. *Geophysical Journal International*, **209**(3), 1629–1647. https://doi.org/10.1093/gji/ggx111

Thomas, W. A. (2018). Tectonic inheritance at multiple scales during more than two complete Wilson cycles recorded in eastern north America. *Geological Society, London, Special Publications*, **470**(1), 337–352. https://doi.org/10.1144/sp470.4

Tromp, J. (2019). Seismic wavefield imaging of earth's interior across scales. *Nature Reviews Earth & Environment*, **1**, 1–14. https://doi.org/10.1038/s43017-019-0003-8

Tsuji, T., Ashi, J., Strasser, M., & Kimura, G. (2015). Identification of the static backstop and its influence on the evolution of the accretionary prism in the Nankai Trough. *Earth and Planetary Science Letters*, **431**, 15–25. https://doi.org/10.1016/j.epsl.2015.09.011

Vannucchi, P., Clarke, A., de Montserrat, A., Ougier-Simonin, A., Aldega, L., & Morgan, J. P. (2022). A strength inversion origin for non-volcanic tremor. *Nature Communications*, **13**(1), 2311. https://doi.org/10.1038/s41467-022-29944-8

Virieux, J., & Operto, S. (2009). An overview of full waveform inversion in exploration geophysics. *Geophysics*, **74**(6), WCC1–WCC26. https://doi.org/10.1190/1.3238367

Von Huene, R., & Lallemand, S. (1990). Tectonic erosion along the Japan and Peru convergent margins. *Geological Society of America Bulletin*, **102**(6), 704–720. https://doi.org/10.1130/0016-7606(1990)102(0704:teatja)2.3.co;2

Wang, C., Ding, W., Schellart, W. P., Li, J., Dong, C., Fang, Y., et al. (2021). Effects of multi-seamount subduction on accretionary wedge deformation: Insights from analogue modelling. *Journal of Geodynamics*, **14**, 101842. https://doi.org/10.1016/j.jog.2021.101842

Wang, K., & Bilek, S. L. (2011). Do subducting seamounts generate or stop large earthquakes? *Geology*, **39**(9), 819–822. https://doi.org/10.1130/g31856.1

Weiss, J. R., Ito, G., Brooks, B. A., Olive, J.-A., Moore, G. F., & Foster, J. H. (2018). Formation of the frontal thrust zone of accretionary wedges. *Earth and Planetary Science Letters*, **495**, 87–100. https://doi.org/10.1016/j.epsl.2018.05.010

Yamashita, M., Miura, S., Moore, G. F., Nakanishi, A., Kodaira, S., & Kaneda, Y. (2017). Bathymetric imaging of protothrust zone along the Nankai Trough. *Island Arc*, **27**(2), e12233. https://doi.org/10.1111/iar.12233

Ye, L., Kanamori, H., & Lay, T. (2018). Global variations of large megathrust earthquake rupture characteristics. *Science Advances*, **4**(3), eaao4915. https://doi.org/10.1126/sciadv.aao4915

Ye, S., Flueh, E. R., Klaeschen, D., & Huene, R. (1997). Crustal structure along the EDGE transect beneath the Kodiak shelf off Alaska derived from OBH seismic refraction data. *Geophysical Journal International*, **130**(2), 283–302. https://doi.org/10.1111/j.1365-246x.1997.tb05648.x