Structural barriers control the spatial extent of slow earthquake slip

Domain: earth_science
Published: 2026-01-03
Editorial outcome: major_revision

Abstract
Recent seismological and geodetic observations have revealed shallow slow earthquakes on subduction megathrusts, offering an opportunity to explore the factors controlling megathrust slip. Identifying these controlling factors helps constrain the potential updip extent of future megathrust ruptures. Here, we investigate the factors controlling the spatial extent of shallow slow earthquakes by integrating tremor locations with uncertainty estimates, detailed bathymetry data, and multichannel seismic data for the 2020–2021 slow earthquake sequence in the Kumano-nada region of the Nankai Trough. Tremor probability maps reveal that the updip, downdip, and lateral limits of tremor activity coincide with distinct structural boundaries. These include the kink or branching of the décollement, a deep-seated strike-slip fault, and the inner–outer wedge boundary. These structures likely act as geometrical or mechanical barriers, further influenced by variations in pore-fluid pressure and by the subducted Paleo–Zenisu ridge. The results indicate that megathrust geometry, material properties of the overriding prism, fluid distribution, and ridge subduction jointly govern the spatial extent of slow earthquakes, emphasizing the need to account for these factors in assessing the potential extent of future megathrust ruptures.

Introduction
Recent seismological and geodetic observations have increasingly detected slow earthquakes located on the shallow portions of subduction zone megathrust faults1–6. Because they occur more frequently than megathrust earthquakes, shallow slow earthquakes offer important insights into megathrust slip behavior7–10. Specifically, delineating the spatial extent of shallow slow earthquakes and interpreting them as indicators of slip tendency along a megathrust may help identify factors that impede the slip propagation of future megathrust events. The exploration of such factors is ongoing. For example, subducted seamounts11 and megathrust curvature12 have been reported to prevent rupture propagation. Understanding whether slip can propagate to the trench is also crucial for assessing tsunami hazards13.

Considerable efforts have been made to elucidate the relationship between slow earthquakes and subsurface structures, primarily to understand the generation mechanisms of slow earthquakes. These studies have identified structural characteristics that may be associated with slow earthquakes, such as fluid-rich sediments14–16, subducted seamounts17–19, smaller-scale topographic irregularities20, and material heterogeneities21. However, these studies have revealed only coarse spatial correlations between the structural factors and the main characteristics of slow earthquake distributions, such as their spatial centroids. Structural causes that delineate the edge of slow earthquake distributions remain elusive.

Slip propagation during slow earthquake episodes, which often span several months, can be traced in detail by analyzing the spatiotemporal evolution of tectonic tremor swarms, each comprising numerous tremors lasting from tens to hundreds of seconds22,23. This approach is based on the working hypothesis that a slow slip event (SSE) generates tectonic tremors as its migration front encounters fine-scale heterogeneities on the fault24,25. This hypothesis is supported by the frequent observation of spatiotemporal synchronization between tremors and the SSE slip front23, although some SSEs have been reported to occur without accompanying tremor activity26. In recent years, such synchronization has also been observed between shallow tremors and SSEs in the Kumano-nada region of southwestern Japan, the focus region of this study27,28.

However, precisely locating tectonic tremors remains challenging because the onset of their seismic signals is often unclear. In addition, for offshore locations, the laterally heterogeneous structures of accretionary prisms can introduce bias in the estimated locations of tectonic tremors29. To tackle these challenges, a recent study30 developed a Bayesian inversion method that uses seismic amplitudes as input data to estimate tremor locations. This approach provides better constraints than conventional time difference data31. This method also accounts for laterally heterogeneous structures by solving for structural parameters that compensate for these variations. Furthermore, the Bayesian framework performs probabilistic sampling of tremor locations, providing formal uncertainty estimates and enabling representation of the source region as a probability map (see Methods).

The previous study30 applied this method to tremors occurring in the Kumano-nada region of the Nankai Trough subduction zone between December 2020 and February 2021 (Fig. 1). The results showed that median location errors across all events are 1.1 km in the east–west direction and 1.2 km in the north–south direction. These errors represent the half-widths of the 95% confidence intervals (Supplementary Fig. 1). Notably, the 2020–2021 tremor episode was the most extensive activity recorded by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET)32,33, a seafloor-cabled network deployed in this region9,34. By comparison, the previous tremor episode in April 2016 exhibited a source region approximately half as wide34. Seismic signals during the 2020–2021 tremor episode were recorded not only by the DONET stations but also by temporarily deployed ocean-bottom seismometers30. The unusually intense activity, together with the enhanced observation network, provided an unprecedented dataset for the present study.

In this study, we combine tremor locations with multichannel seismic (MCS) images acquired along survey lines covering the Kumano-nada region to explore the structural factors that define the spatial extent of slow earthquakes. The location uncertainties of tremors, approximately 1 km, enable meaningful comparisons with MCS images, which typically have spatial resolutions on the order of 100 m. To supplement the two-dimensional nature of the MCS data, we also examine bathymetric features, which closely approximate subsurface structural characteristics35. Together, these datasets show that the distribution of tectonic tremors is correlated with structural boundaries, indicating the structural controls on fault-slip behavior.

Results
Structural features correlated to tremor distribution
The Nankai Trough subduction zone, a typical accretionary system, is characterized by deformation features on the bathymetry, notably fold-and-thrust structures (Fig. 2). These structures are primarily concentrated within an outer wedge located trenchward of the megasplay fault. The outer wedge is divided into two geological zones: the landward, older imbricate thrust zone (ITZ) and the trenchward, younger frontal thrust zone (FTZ)36. These zones are roughly demarcated by a bathymetric depression running parallel to the trench36. In addition, two subparallel right-lateral strike-slip faults are located in the ITZ35,37,38 (two thin black lines with arrows in Fig. 2b). These faults are inferred to be sufficiently deep-rooted to intersect the underlying décollement (i.e., the megathrust)37. The deformation front is also evident in the bathymetry map. In the western region (region A in Fig. 2a), a younger deformation front has been developing36,39, while the older one remains active in other areas.

The 2020–2021 tremor episode probability map indicates that the tremor sources are mostly confined within the ITZ. The downdip limit of tremors correlates with the boundary between the inner and outer wedges. For the central region (region B in Fig. 2a), the updip limit of tremors mostly coincides with the ITZ–FTZ boundary. This close correspondence between tremors and geological zoning implies the presence of a structural factor that exerts primary control on the spatial extent of the tremors. While the absence of tremors is evident in the FTZ of region B, in the western region (region A), the updip limit of the tremors partly crosses the ITZ–FTZ boundary and approaches the old deformation front.

Fold-and-thrust features are poorly expressed in the eastern region (region C), east of the tremor distribution. Instead, the bathymetry is dominated by slump scars35,36,38 commonly attributed to the subduction of the Paleo–Zenisu ridge40. Intriguingly, the transition of bathymetric features from region B, characterized by fold-and-thrust geometry, to region C, lacking such features, aligns with the eastern boundary of the tremor distribution. Furthermore, in region C, tremors occur further landward beneath the inner wedge, highlighting the distinct tectonic setting of region C, compared to regions A and B.

To further explore the correspondence between the tremor distribution and bathymetric features, we analyze and interpret MCS data from seven seismic reflection profiles traversing the tremor zone (Fig. 3 and Supplementary Fig. 2–7). Profiles A1–A2 are located in region A, B1–B3 in region B, and C1–C2 in region C. The most prominent features observed in the resulting seismic images are reflections from the top of the subducting oceanic crust and décollement, which are subparallel within the ITZ. The overriding prism hosts numerous faults, mainly reverse faults, including a few that may be strike-slip faults37.

In regions B and C, the décollement undergoes a sudden change in dip angle and extends upward to the deformation front (profiles B1–B3 and C1–C2 in Fig. 3). Hereafter, we refer to this steepened portion of the décollement as the “steep thrust”. The steep thrust extends westward into region A (profiles A1–A2). However, in this region, the décollement continues to farther seaward beneath the steep thrust and reaches the younger deformation front39. The steep thrust in region A is therefore interpreted as a frontal thrust branching off the décollement.

Comparison between tremor probability and seismic images shows that, in regions A and B, the updip limit of the tremors coincides with the kink and branching points of the décollement associated with the steep thrust (white stars in Fig. 3). This alignment suggests that these kink or branching points may have acted to impede slip propagation during the slow earthquake episode. Accordingly, the reliability of the estimated updip limit of tremors was evaluated through synthetic tests. This test confirmed that the station coverage was adequate for detecting and locating tremors that might have occurred farther seaward of the identified updip limit (Supplementary Fig. 8).

However, in region C, the updip limit of the tremors is distinctly separated from the kink points along survey lines C1 and C2 (Supplementary Fig. 4). This unique characteristic suggests the presence of another structural factor. As discussed later, we attribute this to the subducted Paleo-Zenisu ridge.

The downward continuation of the décollement beneath the inner wedge is difficult to delineate in the seismic reflection profiles because of its weak and relatively incoherent reflections (Fig. 3). Nevertheless, the top of the oceanic crust can still be identified, providing a reasonable proxy for the décollement, given that the separation between the two interfaces remains nearly constant.

The top of the oceanic crust generally dips more steeply beneath the inner wedge than beneath the ITZ. However, the manner in which the dip angle increases with subduction varies among the profiles across the study area. For example, along profile B3, the dip angle increases sharply near the boundary, whereas along profile B2, it remains nearly constant and gradually steepens with subduction. Therefore, the downdip limit of tremors does not correlate with the subduction geometry.

What appears to correlate more strongly with the downdip limit of tremors is the seismic characteristics of the overriding prism. In the ITZ, where tremors occur, the prism exhibits high reflectivity and contains numerous faults. By contrast, within the inner wedge, the prism is characterized by transparent seismic facies, except in the uppermost ~1 km beneath the seafloor, corresponding to the basin-fill sediments of the Kumano Basin.

Geometrical and hydrological barriers controlling the updip limit of slow earthquake slip
Under the hypothesis that the slip front of SSEs generates tremors along the décollement24,25, the spatiotemporal evolution of tectonic tremors effectively approximates the slip propagation of ongoing SSEs along the megathrust23. Within this framework, the findings of the current study indicate that the along-dip propagation of SSEs is impeded at the kink or branching points. In the following, we explore the potential mechanisms for this impediment, and the limitations of this framework will be discussed later.

The geometry of the megathrust can be a primary factor that impedes slip propagation at the kink in region B. Under stress conditions inferred from two-dimensional non-cohesive Coulomb wedge theory41, the steep thrust experiences higher normal stress than the gently dipping section of the décollement (see Methods). In addition, a laboratory friction experiment suggests that the décollement exhibits a higher friction coefficient at this steep thrust than at the landward flat portion42. Consequently, the shear strength, represented by the product of normal stress and friction coefficient, is greater at the FTZ than at the ITZ. The steep thrust acts as a geometrical barrier when the dynamically perturbed shear stress at the slip front fails to exceed this increased shear strength12.

Similar implications have also been reported by a previous investigation43. That study calculated the slip tendency (the ratio of shear stress to normal stress) along the décollement within the relatively narrow area covered by a three-dimensional reflection seismic survey in the Kumano-nada region. By fully accounting for the three-dimensional geometry of the décollement, the study demonstrated that the abrupt increase in the dip at the kink indeed decreased the slip tendency of the megathrust, consistent with our findings.

It remains unclear why slip terminates at the branching point along profiles A1 and A2 in region A, despite the continuation of the décollement further seaward. A possible explanation is that this segment of the décollement undergoes stable sliding. Notably, the reflection pattern in the hanging wall is different across the steep thrust. A stratified pattern observed at the trench side of the steep thrust is absent on the landward side (Fig. 3). The less deformed stratification at the trench side appears to be consistent with the idea of stable sliding. Even if this is not the case, the contrast in the reflection pattern likely reflects differences in physical, material, and/or hydrological properties, which may result in distinct slip behaviors. A more detailed investigation, including core sampling and numerical modeling, would be valuable for understanding the slip behavior near the trench for this region.

Pore fluid pressure also influences effective normal stress and hence slip behaviors44. Although its spatial distribution is not well constrained, the potential role of pore fluid pressure in the study region is worth discussing here. In region B, along survey lines B1–3, a ~1–2 km thick zone of transparent seismic facies is observed immediately above the décollement within the ITZ (Fig. 3 and Supplementary Fig. 3). Independent seismological studies using near-vertical reflection45, wide-angle refraction16, and teleseismic converted waves46 have revealed low seismic velocities for this structure. Because of its high Poisson’s ratio, this low-velocity zone (LVZ) has been interpreted as a zone of high pore fluid pressure46. The fluid is thought to originate mainly from disequilibrium compaction of sediments, and high pore fluid pressure is interpreted to be sustained by a low permeability of the sediment44.

While it is challenging to determine the exact spatial extent of this LVZ from the two-dimensional reflection image alone, its updip limit seems to be near the kink point, and the LVZ is seemingly absent above the steep thrust (Supplementary Fig. 3). Independent studies have similarly shown no supportive evidence for an LVZ above the steep thrust37,45,46. Moreover, due to the high permeability along faults, the steep thrust probably acts as a pathway that leaks fluid from the LVZ to the seafloor. Under this interpretation, the steep thrust likely has lower pore fluid pressure, implying higher shear strength, and help the geometrical barrier terminates slip propagation at the kink point.

In region A, the presence of such an LVZ is not evident in the reflection image from this study. However, a previous study47 identified reflectors with negative polarities slightly above the décollement, which may indicate the top of the LVZ (Supplementary Fig. 2). Further investigation into the properties of the LVZ, including its spatial extent and associated pore fluid pressure, is necessary to better understand its role in fault slip propagation.

Geometrical barrier controlling the along-strike propagation of slow earthquake slip
Another geometrical barrier is found in the along-strike slip propagation. As shown in Fig. 4, the 2020–2021 tremor episode began at the eastern end of the tremor distribution, subsequently migrating southwestward by ~40 km. The migration distance is proportional to the square root of time, as illustrated by the parabolic trajectory in the time–distance plot (Fig. 4c). Such a parabolic, or diffusive, migration pattern can be explained by a heterogeneous fault surface consisting of a ductile background and brittle patches48, where tremors occur at the brittle patches behind the SSE migration front. Notably, this diffusive migration ceased when the migration front encountered a strike-slip fault that intersects the ITZ (Fig. 4a, c). This strike-slip fault, which is inferred to intersect and offset the décollement37, may act as a geometrical barrier, hindering slip propagation. Tremor activity in April 2016 also appears to have terminated when the migration front reached this strike-slip fault47,49 (Supplementary Fig. 9).

For the 2020–2021 episode, however, a cluster of tremors occurred on the opposite side of the strike-slip fault almost simultaneously with the cessation of the initial migration (Fig. 4b). The time-distance plot for these clustered tremors shows a parabolic envelope that represents an isolated diffusive migration from the initial one. The first and second migrations are spatially separated by ~5 km. We interpret that the SSE driving the first migration was impeded at the strike-slip fault, and the resulting remote stress transfer, rather than direct slip propagation, triggered a second SSE ~5 km away. Given that the gap is considerably small compared to the slip area of the first migration, the stress transfer from the first migration appears sufficiently strong to activate the second one.

Mechanical barrier controlling the downdip limit of slow earthquake slip
The downdip limit of the tremor distribution generally corresponds to the boundary between the inner and outer wedges in regions A and B (Fig. 2). Unlike the updip limit, however, the downdip limit does not necessarily appear to correlate with abrupt change in the dip angle. The large-scale tendency for the subduction angle to be greater beneath the inner wedge may exert some geometrical control on slip behavior in the same manner as discussed in the previous section, but the spatial correlation between the downdip limit of tremors and the wedge boundary is much sharper than that between tremors and changes in dip angle, suggesting that the latter is unlikely the primary controlling factor.

Instead, a rigidity contrast within the overriding prism may be responsible for defining the downdip limit. Seismic velocity models suggest that the prism is more rigid in the inner wedge than in the outer wedge50,51. In particular, Fig. 10 of ref. 50 shows that the averaged P-wave velocity within ~3 km above the oceanic crust increases sharply across the boundary between the inner and outer wedges. This sharp velocity increase likely reflects a transition from a less consolidated, faulted prism in the outer wedge to a denser and more lithified prism in the inner wedge, implying a higher rigidity of the overlying material above the downdip limit. This contrast can also be inferred from the seismic reflection profiles, which show transparent and highly reflective facies in the inner and outer wedges, respectively. Such a rigidity contrast at the wedge boundary could act as a mechanical barrier to slip propagation, because a stiffer overlying prism requires greater shear stress to drive slip along the underlying interface.

In addition, multiple lines of evidence, including borehole breakouts52, seismic anisotropy53, and geomorphic signatures35, reveal an abrupt change in the maximum horizontal stress direction across the boundary between the inner and outer wedges: the inner wedge undergoes trench-parallel compression near this boundary, whereas the outer wedge experiences trench-normal compression. The trench-normal compression at the inner wedge misaligns with the slip direction along the megathrust, thereby reducing the resolved shear stress available to drive slip propagation.

It should be noted, however, that the downdip limit for this episode may not necessarily correspond to the boundary between transient-slip and locked zones, as SSEs and tremors occurred further downdip of this boundary during earlier slow earthquake episodes prior to 2020–202154.

Stress shadow induced by ridge subduction limiting slow earthquake propagation
Slump scars that obscure the fold-and-thrust structures in region C indicate the influence of the subducted Paleo–Zenisu ridge on deformation within the overriding prism (Fig. 2). Correspondingly, the reflection image along profile C2 shows relatively well-preserved horizontal sedimentary strata within the prism of the ITZ, on the trailing side of the subducted Paleo–Zenisu ridge (Supplementary Fig. 4). These strata may reflect the effects of a stress shadow caused by ridge subduction. Numerical experiments suggest that seamount or ridge subduction generates a wake of reduced tectonic loading and delayed sediment compaction on its trailing side, forming a stress-shadow zone characterized by lower effective stress and limited deformation18,55. We also find horizontal strata at the FTZ along profile C1, which could be related to the subduction of the smaller, undocumented ridge (Supplementary Fig. 4).

Such stress shadows are thought to affect the generation of slow earthquakes through their influence on hydrological properties, specifically the porosity and permeability, of the prism. However, it remains controversial whether stress shadows either promote18 or inhibit55 slow earthquakes. The spatial correlation identified in this study between the non-tremorgenic zone and the less-deformed prisms suggests that the stress shadow contributed to terminating slip propagation. In a typical accreted prism, progressive tectonic loading reduces porosity, leading to an increase in pore fluid pressure. In contrast, in stress-shadow zones on the trailing side of the ridge, porosity reduction occurs more gradually, leading to a slower buildup of pore fluid pressure55. The resulting relatively low pore-fluid-pressure zone may have prevented the 2020–2021 SSE from propagating into region C.

Limitations and opportunities
The fundamental assumption of this study is that tremors always accompany SSEs. However, SSEs without associated tremors have been reported in several regions26. One possible scenario not captured by this study is that an SSE propagates into the steep thrust toward the deformation front but does not generate tremors there. In such a case, the steep thrust may be characterized by a relatively homogeneous fault surface with fewer brittle patches capable of generating tremors. In line with this scenario, borehole observations during minor SSE episodes other than the 2020–2021 episode have identified slip approximately 5 km landward of the deformation front, possibly along the steep thrust, whereas tremor occurrence there during these episodes remains unclear due to undocumented error estimations28.

In addition, because the present location method provides limited depth resolution, the exact vertical positions of tremor sources cannot be firmly constrained. It is therefore assumed that tremors occur along the décollement. In contrast, centroid moment tensor solutions for VLFEs during the 2020–2021 episode are well constrained: most events have focal depths of 6–8 km below sea level and low-angle (~6°) thrust mechanisms56, suggesting that they occurred along or near the décollement. The overall agreement in the spatiotemporal evolution of tremors and VLFEs (Supplementary Fig. 10) further supports the interpretation that tremors, like VLFEs, occurred along or near the décollement.

Considering the uncertainties in the centroid depths of the VLFEs, which average about 1.3 km56, we cannot rule out the possibility that tremors and VLFEs are generated within a heterogeneous mélange shear zone embedded in the LVZ above the décollement21,47, whereas the SSE may occur along the décollement itself. Even in this scenario, the hypothesis that tremors are generated at the migration front of the SSE remains plausible, as the LVZ would deform in concert with, or be mechanically coupled to, the underlying décollement. The absence of tremors in the FTZ may then reflect the lack of such an LVZ above the steep thrust.

Exploring these possibilities requires further enhanced geodetic and seismic observation networks, as well as more sophisticated analysis methods for better constraining source locations. Such efforts are essential for advancing our understanding of the physical linkages among different modes of slow earthquakes, SSEs, VLFEs, and tremors, and the controlling factors that govern their generation.

Implications for future megathrust earthquakes
Recent megathrust events, such as the 2004 Sumatra-Andaman and 2011 Tohoku earthquakes, have highlighted the significance of coseismic slip extending close to the trench, which produced devastating tsunamis57,58. Understanding the updip limit of megathrust events is therefore important for accurate hazard assessment. In the study region, the latest megathrust event occurred in 1944 (Fig. 1, the Tonankai earthquake), although the inferred updip limit of this event varies among different slip models59–61.

Based on the tremor distribution, three possible scenarios can be proposed for the updip limit of the future megathrust events in the study area. The first scenario is that the megathrust rupture stops at the downdip limit of the tremor zone, that is, the inner and outer wedge boundary. Such spatial complementarity between slow earthquakes and megathrust events has been observed in subduction zones worldwide3,4,9,10, although the underlying mechanism remains debated. Another possibility is that the steep thrust acts as a geometrical barrier, forming the updip limit of the megathrust event. In the most devastating case, the rupture could extend all the way to the trench, as suggested by a study analyzing core samples collected by seafloor drilling62. While multiple numerical simulations have been conducted to assess the hazard of future earthquakes in this region13,63, they have not fully accounted for the mechanical and geometrical barriers identified in this study. Future studies should evaluate the likelihood that these barriers can prevent dynamic rupture during a megathrust event and assess their influence on tsunami generation and height.

A global statistical compilation of megathrust earthquakes also emphasizes the critical role of the geometrical barriers in limiting rupture propagation12. Our study complements this global finding by providing a high-resolution comparison between tremor locations and subsurface structures. Extending the methodologies employed in this study to other regions could advance the understanding of megathrust dynamics. For example, near-trench slip of SSE (or associated tectonic tremors and VLFEs) has been reported for Hyuga-nada6 and Costa Rica3, but the spatial resolution is not sufficient for a detailed comparison to geological structures. This emphasizes the need for enhanced seismic monitoring and surveys that focus on shallow megathrusts beneath the ocean.

Integrated controls on the spatial extent of fault slip
This study revealed the spatial correlation between tremor distribution in the Kumano-nada region and tectonic domains delineated by seafloor topography. Assuming that tremors occur at the migration front of SSEs, this spatial correlation suggests that the subsurface geological structures play a crucial role in controlling the spatial extent of megathrust slip.

The following mechanisms are proposed to impede the propagation of slow earthquake slip, as schematically summarized in Fig. 5:
i. The updip limit of tremors corresponds to the kink of the décollement toward the steep thrust in the central region (region B) and to the branching point where the steep thrust branches off from the décollement in the western region (region A). The steep thrust is considered to experience higher normal stress, acting as a geometrical barrier to slip propagation.
ii. In region A, the décollement continues horizontally seawards beyond the branching point. Less-deformed overriding material may indicate that this section undergoes stable sliding, leading to the absence of tremors and SSEs.
iii. The LVZ observed above the flat portion of the décollement represents a zone of high pore-fluid pressure, which appears to be absent above the steep thrust. This contrast in pore-fluid pressure may further contribute to the slip impediment at the megathrust kink.
iv. A deep-seated strike-slip fault inferred to cut across the décollement may hinder along-strike migration of SSEs.
v. The downdip limit of tremors corresponds to the boundary between the inner and outer wedges, where abrupt changes in material properties and stress orientation within the overriding prism likely define this limit.
vi. A stress shadow caused by the subducted Paleo-Zenisu ridge delays the buildup of pore-fluid pressure, preventing slip from propagating into its trailing side in the eastern region (region C).

These findings indicate that megathrust geometry, the physical properties of the overriding prism, fluid distribution, and ridge subduction jointly govern slip behaviors, emphasizing the importance of considering these factors when assessing the potential extent of future megathrust ruptures.

Methods
Tremor probability map
The Bayesian inversion analysis30 used for locating tectonic tremors provides ensemble solutions sampled from posterior probabilities. For each specific event, we estimate the probability of its epicenter falling within a particular 1×1 km geographical cell using the following formula:

p_i(r_j) = N_j / M

where p_i(r_j) represents the posterior probability of the epicenter of the ith tremor, with respect to the spatial variable r, which denotes the position of the jth cell; N_j represents the count of the ensemble solutions that fall within the jth cell; and M represents the total number of solutions.

We compute the probability of at least one tremor occurring within the corresponding cell by aggregating this probability across multiple events as follows:

p_tremor(r_j) = 1 - \prod_{i=1}^{N_evt} (1 - p_i(r_j))

This formula exhibits two key attributes: reduced sensitivity to the number of tremors and decreased influence from highly uncertain events. Both features are advantageous for delineating the tremor-genic zone compared to conventional visualization using a scatter plot, as shown in Supplementary Fig. 1a. This study refers to this probability as tremor probability for simplicity.

Tremor location reliability test
This study uses the tremor locations determined by a previous study30 that employed a workflow including event prescreening and Bayesian inversion. The prescreening stage estimated the apparent S-wave velocities and attenuation strengths of seismic waves emitted by a tremor, assuming the tremor source was located beneath a station that recorded the highest signal amplitude within the seismic network. The resulting S-wave velocity and attenuation strength were used as quality control factors to retain events with good-quality data. However, this assumption may have increased the spatial variability of tremor detectability and potentially biased the spatial extent of the tremor distribution. To investigate this, we conducted the following synthetic test.

In the synthetic test, virtual sources were evenly distributed 7 km beneath sea level at 5 km horizontal intervals. Synthetic arrival time and amplitude were calculated for every source–receiver path. To simulate real conditions, random noise was introduced to the synthetic data: the arrival times were perturbed by random noise sampled from a zero-mean Gaussian distribution with a standard deviation of 4 s, and the amplitudes were multiplied by a random factor drawn from a Gaussian distribution with a mean of 1 and a standard deviation of ~0.1. These noise levels were chosen empirically based on the observed data.

We applied the same prescreening process used in the previous study30 to the synthetic events. The results showed that the Bayesian inversion process tends to reject events at some locations outside the station coverage, as indicated by open circles in Supplementary Fig. 8. Nevertheless, these rejected events were located far from the updip limit of the tremors identified in the current study. Therefore, we concluded that the updip limit of the tremors is robust. The synthetic test results also demonstrate that any tremor can be located with reasonable uncertainty (i.e., the 95% confidence interval < 10 km) if it occurs outside the coverage area of region C.

Multichannel seismic reflection profile
We use MCS data acquired from the KR02-11 cruise conducted in 2002 by the R/V Kairei of Japan Agency for Marine-Earth Science and Technology. The data were collected using a ~5 km long hydrophone streamer cable with 25-m spaced receivers. An air-gun array with a total volume of 196.6 L (12,000 in3) was fired every 50 m along the survey lines. The towing depths of the streamer cable and the air-gun array were 20 and 10 m, respectively.

The reflection data were reprocessed using the latest standard processing techniques by DownUnder GeoSolutions (DUG) in Kuala Lumpur, Malaysia. Similar to preceding studies64,65, a preprocessing sequence was applied to the data to reduce the noise and multiple reflections, including a tidal static correction, swell-noise attenuation, de-ghosting, de-signature, surface-related multiples suppression, and parabolic Radon transform filtering. Velocity model building using reflection tomography and pre-stack Kirchhoff depth migration was performed to produce the final seismic reflection profiles.

In this study, we investigate seven lines shown in Fig. 3 and Supplementary Figs. 2–7. We track the key horizons of the top of the oceanic crust and the décollement as determined by previous studies20,47,66 and interpret faults in the accretionary prism based on displacement or discontinuity of the seismic reflectors.

Bathymetry data
To effectively extract structural information from the bathymetry of the study region, we utilized a publicly available de-trended bathymetric digital elevation model (DEM) specific to our study area. This de-trended DEM is structured on a 52 m gridded space and is generated by the filling of artificial sinks present in the original DEM and the subsequent application of a 15 km high-pass filter. Further details can be found in Schottenfels and Regalla’s research article35.

Increase in the normal stress across the kink
We use a simplified two-dimensional model to illustrate how a geometrical barrier (i.e., an abrupt change in dip angle) affects slip propagation. The model is defined in an x-z coordinate system, with the x-axis aligned along the décollement (positive downward), and the z-axis oriented perpendicular to it (positive upward), as shown in Supplementary Fig. 11. With this coordinate, the unit normal vector to the décollement is given by n = (0, 1)T.

Assuming that the maximum principal stress axis is oriented at angle ψ_b from the x-axis, the stress tensor is expressed as

Σ = \begin{pmatrix} \sigma_1 \cos^2 \psi_b + \sigma_3 \sin^2 \psi_b & (\sigma_1 - \sigma_3) \sin \psi_b \cos \psi_b \\ (\sigma_1 - \sigma_3) \sin \psi_b \cos \psi_b & \sigma_1 \sin^2 \psi_b + \sigma_3 \cos^2 \psi_b \end{pmatrix} (1)

where σ_1 and σ_3 are the maximum and minimum principal stresses, respectively. A normal stress acting on the décollement is calculated as follows:

σ_n = n^T Σ n = \frac{\sigma_1 + \sigma_3}{2} - \frac{\sigma_1 - \sigma_3}{2} \cos 2\psi_b (2)

This expression shows that σ_n increases monotonically with ψ_b for 0° < ψ_b < 90°.

The stress orientation ψ_b can be estimated using the Coulomb wedge theory41. Assuming a non-cohesive wedge, stress orientation is given by

ψ_b = \frac{1}{2} \arcsin \left( \frac{\sin \phi}{\sin \alpha_0} \right) - \frac{1}{2} \alpha_0 + \alpha + \beta (3)

where the modified surface slope angle α_0 is defined as

α_0 = \arctan \left( \frac{1 - \rho_w}{1 - \lambda} \tan \alpha \right) (4)

Here, α is the surface (seafloor) slope angle; β is the dip angle of the décollement; ρ and ρ_w are the densities of the wedge and seawater, respectively; λ is the pore-fluid pressure ratio; and ϕ is the internal friction angle, defined as

ϕ = \arctan \mu_w (5)

where μ_w is the friction coefficient within the wedge.

Based on parameter values reported for the same region42 (Supplementary Table 1), the resulting ψ_b values are approximately 5° for the ITZ and 26° for the FTZ. From Eq. (2) and assuming that both the mean stress (σ_1 + σ_3)/2 and the differential stress (σ_1 - σ_3) vary only gradually along dip, the normal stress at the FTZ exceeds that at the ITZ by about 20% of the differential stress. This partly supports the interpretation that the kink can act as a geometrical barrier to SSE propagation.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review comments on “Geometrical barrier determines the updip limit of slow earthquake slip”\nThis manuscript provides the time-spacio relationships between precise shallow tremor locations and deformation structures\nfrom seismic profiles in the Nankai Trough off Kumamo, SW Japan. Authors found that the frontal thrusts with higher dip\nangle are the barrier for the tremor, major place for the tremor is the horizontal decollement, tremor is inhibited in the\nseamount subduction area, and strike slip faults has an effect for the tremor migration. Authors show clearly that the shallow\ntremor distributions are strongly related to the deformation structurers in subduction zone, which is very important to assess\nthe scale of earthquake disasters for other subduction zones.\nThere are, however, some unclear descriptions of structures and some unclear interpretations in the discussion section. The\nrelationships between structures, stress and strain (therefore the fluid pressure) are not so clearly mentioned in terms of their\nmechanisms. Because the probability of tremor distribution was already published in the previous study, the new things in\nthis manuscript is the relationship of tremor locations to the deformation features in subduction zones. Therefore, the\ndescriptions for the structures must be carefully conducted. The major purpose of this manuscript is to discuss about the\nmechanism how the tremor locality is controlled by the structure. But discussions do not so much get into the mechanisms\nbut just follow the fact without any logical interpretations about the mechanisms in my impression. Especially, how the fluid\npressure was controlled by the structures is not so clearly mentioned.\nI will show a list of my comments below for the detail. My recommendation is major revision for this manuscript. As I\nmentioned above, the clear spatial relationships between shallow tremor and structures are very interesting and important.\nTherefore, I think this manuscript is worth to be published in Nature communications after the revisions are properly\nconducted.\nL30: Pore fluid pressure contrast is not clear description. Authors need to describe it as that high fluid pressure is expected\nin the underplated structure.\nL39: “occurrences” instead of “occurrence”.\nL91-92: It is not clear for me that the different trends of anticlines between ITZ and FTZ (Only in R3?). Please show them in\nFig. 2. This difference in the trends is important in the discussion?\nL114-115: Please show the reflections from oceanic crust and decollement in Fig. 3 to emphasize the one from oceanic crust\nand the other from decollement. The difference is very important. Authors want to say the difference the frontal thrusts\nbetween branching from the decollement (D1-D2) and not branching but the angle getting higher (D3-D5)?\nL119: I agree that the reflectors branching from the decollement are the frontal thrusts which can not be active currently\nbecause the active decollement exists below. But the reflectors without decollement from which they are branching can be\ncalled decollment as a currently active plate boundary fault. Please define the frontal thrusts and decollement. Could be\nbetter to call the highly dipping thrust as a descriptive term.\nL120: It is better to delete “may”. It is clear to have decollement to the current deformation front in D1 and D2. But I am\nwondering that authors can say the proto-decollement is there in D3-D5. If there, you can say the reflectors with high dip\nangle in D3-D5 as frontal thrusts.\nL142: Branching points are there only in D1 and D2. Authors need to describe precisely these for all from D1 to D5.\nL149: For the interpretations about increase in shear strength, explain it more detail. You need to say it with the regional\nstress state that has sigma1 is sub-horizontal. And the increase in shear strength itself does not mean the factor to prevent\nslip. Discussion for magnitude of differential stress is needed to evaluate enhancement or prevention of slip. Slip tendency is\nalso another expression to evaluate slip enhancement or prevention. Rephrase the sentence to say how the slip is\nprevented with the higher angle of the thrusts.\nL155: For the interpretations of less strain accumulation in the frontal area, why you can expect the less strain accumulation\nas a mechanism controlled by geometry? Please explain it. This is one of my concerns that authors say just a phenomenon\nwithout any mechanical interpretation.\nL157: Delete “may”.\nL168-179: Please give a logical explanation with evidence for the interpretation.\nL173: You do not need the interpretation for the LVZ as underplating. No evidence for underplating because duplex structure\nis not clearly observed in the sturcure and this interpretation can not be useful for your discussion. It is better just to say the\ntransparent area has low velocity suggesting the high fluid pressure. It is enough.\nL177: Why authors can say that the underplating occurs at the horizontal area? And such low velocity zone is not common\nevery seismic profiles. If so, authors can not say that underplating structures correspond to the tremor distribution. As authors\nalso pointed out below, the LVZ is not less visible in the eastern side. Really less visible but is it there? Show it.\nL179-180: How to explain the fluid pressure controlled by the structures mechanically? Authors just describe the velocity\ndistribution. No mechanical interpretation is provided for the relationship between structure and fluid pressure. I was\nconfused between the horizontal decollement, underplating structure, high fluid pressure, frontal thrusting and less strain\naccumulation in the frontal area. What are the causes and the results? And how they are mechanically related?\nL192: Why the stress shadow makes fluid pressure increased? Please explain it or cite previous studies.\nL199: What does “locally overwhelmed the stress shadow condition” mean? Fluid pressure was increased in the stress\nshadow area locally by the intra slab earthquake? Why? Liquefaction?", "reviewer_2": "The authors present high-resolution tremor locations for the Nankai Trough of Japan dating to 2020-2021, obtained using a\ndense networks of ocean-bottom seismometers and their own recently-developed technique for improved tremor detection\nand location. This region is extremely well characterized using geophysical and bathymetric imaging, among the best-\nknown offshore subduction zones in the world. They interpret the boundaries of the tremor episodes in light of the well-\ndelineated tectonic structure of the wedge sediments and underlying plate topography, including a prominent subducted\nridge that has been imaged previously and has a major effect on the structural development and geometry of the\naccretionary wedge.\nThey show clearly and effectively that the tremor patch appears to be confined to the “ITZ,” the portion of the outer wedge\ninboard of the frontal thrust zone. The updip limit of tremor in particular is spatially correlated with the branch point of a major\nfrontal thrust splay fault system, and furthermore the along-strike propagation of the tremor episode appears to be limited or\naffected by the presence of a system of prominent strike-slip faults crossing the wedge. I find the argument to be clear and\nconvincing. It is presented well, with an economy of data and figures. The result is important for Nankai structure and hazard\nspecficially, but also for our global understanding of the nature of subduction zone megathrust strain accumulation and\nrelease, and the possible limits to co-seismic rupture.\nThere is essentially nowhere else in the world besides Nankai where tremor and SSE at the up-dip end of the locked system\nhave been seen to anything approaching this extent and level of detail and resolution. I would enthusiastically support\npublication of this manuscript with only minor editorial revisions. Here are few comments for the authors and editors to\nconsider:\nThe authors don’t say much about the downdip limit of the tremor patch, but it seems to be correlated in map view with the\nseaward edge of the Kumano forearc basin, which has previously been identified as a potential mega-splay fault and up-dip\nlimit of the main locked zone. Perhaps it would strengthen this manuscript to discuss the down-dip limit briefly as well as the\nup-dip limit? I think the case is compelling that the patch corresponds to the low-velocity, high-porosity zone identified in\nother papers that are cited here. To me, this is the most likely reason the tremor patch exists.\nThroughout the manuscript: In the majority of papers on the region, the ridges are referred to as the “Zenisu Ridge” and the\n“Paleo-Zenisu Ridge.” I realize of course that zenisu is a Japanese form of the foreign word zenith, so the usage in this\nmanuscript is not incorrect. However, perhaps for clarity it would be useful to call it Paleo-Zenisu, because many\nresearchers are more familiar with that name?\nLine 40: maybe not say “better” insights? Maybe “important” or “useful” insights would be more precise word choice.\nLines 158 – 164 and figure 4: There is a mention here of a 2016 episode of activity, in addition to the analyzed 2020-2021\nsequence, but as far as I can see it is not mentioned elsewhere in the manuscript at all, so we don’t have much information\non it, except two cited references. Perhaps another sentence or two on this sequence could be added, for better context and\ncomparison? Otherwise, it could perhaps be omitted, since it doesn't add to the point of this manuscript without context.\nLines 207-210: Papers by Sakaguchi et al. (2011) and Yamaguchi et al. (2011) analyzing core samples from IODP drilling of\nthe Nankai frontal thrust both concluded that coseismic slip inducing frictional heating has affected very shallow regions in\nthis zone (equivalent to along line D5, approximately). This is evidence that megathrust earthquake slip does propagate\nthrough the tremor zone, and should be mentioned here as well.\nFigure 2. The black contours of tremor probability overlain on the rainbow bathymetry and heavy black fault traces in 2A are\nhard to clearly see and interpret. Perhaps a modified color scheme would be useful here. It’s not clear to me that the color\nrepresentation of bathymetry in A conveys much information that is not also shown in B as a grayscale, so maybe just use\nthat palette for both, with color contours of tremor for A? Also, in 2B, the blue solid and dashed lines are not identified in the\ncaption.\nFigure 4. The “slip-inhibitive” purple dashed line is described in the key as “on the megathrust” but is shown overlain on the\nfrontal thrust, not the megathrust, except for in the panel labeled D7. This is a bit confusing, although I think that I understand\nwhat the authors intended, which is that slip transfer to the branching frontal thrust inhibits the tendency for slip on the\nunderlying, lower-dip megathrust (décollement). I suggest modifying this figure and its key to make a bit more clear what\nis being highlighted.\nFigure 5. The blue dashed hyperbola in panel C appeared at first look to be a set of blue dots, about the same size as the\nred and orange dots, at least in the provided pdf file. I could only see that it was actually a dashed line when I enlarged the\nfigure to a very large size on screen. Perhaps the figure can be edited to more clearly distinguish the dashed line from the\ndots even at low magnification.", "reviewer_3": "Review of Nature Communications ms. NCOMMS-24-45146-T – “Geometrical barrier determines the updip limit of slow earthquake slip” by Akuhara et al.\nIn this well-written manuscript the authors present high-resolution tremor locations, bathymetry, and seismic reflection\nimages that place new constraints on the up-dip limit of shallow slow slip earthquakes in the Nankai subduction zone. In\nmost of the study region, the up-dip limit of tremor is spatially correlated with the intersection of the subduction decollement\nand the main frontal thrust suggesting that changes in fault geometry and/or associated changes in fluid pressure may act as\na barrier to slow slip. This finding has important implications for the up-dip limit of megathrust earthquakes and tsunami\ngeneration I recommend publication of this manuscript in Nature Communications after considering my minor comments on\nthe manuscript and figures below.\n• Labelling of regions R1-R3 and profiles D1-D7 in text and figures -- I suggest modifying the labelling of the regions and\nprofiles to make it easier for the reader to understand which profiles correspond to which regions. Based on my\nunderstanding, profiles D1 and D2 occur in region R1; profiles D3, D4, and D5 occur in region R2; and profiles D6 and D7\noccur in region R3. As one possible approach, if the regions were labelled A, B, and C, then the seven profiles could be\nlabelled A1, A2, B1, B2, B3, C1, and C2 making it easy to see which profiles correspond to which regions.\nLine 95 – I suggest changing “crosscut” to “intersect”. From the information presented in this paper, it is not clear to me that\nthe right-lateral strike-slip faults offset the decollement or are restricted to the upper plate.\nLine 135 “Influence of geometry and pore fluid pressure variations on slip propagation” section -- In this section, I\nrecommend adding a few sentences comparing the new results from Nankai presented in this manuscript with what has\nbeen discovered about the up-dip limit of shallow slow-slip earthquakes in other subduction zones, such as the Hikurangi,\nNew Zealand (e.g., Wallace et al., 2016, Science) and Costa Rica (e.g., Davis et al., 2015, EPSL) subduction zones.\nLines 147-149 – I was not able to follow why the change in dip angle would increase the normal stress. I can understand\nhow the change in geometry would result in a geometric barrier to slip and arguably the shear stress that would need to be\novercome would increase from the decollement to the frontal thrust. But with the steeper dip (and shallower depth) of the\nfrontal thrust I would expect the normal stress on the faults to decrease toward the trench. I recommend the authors’ clarify\nthis point to the extent possible\nParagraph starting at line 171 – I suggest adding a reference to Wallace et al. (2012, Geology) or similar paper that has\nproposed variations in fluid pressure as affecting the slip behavior of subduction plate interfaces. If high fluid pressures\nalong the decollement are required for slow slip to occur, then it seems plausible that fluid could be leaking up the frontal\nthrust, reducing fluid pressures at the intersection with the decollement and therefore providing an up dip limit to slow slip.\nLine 184 – I suggest changing “overwrite” to “cut across” or “obscure”.\nLine 211 – I agree with the authors that the geometrical barrier () identified in their study might prevent a future megathrust\nearthquake from reaching the trench, but it is also possible megathrust displacement along the decollement may track up the\nfrontal thrust and still displace the seafloor.\nFigure 2 – The tremor probability contours in Figure 2a cannot be deciphered at the scale of the figure. If this figure cannot\nbe expanded in the main paper, I recommend that only 1-2 key contours be shown and the expanded figure with more\ncontours be included in the supplemental material.\nFigure 2 and 5 – I recommend adding horizontal scales to these figures."}, "author_response": "Reply to reviewers’ comments on the manuscript\n“Geometrical barrier determines the updip limit of slow earthquake slip\n(NCOMMS-24-45146-T)”\nResponse to Reviewer #1:\nComment 1-1: This manuscript provides the time-spacio relationships between precise\nshallow tremor locations and deformation structures from seismic profiles in the Nankai\nTrough off Kumamo, SW Japan. Authors found that the frontal thrusts with higher dip\nangle are the barrier for the tremor, major place for the tremor is the horizontal\ndecollement, tremor is inhibited in the seamount subduction area, and strike slip faults\nhas an effect for the tremor migration. Authors show clearly that the shallow tremor\ndistributions are strongly related to the deformation structurers in subduction zone, which\nis very important to assess the scale of earthquake disasters for other subduction zones.\nThere are, however, some unclear descriptions of structures and some unclear\ninterpretations in the discussion section. The relationships between structures, stress and\nstrain (therefore the fluid pressure) are not so clearly mentioned in terms of their\nmechanisms. Because the probability of tremor distribution was already published in the\nprevious study, the new things in this manuscript is the relationship of tremor locations to\nthe deformation features in subduction zones. Therefore, the descriptions for the\nstructures must be carefully conducted. The major purpose of this manuscript is to discuss\nabout the mechanism how the tremor locality is controlled by the structure. But\ndiscussions do not so much get into the mechanisms but just follow the fact without any\nlogical interpretations about the mechanisms in my impression. Especially, how the fluid\npressure was controlled by the structures is not so clearly mentioned.\nI will show a list of my comments below for the detail. My recommendation is major\nrevision for this manuscript. As I mentioned above, the clear spatial relationships between\nshallow tremor and structures are very interesting and important. Therefore, I think this\nmanuscript is worth to be published in Nature communications after the revisions are\nproperly conducted.\nReply 1-1: We would like to express our gratitude to the reviewer for the insightful\ncomments. As the reviewer pointed out, the previous manuscript lacked detailed\ninformation about the mechanisms of how slip propagation, structure, and stress interact\nwith each other. In the revised manuscript, we have attempted to describe this as much as\npossible. Please refer to our point-by-point responses below.\nComment 1-2: L30: Pore fluid pressure contrast is not clear description. Authors need to\ndescribe it as that high fluid pressure is expected in the underplated structure.\nReply 1-2: We have rephrased the sentence for clarity (Lines 32–33 in the revised\nmanuscript).\nComment 1-3: L39: “occurrences” instead of “occurrence”.\nReply 1-3: We have changed it to “occurrences” (Line 41).\nComment 1-4: L91-92: It is not clear for me that the different trends of anticlines between\nITZ and FTZ (Only in R3?). Please show them in Fig. 2. This difference in the trends is\nimportant in the discussion?\nReply 1-4: The change in the trend is only seen in region C (previously referred to as R3),\nas the reviewer pointed out. Since this sentence is not necessary for our conclusion, we\nhave removed this sentence.\nComment 1-5: L114-115: Please show the reflections from oceanic crust and\ndecollement in Fig. 3 to emphasize the one from oceanic crust and the other from\ndecollement. The difference is very important. Authors want to say the difference the\nfrontal thrusts between branching from the decollement (D1-D2) and not branching but\nthe angle getting higher (D3-D5)?\nReply 1-5: We consider that the reflections from the oceanic crust and décollement are\nalready clearly indicated in the original Figure 3 using thick solid and dashed lines,\nrespectively. The original sentence merely introduces to readers the primary features seen\nin the profile, that is, reflections from the top of oceanic crust and the décollement. To\navoid potential confusion, we have rephrased the sentence for clarity (Lines 123–125).\nComment 1-6: L119: I agree that the reflectors branching from the decollement are the\nfrontal thrusts which can not be active currently because the active decollement exists\nbelow. But the reflectors without decollement from which they are branching can be\ncalled decollment as a currently active plate boundary fault. Please define the frontal\nthrusts and decollement. Could be better to call the highly dipping thrust as a descriptive\nterm.\nReply 1-6: We are grateful for this suggestion, and we have reorganized the usage of\nsome geological terms. In the revised manuscript, the term “décollement” is used only for\nthe active plate boundary. Consequently, what was previously referred to as “frontal thrust”\nalong lines B1-B3, C1, C2 (formerly D3-D7) is now regarded as part of the “décollement”.\nThe term “steep thrust” is used to denote the steeply dipping portion of the decollement\n(Lines 127–133).\nComment 1-7: L120: It is better to delete “may”. It is clear to have decollement to the\ncurrent deformation front in D1 and D2. But I am wondering that authors can say the\nproto-decollement is there in D3-D5. If there, you can say the reflectors with high dip\nangle in D3-D5 as frontal thrusts.\nReply 1-7: We have rephrased the sentence. Since the proto-decollement is not clear in\nthe image, we have decided to use the term “steep thrust” for lines B1-B3 (Lines 127–\n133).\nComment 1-8: L142: Branching points are there only in D1 and D2. Authors need to\ndescribe precisely these for all from D1 to D5.\nReply 1-8: Now the revised manuscript uses “kink” points instead of “branching” points\nfor the eastern profiles B1-3 and C1-2. For profiles A1-2, we use “branching” points\nThe concerning sentence has been modified accordingly (Lines 153–155).\nComment 1-9: L149: For the interpretations about increase in shear strength, explain it\nmore detail. You need to say it with the regional stress state that has sigma1 is sub-\nhorizontal. And the increase in shear strength itself does not mean the factor to prevent\nslip. Discussion for magnitude of differential stress is needed to evaluate enhancement or\nprevention of slip. Slip tendency is also another expression to evaluate slip enhancement\nor prevention. Rephrase the sentence to say how the slip is prevented with the higher\nangle of the thrusts.\nReply 1-9: We have renewed the corresponding paragraph (Lines 159–171) and explicitly\nstated that the maximum principal stress is sub-horizontal. Moreover, new paragraphs\nhave been added to the Methods section (Lines 423–446) to provide a brief mathematical\nmodel explaining how the steep thrust can act as a geometrical barrier.\nRegarding the use of slip tendency (or static differential stress), while it is a useful\nmeasure for assessing the likelihood of fault slip under static conditions, it may not be\ndirectly applicable to discussions of rupture termination, where dynamic shear stress\nplays a crucial role. Instead, we focus on shear strength following Bletery et al. (2016).\nFor readers interested in the slip tendency, we retain the sentence mentioning a previous\nstudy that investigated the slip tendency of this region (Hashimoto et al. 2022) (Lines\n166–171). Overall, their results are consistent with our interpretation.\nComment 1-10: L155: For the interpretations of less strain accumulation in the frontal\narea, why you can expect the less strain accumulation as a mechanism controlled by\ngeometry? Please explain it. This is one of my concerns that authors say just a\nphenomenon without any mechanical interpretation.\nReply 1-10: In the previous manuscript, we regrettably did not dedicate sufficient\ndiscussion to this interpretation. By referring to 'less strain accumulation,' the intension\nwas to suggest the possibility that the décollement undergoes stable sliding. The well-\nstratified pattern observed in the hanging wall (Supplementary Fig. 2) appears to be\nconsistent with stable sliding. We have incorporated this information into the main text\n(Lines 172–182) while ensuring that the interpretation remains non-conclusive.\nComment 1-11: L157: Delete “may”.\nReply 1-11: We have deleted it (Line 183).\nComment 1-12: L168-179: Please give a logical explanation with evidence for the\ninterpretation.\nReply 1-12: The separation distance between the termination of the first migration and\nthe initiation of the second migration is approximately 5 km (Line 196). This separation\ndistance is relatively small in comparison to the length of the first migration (~40 km;\nLine 185), thereby indicating the potential for stress transfer from the first migration to\ntrigger the subsequent migration (Lines 197–201). To enhance clarity, the relevant\nparagraph has been revised (Lines 192–201).\nComment 1-13: L173: You do not need the interpretation for the LVZ as underplating.\nNo evidence for underplating because duplex structure is not clearly observed in the\nsturcure and this interpretation can not be useful for your discussion. It is better just to\nsay the transparent area has low velocity suggesting the high fluid pressure. It is enough.\nReply 1-13: In accordance with the reviewer’s suggestion, we have decided not to use\nthe word “underplating” for this structure, acknowledging that the geological\ninterpretation of this structure is still debated. We instead refer to it as LVZ in the revised\nmanuscript.\nComment 1-14: L177: Why authors can say that the underplating occurs at the horizontal\narea? And such low velocity zone is not common every seismic profiles. If so, authors\ncan not say that underplating structures correspond to the tremor distribution. As authors\nalso pointed out below, the LVZ is not less visible in the eastern side. Really less visible\nbut is it there? Show it.\nReply 1-14: We have substantially reorganized the paragraph concerning the spatial\nextent of the LVZ (Lines 217–230). We consider that the one-to-one spatial\ncorrespondence between tremor and high pore-fluid pressure (or LVZ) remains\nchallenging, as the fluid pressure distribution is not well constrained even with the\ncomprehensive set of reflection profiles. Nevertheless, the discussion of the potential role\nof fluid pressure would be still valuable for readers because fluid pressure is recognized\nto regulate the slip behaviors of faults. This position is explicitly stated in the first line of\nthe renewed paragraph.\nWith regard to the updip limit of the LVZ, we have removed the speculative sentence\n“underplating primarily occurs in areas where the thrust is horizontal”. Instead, we\nmention a possibility of the absence of LVZ above the steep thrust, by referring to some\ngeophysical studies (Akuhara et al. 2020; Park et al. 2010; Azevedo et al. 2018; Lines\n220–221).\nFor the LVZ in region A, we have included a detailed description of a previous study\n(Fahrudin et al. 2022; Lines 224–227), which interprets a negative reflector above the\ndecollement as the top of the LVZ (or what they call “shear zone”). We have clarified this\nreflector in Supplementary Fig. 2. It is acknowledged that the clarity of this reflector is\ninadequate on the reflection image used in this study, and thus we avoid making\nconclusive statements in this paragraph.\nComment 1-15: L179-180: How to explain the fluid pressure controlled by the structures\nmechanically? Authors just describe the velocity distribution. No mechanical\ninterpretation is provided for the relationship between structure and fluid pressure. I was\nconfused between the horizontal decollement, underplating structure, high fluid pressure,\nfrontal thrusting and less strain accumulation in the frontal area. What are the causes and\nthe results? And how they are mechanically related?\nReply 1-15: We have included information about the relationship between fluid pressure\nand structure. Added information includes fluid source (Lines 210–211); how high pore\nfluid pressure is sustained (Lines 211–212); effects of basement high (Lines 212–213);\nand evidence showing that high pore fluid pressure is linked to the slow earthquake\ngenerations (Lines 213–216).\nComment 1-16: L192: Why the stress shadow makes fluid pressure increased? Please\nexplain it or cite previous studies.\nReply 1-16: Throughout the manuscript, we interpret that the stress shadow “decreases”\nthe fluid pressure. We guess that the “increased” in the reviewer’s comment is merely a\ntypo of “decreased” and therefore have added a detailed description how the stress\nshadow decreases fluid pressure compared to other regions (Lines 241–246).\nComment 1-17: L199: What does “locally overwhelmed the stress shadow condition”\nmean? Fluid pressure was increased in the stress shadow area locally by the intra slab\nearthquake? Why? Liquefaction?\nReply 1-17: The sentence was merely intended to convey that slow earthquakes were\ntriggered by stress perturbation from the M7.4 earthquake. There is no requirement for\nfluid pressure. We have rephrased the sentence for clarity (Lines 251–253).\nResponse to Reviewer #2:\nComment 2-1: The authors present high-resolution tremor locations for the Nankai\nTrough of Japan dating to 2020-2021, obtained using a dense networks of ocean-bottom\nseismometers and their own recently-developed technique for improved tremor detection\nand location. This region is extremely well characterized using geophysical and\nbathymetric imaging, among the best-known offshore subduction zones in the world.\nThey interpret the boundaries of the tremor episodes in light of the well-delineated\ntectonic structure of the wedge sediments and underlying plate topography, including a\nprominent subducted ridge that has been imaged previously and has a major effect on the\nstructural development and geometry of the accretionary wedge.\nThey show clearly and effectively that the tremor patch appears to be confined to the\n“ITZ,” the portion of the outer wedge inboard of the frontal thrust zone. The updip limit\nof tremor in particular is spatially correlated with the branch point of a major frontal thrust\nsplay fault system, and furthermore the along-strike propagation of the tremor episode\nappears to be limited or affected by the presence of a system of prominent strike-slip\nfaults crossing the wedge. I find the argument to be clear and convincing. It is presented\nwell, with an economy of data and figures. The result is important for Nankai structure\nand hazard specficially, but also for our global understanding of the nature of subduction\nzone megathrust strain accumulation and release, and the possible limits to co-seismic\nrupture.\nThere is essentially nowhere else in the world besides Nankai where tremor and SSE at\nthe up-dip end of the locked system have been seen to anything approaching this extent\nand level of detail and resolution. I would enthusiastically support publication of this\nmanuscript with only minor editorial revisions. Here are few comments for the authors\nand editors to consider:\nThe authors don’t say much about the downdip limit of the tremor patch, but it seems to\nbe correlated in map view with the seaward edge of the Kumano forearc basin, which has\npreviously been identified as a potential mega-splay fault and up-dip limit of the main\nlocked zone. Perhaps it would strengthen this manuscript to discuss the down-dip limit\nbriefly as well as the up-dip limit? I think the case is compelling that the patch\ncorresponds to the low-velocity, high-porosity zone identified in other papers that are\ncited here. To me, this is the most likely reason the tremor patch exists.\nReply 2-1: We thank the reviewer for the positive feedback. We agree with the importance\nof discussing the downdip limit. Thus, we have added a paragraph that briefly discusses\nthe downdip limit (Lines 255–266).\nComment 2-2: Throughout the manuscript: In the majority of papers on the region, the\nridges are referred to as the “Zenisu Ridge” and the “Paleo-Zenisu Ridge.” I realize of\ncourse that zenisu is a Japanese form of the foreign word zenith, so the usage in this\nmanuscript is not incorrect. However, perhaps for clarity it would be useful to call it\nPaleo-Zenisu, because many researchers are more familiar with that name?\nReply 2-2: We have changed “Zenith” to “Zenisu”.\nComment 2-3: Line 40: maybe not say “better” insights? Maybe “important” or “useful”\ninsights would be more precise word choice.\nReply 2-3: We have changed it to “important” (Line 42).\nComment 2-4: Lines 158 – 164 and figure 4: There is a mention here of a 2016 episode\nof activity, in addition to the analyzed 2020-2021 sequence, but as far as I can see it is not\nmentioned elsewhere in the manuscript at all, so we don’t have much information on it,\nexcept two cited references. Perhaps another sentence or two on this sequence could be\nadded, for better context and comparison? Otherwise, it could perhaps be omitted, since\nit doesn't add to the point of this manuscript without context.\nReply 2-4: We have added an introductory sentence for the 2016 episode in the intorcition\n(Lines 75–76).\nComment 2-5: Lines 207-210: Papers by Sakaguchi et al. (2011) and Yamaguchi et al.\n(2011) analyzing core samples from IODP drilling of the Nankai frontal thrust both\nconcluded that coseismic slip inducing frictional heating has affected very shallow\nregions in this zone (equivalent to along line D5, approximately). This is evidence that\nmegathrust earthquake slip does propagate through the tremor zone, and should be\nmentioned here as well.\nReply 2-5: We thank the reviewer for this suggestion. The revised manuscript now clearly\nstates the possibility of slip propagating through the tremor zone by referring to Sakaguchi\net al. (2011) (Lines 281–283).\nComment 2-6: Figure 2. The black contours of tremor probability overlain on the\nrainbow bathymetry and heavy black fault traces in 2A are hard to clearly see and interpret.\nPerhaps a modified color scheme would be useful here. It’s not clear to me that the color\nrepresentation of bathymetry in A conveys much information that is not also shown in B\nas a grayscale, so maybe just use that palette for both, with color contours of tremor for\nA? Also, in 2B, the blue solid and dashed lines are not identified in the caption.\nReply 2-6: We have changed the contour color to blue for visibility. We have added an\nexplanation for the purple dashed line in the figure caption (Lines 314–315).\nComment 2-7: Figure 4. The “slip-inhibitive” purple dashed line is described in the key\nas “on the megathrust” but is shown overlain on the frontal thrust, not the megathrust,\nexcept for in the panel labeled D7. This is a bit confusing, although I think that I\nunderstand what the authors intended, which is that slip transfer to the branching frontal\nthrust inhibits the tendency for slip on the underlying, lower-dip megathrust\n(décollement). I suggest modifying this figure and its key to make a bit more clear what\nis being highlighted.\nReply 2-7: We have decided to remove the key saying “megathrust”, to avoid potential\nconfusing.\nComment 2-8: Figure 5. The blue dashed hyperbola in panel C appeared at first look to\nbe a set of blue dots, about the same size as the red and orange dots, at least in the provided\npdf file. I could only see that it was actually a dashed line when I enlarged the figure to a\nvery large size on screen. Perhaps the figure can be edited to more clearly distinguish the\ndashed line from the dots even at low magnification.\nResponse 2-8: We have increased the length of the line segment along the dashed line for\nbetter visibility.\nResponse to Reviewer #3:\nComment 3-1: In this well-written manuscript the authors present high-resolution tremor\nlocations, bathymetry, and seismic reflection images that place new constraints on the up-\ndip limit of shallow slow slip earthquakes in the Nankai subduction zone. In most of the\nstudy region, the up-dip limit of tremor is spatially correlated with the intersection of the\nsubduction decollement and the main frontal thrust suggesting that changes in fault\ngeometry and/or associated changes in fluid pressure may act as a barrier to slow slip.\nThis finding has important implications for the up-dip limit of megathrust earthquakes\nand tsunami generation I recommend publication of this manuscript in Nature\nCommunications after considering my minor comments on the manuscript and figures\nbelow.\nReply 3-1: We thank the reviewer for the positive feedback.\nComment 3-2: Labelling of regions R1-R3 and profiles D1-D7 in text and figures -- I\nsuggest modifying the labelling of the regions and profiles to make it easier for the reader\nto understand which profiles correspond to which regions. Based on my understanding,\nprofiles D1 and D2 occur in region R1; profiles D3, D4, and D5 occur in region R2; and\nprofiles D6 and D7 occur in region R3. As one possible approach, if the regions were\nlabelled A, B, and C, then the seven profiles could be labelled A1, A2, B1, B2, B3, C1,\nand C2 making it easy to see which profiles correspond to which regions.\nReply 3-2: In accordance with the reviewer’s suggestion, we have changed the name of\nof regions and profiles (Figures 2 and 3). Also, to include lines profile A2 into region A,\nwe have slightly shifted the boundary between regions A and B eastward (Figure 2).\nComment 3-3: Line 95 – I suggest changing “crosscut” to “intersect”. From the\ninformation presented in this paper, it is not clear to me that the right-lateral strike-slip\nfaults offset the decollement or are restricted to the upper plate.\nReply 3-3: We have changed it into “intersect” (Line 97).\nComment 3-4: Line 135 “Influence of geometry and pore fluid pressure variations on\nslip propagation” section -- In this section, I recommend adding a few sentences\ncomparing the new results from Nankai presented in this manuscript with what has been\ndiscovered about the up-dip limit of shallow slow-slip earthquakes in other subduction\nzones, such as the Hikurangi, New Zealand (e.g., Wallace et al., 2016, Science) and Costa\nRica (e.g., Davis et al., 2015, EPSL) subduction zones.\nReply 3-4: We agree with the importance of a comparison to other subduction zones. We\nnotice that the slow earthquakes in Hikurangi and Costa Rica have potentially propagated\nup to the trench, but the spatial resolution of their source locations would not be sufficient\nto discuss structural controls on their updip limit. We have added a sentence for this\ndiscussion in the last section (Lines 292–295).\nComment 3-5: Lines 147-149 – I was not able to follow why the change in dip angle\nwould increase the normal stress. I can understand how the change in geometry would\nresult in a geometric barrier to slip and arguably the shear stress that would need to be\novercome would increase from the decollement to the frontal thrust. But with the steeper\ndip (and shallower depth) of the frontal thrust I would expect the normal stress on the\nfaults to decrease toward the trench. I recommend the authors’ clarify this point to the\nextent possible\nReply 3-5: Since the first principal stress axis is sub-horizontal, kink in the megathrust\ncan increase the normal stress. We have added a detailed explanation for it, as another\nreviewer has a similar concern (Lines 159–171; Lines 423–446).\nComment 3-6: Paragraph starting at line 171 – I suggest adding a reference to Wallace\net al. (2012, Geology) or similar paper that has proposed variations in fluid pressure as\naffecting the slip behavior of subduction plate interfaces. If high fluid pressures along the\ndecollement are required for slow slip to occur, then it seems plausible that fluid could be\nleaking up the frontal thrust, reducing fluid pressures at the intersection with the\ndecollement and therefore providing an up dip limit to slow slip.\nReply 3-6: We have renewed the corresponding paragraph regareding LVZ in response\nwith another reviewer’s comment (Lines 202–230). In the renewed paragraph, we have\nmentioned the possibility that the steep thrust could act as a fluid pathway that leaks fluid\nfrom the LVZ, which was suggested by the reviewer (Lines 221–222).\nComment 3-7: Line 184 – I suggest changing “overwrite” to “cut across” or “obscure”.\nReply 3-7: We have changed it into “obscure” (Line 233).\nComment 3-8: Line 211 – I agree with the authors that the geometrical barrier ()\nidentified in their study might prevent a future megathrust earthquake from reaching the\ntrench, but it is also possible megathrust displacement along the decollement may track\nup the frontal thrust and still displace the seafloor.\nReply 3-8: We have reorganized the paragraph and mentioned that possibility (Lines 275–\n287).\nComment 3-9: Figure 2 – The tremor probability contours in Figure 2a cannot be\ndeciphered at the scale of the figure. If this figure cannot be expanded in the main paper,\nI recommend that only 1-2 key contours be shown and the expanded figure with more\ncontours be included in the supplemental material.\nReply 3-9: We have changed the contour color and reduced the number of contours to\nenhance visibility.\nComment 3-10: Figure 2 and 5 – I recommend adding horizontal scales to these figures.\nReply 3-10: We have added horizontal scales to these figures."}, {"round": 2, "reviewer_comments": {"reviewer_3": "All of my concerns with the original manuscript have been appropriately addressed by the authors in the revised manuscript and I recommend publication in Nature Communications.", "reviewer_4": "Dear Editor,\nAs was pointed by previous reviewers, the manuscript by Akuhara et al. presents an interesting correlation between a slow slip-tremors event in Nankai Trough and the geometrical features of the interplate boundary and continental wedge structures observed in high quality active seismic lines. This observation is important for the open research line associated to the interplay between seismotectonics and structural/geodynamical/geological evolution of subduction zones. As I’m participating in a second round of revision, I give some comments of the last version of the manuscript, considering only the points that, in my view, were not previously solved in the revision process. Below, I give comments after an R, referencing the corresponding lines in the text and the beginning of specific paragraphs.\nLines 75-76: “For comparison, the previous tremor episode….” R: Please include the location of this previous tremor episodes in a map figure. This should be important for interpretation.\nLines 92-93: “… outer wedge situated on the trenchward side of a megasplay fault….” R: In Fig.2 it is clear that the correlation between the magasplay fault and the downdip of tremor events is even better than the correlation between the updip of this activity and the “kink” barrier. In my opinion profiles figures and text should be modified to include a more complete discussion of the downdip relation between tremors and continental wedge structure.\nLines 93-95: “… The outer wedge is divided into two geological 93 zones…” R: Please include the limit between ITZ and FTZ in figure 2. This should help to note that the updip limit of slow earthquake is correlated with this limit only in the zone B.\nLines 96-97: ..two subparallel right-lateral strike-slip faults located in the ITZ… R: Where are this right-lateral structures?. Please include the reference to the corresponding Figures.\nLines 114-119: “…to region C, which lacks such features, aligns with the eastern boundary of the tremor distribution. Furthermore…” R: C is the estern limit of this tremor episode, but this does not mean that future tremors episodes could not be located seaward in the segment C. To interpret that, the authors should analyze a much longer period of seismicity along the margin. In my view the analysis do not prove the tectonic control of the updip seismicity in C.\nLines 154-155:….current study indicate that the propagation of SSEs along the décollement is hindered at the kink or branching points…” R: Respecting to figure, the correlation between the updip of SSEs and the kink or branching points is clearly observed at the profiles B and in the profile A2. However, analyzing the profile A1 (in the supplementary material) the “kink” seems to be located landward from the point interpreted by the authors, then at the profile A1 the correlation with the updip of SSEs is not clear. Geometrically, the “kink” points seem to be correlated with the landward limit of the FTZ high. Maybe this observation is debatable, but in any case, it is important the inclusion of figures of the seismic profiles without interpretation (in the supplementary material) to facilitate the observation of reflectors near the decollement.\nLines 159-164: “…the steep thrust experiences higher normal stress than the gently dipping section of the décollement…” R: Respecting the discussion of Normal stresses, the authors include an analysis that is correct under the assumptions of the horizontality of the maximum stress and constant frictional conditions. However, both assumptions are difficult to associate with an accretionary wedge deformed under compressive forces, gravitational forces and different internal an basal effective friction coefficients. In fact, the formation of a kink should be related precisely with a more complex stress field inside the wedge (see for instance the simple model of Dahlen et al. 1984, Noncohesive critical Coulomb wedges: An exact solution). On the other hand, the observation of the current and paleo deformation fronts should be related with variations of the basal effective friction coefficient and maybe with the aseismic deformation of sedimentary units near the deformation front. In my opinion, a more detailed discussion of the stress and strain field in accretionary wedges should be considered for the interpretation of structures and the SSEs episode.\nLines 237-239: “…This lack of deformation is likely due to a stress shadow caused by the subduction of the Paleo–Zenisu ridge..” R: where is the Palo Zenisu_Ridege? Please indicate in maps and profiles. What is the definition of stress shadow used here? The authors of the reference 47 defined shadows zones seaward from subducted oceanic features and below inactive decollements, which seems to be not the case of profiles C. What is the subducted oceanic feature (observed in the profile) that generates the “stress shadow”? Please indicate in maps and figures.\nSupplementary material: There are some signs “Error!” in figure captions.", "reviewer_5": "Review by Madison Bombardier of the manuscript “Geometrical barrier determines the updip limit of slow earthquake slip” by Akuhara et al.\nThis manuscript uses a previously-published tremor catalogue to examine the relationship of shallow slow slip (using tremor as a proxy) to subduction zone structures imaged by seismic reflection in Nankai. Their main finding is the interpretation that the updip limit of slow slip is bounded by an abrupt steepening of the main thrust fault (top of decollement) in most of the region. They infer that the impedance of slow slip on the steep frontal thrust segment may imply impedance of coseismic slip during regular earthquakes, which may have consequences on tsunami generation and associated hazards.\nThree reviewers during the first round of peer review requested: (1), clearer and more robust mechanistic explanations of slip impedance along the steep frontal thrust in terms of pore-fluid pressure and stress; (2), discussion of the downdip limits of tremor; and (3), many editorial changes. The authors have reasonably addressed most of the comments from these reviewers, with some exceptions (see my comment number 29 below); however, some explanations and claims are vague or poorly supported.\nThe work presented in this manuscript is important to the understanding of subduction zone science, and represents an effective use of tremor catalogues for the study of this topic. After minor revision considering my comments below, this manuscript is worthy to be published. I have two main comments, and more than 3 dozen minor comments.\n(1) Main comment:\nThe authors claim in multiple locations throughout the manuscript that the tremor catalogue they use contains “precise” tremor locations; however, evidence for this claim is not provided. The only relevant information provided is that the horizontal uncertainty on each tremor epicentre is estimated by one half the width of the 95% confidence interval to be “a few kilometres” (lines 72–73). It is not clear to me that this is precise, or even more precise than other methods, nor do I think that such a claim is necessary. The primary advantage of Bayesian inference is that solutions are provided as probability distributions, which gives unique 3D probability distributions of each tremor event from which uncertainty can be quantified; there is no guarantee of precision. In other words, this catalogue provides robust uncertainty estimates. Because of this, the tremor catalogue may be considered reliable or credible, but not precise. This is especially true in the context of this analysis where multi-channel seismic reflection data is used with resolution on the order of 100 m (line 82).\nDespite that the primary strength of this tremor catalogue is the detailed uncertainty estimation, the authors do not provide quantitative estimates or distributions of these uncertainties, nor do they consider them in their analysis. Supplementary figure 1 shows all tremor epicentres with 95% confidence interval error bars, but the superposition of the densely-clustered events obscures most of this information.\nHere I make several suggestions to address these issues:\n- remove descriptors such as “precise” everywhere related to the tremor catalogue\n- in the main text, replace “a few kilometres” (lines 72–73) with mean or median horizontal uncertainty estimates, and/or state the range within which 50% or 75% or 90% (or whatever) of horizontal uncertainties are in.\n- at the location where the tremor catalogue is introduced in the main text, briefly explain the advantage that Bayesian inference provides to this catalogue in terms of robust uncertainty estimation. Probabilistic solutions are used to represent the entire catalogue as one probability map, which provides more information than a scatterplot of point sources. A probability map is an unusual way to represent a catalogue of seismic events, and you should clearly explain the importance and novelty of this to encourage readers to adopt similar methods in future work.\n- change supplementary figure 1 to better represent the geographic distribution of uncertainties. I suggest to segment the area into 1 km (or 5 km, or whatever) squared cells and colour each cell according to the average (or median) uncertainty of tremor events in that cell. In this case, you would need two panels in the figure: one for NS uncertainty and one for EW uncertainty. This method would allow the reader to view the heterogeneous geographic distribution of uncertainty over the study area. I expect average uncertainties vary systematically over the region, which may be useful information for the main analysis. In particular, I wonder about the EW uncertainties on the downdip tremor patch in the far north of the study area (region C). This type of information should be clearly discernible from this figure. An example can be found in the supplementary information of Bombardier et al 2025 (doi: 10.1016/j.tecto.2025.230752).\n(2) Main comment:\nInterpretations made in this analysis require a framework of SSEs wherein slow slip directly generates/triggers tremor, and wherein both tremor and slow slip occur on the megathrust fault (in this manuscript, the top of decollement). It is not my understanding that this framework is universally accepted as certain fact, thus the high degree of certainty with which it is presented is not warranted.\nRegarding lines 60–62: it has been shown that tremor does indeed tend to occur near the slip front in Cascadia (eg Hall et al 2019, Bartlow et al 2011, Luo and Liu 2019), but I do not think there is sufficient evidence to use the word “triggered”, since there may be multiple processes occurring at the slip front that may cause, generate, or trigger tremor. In addition, shallow slow slip may not be perfectly analogous to deep slow slip where this has been studied more extensively. The brittle-asperity-in-ductile-matrix model (Luo and Liu 2019) is a plausible hypothesis (among others) to explain some elements of tremor and slow slip, and using this framework is justifiable, but it should be presented as a framework or base-level assumption rather than a known fact about the physical mechanisms.\nRegarding lines 149–153: this reasoning is fuzzy. If you assume tremor is directly caused by slow slip, as the previous sentence states, how can the depth of tremor “most likely” be “similar” to that of SSEs? Would it not be certain that the depths are exactly the same? Reference 19 (Toh 2018) states that some VLFEs likely occurred on a splay fault, which indicates that the depths of these VLFEs are not localized entirely to the megathrust fault (insofar as depth constraints on VLFEs support such an observation); or that the location/structure of megathrust fault itself is uncertain or complex. In addition, deep tremor in Cascadia represented as probability resulting from a Bayesian location method (Bombardier et al 2025) indicates that the depth of tremor can deviate significantly from the top of the subducting oceanic crust and is inconsistent along the margin. Again, deep tremor/SSEs may not be so analogous to shallow tremor/SSEs, but given the difficulty of adequately constraining depths of tremor/LFEs/VLFEs/slow slip, and the uncertainty regarding the relationship between these phenomena, consideration that these processes may occur beyond the megathrust fault is warranted. The possibility of tremor and SSE activity beyond the megathrust fault has important implications for discerning active structures and understanding hazards, and the study of this topic should not be discouraged by premature or simplifying assertions.\nMy suggestion to address this issue is to clearly explain that the analysis and interpretations presented in this manuscript requires the assumption of a specific framework/hypothesis about the nature of the relationship between tremor and slow slip, and about which structures host this activity due to the fact that tremor depths are not resolved in this catalogue. You already have many references to use to justify the use of such a framework. Also explain that variation exist between regions globally and uncertainty regarding the depth and host structures for such activity is prevalent, more work on these topics is needed. These changes largely involve reframing and rephrasing at various locations in the manuscript.\n(3) The abstract should include why your results matter. I suggest simplifying the summary of results in the abstract to make space for the broader interpretations/impact of the results mentioned in the “introduction” and “impacts” sections.\n(4) Lines 51–53 it may be relevant to the present manuscript that the forearc mantle corner has also been shown as a boundary to slow slip by Sawaki et al (2021) and Bombardier et al (2024).\n(5) Lines 99–100 regarding a new deformation front, this should have a citation.\n(6) Line 101 the tremor probability map is mentioned for the first time without explanation of what it is. A reference to the methods section where the probability map is mathematically defined is missing.\n(7) The ITZ-FTZ boundary denoted in Figure 2b by the purple dashed line would be helpful in panel (a) of this figure, so that the spatial correspondence between this boundary and the updip limit of tremor is very clear.\n(8) It is not that easy to discern the annotations in Figure 2b. The dark colour of the bathymetry shading makes it difficult to clearly see the thin black font of the text and transparent blue/red lines. It would be better to make these annotations more pronounced, possibly by decreasing their transparency, making the text font bolder, and/or increasing the transparency of the grey bathymetry shading.\n(9) Lines 115–117 here is an example where it is important to know the uncertainties on tremor in region C. “much further landward” may not be a well-supported observation depending on the uncertainties.\n(10) Lines 118–121 This sentence is long and awkward, and should be in present tense rather than in a past tense narrative-style description of what you did. For example, try “To explore the apparent agreement between the tremor distribution and bathymetric features, we [analyze]…”\n(11) Supplementary figures 2–4 have a “Error! Reference not found” note in the captions. Also the caption in supplementary figure 2 should reference “inverted triangles” (plural), not “inverted triangle”. These inverted triangles in profile A1 are difficult to see clearly; I did not realize they were triangles at first. Perhaps make them bigger with a black outline?\n(12) Lines 124–125 “almost subparallel” doesn’t really make sense. I suggest rephrasing as “almost parallel” or simply “subparallel”. In addition, this short sentence could be joined with the previous to make it less awkward. For example, “The most prominent features … oceanic crust and decollement, which are subparallel within the ITZ.”\n(13) Line 131 I suggest changing “new” and “old” deformation front to “deformation front” and “mega splay”. To say “new” implies it is a new interpretation in this manuscript, and it can confuse the reader when there is no discussion of this new feature. This requires changes to annotations in figures as well.\n(14) Lines 143–144 Supplementary figure 4 does not actually show the tremor distribution along profile C1 and C2. The map in Figure 3 is a more appropriate reference for this observation. However, as a reader, I find that I need to refer to both figure 3 and supplementary figure 4 to fully understand this observation, which is cumbersome. It would help to include the tremor probability distribution along each profile in supplementary figures 2–4 (as is done in figure 3).\n(15) I suggest including an annotation in supplementary figure 4 to identify the subducting ridge. I can guess what constitutes the ridge, but better to identify it clearly because I may be wrong.\n(16) Lines 153–155 this is the only location figure 4 is referenced. Perhaps this early reference to this figure should be removed so the figures can be ordered logically; I do not think it adds much at this point in the manuscript. Figure 4 represents a summary of your interpretations, so it should be explained near the end of the manuscript and used to support such a summary. Notably, a brief summary of the new observations and interpretations do not really exist, but are apt.\nI think lines 153–155 should say “… the propagation of SSEs along dip is hindered…” since figure 4 also shows the ridge stress shadow and the strike slip faults hinder along-strike SSE propagation.\n(17) Lines 155–158 reference 40 (Wech and Bartlow 2014) is a study from Cascadia that observed one instance of an SSE occurring without tremor for a short time. While I agree that these cases seem infrequent in Cascadia, I do not think this is relevant to shallow SSEs in Nankai. It is reasonable to exclude SSEs without tremor, but this should be explained and justified near line 60 where the framework, assumptions, and unknowns are explained. And this choice should be supported with observations for the relevant region. Also, related to my main comment (2), SSEs occurring without tremor are incompatible within a framework wherein slip generates tremor, as stated just three sentences before this. If there are SSEs that occur without tremor, such that they are excluded from this analysis, maybe the framework needs to be revised?\n(18) Line 175 “distinctly different” is a redundant phrase. “Distinct” and “different” are synonyms.\n(19) Line 176 rephrase this as “A stratified pattern observed at the trench side of the steep thrust is absent on the landward side.” and it would be better to reference figure 3 rather than supplementary figure 2, since this feature can be seen in the main-text figure.\n(20) Line 185 “in distance” is unnecessary because kilometre is a unit of distance.\n(21) Line 186 briefly explain why the slip is diffusive, and why diffusive slip is characterized by a hyperbola.\n(22) Line 191 “bounded” is different from “impeded”. In this manuscript, the tremor migration is impeded, not bounded by the strike slip faults.\n(23) Line 197 “horizontally” is unnecessary.\n(24) Lines 197–198 could slip have propagated along one of the strike slip faults? This could explain why the second small SSE initiated landward of the termination of the larger SSE (roughly along the strike of the strike slip faults).\n(25) grey dots in figure 5b are not explained anywhere.\n(26) Line 203 rephrase to “Although its spatial distribution is not well constrained, the potential role of pore-fluid pressure in the study region is worth discussing here.”\n(27) Lines 205–206 the transparent facies are observed immediately above the decollement, not oceanic crust, according to figure 3.\n(28) Line 211 rephrase to “… high pore-fluid pressure is interpreted to be sustained by …”\n(29) Lines 212–216 this section was added in response to comment 1-15 from the first review. It is awkward and oddly written as a string of short facts that are not presented cohesively. How exactly are these inferences relevant to the analysis?\nExplain “tectonic push” and what “these structures” are that may create tectonic push; does this refer to basement highs? Not clear.\n(30) Lines 237–238 it is my understanding that subducting ridges and seamounts lead to highly fractured/deformed accretionary prisms/forearcs. In order for a lack of deformation in the prism to result from the stress shadow of a subducted ridge, it seems to me that the undeformed prism material would have to have been accreted after the ridge (in its shadow) in order to remain relatively undeformed. If the prism material was accreted first, then the ridge would have subducted under it, and the explanation given in the manuscript does not make sense to me. In this case, lack of deformation within the prism would not be a reliable indicator of the existence of a stress shadow. Please clarify the accretionary history and the relationship between subducted ridges/seamounts and prism deformation. Maybe there is an alternative explanation for the reflective strata?\n(31) Line 252 rephrase to “… large earthquake may have facilitated tremor ...”\n(32) Line 257 I believe the boundary between the inner and outer wedges is the megasplay fault shown in figure 2b and mentioned on line 93. I suggest including the “megasplay” term here with reference to figure 2b so it is clear.\n(33) Line 258 “distinctly differ” is redundant\n(34) Lines 262–263 please explain how a discontinuous stress state in the outer forearc wedge would affect slip on the megathrust.\n(35) Line 264 the use of the term “separate” is confusing here. The tremor episode that is the focus of this manuscript occurs in the transient slip zone, so how can it separate the transient slip zone from the fully locked zone? I think this should be phrased “the downdip limit of this tremor episode may not be a universal representation, as SSEs and tremors have occurred further downdip in earlier episodes.”\n(36) Line 283 “numerous numerical” is a slightly awkward phrase. I suggest “multiple”, “several”, or any other synonym for “numerous”.\n(37) Line 294 I do not think Hikurangi is a relevant example here because tremor is not closely associated in space or time with shallow SSEs (eg Todd et al, 2018). As such, the tremor location methods used in this paper would be useful for locating tremor in Hikurangi, but not be useful for approximating slow slip.\n(38) There are numerous grammatical errors and typos throughout the main text and supplementary material, in addition to the ones mentioned above. Text added in response to the earlier reviewers could be integrated better so that the text flows well and seems cohesive.\n(39) Inconsistent use of “Figure” and “Fig” (eg line 212), and bolded text for figure references (eg line 206).", "reviewer_author_response": "Reply to reviewers’ comments\nReviewer #3 (Remarks to the Author):\nAll of my concerns with the original manuscript have been appropriately addressed by\nthe authors in the revised manuscript and I recommend publication in Nature\nCommunications.\nReply: We thank the reviewer for the positive and encouraging comments.\nReviewer #4 (Remarks to the Author):\nDear Editor,\nAs was pointed by previous reviewers, the manuscript by Akuhara et al. presents an\ninteresting correlation between a slow slip-tremors event in Nankai Trough and the\ngeometrical features of the interplate boundary and continental wedge structures observed\nin high quality active seismic lines. This observation is important for the open research\nline associated to the interplay between seismotectonics and\nstructural/geodynamical/geological evolution of subduction zones. As I’m participating\nin a second round of revision, I give some comments of the last version of the manuscript,\nconsidering only the points that, in my view, were not previously solved in the revision\nprocess. Below, I give comments after an R, referencing the corresponding lines in the\ntext and the beginning of specific paragraphs.\nReply: We thank Reviewer #4 for joining the review process and for providing thoughtful\nand constructive comments. We appreciate the reviewer’s recognition of the importance\nof our study and have carefully addressed all the points raised in the following responses.\nLines 75-76: “For comparison, the previous tremor episode….” R: Please include the\nlocation of this previous tremor episodes in a map figure. This should be important for\ninterpretation.\nReply: We appreciate the reviewer’s suggestion. While we understand the importance of\nshowing the location of the 2016 tremor episode, we are concerned that adding these data\nto Fig. 1 could cause unnecessary confusion due to the difference in the quality between\nthe catalogues. Instead, we have presented the tremor distribution of the 2016 episode in\nSupplementary Fig. 9 and referred to this figure in a later section (Line 251), where the\nprevious tremor locations are more relevant to the discussion.\nLines 92-93: “… outer wedge situated on the trenchward side of a megasplay fault….”\nR: In Fig.2 it is clear that the correlation between the magasplay fault and the downdip of\ntremor events is even better than the correlation between the updip of this activity and the\n“kink” barrier. In my opinion profiles figures and text should be modified to include a\nmore complete discussion of the downdip relation between tremors and continental\nwedge structure.\nReply: We agree with the reviewer’s comment. In fact, Reviewer #2 in the previous round\nmade a similar suggestion. Considering these comments seriously, we have substantially\nrevised the manuscript to expand the discussion on the downdip limit of tremors (Lines\n159–175, 262–293). We have also modified the manuscript title to emphasize both the\ndowndip and updip limits.\nLines 93-95: “… The outer wedge is divided into two geological 93 zones…” R: Please\ninclude the limit between ITZ and FTZ in figure 2. This should help to note that the updip\nlimit of slow earthquake is correlated with this limit only in the zone B.\nReply: We have added the ITZ–FTZ boundary to Fig. 2a, as suggested by the reviewer.\nLines 96-97: ..two subparallel right-lateral strike-slip faults located in the ITZ… R:\nWhere are this right-lateral structures?. Please include the reference to the corresponding\nFigures.\nReply: We have added references to the corresponding figures as suggested (Line 110).\nLines 114-119: “…to region C, which lacks such features, aligns with the eastern\nboundary of the tremor distribution. Furthermore…” R: C is the estern limit of this tremor\nepisode, but this does not mean that future tremors episodes could not be located seaward\nin the segment C. To interpret that, the authors should analyze a much longer period of\nseismicity along the margin. In my view the analysis do not prove the tectonic control of\nthe updip seismicity in C.\nReply: We appreciate the reviewer’s insightful comment. Indeed, our analysis focuses\nonly on the most recent tremor episode. Because the tremor distributions from earlier\nepisodes are not well constrained, we decided not to extend our interpretation to previous\nor future episodes. Accordingly, we have deleted the relevant paragraph from the\nmanuscript.\nLines 154-155:….current study indicate that the propagation of SSEs along the\ndécollement is hindered at the kink or branching points…” R: Respecting to figure, the\ncorrelation between the updip of SSEs and the kink or branching points is clearly\nobserved at the profiles B and in the profile A2. However, analyzing the profile A1 (in\nthe supplementary material) the “kink” seems to be located landward from the point\ninterpreted by the authors, then at the profile A1 the correlation with the updip of SSEs is\nnot clear. Geometrically, the “kink” points seem to be correlated with the landward limit\nof the FTZ high. Maybe this observation is debatable, but in any case, it is important the\ninclusion of figures of the seismic profiles without interpretation (in the supplementary\nmaterial) to facilitate the observation of reflectors near the decollement.\nReply: While we are confident in the location of the kink, which we define as the\nintersection between the décollement and the steep thrust for profile A1, we acknowledge\nthat its interpretation may be debatable. Following the reviewer’s suggestion, we have\nadded the profiles without interpretation to Supplementary Figs. 5–7 to facilitate direct\nobservation of the reflectors.\nLines 159-164: “…the steep thrust experiences higher normal stress than the gently\ndipping section of the décollement…” R: Respecting the discussion of Normal stresses,\nthe authors include an analysis that is correct under the assumptions of the horizontality\nof the maximum stress and constant frictional conditions. However, both assumptions are\ndifficult to associate with an accretionary wedge deformed under compressive forces,\ngravitational forces and different internal an basal effective friction coefficients. In fact,\nthe formation of a kink should be related precisely with a more complex stress field inside\nthe wedge (see for instance the simple model of Dahlen et al. 1984, Noncohesive critical\nCoulomb wedges: An exact solution). On the other hand, the observation of the current\nand paleo deformation fronts should be related with variations of the basal effective\nfriction coefficient and maybe with the aseismic deformation of sedimentary units near\nthe deformation front. In my opinion, a more detailed discussion of the stress and strain\nfield in accretionary wedges should be considered for the interpretation of structures and\nthe SSEs episode.\nReply: We greatly appreciate this constructive comment. Following the reviewer’s\nsuggestion, we estimated the orientation of the maximum principal stress using the\nCoulomb wedge theory. In this calculation, reasonable parameters for the friction\ncoefficient and related values were adopted (see Supplementary Table 1), based on\nlaboratory experiments conducted for the study region (Okuda et al., 2021, JGR). Even\nwhen considering the tilted stress orientation estimated in this way, our interpretation—\nthat the steep thrust experiences higher normal stress than the gently dipping section—\nremains valid (Lines 547–577). Furthermore, Okuda et al. (2020) reported a higher\nfriction coefficient for the décollement beneath the FTZ than that beneath the ITZ, which\nwould further increase fault strength along the steep thrust, consistent with our\ninterpretation (Lines 188–190).\nLines 237-239: “…This lack of deformation is likely due to a stress shadow caused by\nthe subduction of the Paleo–Zenisu ridge..” R: where is the Palo Zenisu_Ridege? Please\nindicate in maps and profiles. What is the definition of stress shadow used here? The\nauthors of the reference 47 defined shadows zones seaward from subducted oceanic\nfeatures and below inactive decollements, which seems to be not the case of profiles C.\nWhat is the subducted oceanic feature (observed in the profile) that generates the “stress\nshadow”? Please indicate in maps and figures.\nReply: We acknowledge that the previous version of the manuscript was somewhat\nunclear. We have now indicated the location of the subducted Paleo–Zenisu ridge in the\nprofile (Supplementary Fig. 4). The map view of the Paleo–Zenisu ridge was already\npresented in previous figures (e.g., Fig. 1), but we have improved clarity by explicitly\nlabeling it as the “subducted Paleo–Zenisu ridge.”\nThe numerical setup of Miyakawa et al. (2022) may not fully represent the situation along\nprofiles C1 and C2, as the height of the subducting ridge (or topographic high) in their\nmodel is much smaller (~100 m) than that in our study area. In this study, we consider\nthat the stress-shadow zone (or zone of reduced tectonic loading) extends up to the\nshallow portion near the seafloor—at least to the depth where well-preserved horizontal\nstrata are observed. This broader effect is consistent with the simulation results of Sun et\nal. (2020), who modeled a subducting seamount of comparable scale to that in our study.\nIn addition, we found that the horizontal strata observed along profile C1 may be related\nto another undocumented ridge, separate from the Paleo–Zenisu ridge. We have clarified\nthis interpretation in the main text (Lines 304–306) and in Supplementary Fig. 4.\nSupplementary material: There are some signs “Error!” in figure captions.\nReply: All figure caption errors in the Supplementary Material have been corrected.\nReviewer #5 (Remarks to the Author):\nReview by Madison Bombardier of the manuscript “Geometrical barrier determines the\nupdip limit of slow earthquake slip” by Akuhara et al.\nThis manuscript uses a previously-published tremor catalogue to examine the relationship\nof shallow slow slip (using tremor as a proxy) to subduction zone structures imaged by\nseismic reflection in Nankai. Their main finding is the interpretation that the updip limit\nof slow slip is bounded by an abrupt steepening of the main thrust fault (top of\ndecollement) in most of the region. They infer that the impedance of slow slip on the steep\nfrontal thrust segment may imply impedance of coseismic slip during regular earthquakes,\nwhich may have consequences on tsunami generation and associated hazards.\nThree reviewers during the first round of peer review requested: (1), clearer and more\nrobust mechanistic explanations of slip impedance along the steep frontal thrust in terms\nof pore-fluid pressure and stress; (2), discussion of the downdip limits of tremor; and (3),\nmany editorial changes. The authors have reasonably addressed most of the comments\nfrom these reviewers, with some exceptions (see my comment number 29 below);\nhowever, some explanations and claims are vague or poorly supported.\nThe work presented in this manuscript is important to the understanding of subduction\nzone science, and represents an effective use of tremor catalogues for the study of this\ntopic. After minor revision considering my comments below, this manuscript is worthy to\nbe published. I have two main comments, and more than 3 dozen minor comments.\nReply: We thank Reviewer #5 for joining the review process and for providing thoughtful\nand constructive comments. We appreciate the reviewer’s recognition of the importance\nof our study and have carefully addressed all the points raised in the following responses.\n(1) Main comment:\nThe authors claim in multiple locations throughout the manuscript that the tremor\ncatalogue they use contains “precise” tremor locations; however, evidence for this claim\nis not provided. The only relevant information provided is that the horizontal uncertainty\non each tremor epicentre is estimated by one half the width of the 95% confidence interval\nto be “a few kilometres” (lines 72–73). It is not clear to me that this is precise, or even\nmore precise than other methods, nor do I think that such a claim is necessary. The\nprimary advantage of Bayesian inference is that solutions are provided as probability\ndistributions, which gives unique 3D probability distributions of each tremor event from\nwhich uncertainty can be quantified; there is no guarantee of precision. In other words,\nthis catalogue provides robust uncertainty estimates. Because of this, the tremor catalogue\nmay be considered reliable or credible, but not precise. This is especially true in the\ncontext of this analysis where multi-channel seismic reflection data is used with\nresolution on the order of 100 m (line 82).\nDespite that the primary strength of this tremor catalogue is the detailed uncertainty\nestimation, the authors do not provide quantitative estimates or distributions of these\nuncertainties, nor do they consider them in their analysis. Supplementary figure 1 shows\nall tremor epicentres with 95% confidence interval error bars, but the superposition of the\ndensely-clustered events obscures most of this information.\nHere I make several suggestions to address these issues:\n- remove descriptors such as “precise” everywhere related to the tremor catalogue\n- in the main text, replace “a few kilometres” (lines 72–73) with mean or median\nhorizontal uncertainty estimates, and/or state the range within which 50% or 75% or\n90% (or whatever) of horizontal uncertainties are in.\n- at the location where the tremor catalogue is introduced in the main text, briefly\nexplain the advantage that Bayesian inference provides to this catalogue in terms of\nrobust uncertainty estimation. Probabilistic solutions are used to represent the entire\ncatalogue as one probability map, which provides more information than a scatterplot\nof point sources. A probability map is an unusual way to represent a catalogue of\nseismic events, and you should clearly explain the importance and novelty of this to\nencourage readers to adopt similar methods in future work.\n- change supplementary figure 1 to better represent the geographic distribution of\nuncertainties. I suggest to segment the area into 1 km (or 5 km, or whatever) squared\ncells and colour each cell according to the average (or median) uncertainty of tremor\nevents in that cell. In this case, you would need two panels in the figure: one for NS\nuncertainty and one for EW uncertainty. This method would allow the reader to view\nthe heterogeneous geographic distribution of uncertainty over the study area. I expect\naverage uncertainties vary systematically over the region, which may be useful\ninformation for the main analysis. In particular, I wonder about the EW uncertainties\non the downdip tremor patch in the far north of the study area (region C). This type\nof information should be clearly discernible from this figure. An example can be\nfound in the supplementary information of Bombardier et al 2025 (doi:\n10.1016/j.tecto.2025.230752).\nReply: We appreciate the reviewer’s insightful comments. As suggested, we have\nremoved subjective descriptors related to tremor locations, such as “precise,” throughout\nthe manuscript and have emphasized the primary advantage of the method—its ability to\nprovide reliable uncertainty estimates—in the Introduction section (Lines 73–78).\nAlthough this may differ slightly from the reviewer’s perspective, we have retained the\nphrase “which provide better constraints than conventional time-difference data,” as the\nsuperiority of amplitude data over time-difference data has been demonstrated for this\ndataset (Akuhara et al., 2023, GJI).\nIn addition, following the reviewer’s suggestion, we have added panels showing the\nmedian uncertainties within each geographic cell to Supplementary Fig. 1, and we now\nprovide a more quantitative description of the uncertainty in the main text (Lines 81–83).\n(2) Main comment:\nInterpretations made in this analysis require a framework of SSEs wherein slow slip\ndirectly generates/triggers tremor, and wherein both tremor and slow slip occur on the\nmegathrust fault (in this manuscript, the top of decollement). It is not my understanding\nthat this framework is universally accepted as certain fact, thus the high degree of\ncertainty with which it is presented is not warranted.\nRegarding lines 60–62: it has been shown that tremor does indeed tend to occur near the\nslip front in Cascadia (eg Hall et al 2019, Bartlow et al 2011, Luo and Liu 2019), but I do\nnot think there is sufficient evidence to use the word “triggered”, since there may be\nmultiple processes occurring at the slip front that may cause, generate, or trigger tremor.\nIn addition, shallow slow slip may not be perfectly analogous to deep slow slip where this\nhas been studied more extensively. The brittle-asperity-in-ductile-matrix model (Luo and\nLiu 2019) is a plausible hypothesis (among others) to explain some elements of tremor\nand slow slip, and using this framework is justifiable, but it should be presented as a\nframework or base-level assumption rather than a known fact about the physical\nmechanisms.\nRegarding lines 149–153: this reasoning is fuzzy. If you assume tremor is directly caused\nby slow slip, as the previous sentence states, how can the depth of tremor “most likely”\nbe “similar” to that of SSEs? Would it not be certain that the depths are exactly the same?\nReference 19 (Toh 2018) states that some VLFEs likely occurred on a splay fault, which\nindicates that the depths of these VLFEs are not localized entirely to the megathrust fault\n(insofar as depth constraints on VLFEs support such an observation); or that the\nlocation/structure of megathrust fault itself is uncertain or complex. In addition, deep\ntremor in Cascadia represented as probability resulting from a Bayesian location method\n(Bombardier et al 2025) indicates that the depth of tremor can deviate significantly from\nthe top of the subducting oceanic crust and is inconsistent along the margin. Again, deep\ntremor/SSEs may not be so analogous to shallow tremor/SSEs, but given the difficulty of\nadequately constraining depths of tremor/LFEs/VLFEs/slow slip, and the uncertainty\nregarding the relationship between these phenomena, consideration that these processes\nmay occur beyond the megathrust fault is warranted. The possibility of tremor and SSE\nactivity beyond the megathrust fault has important implications for discerning active\nstructures and understanding hazards, and the study of this topic should not be\ndiscouraged by premature or simplifying assertions.\nMy suggestion to address this issue is to clearly explain that the analysis and\ninterpretations presented in this manuscript requires the assumption of a specific\nframework/hypothesis about the nature of the relationship between tremor and slow slip,\nand about which structures host this activity due to the fact that tremor depths are not\nresolved in this catalogue. You already have many references to use to justify the use of\nsuch a framework. Also explain that variation exist between regions globally and\nuncertainty regarding the depth and host structures for such activity is prevalent, more\nwork on these topics is needed. These changes largely involve reframing and rephrasing\nat various locations in the manuscript.\nReply: We thank the reviewer for this constructive and thoughtful comment. In the\nrevised manuscript, we have clarified the working hypothesis in the Introduction\nsection—that tremors accompany SSEs—and immediately followed it by noting the\npresence of observations that do not conform to this framework, in order to clarify its\nlimitations (Lines 61–65). We agree that synchronization among tremors, VLFEs, and\nSSEs has been most frequently reported in deep slow-earthquake regions (i.e., downdip\nof the locked zone). However, in the Kumano-nada region, synchronization between\nVLFEs and SSEs has also been observed (Nakano et al., 2018; Edgington et al. 2025),\nowing to the high-sensitivity pressure gauges deployed by the DONET network (Lines\n65 – 67).\nIn the Results section (Lines 319–350) , we have added a subsection titled “Limitations\nand opportunities” to discuss two key limitations not considered within the current\nframework: (1) the possibility of SSEs without associated tremors, and (2) the possibility\nthat tremors occur on structures other than the décollement. For the latter, we refer to the\nrecently published centroid moment tensor catalog of VLFEs (Yamamoto et al., 2022),\nwhich shows focal depths and mechanisms indicating that most VLFEs occur near the\ndécollement. Because tremors and VLFEs are spatiotemporally synchronized\n(supplementary Fig. 10), we infer that tremors also occur near the décollement, possibly\nwithin a mélange shear zone embedded in the LVZ. This interpretation has been explicitly\nstated in the new subsection.\nOverall, we have designed the added section not only to acknowledge the limitations of\nthe current framework but also to highlight potential future research directions. We firmly\nbelieve that this addition strengthens the manuscript by clarifying the scope of our\ninterpretation while pointing toward avenues for further investigation.\n(3) The abstract should include why your results matter. I suggest simplifying the\nsummary of results in the abstract to make space for the broader interpretations/impact of\nthe results mentioned in the “introduction” and “impacts” sections.\nReply: We have revised and reorganized the abstract accordingly to better emphasize the\nbroader implications and significance of our results.\n(4) Lines 51–53 it may be relevant to the present manuscript that the forearc mantle corner\nhas also been shown as a boundary to slow slip by Sawaki et al (2021) and Bombardier\net al (2024).\nReply: While we recognize the importance of these studies, we decided not to include\nthem because they focus on deep slow earthquakes, which are beyond the scope of the\npresent study.\n(5) Lines 99–100 regarding a new deformation front, this should have a citation.\nReply: We have added the appropriate citations to support this statement (Line 113).\n(6) Line 101 the tremor probability map is mentioned for the first time without\nexplanation of what it is. A reference to the methods section where the probability map is\nmathematically defined is missing.\nReply: We have added a reference in the Introduction to the Methods section that defines\nthe tremor probability map (Line 78).\n(7) The ITZ-FTZ boundary denoted in Figure 2b by the purple dashed line would be\nhelpful in panel (a) of this figure, so that the spatial correspondence between this\nboundary and the updip limit of tremor is very clear.\nReply: We have added the ITZ–FTZ boundary to Fig. 2a, as suggested by the reviewer.\n(8) It is not that easy to discern the annotations in Figure 2b. The dark colour of the\nbathymetry shading makes it difficult to clearly see the thin black font of the text and\ntransparent blue/red lines. It would be better to make these annotations more pronounced,\npossibly by decreasing their transparency, making the text font bolder, and/or increasing\nthe transparency of the grey bathymetry shading.\nReply: We have adjusted the background color of Fig. 2b to a lighter tone so that all\nannotations are more clearly visible.\n(9) Lines 115–117 here is an example where it is important to know the uncertainties on\ntremor in region C. “much further landward” may not be a well-supported observation\ndepending on the uncertainties.\nReply: We have added a new panel showing the median uncertainty within each\ngeographic cell to Supplementary Fig. 1. This figure confirms that tremor occurrence\nextends farther landward in region C.\n(10) Lines 118–121 This sentence is long and awkward, and should be in present tense\nrather than in a past tense narrative-style description of what you did. For example, try\n“To explore the apparent agreement between the tremor distribution and bathymetric\nfeatures, we [analyze]…”\nReply: We have rephrased the sentence to “To further explore the correspondence\nbetween the tremor distribution and bathymetric features” and changed it to the present\ntense (Lines 132–134).\n(11) Supplementary figures 2–4 have a “Error! Reference not found” note in the captions.\nAlso the caption in supplementary figure 2 should reference “inverted triangles” (plural),\nnot “inverted triangle”. These inverted triangles in profile A1 are difficult to see clearly;\nI did not realize they were triangles at first. Perhaps make them bigger with a black\noutline?\nReply: We have corrected the erroneous references to Fig. 3 and changed the color of the\ninverted triangles to a darker tone to improve their visibility.\n(12) Lines 124–125 “almost subparallel” doesn’t really make sense. I suggest rephrasing\nas “almost parallel” or simply “subparallel”. In addition, this short sentence could be\njoined with the previous to make it less awkward. For example, “The most prominent\nfeatures … oceanic crust and decollement, which are subparallel within the ITZ.”\nReply: We have changed “almost subparallel” to “subparallel” and combined the two\nsentences, following the reviewer’s suggestion (Line 137).\n(13) Line 131 I suggest changing “new” and “old” deformation front to “deformation\nfront” and “mega splay”. To say “new” implies it is a new interpretation in this manuscript,\nand it can confuse the reader when there is no discussion of this new feature. This requires\nchanges to annotations in figures as well.\nReply: We agree that the terms “new” and “old” could be misleading. We have replaced\nthem with “younger” and “older” to better reflect their relative development stages while\navoiding the implication of a new interpretation (e.g., Line 113). We did not adopt the\nterm “megasplay” suggested by the reviewer, because in this region the term is usually\nused for the fault along the boundary between the inner and outer wedges. The figure\nannotations have been updated accordingly.\n(14) Lines 143–144 Supplementary figure 4 does not actually show the tremor\ndistribution along profile C1 and C2. The map in Figure 3 is a more appropriate reference\nfor this observation. However, as a reader, I find that I need to refer to both figure 3 and\nsupplementary figure 4 to fully understand this observation, which is cumbersome. It\nwould help to include the tremor probability distribution along each profile in\nsupplementary figures 2–4 (as is done in figure 3).\nReply: We have added the tremor probability distributions to the profiles in\nSupplementary Figs. 2–7, as suggested. This addition allows readers to understand the\nobservations without referring to both Figure 3 and the supplementary figures.\n(15) I suggest including an annotation in supplementary figure 4 to identify the subducting\nridge. I can guess what constitutes the ridge, but better to identify it clearly because I may\nbe wrong.\nReply: We have added annotations indicating the location of the subducted ridge in\nSupplementary Fig. 4, so that readers can clearly identify it.\n(16) Lines 153–155 this is the only location figure 4 is referenced. Perhaps this early\nreference to this figure should be removed so the figures can be ordered logically; I do\nnot think it adds much at this point in the manuscript. Figure 4 represents a summary of\nyour interpretations, so it should be explained near the end of the manuscript and used to\nsupport such a summary. Notably, a brief summary of the new observations and\ninterpretations do not really exist, but are apt.\nReply: We appreciate this helpful suggestion. We have moved the figure to a newly added\nsubsection titled “Conclusions” (Lines 383 – 414). In this section, each finding is\nsummarized in bullet-point form, and the corresponding portions are displayed in the\nfigure with matching numbering for clarity. We firmly believe that this revision better\ndemonstrates the necessity and relevance of the figure than in the previous version.\nI think lines 153–155 should say “… the propagation of SSEs along dip is hindered…”\nsince figure 4 also shows the ridge stress shadow and the strike slip faults hinder along-\nstrike SSE propagation.\nReply: We have added “along-dip” to this sentence (Line 181).\n(17) Lines 155–158 reference 40 (Wech and Bartlow 2014) is a study from Cascadia that\nobserved one instance of an SSE occurring without tremor for a short time. While I agree\nthat these cases seem infrequent in Cascadia, I do not think this is relevant to shallow\nSSEs in Nankai. It is reasonable to exclude SSEs without tremor, but this should be\nexplained and justified near line 60 where the framework, assumptions, and unknowns\nare explained. And this choice should be supported with observations for the relevant\nregion. Also, related to my main comment (2), SSEs occurring without tremor are\nincompatible within a framework wherein slip generates tremor, as stated just three\nsentences before this. If there are SSEs that occur without tremor, such that they are\nexcluded from this analysis, maybe the framework needs to be revised?\nReply: As mentioned in our response to comment (2), the limitation of our framework—\nparticularly the possibility of SSEs occurring without accompanying tremors—is now\ndiscussed in a newly added subsection (Lines 319–350). We also briefly note this\nlimitation in the Introduction (Lines 64–65) and at the beginning of the discussion section\n(Lines 182–184).\n(18) Line 175 “distinctly different” is a redundant phrase. “Distinct” and “different” are\nsynonyms.\nReply: We have removed “distinctly”, as suggested (Line 205).\n(19) Line 176 rephrase this as “A stratified pattern observed at the trench side of the steep\nthrust is absent on the landward side.” and it would be better to reference figure 3 rather\nthan supplementary figure 2, since this feature can be seen in the main-text figure.\nReply: We have revised the sentence as suggested (Lines 205–206) and now refer to\nFigure 3 instead of Supplementary Figure 2.\n(20) Line 185 “in distance” is unnecessary because kilometre is a unit of distance.\nReply: We have removed “in distance”, as suggested (Line 242).\n(21) Line 186 briefly explain why the slip is diffusive, and why diffusive slip is\ncharacterized by a hyperbola.\nReply: We acknowledge that the term “hyperbola” was erroneously used and have\ncorrected it to “parabola,” where the propagation distance is proportional to the square\nroot of time. This parabolic feature commonly appears in diffusion processes. Thus, the\nterm “diffusive” is used here in a descriptive sense, rather than implying a specific\nphysical mechanism. We have added an explanation in the main text that this parabolic\npattern can be interpreted as the result of an SSE propagating along a heterogeneous fault\nsurface, where tremors occur on small-scale brittle patches (Ando et al., 2012) (Lines\n244–246).\n(22) Line 191 “bounded” is different from “impeded”. In this manuscript, the tremor\nmigration is impeded, not bounded by the strike slip faults.\nReply: We have rephrased this sentence to “Tremor activity in April 2016 also appears to\nhave terminated when the migration front reached this strike-slip fault” to more accurately\nconvey our intention (Lines 250–251).\n(23) Line 197 “horizontally” is unnecessary.\nReply: We have removed it, as suggested (Line 256).\n(24) Lines 197–198 could slip have propagated along one of the strike slip faults? This\ncould explain why the second small SSE initiated landward of the termination of the larger\nSSE (roughly along the strike of the strike slip faults).\nReply: This is an interesting possibility, but assessing whether slip propagated along the\nstrike-slip faults is beyond the scope of this study, as it would require detailed evaluation\nof the errors and the definition of the projection profiles.\n(25) grey dots in figure 5b are not explained anywhere.\nReply: We have added the explanation to the figure caption.\n(26) Line 203 rephrase to “Although its spatial distribution is not well constrained, the\npotential role of pore-fluid pressure in the study region is worth discussing here.”\nReply: We have rephrased it, as suggested (Lines 214–215).\n(27) Lines 205–206 the transparent facies are observed immediately above the\ndecollement, not oceanic crust, according to figure 3.\nReply: We have changed it to “above the décollement” (Line 217).\n(28) Line 211 rephrase to “… high pore-fluid pressure is interpreted to be sustained by …”\nReply: We have rephrased it, as suggested (Lines 222–223).\n(29) Lines 212–216 this section was added in response to comment 1-15 from the first\nreview. It is awkward and oddly written as a string of short facts that are not presented\ncohesively. How exactly are these inferences relevant to the analysis?\nExplain “tectonic push” and what “these structures” are that may create tectonic push;\ndoes this refer to basement highs? Not clear.\nReply: We agree that this paragraph was not cohesively written and only loosely related\nto the main discussion. As it was an overreaction to Comment 1-15 from the first review,\nwe have decided to remove it in the revision.\n(30) Lines 237–238 it is my understanding that subducting ridges and seamounts lead to\nhighly fractured/deformed accretionary prisms/forearcs. In order for a lack of deformation\nin the prism to result from the stress shadow of a subducted ridge, it seems to me that the\nundeformed prism material would have to have been accreted after the ridge (in its shadow)\nin order to remain relatively undeformed. If the prism material was accreted first, then the\nridge would have subducted under it, and the explanation given in the manuscript does not\nmake sense to me. In this case, lack of deformation within the prism would not be a\nreliable indicator of the existence of a stress shadow. Please clarify the accretionary history\nand the relationship between subducted ridges/seamounts and prism deformation. Maybe\nthere is an alternative explanation for the reflective strata?\nReply: We appreciate the reviewer’s thoughtful comment. According to Kimura et al.\n(2011), the ITZ had already formed at the initiation of ridge subduction. Nevertheless,\neven if the ridge subducted later, the tectonic loading subsequently applied to the prism\ncould have been reduced by the stress shadow of the subducted ridge, as demonstrated by\nnumerical simulations (e.g., Sun et al., 2020). This stress-shadow effect provides a\nreasonable explanation for the relatively weak deformation observed in this region\ncompared to adjacent areas.\nWe do not mean that the overriding prism does not deform at all; rather, the deformation\nmay occur only in a limited portion, as indicated by several sparsely distributed thrusts\nobserved in the profile.\n(31) Line 252 rephrase to “… large earthquake may have facilitated tremor ...”\nReply: The paragraph containing this sentence has been removed from the revised\nmanuscript in accordance with another reviewer’s suggestion.\n(32) Line 257 I believe the boundary between the inner and outer wedges is the megasplay\nfault shown in figure 2b and mentioned on line 93. I suggest including the “megasplay”\nterm here with reference to figure 2b so it is clear.\nReply: We have explicitly indicated the location of the boundary between the inner and\nouter wedges in Figure 2b, which indeed corresponds to the megasplay fault.\n(33) Line 258 “distinctly differ” is redundant\nReply: The paragraph containing this sentence has been revised in accordance with\nanother reviewer’s suggestion, and the corresponding phrase is no longer used in the\nmanuscript.\n(34) Lines 262–263 please explain how a discontinuous stress state in the outer forearc\nwedge would affect slip on the megathrust.\nReply: The revised text now elaborates on how an abrupt change in the stress state can\nimpede slip propagation along the megathrust (Lines 287–289).\n(35) Line 264 the use of the term “separate” is confusing here. The tremor episode that is\nthe focus of this manuscript occurs in the transient slip zone, so how can it separate the\ntransient slip zone from the fully locked zone? I think this should be phrased “the downdip\nlimit of this tremor episode may not be a universal representation, as SSEs and tremors\nhave occurred further downdip in earlier episodes.”\nReply: We have revised the phrase to “the downdip limit for this episode may not\nnecessarily correspond to the boundary between the transient-slip and locked zones”\n(Lines 290–291).\n(36) Line 283 “numerous numerical” is a slightly awkward phrase. I suggest “multiple”,\n“several”, or any other synonym for “numerous”.\nReply: We have replaced “numerous” with “multiple,” as suggested (Line 367).\n(37) Line 294 I do not think Hikurangi is a relevant example here because tremor is not\nclosely associated in space or time with shallow SSEs (eg Todd et al, 2018). As such, the\ntremor location methods used in this paper would be useful for locating tremor in\nHikurangi, but not be useful for approximating slow slip.\nReply: We have removed the reference to Hikurangi from this sentence, as suggested\n(Line 379).\n(38) There are numerous grammatical errors and typos throughout the main text and\nsupplementary material, in addition to the ones mentioned above. Text added in response\nto the earlier reviewers could be integrated better so that the text flows well and seems\ncohesive.\nReply: We have carefully checked the entire manuscript for grammatical errors and typos.\nIn addition, rather than addressing each reviewer’s comment in isolation, we have\nreorganized the text to improve the overall flow and coherence of the manuscript.\n(39) Inconsistent use of “Figure” and “Fig” (eg line 212), and bolded text for figure\nreferences (eg line 206).\nReply: We have made the usage of “Figure” and “Fig.” consistent throughout the\nmanuscript and removed bold formatting from figure references."}, "author_response": ""}]

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