Electromagnetic imaging reveals insufficient fluids to explain shallow megathrust creep at the Shumagin Gap

Domain: biology
Published: 2026-03-28
Editorial outcome: minor_revision

Abstract
The Shumagin Gap, a creeping segment of the Alaska subduction zone characterized by tsunamigenic structures, experienced a deep rupture during the July 2020 M7.8 earthquake. However, shallow slip behavior and the upper boundary of the rupture remain poorly understood. Here we utilize controlled-source electromagnetic data to image subsurface electrical resistivity, investigating the role of fluids in modulating megathrust locking state within the Shumagin Gap. Results reveal pronounced trench-normal heterogeneity in electrical resistivity both along the shallow plate interface and within the overriding plate, showing fluid presence but low overall porosity at the interface. An observed conductive channel extending into the overriding plate may facilitate upward fluid drainage. Our findings suggest that the volumes of fluids and inferred pore pressures are not sufficient to explain megathrust creep at the Shumagin Gap. Rather, the intricate interplay between heterogeneous structure and fluid distribution contributes to the region's seismogenic behavior and tsunami hazards, particularly in the shallow portion of the megathrust.

Introduction
Understanding the key mechanisms that govern megathrust slip behavior and seismic activity is essential for advancing our knowledge of subduction zone processes and assessing related seismic hazards. Fluids have been thought to profoundly influence or control many subduction zone processes1–5. Fluids are primarily released from sediment compaction in the outermost forearc6 and through dehydration reactions farther downdip7. Along megathrust faults, these fluids affect the mechanical behavior of faults by altering pore pressure1. One prevailing hypothesis suggests that excess pore pressure along the megathrust reduces the effective normal stress and therefore promotes creep8,9. Yet, the extent to which fluid-driven processes influence plate interface slip behavior remains poorly understood.

The Shumagin Gap of the Alaska-Aleutian Subduction Zone (AASZ) offers an ideal target to investigate the relationship between fluids and the nature of megathrust slip (Fig. 1). This region behaves differently from its neighboring segments2,10,11 and is recognized as a seismic gap because no significant earthquakes (Mw > 8) had occurred for at least a century12. Specifically, the Shumagin Gap is distinguished by abundant interplate and intermediate-depth seismicity2, extensive faulting associated with plate bending2,13, a rugged lower-plate relief, and a sediment-starved trench2,3,14. Despite the lack of prominent seamounts on the incoming plate and the absence of clearly resolved subducted seamounts, the plate interface remains highly rugged, characterized by extensive topographic relief associated with bend faulting. In addition, geodetic observations show a weakly locked plate interface in this region10. Past studies present conflicting evidence regarding fluid accumulation in the Shumagin Gap. Seismic imaging suggests that intense bend faulting in the incoming plate promotes hydration in the crust and upper mantle and introduces a substantial amount of fluid to the system2,15,16. Yet this additional fluid likely impacts intraslab seismicity and subduction processes farther downdip, rather than influencing effective stress and fault creep within the seismogenic zone2,17. Multi-channel seismic reflection data have revealed thin and irregular subducted sediments, leading to the inference of low pore fluid pressure in the shallow portion (within 12 km of the trench) of the Shumagin Gap3. Furthermore, magnetotelluric (MT) data used to image the deeper resistivity structure beneath the forearc shelf18 observed a similarly fluid-starved plate interface at depths shallower than 25 km (between 60 and 90 km from the trench), with a segment of larger fluid accumulations at greater depths (100–140 km from the trench). In contrast, Vp/Vs ratios at 15–35 km depth have been used to infer that both the plate interface and overriding plate are fluid-rich throughout the Shumagin Gap5, although the shallow portion (<10 km) of the megathrust near the trench remains poorly imaged. Contributing further complexity is the rugged plate interface geometry seen in seismic reflection imaging3,14, which might be causally related to fault creep. In addition, the slip behavior and degree of geodetic locking of the shallow plate interface remains enigmatic19–22,5, with incomplete understanding of along dip variations in slip behavior.

The recent sequence of seismic events highlights the complex along-strike segmentation in megathrust locking and slip behavior within the AASZ. The July 2020 M7.8 Simeonof earthquake initiated near the border between the Semidi segment and Shumagin Gap and propagated westward to the eastern Shumagin Gap23, but ruptured the deeper portion of the seismogenic zone and therefore only partially filled the gap11. This event was followed by an unusual M7.6 strike-slip intraplate earthquake in October 2020 that also involved slip on the megathrust and/or an upper plate fault24,25. Subsequently, a M8.2 megathrust earthquake struck to the east of the Shumagin Gap on 29 July 2021, primarily rupturing the western part of the 1938 M8.2 rupture zone. This event exhibited distinct slip distributions and seismic characteristics, indicating that it was not a repeat of the 1938 earthquake26.

According to MT imaging, the July 2020 rupture propagated through a deeper, fluid-rich segment of the plate interface around 30 km depth, rather than rupturing updip into more resistive, fluid-poor areas18, possibly because the fluid-rich portion of the plate interface is conditionally stable20,27. These fluid-rich regions, where high pore pressures may exist, can promote stable fault creep during interseismic periods. However, they can become unstable and experience dynamic rupture when triggered by nearby stress concentrations which initiated rupture elsewhere.

The structural and compositional properties of the overriding plate also appear to affect megathrust slip behavior28–30. In the Shumagin Gap, an active deep-rooted normal fault separates the Cretaceous Chugach Terrane from the younger, Paleocene Prince William Terrane (PWT), potentially impacting tsunami hazards14. Variations in the seismic velocity of the overriding plate also suggest changes in rigidity that could influence megathrust slip behavior, segmentation, and seismicity patterns30,31. The overriding forearc crust offers key insights into fluid release and redistribution, fluid-driven fault weakening, and pore pressure variations4,32,33. Understanding the fate of these fluids within the forearc crust is especially critical for constraining shallow slip behavior and tsunamigenesis.

In this study, we use controlled-source electromagnetic (CSEM) data to image the electrical resistivity of the shallow megathrust and overriding plate in the Shumagin Gap. Our CSEM profile extends from ~10 to 120 km inboard of the trench axis, revealing the resistivity structure of the shallow megathrust updip of the collocated MT study18 and providing higher resolution imaging in the upper plate. Electrical resistivity is notably sensitive to fluids (e.g., Naif et al.34) and hence serves as a powerful proxy for fluid volume and distribution at depth. Our inverted resistivity model and subsequent porosity estimates and analyses identify a shallow, fluid-poor plate interface with low fluid pressures, indicating that high pore pressure does not fully account for its creeping behavior. Instead, slip mode and seismic hazard in this region are likely governed by the interplay among multiple factors, such as limited fluid availability along the plate interface, interface roughness, and heterogeneities in the rigidity, stress state, and permeability of the overriding plate (e.g., Bassett et al.35). By offering constraints on near-surface fluid distributions and bridging the gap between prior seismic and geodetic observations, the CSEM data can advance our understanding of how along-dip changes in fluids, structure, and slip behavior influence both seismic hazard and the broader subduction system.

Results
Heterogeneous outer forearc and shelf electrical structure
We present our preferred resistivity model obtained through vertically transverse anisotropic modeling of the CSEM data collected along the survey line in this study (Figs. 1, 2, Methods). In the outer forearc, a dipping conductive zone (C1) extends from the frontal prism to approximately 50 km landward of the trench at about 10 km below the seafloor. The along-dip variation in electrical resistivity suggests scattered patches of moderately conductive structure along the plate interface, likely reflecting irregularities at the plate interface and heterogeneity along the subduction zone boundary. Given the paucity of subducting sediments3,14 and the abundant bending faults in the incoming plate2,13, the observed elevated conductivity along the slab perhaps implies a complex interaction between subducting and overriding plates, in which fluids derived from the subducting oceanic crust can migrate into and accumulate within the overlying forearc crust.

Close to the trench, a small and relatively high-conductivity frontal prism is bounded by possible backstop splay fault zones. Such structures can be prone to tsunamigenesis36. Farther landward, between the East Sanak Basin and the frontal prism, conductive channels cut across the resistive margin framework. These channels suggest the presence of localized permeable zones, possibly related to faults, that act as fluid pathways from the plate interface up into the upper plate, similar to conductive features observed on the Nicaragua forearc slope37. One notable conductive channel appears at 50–70 km landward of the trench (C2), stretching from the downdip edge of C1 to the seafloor slope sediments. Given the complex near-surface electrical resistivity structures in this forearc slope, additional geochemical sampling of fault-associated seeps is needed to determine whether slab fluids escape to the seafloor through this channel. This large, seaward-dipping conductive zone (C2) also separates the electrically heterogeneous, relatively shallow framework material from a deeper, more resistive Prince William Terrane (PWT). A localized high-velocity anomaly (Vp > 5.5 km/s) delineated by white contours in Fig. 2c30 spatially aligns with a high-resistivity anomaly (R1). The sharp contrast between R1 and conductive frontal prism suggests a boundary separating the strong, older framework materials from the younger frontal prism. The geometry of the East Sanak Basin, which thins to the north, is well resolved in our resistivity model (C3; Fig. 2a, b). This thick (~ 6 km) and electrically conductive sedimentary basin attenuates the electromagnetic signal penetration, reducing the data sensitivity and resolution to deeper structure below the basin (Methods). This limits our ability to determine whether fluids are associated with the large normal fault (Fig. 2a, b) which extends upwards from the plate interface and separates Chugach and Prince William Terranes, though this fault does extend into the East Sanak Basin and offsets several sedimentary units as seen by active-source seismic reflection imaging14.

Low porosity and fluid pressure along the plate interface
We apply the empirical Archie’s law38 to estimate the porosity from our preferred electrical resistivity model (Fig. 2, Fig. S2, Methods). We show the estimated along-dip porosity under three scenarios of thermal gradient (𝐺) and cementation exponent (𝑚) in Fig. 3. Beyond ~10 km landward of the trench, the baseline estimate (𝑚 = 2, 𝐺 = 10 °C km−1 ) shows that the maximum porosity stays around 15 at approximately 2 km below the seafloor. Even for the upper bound porosity endmember (𝑚 = 2.2, 𝐺 = 5 °C km−1), our estimate remains below 20 . Within ~10 km landward of the trench, we did not deploy receivers, and water depths exceed the maximum operational rating of our deep-towed electromagnetic transmitter system. Consequently, data coverage is insufficient to resolve plate interface porosity in the near-trench region. However, seismic reflection data3 suggest only thin or highly compacted sediments with low porosity and low pore pressures subducting in this region. Furthermore, porosity inferred from pre-stack depth migration derived P-wave velocity39 in the Shumagin Gap agrees well with our CSEM-based estimates at around 11 km landward of the trench3. This independent observation lends confidence to our inferred porosity trend farther downdip from the near-trench region in this study. Combining the near-trench porosity estimates from P-wave velocity3 with our observations beyond ~10 km, the porosity estimates in the near-trench region are considerably lower than the compaction trends of many other subduction zones at comparable depths40 and even from the neighboring Semidi segment3. Furthermore, extrapolating a global generic porosity compaction trend41 towards the seafloor as a lower bound still results in higher porosity values than the near-trench estimates obtained for this region.

To investigate the range of pore pressures that are consistent with our porosity estimates, we follow Li et al.3 and compare our estimated porosities with those predicted from sediment consolidation for a range of fluid pressure ratios * (0, 0.2, 0.4, 0.6, and 0.8) in Fig. 3 (Methods), where the value of zero represents hydrostatic fluid pressure. For the near-trench region where our data coverage is limited, we convert the predicted P-wave velocities from Li et al.3 into porosities using the relationship of Hoffman & Tobin39, for the same range of assumed pore pressure ratios. The nearly identical porosity estimates from seismic and CSEM at 12 km landward of the trench highlight how our work complements the findings of Li et al.3. Comparing the predicted porosities for different fluid pressures with the porosity estimated from our resistivity model demonstrates that the shallow plate interface in the Shumagin Gap is primarily consistent with low pore pressure ratios (𝜆∗ ≈ 0–0.4). We note that some porosity estimates may imply unlikely negative 𝜆∗ values, which may be an artifact of our calculation. Our pore pressure calculation assumes that porosity is controlled by sediment compaction. However, the material along the plate interface may not be solely subducting sediments and could contain material of igneous origin (e.g., igneous crust) which has a lower, inherent porosity not governed by a sediment compacting model. In addition, sediments likely endure shear deformation distinct from vertical compaction, which is challenging to constrain with our data and beyond the scope of our study. Overall, despite uncertainties in thermal gradient and cementation exponent (Methods), our results indicate that the shallow plate interface in this region is characterized by limited porosity and low pore pressure, with maximum porosity not exceeding about 20 at ~10 km landward of the trench.

Beyond approximately 17 km landward of the trench, our porosity estimates drop below 11 , which corresponds to the average porosity of the uppermost oceanic crust of the incoming plate in this region15. Considering that the abundant trench parallel bending faults in the incoming plate of the Shumagin Gap2,13 may facilitate fluid penetration into the incoming plate, we interpret the porosity farther downdip as being associated with a broader zone that encompasses the base of the upper plate, the plate interface fault zone, and the extrusive oceanic crust of the down-going plate. Beyond 45 km from the trench, where the plate interface reaches depths of approximately 10 km below the seafloor, the sensitivity of the CSEM data to deeper structures diminishes (Methods).

Methods
CSEM data collection
As part of the ElectroMagnetic Alaskan GeoPRISMS Experiment (EMAGE), we collected CSEM data along a profile in the Shumagin Gap (Fig. 1). This profile extends from the shelf to outer forearc and is co-located with the ALEUT active-source seismic profile Line 52. Ocean-bottom electromagnetic (OBEM) receivers were deployed at 4 km intervals along the profile. In addition, a local array of receivers was deployed on the forearc slope for 3D imaging, which is beyond the scope of this study. Each OBEM receiver was equipped with electrodes and induction coil magnetometers to measure horizontal electric and magnetic field time series in two orthogonal directions with a 62.5 Hz sampling rate. These broadband OBEM receivers can simultaneously capture both high- frequency CSEM and longer-period MT signals55. For CSEM data acquisition, we deep-towed a transmitter55 that injected a 250–300 A source into the water column through a 293 m long horizontal electric dipole antenna. The electric current was generated in a doubly symmetric waveform with a 0.25 Hz fundamental frequency56, which provides higher signal-to-noise ratios for higher frequencies, allowing for a wide frequency spectrum to better constrain the crustal resistivity structure. During deep-tow operations, the transmitter was maintained at an approximate altitude of 100 m above the seafloor (measured by an acoustic altimeter) to ensure energy coupling with the seafloor while avoiding rugged topography. On the continental shelf section (> 83 km from trench) of the profile, where water depths are less than 100 m, the transmitter was towed a few meters below the surface approximately 4 m astern of the vessel. Seawater conductivity, temperature, depth and sound velocity data were measured by sensors attached to the transmitter throughout the operation, as these parameters are critical for navigation accuracy and CSEM inversion. With the exception of the OBEM receivers deployed on the shelf, whose positions were estimated to be the same as their drop locations, the positions of OBEM receivers were determined using classical long baseline acoustic ranging, while the transmitter positions were determined using an inverted long-baseline acoustic navigation system57.

CSEM data processing
We apply a robust stacking method56 to enhance the signal-to-noise ratio (SNR) across multiple frequencies and estimate the variance for the data. First, the raw CSEM time series data were segmented into 4-s windows, corresponding to the fundamental period of the transmitted waveform. The segmented data were transformed into frequency-domain transfer function (TF) estimates of the Earth response by pre-whitening, Fourier transformation, and post-darkening. After normalization by the source dipole moment, the short window (4 s) TF estimates were robustly stacked in 120 s intervals to increase the SNR. After processing, the TF estimates are converted into amplitude and phase responses for each receiver as a function of transmitter-receiver offset at each frequency. The variance was calculated for each stacked data point from residuals after removing linear trends, providing an estimate of the data error. The navigational information of the transmitter and receivers was merged with the stacked amplitude and phase responses. Note that accurate navigation is critical for CSEM inversion and represents the largest source of error. To account for this, we apply a 3 error floor to the data.

For our inversion, we used CSEM data at the first, third, and seventh waveform harmonics (0.25, 0.75, and 1.75 Hz) which provide peak energy, i.e., the highest sensitivity to the depth range of interest while maintaining a good SNR. We omitted the data with a SNR below 2 and those with transmitter-receiver offsets ≤ 2 km due to navigation uncertainties58. We removed any obvious outliers after stacking. Example data responses from site 540b on the shelf (approximately 106 km landward of the trench) and site 533 on the forearc slope (approximately 34 km landward of the trench) are presented in Fig. S4.

CSEM inversion and mesh design
We applied the 2D inversion code MARE2DEM59 to image the 2D electrical conductivity structure from the CSEM data. MARE2DEM does not require prior knowledge about the model, so we set a 10 Ω m uniform half space as the initial model for the inversion. The core volume of particular interest starts from trench to approximately 120 km landward along the profile, extending to about 18 km below the seafloor. The quadrilateral cells at the surface are about 50 m height × 150 m width, with height following growth factor of 1.05 to about 10 km below the seafloor. For depths between ~10 km and ~18 km, a coarser horizontal resolution of 300 m per cell was applied while maintaining the same vertical growth factor. High-resolution bathymetry recorded by deep-towed SUESI was incorporated in the core area of interest in our model. The mesh was refined beneath the CSEM receivers. To avoid the boundary effect, a triangle-meshed outer padding region extends from -500 km and 600 km relative to the trench axis, from 100 km above sea level and to a depth of 1000 km, with largest possible triangles to keep computation efficiency.

Resistivity values for the air and seawater were determined and subsequently held fixed during the inversion. A seawater resistivity model was discretized using near-seafloor conductivity measurements from the CSEM transmitter and background climatological data from WOA18 (Northern Hemisphere summer, 2005–2017 average). The air was assigned a resistivity of 1013 Ω m.

Our preferred model (Fig. 2) converged to a root mean square (RMS) misfit of 0.9025. Fig. S5 summarizes the CSEM data and response matrices for each frequency from all receivers. Another detailed misfit distribution is provided in Fig. S6. The elevated misfits near ~62 km and ~50 km landward of the trench axis are likely attributed to rapid bathymetry changes and rapid transmitter altitude variations during re-diving, respectively. As inline data is primarily sensitive to the vertical resistivity60, we present the vertical resistivity in Fig. 2, while horizontal resistivity and the anisotropy ratio are shown in Fig. S7. The inherent anisotropy appears to be minor, as demonstrated by the isotropic inversion (Fig. S8), which can achieve a similar RMS misfit even with less degrees of freedom and recover essentially the same first-order conductivity structures as the anisotropic inversion. Variations in the anisotropy penalty weight introduce only marginal changes to the model, and the anisotropy ratios remain near zero except near the surface (Fig. S7). These suggest that strong anisotropy is not required to explain the data. We choose the vertical resistivity from the anisotropic model because it converges to the similar target RMS misfit more rapidly and robustly without sacrificing model smoothness, providing better near-surface fits. Note that OBEM sites on the shelf were not precisely located in shallow water environment, with drop locations used as site positions. In addition, the error floor might affect the convergence and the imaging of the shallow structures, especially for the northernmost data points on the shelf.

Resolution and sensitivity tests of forearc conductors
We first tested whether our CSEM data can resolve extending C1 farther downdip. Starting from our inverted vertical electrical resistivity model (Fig. 2, Fig. S1), we generated synthetic data by directly introducing a thick conductive layer (5 Ω m) along the plate interface, extending downdip to 20 km depth (Fig. S9a). Assuming the same uncertainty distribution as the observed real data, the inversion of this synthetic dataset converged to a misfit of 0.9047. However, we cannot identify clear anomalies attributable to the added structure; as depth increase s, the data sensitivity diminishes. Although the introduced structure causes measurable RMS changes up to 60 km landward of the trench, this effect is generated by a substantial 80 decrease in resistivity, far greater than the 10–20 changes typically used in our sensitivity tests. Overall, our CSEM data have sufficient sensitivity to resolve a conductive feature along the plate interface up to ~45 km landward of the trench (~10 km below seafloor), but lose resolution farther downdip. Although our image (Fig. 2) extends to 17 km below sea-level, the data have only limited sensitivity and insufficient resolution at this depth. Therefore, using only CSEM data, we cannot definitively determine whether a conductive structure exists farther downdip along the plate interface in the Shumagin Gap. However, collocated MT data18 are sensitive to the downdip portion of the plate interface and image an electrically resistive segment (Fig. S3). This indicates a lack of fluids along the plate interface between approximately 50 km and 90 km distance from the trench. We speculate that C2 (Fig. 2) may facilitate sufficient fluid drainage from the plate interface to render it fluid-poor.

We also assessed whether our data can resolve features beneath the Sanak Basin (C3), where a deep-rooted, large normal fault is identified by collocated seismic studies14,30. Beneath the Sanak Basin, we add a conductive anomaly along the fault, extending from approximately 5 km to 20 km depth (Fig. S9a). Following the same strategy as mentioned above, the inversion converged with a misfit of 0.9045. Yet again, the inverse model does not exhibit distinct anomalies corresponding to the introduced structure, indicating that we are unable to resolve a conductor beneath the basin. Given the presence of the thick Sanak sedimentary basin, it is reasonable that our data lack the sensitivity required to resolve deeper structures such as conductive fault zones. It is noteworthy that we do not expect a similar loss of sensitivity in the conductive frontal prism because it is thinner, and the target of interest—the plate interface—is shallower (< 3.5 km below the seafloor) compared to the deep conductor tested beneath the Sanak Basin (> 5 km). While transmitter coverage is limited near the trench (effective coverage starts at ~18 km landward), the high SNR of our CSEM receiver data ensures that the data retain sufficient resolution to estimate porosity along the plate interface from approximately 10 to 20 km landward of the trench.

In our model (Fig. 2), C1 and C2 represent two weak conductive structures. C1 exhibits resistivity around 10 to 40 Ω m and C2 ranges from approximately 30 to 55 Ω m. To evaluate the data sensitivity to these structures, we increased their resistivity values by 10 Ω m and forward modelled the data responses. Because our CSEM data lack sufficient sensitivity to the plate interface below 13 km depth or beyond 45 km landward of the trench, we only use the shallower, more reliable portion of C1 in this sensitivity test. The approximate geometry of C2 is shown in Fig. S9a as well. Both C1 (Fig. S9c) and C2 (Fig. S9b) show clear local RMS changes, demonstrating that, despite the inherent differences in resistivity and depth, our CSEM data display sufficient sensitivity to perturbations in both C1 and C2.

Porosity estimation
To estimate the porosity from our preferred electrical resistivity model (Fig. 2), we apply the empirical Archie’s law38:

𝜙=(1/𝜌𝑓)1/𝑚 (1)

where 𝜙 is the porosity, 𝜌 is the bulk resistivity, 𝜌 is the estimated pore fluid resistivity, and 𝑚 is the cementation exponent. Since direct salinity measurements are unavailable and the potential impact of pore fluid freshening on porosity estimates is considered minor37, we assume the pore fluid has a seawater salinity of 3.5 wt , where the fluid resistivity 𝜌 depends on temperature. For lower-temperature regions (𝑇 < 200 °C), we use a cubic relationship to calculate the pore fluid resistivity61:

𝜌 𝑓 (𝑇)=[2.0903916(1+2.97175×10−2𝑇+1.5551×10−4𝑇2−6.7×10−7𝑇3)]−1 (2)

valid for temperatures between 0 °C and 200 °C. At higher temperatures possibly considered in our analyses, we smoothly interpolate the pore fluid resistivity by applying an empirical relationship62, which is also depth (pressure) dependent.

We assume a linear geothermal gradient with depth and set a geothermal structure of the plate interface following

𝑇=𝑇 0+𝐺Δ𝑧 (3)

where 𝑇 is the temperature at the seafloor which is extrapolated from the temperature data recorded by the CSEM transmitter, 𝐺 is an assumed geothermal gradient, and Δ𝑧 is the depth below the seafloor. In our baseline case, we use a constant geothermal gradient of 𝐺 = 10 °C km−1 and adopt a cementation exponent in Archie’s law of 𝑚 = 2 (corresponding porosity model is shown in Fig. S2).

We employ this simplified geothermal gradient assumption primarily due to the considerable uncertainties inherent in thermal modeling of shallow forearc subduction zones. Unlike deeper regions, where slab geometry and geodynamic modeling help constrain the thermal structure, the shallow forearc in this region lacks heat flow data on the seafloor, which hinders a clear understanding of the shallow (upper tens of kilometers) thermal structure near the trench. While the average geothermal gradient from a reference thermal model63 is less than 10 °C km−1 for shallow subduction zones, the accuracy of this estimate is not well constrained. For example, the potential influence of shear heating and fluid circulation in the subducting crust64–66 suggests that subduction zones can be warmer than predicted in thermal models that ignore these effects. Complex shallow forearc thermal conditions also make this question more complicated. Furthermore, while more complex thermal models are available in other regions, applying them to this data-limited, shallow region requires local heat flow observations and would likely yield only a minor impact on our porosity estimates because we attempt to account for these uncertainties within our bounds. Indeed, our comparative calculation using the plate interface temperature from the van Keken and Wilson67 model reveals only a minor effect on the resulting porosity trends (Fig. 3). Our simplified approach for calculating the plate interface thermal structure, therefore, allows for greater transparency regarding its assumptions and limitations. To explore end-member scenarios and take account of the uncertainties, we vary the thermal gradient from 𝐺 = 5 °C km−1 up to 𝐺 = 15 °C km−1 and vary 𝑚 between 1.8 and 2.2 to represent differences in pore geometry. We note that our porosity estimates likely represent a conservative upper bound. The presence of clay minerals (e.g., smectite) would enhance bulk conductivity via surface conduction, while the presence of fracture porosity would imply a lower cementation exponent (𝑚 < 2 ). Accounting for either of these factors would result in porosity estimates even lower than those reported here. Thus, the potential influence of clays or fractures reinforces our conclusion of a low-porosity, fluid-poor plate interface.

From porosity to fluid pressure
We present fluid pressure using the modified pore pressure ratio, 𝜆∗ = (𝑃 𝑓 − 𝑃 ℎ) / (𝜎 𝑣 − 𝑃 ℎ), where 𝜎 𝑣 is the total vertical stress, 𝑃 ℎ is the hydrostatic pressure, and 𝑃 𝑓 is the pore fluid pressure. We assume hydrostatic pore fluid pressure for normally consolidated sediments on the incoming oceanic plate. Since the pore fluid pressure is defined by 𝑃 𝑓 = 𝜎 𝑣 −𝜎′ 𝑣, where 𝜎′ 𝑣 is the vertical effective stress, we can rearrange the expression of 𝜆∗ to solve for the vertical effective stress as 𝜎′ 𝑣 = (1−𝜆∗) (𝜎 𝑣 −𝑃 ℎ). The total vertical stress 𝜎 𝑣 is calculated by integrating the bulk density downward, which is estimated from our porosity structure, assuming a grain density of 2,700 kg m−3 . The hydrostatic pressure 𝑃 ℎ is calculated along the plate interface, assuming a fluid density of 1,030 kg m−3. By using a compression trend that links void ratio (and thus porosity) and vertical effective stress68,69 and the specific parameters (compression index and void ratio projected at zero effective stress) reported by Li et al.3 for our study region, we can predict the porosity along the plate interface as a function of distance landward of the trench for a given presumed pore pressure ratio 𝜆∗.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review report NCOMMS-25-77104\nThe authors carried out controlled-source EM profiling along a 200km profile over one of the rupture areas of the July 2020\nMw 7.8 earthquake in the Shumagin Gap, to clarify shallow slip behaviour and the upper boundary in terms of fluid\ndistribution. The authors successfully imaged the resistivity section in the near-trench forearc region to a depth of 16km,\nrevealing trench-normal heterogeneity in the prism and insufficient fluid to explain the weak plate coupling.\nThe topic is interesting, and the objective is clear. The measurement methodology and inversion schemes are state-of-the-\nart, and the resistivity model is robust.\nThe Shumagin Gap is known to have a weakly coupled interface from geodesy, but the fluid distribution at the Gap has been\ndebated in seismology and magnetotellurics. The present paper reports that the near-trench sedimentary fluid supply does\nnot extend to greater depths within the plate interface. This conclusion is essential and casts new questions about the\nweakly coupled plate interface to the Shumagin Gap.\nThe reviewer has the following comments.\nRegarding the influencing factor in plate coupling, fluid is a candidate. However, the existence of a seamount or the\ntopography (plate-bending geometry) of the downgoing plate is another. Can the authors say whether the Shumagin Gap\nlacks seamounts or has a smoother plate geometry than the surroundings?\nIs there any other unique geophysical nature (excluding the geodesy) of the Shumagin Gap, as compared with the\nsurrounding past-rapture area (M9 class)? To this reviewer, the Shumagin Gap is not as earthquake-prone as the\nsurrounding areas, as it has had multiple M7-class earthquakes, as shown in Figure 1.\nAs trench-normal heterogeneity exists, the deeper part of the plate (where M7-class ruptures occurred) may contain fluids\nand have triggered the rupture. Will this idea conflict with the previous MT result?\nThe concluding figure, together with the previous magnetotelluric results, should emphasize the conclusion.\nIn the future, another CSEM profile over the surrounding locked segments (e.g., Semidi) will directly clarify the authors'\nargument for water-starved subduction at the Shumagin Gap. Instead, the supply of fluids for the \"locked\" segment\nsurrounding the Shumagin Gap may have triggered the large ruptures.\nA graphical comparison or compilation of past similar studies (e.g., Nicaragua, Hikurangi) on the strongly/weakly coupled\ninterface and the incoming sediments near the trench will strengthen the authors' argument.\nRegarding the interpretation of the porosity from the resistivity, how about the influence of clay minerals, like smectite? How\nis the fracture porosity?\nSpecific comments:\nFig.1: The contour lines of the slab depth are shown with a 10km interval, but the depths should also be annotated in the\nfigure.\nFig. 1 and L. 283: The SSEs can be mapped in Fig. 1.\nFig.3 Caption: Lambda should be defined, or the definition should be referenced.", "reviewer_2": "This manuscript presents a new 2-D high-resolution image of electrical resistivity in the Shumagin seismic gap, using the\nlatest collected controlled-source electromagnetic data from the seafloor. The authors further interpret the low-resistivity\nanomalies as rock porosity filled with fluids. The results show that the plate interface in the top 10 km has a lower porosity\nthan previously expected. This is inconsistent with previous suggestions that abundant fluids on the plate interface result in\na weak interseismic plate coupling in the Shumagin Gap. The authors suggest that it is the interplay of the whole system,\nrather than simply the fluids on the plate interface, that modulates the megathrust in this region.\nSince electromagnetic data analysis is not my expertise, my comments mainly focus on interpretations and defer the\ntechnical part to other reviewers.\nI think this is an exciting discovery. It, together with some recent studies, challenges a long-standing simplified model that\nthe fluids on the plate interface dominate the megathrust. I recommend a minor revision. Here are my comments, as the\nauthors had addressed many of my early concerns.\nLine 39: The hypothesis needs a reference, ideally in rock physics.\nLine 59: I suggest adding “…, although the shallow portion (<10 km) of the megathrust near the trench remains poorly\nimaged.”\nLines 260–276: These sentences need to be more precise on the depths. Earthquake tomography (Wang et al., 2024) and\nMT images (Cordell et al., 2023) show fluids on the interface deeper than 25 km, but CSEM (this study) and active-source\nseismic imaging (Li et al., 2018) show a fluid-starving interface shallower than 25 km. In contrast, interseismic plate coupling\nmodels usually extend to 45 km depth, and have very low resolution near the trench (<15 km depth). So do the authors\npropose that the entire megathrust weak coupling is not related to fluids on the interface or only the shallow portion of the\nmegathrust? Please be more specific on the depths.", "reviewer_3": "Although I am unable to scrutinize fine technical details of the data collection and processing reported in this paper, there is\nenough information in the paper to help me assess the validity and importance of the scientific story. As far as I can tell, the\ndata and the research work are of high quality, and the scientific conclusions are important. The paper thus deserves timely\npublication in Nature Communications with some minor revision. I am returning an annotated pdf copy of the manuscript\nmarked with my comments. I am only reiterating a few of them here.\nThe title of the paper reads like that of a newspaper report. This is unfortunately what journals like Nature and Science are\npromoting today. I wish these journals could understand that objectivity is the lifeline of science, and an assertion or claim is\ninappropriate for the title of a scientific paper. I don’t know what the authors can do about it given the (strangely) tight word\nlimit imposed by these journals, but to the minimum I think readers deserve to know from the title what scientific approach is\nused (e.g., electromagnetic imaging, or at least geophysical imaging).\nThe meaning of “weak coupling” is unclear. Obviously, this paper is trying to address whether fluid controls slip behavior,\ncreeping vs. stick-slip. A decrease in effective normal stress leads to a lower shear strength, but it does not translate to slip\nbehavior. See Wang and Dixon (2004, https://doi.org/10.1029/2004EO180005) and Wang and Bilek (2014, ref 45 in this\nmanuscript)). Instead of weak or strong coupling, I suggest that clear wording is used as needed such as \"lower shear\nstress\" (force), \"velocity-strengthening\" (property or behavior), \"creeping\" (kinematics), etc.\nI suggest that the authors make a better effort in linking figures to text and connecting different figures. See comments in the\nannotated manuscript.\nTo me, having pore fluid pressure lower than hydrostatic is not physically unrealistic, although one can consider it \"unlikely\".\nI made some suggestion to rephrase line 222.\nIn the discussion of applicability of the sediment compaction model in lines 221-226, only the presence basalt is mentioned.\nI made a suggestion to consider sediment deformation (shear vs. in situ compaction).\nKelin Wang\nNovember 2025"}, "author_response": "Response to Reviewers’ comments on Nature Communications manuscript NCOMMS-25-\n77104\n\"Insu@icient fluids to explain weak plate coupling along the shallow megathrust at the\nShumagin Gap\" by Li et al.\nReviewers’ comments are in dark gray and our responses are in dark blue. All line\nnumbers in our responses refer to the revised manuscript (clean version). We have also\nmade some edits to meet the Nature Communications format requirements. The\nsupplementary information file has been updated accordingly.\nWe thank the reviewers for their insightful comments which we believe have significantly\nimproved the clarity and robustness of the manuscript.\nReviewer #1\nThe authors carried out controlled-source EM profiling along a 200km profile over one of\nthe rupture areas of the July 2020 Mw 7.8 earthquake in the Shumagin Gap, to clarify\nshallow slip behaviour and the upper boundary in terms of fluid distribution. The authors\nsuccessfully imaged the resistivity section in the near-trench forearc region to a depth of\n16km, revealing trench-normal heterogeneity in the prism and insu@icient fluid to explain\nthe weak plate coupling.\nThe topic is interesting, and the objective is clear. The measurement methodology and\ninversion schemes are state-of-the-art, and the resistivity model is robust.\nThe Shumagin Gap is known to have a weakly coupled interface from geodesy, but the fluid\ndistribution at the Gap has been debated in seismology and magnetotellurics. The present\npaper reports that the near-trench sedimentary fluid supply does not extend to greater\ndepths within the plate interface. This conclusion is essential and casts new questions\nabout the weakly coupled plate interface to the Shumagin Gap.\nWe thank the reviewer for the positive remarks. We appreciate the recognition of our work’s\nnovelty and contributions.\nThe reviewer has the following comments.\nRegarding the influencing factor in plate coupling, fluid is a candidate. However, the\nexistence of a seamount or the topography (plate-bending geometry) of the downgoing\nplate is another. Can the authors say whether the Shumagin Gap lacks seamounts or has a\nsmoother plate geometry than the surroundings?\nWe thank the reviewer for this comment regarding alternative controlling mechanisms for\nplate coupling. The Shumagin Gap is indeed characterized by rough plate interface (Bécel\net al., 2017; Li et al., 2018) and extensive bend faulting at the incoming plate (Shillington et\nal., 2015). Although specific seamounts are not clearly resolved, the overall lower-plate\nrelief is rugged. We have revised the text (Line 46-51) to explicitly describe this topographic\nroughness. This roughness is a critical component of the “interplay” we conclude: it is the\ninterplay of the whole system, rather than simply the fluids on the plate interface, that\nmodulates the megathrust in this region.\nIs there any other unique geophysical nature (excluding the geodesy) of the Shumagin Gap,\nas compared with the surrounding past-rapture area (M9 class)? To this reviewer, the\nShumagin Gap is not as earthquake-prone as the surrounding areas, as it has had multiple\nM7-class earthquakes, as shown in Figure 1.\nThe Shumagin Gap was identified as a seismic gap since it has not ruptured in a great\nearthquake (Mw > 8) for at least a century (Davies et al., 1981). Excluding geodetic\nobservations, several unique geophysical and geological features distinguish the Shumagin\nGap from the surrounding regions or segments:\n• Abundant interplate and intermediate-depth seismicity is observed at the Shumagin\nGap, whereas the Semidi segment has relatively sparse seismicity (Shillington et al.,\n2015).\n• Extensive plate bending-related faulting at incoming plate (Clarke et al., 2024;\nShillington et al., 2015) and a rough lower-plate relief (Bécel et al., 2017; Li et al.,\n2018).\n• The lack of a thick sediment supply (Li et al., 2018; Shillington et al., 2015).\nWhile we briefly mentioned the Shumagin Gap's unique geophysical nature in the original\nmanuscript, we have expanded this description in the revised version to clearly highlight\nother unique characteristics (Lines 46-49).\nAs trench-normal heterogeneity exists, the deeper part of the plate (where M7-class\nruptures occurred) may contain fluids and have triggered the rupture. Will this idea conflict\nwith the previous MT result?\nThis idea does not conflict with the previous collocated MT results (Cordell et al., 2023). In\nfact, the MT model explicitly images a conductive anomaly at depths > 25 km, which\nsupports the reviewer’s suggestion that fluids are present in the deeper part of the plate\ninterface where M7-class ruptures have occurred. Crucially, the same MT model shows a\nresistive structure at depths shallower than 25 km. This independent constraint agrees well\nwith our CSEM results, which infer a fluid-poor (low porosity) environment along the\nshallow plate interface. These are illustrated in the newly added Fig. 4 and Fig. S3, which\nare discussed in detail in next response. Please also see our response to Reviewer #2.\nThe concluding figure, together with the previous magnetotelluric results, should\nemphasize the conclusion.\nWe appreciate this suggestion to visualize our findings in the context of previous studies.\nWe have added Fig. 4 (Line 331-344, 276, 311, 355) as a new conceptual model, which\ncombines the MT model (Cordell et al., 2023) with our CSEM results into a unified\nschematic. It clearly illustrates the heterogeneous forearc structure, emphasizing our\nconclusion.\nFig. 4. Schematic diagram illustrating a heterogeneous forearc and a fluid-poor shallow\nmegathrust with low fluid pressure in the Shumagin Gap. The red box outlines the schematic\ninterpretation derived from the electrical resistivity model in Fig. 2a. The shallow megathrust is\ncharacterized by thin and irregular subducted sediments and a rugged plate interface. The\ninterface is depicted as a diffuse fractured zone, incorporating the base of the overriding plate\nand the uppermost down-going oceanic crust, where dipping conductive fault zones (blue\narrows) likely facilitate upward fluid migration. The observed fluid-starved distribution and\ninferred low fluid pressures indicate that elevated pore pressure cannot explain the low geodetic\nlocking in this region. Instead, the structural complexity highlighted by the pronounced electrical\nresistivity heterogeneity both along the plate interface and within the overriding plate suggests\nthat the intricate interplay among fluids, geological structures, and dynamic processes controls\nthe region’s seismogenic behavior. Structural interpretations outside the red box are based on\nCordell et al. (2023).\nWe have also included a supplementary figure (Fig. S3) to show the comparison between\nprevious MT and our CSEM resistivity model together. This comparison highlights the\nconsistency between the two independent datasets, thereby strengthening the robustness\nof our interpretation and conclusions. Please note that the sequence of supplementary\nfigures and corresponding citations have been updated accordingly.\nFig. S3. Comparison between our preferred resistivity model and the MT result (Cordell et\nal., 2023). Background model is our preferred resistivity model shown in Fig. 2 and Fig. S1.\nBlack solid lines are the contour line at 0.5 log unit intervals from the resistivity model from\n(Cordell et al., 2023). Red dashed rectangle outlines the spatial extent of P-wave velocity model\nfrom (Li et al., 2018). Black dashed line indicates the plate interface from (Bécel et al., 2017).\nPurple dashed lines denote slab crust boundaries from Shillington et al. (2022). The uppermost\nsegment located at >135 km represents the overriding plate Moho.\nIn the future, another CSEM profile over the surrounding locked segments (e.g., Semidi) will\ndirectly clarify the authors' argument for water-starved subduction at the Shumagin Gap.\nInstead, the supply of fluids for the \"locked\" segment surrounding the Shumagin Gap may\nhave triggered the large ruptures.\nA graphical comparison or compilation of past similar studies (e.g., Nicaragua, Hikurangi)\non the strongly/weakly coupled interface and the incoming sediments near the trench will\nstrengthen the authors' argument.\nThis is a valid point. However, these comparisons present significant interpretative\nchallenges. For the Nicaragua margin, while high porosities have been observed, the\nlocking state of the shallow megathrust remains unconstrained due to a lack of seafloor\ngeodetic data. Additionally, the region’s history of anomalous tsunami earthquakes\ncomplicates the relationship between porosity and interplate coupling. Regarding the\nHikurangi margin, the resistivity structure is highly heterogeneous, making it di@icult to\nreliably identify the plate interface for comparable porosity estimation.\nNevertheless, to address the reviewer's point, we have compiled a comparison figure here\n(Fig. R1) plotting porosity estimates along the plate interface against depth below the\nseafloor. To minimize bias and maximize comparability between the datasets, we\nstandardized the axis to depth below the seafloor rather than distance from the trench. As\nshown in Fig. R1, porosity estimates at the Shumagin Gap are significantly lower than those\nobserved at the Nicaragua margin (Naif et al., 2016), which align closely with the global\naverage trends (Bray and Karig, 1985). We excluded Hikurangi because in that study area\nthe resistivity structure is highly heterogeneous, making it challenging to identify the plate\ninterface where a porosity can be estimated. Therefore, given the uncertainties regarding\nthe locking state in Nicaragua and heterogeneity in Hikurangi, we think sticking with the\ncomparison to the global average values from Bray and Karig (1985) is su@icient to ensure a\nmore robust and unbiased baseline for our study region (Fig. 3).\nFig. R1. Comparison of porosity estimates along the plate interface versus depth\nbelow the seafloor. The green shaded area represents porosity estimates derived from\nCSEM data at the Nicaragua margin (Naif et al., 2016). The other curves represent data\nfrom this study, following the same conventions as Fig. 3. Note that to facilitate direct\ncomparison between margins, the vertical axis represents the depth below the seafloor,\ndi@ering from the distance-to-trench axis used in the main text (Fig. 3).\nWe acknowledge the importance of placing our results in the context of other marine EM\nstudies. Fortunately, the recent review by Chesley (2025), which has been cited in our\nrevised manuscript (ref 54), o@ers a comprehensive summary of global marine EM findings.\nA compilation study specifically focused on forearc regions for these EM studies in the\nfuture would be the most appropriate venue to investigate the relationship between\ninterseismic coupling state and fluids across di@erent margins.\nRegarding the interpretation of the porosity from the resistivity, how about the influence of\nclay minerals, like smectite? How is the fracture porosity?\nThe presence of conductive clay minerals (e.g., smectite) would contribute to the bulk\nconductivity via surface conductivity [see Naif et al. (2016) and Chesley et al. (2023) for\nfurther discussion on this e@ect]. If we were to account for this additional conductive\nmechanism, the fluid-filled porosity required to explain the observed resistivity would be\nlower than the estimates derived from the Archie’s law. Therefore, our current estimates\nrepresent an upper bound (or a conservative maximum) for porosity. Since our study\nfocuses on the lack of fluid or low porosity along the plate interface, using this upper-\nbound estimate ensures our conclusions are conservative and robust.\nRegarding the fracture porosity, if we acknowledge the presence of fractures, the\ncementation exponent should be lower. Similarly, the bulk porosity required to match the\nobserved resistivity would again be lower than what we calculated using standard\ncementation exponent (m ≈ 2).\nWe have clarified this in the revised manuscript (Line 535-540).\nSpecific comments:\nFig.1: The contour lines of the slab depth are shown with a 10km interval, but the depths\nshould also be annotated in the figure.\nLine 110-124 (Fig. 1): Thanks for helping improving readability of Fig. 1. We have explicitly\nannotated the depth values on the Slab2.0 contours (in km below sea level). To further\nvisualize the slab interface, as also requested by Reviewer #3, we have incorporated the\nSlab2.0 interface into Figure 2a and 2c (Line 171).\nFig. 1 and L. 283: The SSEs can be mapped in Fig. 1.\nThe SSE patches identified in recent studies (He et al., 2023) are located in the Semidi\nsegment, which lies to the northwest of our survey area. Since these events fall outside the\nspatial extent of Fig. 1, we are unable to display them on the current map.\nFig.3 Caption: Lambda should be defined, or the definition should be referenced.\nWe have referenced the definition of l* (Line 543 in Methods) in Fig. 3 caption (Line 212).\nReviewer #2\nThis manuscript presents a new 2-D high-resolution image of electrical resistivity in the\nShumagin seismic gap, using the latest collected controlled-source electromagnetic data\nfrom the seafloor. The authors further interpret the low-resistivity anomalies as rock\nporosity filled with fluids. The results show that the plate interface in the top 10 km has a\nlower porosity than previously expected. This is inconsistent with previous suggestions that\nabundant fluids on the plate interface result in a weak interseismic plate coupling in the\nShumagin Gap. The authors suggest that it is the interplay of the whole system, rather than\nsimply the fluids on the plate interface, that modulates the megathrust in this region.\nSince electromagnetic data analysis is not my expertise, my comments mainly focus on\ninterpretations and defer the technical part to other reviewers.\nI think this is an exciting discovery. It, together with some recent studies, challenges a long-\nstanding simplified model that the fluids on the plate interface dominate the megathrust.\nI recommend a minor revision. Here are my comments, as the authors had addressed many\nof my early concerns.\nWe are grateful for the reviewer’s positive remarks and recognition of our work’s novelty and\ncontributions. We believe the comments have further improved the manuscript.\nLine 39: The hypothesis needs a reference, ideally in rock physics.\nLine 41: We have added two references (Cappa et al., 2019; Scuderi et al., 2017) to support\nthe hypothesis.\nLine 59: I suggest adding “…, although the shallow portion (<10 km) of the megathrust near\nthe trench remains poorly imaged.”\nLine 64-65: Agreed. We have accepted this suggestion to improve the precision of our\ndescription of Wang et al. (2024).\nLines 260–276: These sentences need to be more precise on the depths. Earthquake\ntomography (Wang et al., 2024) and MT images (Cordell et al., 2023) show fluids on the\ninterface deeper than 25 km, but CSEM (this study) and active-source seismic imaging (Li\net al., 2018) show a fluid-starving interface shallower than 25 km. In contrast, interseismic\nplate coupling models usually extend to 45 km depth, and have very low resolution near the\ntrench (<15 km depth). So do the authors propose that the entire megathrust weak\ncoupling is not related to fluids on the interface or only the shallow portion of the\nmegathrust? Please be more specific on the depths.\nWe have rewritten portions of this paragraph (Line 270-287) to be precise about depth. We\nagree that our CSEM data primarily constrains the shallow interface. We would like to\nclarify that while the MT model (Cordell et al., 2023) shows a conductive segment at\ndepths > 25 km (attributed to slab mantle fluids), the model becomes resistive again\nfarther downdip, rather than indicating continuous fluids all the way along the deep\ninterface (see Fig. S3 and newly added Fig. 4, in responses to Reviewer #1). The MT results\nalso show a resistive region updip of 25 km indicating a lack of fluids, consistent with the\nshallower CSEM result (Fig. S3). By integrating the MT, CSEM, and active-source seismic\nimaging results, we can infer the fluid distribution along the plate interface from the trench\naxis down to 40 km depth. Based on the cumulative result, we suggest that fluids are\nunlikely to be the sole or primary governing factor for the megathrust locking state across\nthe entire megathrust in the Shumagin Gap.\nReviewer #3 Kelin Wang\nAlthough I am unable to scrutinize fine technical details of the data collection and\nprocessing reported in this paper, there is enough information in the paper to help me\nassess the validity and importance of the scientific story. As far as I can tell, the data and\nthe research work are of high quality, and the scientific conclusions are important. The\npaper thus deserves timely publication in Nature Communications with some minor\nrevision. I am returning an annotated pdf copy of the manuscript marked with my\ncomments. I am only reiterating a few of them here.\nWe are grateful for the reviewer’s encouraging assessment of our work and the recognition\nof the high quality of the marine controlled-source EM data and the importance of this\nstudy. We have carefully reviewed the annotated PDF and incorporated the suggestions\ninto the revised manuscript. We believe that these constructive comments have helped\nimprove the clarity and precision of the manuscript.\nThe title of the paper reads like that of a newspaper report. This is unfortunately what\njournals like Nature and Science are promoting today. I wish these journals could\nunderstand that objectivity is the lifeline of science, and an assertion or claim is\ninappropriate for the title of a scientific paper. I don’t know what the authors can do about it\ngiven the (strangely) tight word limit imposed by these journals, but to the minimum I think\nreaders deserve to know from the title what scientific approach is used (e.g.,\nelectromagnetic imaging, or at least geophysical imaging).\nWe appreciate the reviewer’s perspective on scientific objectivity of the title. Given the\nmaximum title length (15 words), we have changed the title to “Electromagnetic imaging\nreveals insu@icient fluids to explain shallow megathrust creep at the Shumagin Gap”\nThe meaning of “weak coupling” is unclear. Obviously, this paper is trying to address\nwhether fluid controls slip behavior, creeping vs. stick-slip. A decrease in e@ective normal\nstress leads to a lower shear strength, but it does not translate to slip behavior. See Wang\nand Dixon (2004, https://doi.org/10.1029/2004EO180005) and Wang and Bilek (2014, ref 45\nin this manuscript)). Instead of weak or strong coupling, I suggest that clear wording is used\nas needed such as \"lower shear stress\" (force), \"velocity-strengthening\" (property or\nbehavior), \"creeping\" (kinematics), etc.\nWe appreciate this terminology suggestion. By weak coupling, we meant specifically the\ngeodetic locking constrained in this region, which show values near zero indicating that the\nsubducting slab is creeping. Acknowledging the potential ambiguity between mechanical\nstrength and kinematic behavior, we have adopted the reviewer’s suggestion and have\nreplaced “weak (plate) coupling” with “(megathrust) creep” or “low geodetic locking”\nthroughout the manuscript (Title, Line 29, 41, 67, 83, 253, 274, 362). We have also replaced\n“plate coupling” with “megathrust locking (state)” (Line 24, 57, 71, 250, 288, 308, 358, 367).\nI suggest that the authors make a better e@ort in linking figures to text and connecting\ndi@erent figures. See comments in the annotated manuscript.\nWe have gone through the manuscript and improved the cross-referencing between the\nmain text and the figures. Here is a quick summary. Details can be seen in point-by-point\nresponse regarding each figure.\n1. As also recommended by Reviewer # 1, we have added labels of Slab2.0 model\n(Hayes et al., 2018) in Fig. 1 and its geometry in Fig. 2.\n2. We have marked the profile (thick orange line) in Fig. 1 to show the site range in Fig.\n2a to link map and model better.\n3. We have revised Figs 1 and 2’s captions to consistently use the term “OBEM\nreceiver” for stations to keep a better link between Fig. 1 and Fig. 2.\nTo me, having pore fluid pressure lower than hydrostatic is not physically unrealistic,\nalthough one can consider it \"unlikely\". I made some suggestion to rephrase line 222.\nLine 221-222: Sub-hydrostatic pore fluid pressure is not physically unrealistic. Perhaps\n\"unlikely\" is a better word. Then the rest of the sentence should be less definitive: \"which\nmay be an artifact ...\".\nLine 230-231: We have accepted the suggestions and rewritten this sentence in the revised\nmanuscript.\nIn the discussion of applicability of the sediment compaction model in lines 221-226, only\nthe presence basalt is mentioned. I made a suggestion to consider sediment deformation\n(shear vs. in situ compaction).\nLine 225-226: Composition is just one aspect. Deformation is also an issue. Subducted\nsediment likely endures much shear deformation, very di@erent from sediment that only\nendures compaction.\nWe agree that there are many possible complicating factors besides composition. We now\npoint this out, but we prefer not to get into expand the discussion or get into specifics since\nthese factors are challenging to constrain with our observations and therefore beyond the\nscope of our study. We have revised the text to explicitly acknowledge shear deformation\nas an unconstrained complicating factor when discussing the applicability of the sediment\ncompaction model (Line 234-236).\nOther Annotated PDF comments\nWe are thankful for the reviewer’s detailed editorial suggestions marked in the annotated\nPDF. We have accepted these suggestions to improve the clarity and flow of the text.\nLine 51: ? Sounds like all intraslab seismicity is seaward of the arc.\nLine 55: We have deleted “subarc”.\nFig.1\n• Please mark the range of the section in Fig. 2a.\nTo clarify the location of the section shown in Fig. 2, we have added a profile line on the\nmap figure (Fig. 1). Correspondingly, we have added a sentence “The thick orange line\nmarks the location and extent of the cross-section shown in Fig. 2a.” in Fig. 1’s caption.\n• Please label a few of them on map. Also, please explain whether the depth is below\nsea level or seafloor.\nWe have added labels of Slab2.0 depth contours in Fig. 1. The depth is below sea level.\nLine 140: Not an accurate description given the big blue patches seaward of it. Also, \"rigid\nblock\" means infinite rigidity which neither is true nor can be inferred from resistivity\nsignals.\nLine 149-151: We have rephrased and clarified this sentence.\nLine 144: Do we know the rock types to make this statement? Or is it an inference based on\nsome reference? Also, if this thing marks this separation, what do the backstops marked in\nFig. 2a do?\nLine 151-152: We clarify that this statement is an inference based on the distinct contrast\nin high velocity/resistivity vs. low velocity/resistivity. The high-resistivity anomaly (R1) is\ninterpreted as the older and stronger framework materials. So, the boundary between R1\nand the conductive frontal prism spatially corresponds to the backstops marked in Fig. 2a.\nTo resolve the confusion and align the text with the figure, we have revised the sentence to\n“The sharp contrast between R1 and conductive frontal prism suggests a boundary\nseparating the strong, older framework materials from the younger frontal prism.”\nFig.2\n• Line 149: Please label this feature in Fig. 2 and quote the figure here.\nLine 157: We have labeled this feature in Fig. 2 and quoted the figure here.\n• Please also show Slab2.0 (shown in Fig. 1) to allow the reader to build a connection\nwith Fig. 1.\nLine 171: Agreed. We have added the Slab2.0 interface and labeled it in the revised Fig.\n2. We have updated the caption accordingly.\n• Please make a better link with Fig. 1 by using the same circle color and same\ndescription for stations (e.g., Fig. 1 does not mention OBEM).\nWe didn’t update the color of receivers in Fig. 2. But we have revised Figs 1 and 2’s\ncaptions to consistently use the term “OBEM receiver” instead of “station” that was\nused in Fig. 1’s caption previously. We have corrected this to avoid confusion as they\nrepresent the same sites.\nLine 185: Please use a separate sentence to clarify meaning.\nLine 193: We have removed this part to increase clarity.\nLine 193: in the Semidi segment? If so, I suggest to use clearer writing like \"Even if we\nextrapolate ... at the Semidi segment, the inferred ... would still be higher than the near-\ntrench ... for our study area.\"\nLine 200-201: We thank the reviewer for pointing out this potential ambiguity. We would like\nto clarify that the “generic porosity compaction trend” refers to a global averaged\ncompaction model (Bray and Karig, 1985), rather than the specific trend from the Semidi\nsegment. To prevent this confusion for future readers, we have added “global” and used the\nconnector “Furthermore” in revised sentence: “Furthermore, extrapolating a global generic\nporosity compaction trend (Bray and Karig, 1985) towards the seafloor as a lower bound\nstill results in higher porosity values than the near-trench estimates obtained for this\nregion.”\nLine 226-227: refer to Methods? Otherwise unexplained.\nLine 237: Yes. We have added “(Methods)” to the sentence.\nLine 261-262: This seems to imply that the fluids facilitated the 2020 earthquake. Is this\nwhat you mean? If not, revise the text to clarify.\nLine 271: That was not our intension. We agree that the phrasing was misleading. To avoid\nany further confusion or misinterpretation, we have decided to delete this sentence in the\nrevised manuscript.\nLine 328: Please clarify whether you mean \"we think ...\" or the imaging can resolve this.\nLine 353: We have rewritten the sentence in the revised manuscript.\nOther Changes\nTechnical Correction\nSupplementary Materials: We noticed a minor typo regarding the figure label in the\nSupplementary Materials where Fig. S1 was inadvertently labeled as \"Fig. S6\". The figure\ncontent, order, and its citation in the main text were correct. We have corrected this label in\nthe revised Supplementary Information file.\nFig. 1: We corrected unit typo in colorbar of Fig. 1.\nAddition of Recent Event on Map\nTo ensure that the Fig. 1 provides the most up-to-date representation of the regional\nseismic activity, we have updated Fig. 1 to include the GCMT focal mechanism of the M7.3\nstrike slip Sand Point earthquake that occurred in July 2025.\nColormap of Fig. S2\nWe have updated the colormap in Supplementary Fig. S2 (showing porosity) to distinguish\nit from the resistivity models in Fig. 2 of the main text to ensures that the two di@erent\nphysical properties (porosity vs. resistivity) are visually distinct to the reader."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review report on NCOMMS-25-77104A\nThe previous comments from this reviewer were answered point by point, and the manuscript was revised accordingly. The\npaper is publishable."}, "author_response": ""}]

Decision letter