Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity

Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity Paul Caesar M. Flores, Shuichi Kodaira, Kazuya Shiraishi, Gou Fujie, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7966953/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Feb, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract (192 words) High pore pressure ratio (λ*) has been associated with the occurrence of slow earthquakes. Many studies have estimated the λ* using taper angle, P-and S-wave velocities, and drilling but the extent is not yet clearly defined. This study utilized a recently published high-resolution P-wave velocity model off Muroto derived from a two-step tomographic inversion of ocean-bottom seismograph data to determine the λ* using empirical relationships between velocity, porosity, and effective mean stress. We determined an extensive zone of high λ* (>0.4) from the frontal thrust up to ~65 km landward and to a depth of 8 km with three characteristic observations. First, the underthrust sediments in the outer wedge show patches of overpressured aquifers where λ* >0.6, consistent with previous drilling results. Second, the high λ* (>0.6) region in the inner wedge coincides with previously reported underplated sediments composed of fluid-rich trench-fill sediments dragged down by seamounts. The high λ* may be caused by tectonic compression from the newly subducted seamounts. Lastly, vertical columns of high λ* were observed in many of the thrust faults with some faults showing a negative polarity, which are interpreted to be evidence of fluid flow. Earth and environmental sciences/Natural hazards Earth and environmental sciences/Solid earth sciences Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Fluids and tectonic processes have interconnected processes at subduction zones 1 . Pore fluid pressure is an important factor in the development of accretionary complexes 2 and fault movements that can cause a large slip during megathrust earthquakes 3 , 4 or facilitate slow earthquakes 5 . Tectonic processes drive pore fluid pressures and fluid geochemical evolution through loading and heating, and fluid escape is modulated by modifying permeability through faulting and deformation 1 . Slow earthquakes are characterized by their seismic moment that is proportional to its source duration 6 , 7 . In the Nankai Trough, high pore pressure has been commonly invoked to explain the occurrence of shallow (< 10 km) slow earthquakes 8 . Research on slow earthquakes has significantly improved our understanding of slip phenomena but remains a complex topic because slow earthquakes have been linked to both rupture initiation and termination 8 – 10 . The association of high pore fluid pressure and slow earthquake activity is also not a direct relationship because other factors have also been proposed to possibly explain the occurrence of slow earthquakes such as seamount subduction 11 – 14 , plate coupling 11 , 15 , 16 , sediment thickness and lithology 17 , 18 , friction 16 , 19 , dynamic stress changes from earthquakes 20 , 21 , and tides 20 . Many previous studies have estimated the pore pressure in Nankai Trough off Muroto using different methods such as drilling 22 – 26 , taper angle 27 – 29 , P-wave velocity 29 – 31 , and S-wave velocity 32 . The existence of fluid overpressure off Muroto has only been recently confirmed because of mud outflow during drilling at site C0023 26 (Fig. 1 ). However, the cause and extent of fluid overpressure is not yet clearly defined, and each method has its limitations. Flores et al. 29,33 proposed a conceptual model integrating multi-seamount subduction, sediment underplating, and lithologic heterogeneity to explain fluid overpressure and slow earthquake activity, but quantitative pore pressure estimates beyond ~ 10 km from the frontal thrust were lacking. A recently published high-resolution P-wave velocity model along the Muroto transect that spans 100 km and extends to 20 km depth achieved vertical and horizontal resolutions of ~ 0.5 and 1.0 km above the oceanic crust through full waveform inversion of wide-angle ocean-bottom seismograph data 34 . A laterally continuous low-velocity band within subducted sediments was identified and interpreted as indicative of high pore pressures 34 (Fig. 2 ). This study quantified the pore pressure ratio (λ*), also called normalized pore pressure, from the P-wave velocity model of Shiraishi et al. 34 (Fig. 2 a) using empirical relationships between velocity, porosity, and effective mean stress 35 . The resulting λ* model was then compared with structural and tectonic features to infer the processes controlling fluid overpressure and explore its spatial correlation with shallow slow earthquake activity. Results and Discussion Pore pressure distribution Underthrust sediments in the outer wedge (131 words) The λ* in the underthrust sediments in the outer wedge is generally greater than 0.4 but there are patches of higher λ* (> 0.6) (Fig. 3 , Fig. 4 ). This is consistent with the results of Hirose et al. 26 where they inferred the presence of sub-kilometer scale overpressured aquifers beneath the Nankai Trough decollement off Muroto by modeling the transient borehole mud flow at Site C0023 (Fig. 1 ). Our results indicate the possible widespread existence of these overpressured aquifers in Nankai Trough especially in Muroto where a low taper angle has been reported by previous studies 26 – 29 , 36 . However, the cause of this patchy distribution is still unknown because there is no clear change in the seismic reflection signature of the underthrust sediments that coincides with the occurrence of the overpressured aquifers (Fig. 3 e). Underplated sediments in the inner wedge A wide region of high λ* (> 0.6) coincides well with the location of the underplated sediments (Fig. 3 b). The underplated sediments are generally composed of strong and discontinuous reflections while the underthrust sediments in the outer wedge are transparent. These 2 units are separated by the decollement step down (Fig. 3 c). Our results contradict the earlier studies where they attributed the decollement step down to the increasing shear strength of the decollement because of drained conditions in the inner wedge 31 . Recent studies on the other hand using S-wave velocities showed a large velocity reduction (> 20%) in this region and a λ* of 0.75 32 (Fig. 1 ). The presence of the high λ* can be explained by the deformation caused by multiple seamount subduction proposed by Flores et al. 29,33 . They interpreted that the underplated sediment is composed of fluid-rich trench-fill sediments that were dragged down or underplated by Seamount A or any older seamount (Fig. 1 ). Tectonic compression from newly-subducted seamounts then generated this high pore pressure region. Thrust Faults Vertical columns of high λ* can be observed in many of the thrust faults. A prominent region is observed between the 60 to 65 km mark which is interpreted as evidence of upward fluid flow (Fig. 3 b). This is supported by the presence of cold seeps near fault zones reported from deep-sea observations 37 . Some of the thrust faults in the outer wedge show a negative polarity and high λ* (Fig. 3 b-d) but the dip angle of the high λ* and the thrust faults do not align well. The vertical orientations of the high λ* areas may be an artefact of the full waveform inversion, however, their co-location with the thrust faults can be indicative of their possible existence. Previous studies also reported a negative polarity decollement in Muroto 27 , 29 , 33 which can be explained with high pore pressure along the fault 36 . Thus, the high λ* in the thrust faults is also interpreted as evidence of fluid flow where faults act as permeable pathways. Association of pore pressure with slow earthquake activity (499 words) The reference porosity model decreases from ~ 0.25 at the frontal thrust to 0.05 at ~ 20 km landward, while the calculated porosity from P-wave velocity shows a modest decrease from ~ 0.20 to 0.10 from the frontal thrust up to ~ 50 km landward (Fig. 4 a). The pore pressure line nearly parallels the lithostatic line for most of the profile (Fig. 4 b) and this indicates that the increasing lithostatic load associated with progressive underthrusting is borne by trapped pore fluids 31 . The effect of fluid pressure is further emphasized by the significantly lower effective mean stress calculated from the empirical relation with P-wave velocity than the effective mean stress calculated from the reference porosity (Fig. 4 c). The number of slow earthquakes increases with distance from the deformation and is maximized at around 50–55 km (Fig. 2 , Fig. 3 ). This region corresponds to the underplated sediments discussed in the previous section where cold seeps have also been reported 37 . Fluid migration plays an important role in activating slow earthquakes 38 . This implies that the underplated sediments can be a source region for fluids 33 and episodic fluid flow from the underplated sediments may also activate slow earthquakes. This episodic fluid flow may be modulated by the tectonic compression from the subducted seamounts 29 , 33 . A recent study using borehole data in the Kumano region has documented that slow earthquakes can migrate seaward and breach the trench 39 . The negative polarity decollement and negative polarity thrust faults near the trench showing high λ* (Fig. 2 ) may indicate that slow earthquakes can also migrate and breach the trench in this region. Additionally, since drilling data in Kumano also indicates that the fault may experience both fast and slow slip at different times 40 , this brings up important questions if it is also possible for future megathrust slip to breach the trench in Muroto region considering that subducted seamounts (Fig. 1 ) have been proposed to have functioned as a rupture barrier during the 1946 Nankai earthquake 41 . Other studies on the other hand using joint inversion of seismic waveforms and geodetic data from the 1946 Nankai earthquake identified two asperities separated by Seamount A and maximum slip of 5.1 m occurring at a point south of Cape Muroto 42 (Fig. 1 ). Hence, we infer that it may be possible for trench breaching slip to occur in the future. However, factors controlling whether the subducted seamount will function as a barrier or asperity is still unknown. The λ* from the deformation front up to ~ 65 km landward is generally greater than 0.4 (Fig. 4 c), which agrees with previous observations that slow earthquakes appear to only occur in overpressured areas where λ* ranges from ~ 0.4 to 0.9 43 . However, it is also important to note the heterogenous pore pressure distribution especially in the outer wedge where patches of overpressured aquifers were observed (Fig. 2 b, Fig. 2 c). Overall, the generated pore pressure map using the high-resolution P-wave velocity model derived from full waveform inversion of ocean seismometer data 34 provided new insights into the fluid and tectonic processes in Nankai Trough, and supports previous studies on the existence of overpressured aquifers 26 and the conceptual models for creating a high pore pressure environment from multiple seamount subduction off Muroto 29 , 33 . Materials and Methods The pore pressure ratio (λ * ), also called normalized pore pressure, was calculated using empirical relationships between P-wave velocity \(\:\left({V}_{p}\right)\) , porosity \(\:\left(\varphi\:\right)\) , and effective mean stress \(\:\left({p}^{{\prime\:}}\right)\) 35 .This study used the latest \(\:{V}_{p}\) model off Cape Muroto which has a vertical resolution of 0.5 km and a horizontal resolution of 1.0 km above the top of the oceanic crust 34 . The P-wave velocity is first converted to porosity 44 : $$\:{V}_{p}=A+B\varphi\:+\:\frac{0.305}{{(\varphi\:\:+C)}^{2}\:+\:D}+E\left({v}_{sh}-F\right)[\text{tanh}\left(G\left(\varphi\:-{\varphi\:}_{c}\right)\right)-\left|\text{tanh}\left(G\left(\varphi\:-{\varphi\:}_{c}\right)\right)\right|]$$ Where \(\:{v}_{sh}\) is the shale volume equal to 1.057; \(\:{\varphi\:}_{c}\) is the critical porosity equal to 0.295; A,B,C,D,E,F, and G are constants equal to 0.746, 0.532, 0.124, 0.0513, 0.61, and 1.123, respectively. The calculated \(\:\varphi\:\) is then converted to \(\:{p}^{{\prime\:}}\) using the equation: $$\:\frac{\varphi\:}{1\:-\:\varphi\:}=0.79-\:0.40\text{log}{p}^{{\prime\:}}$$ Then, the pore fluid pressure \(\:\left({P}_{f}\right)\) is calculated using the equation: $$\:{P}_{f}=\:{\sigma\:}_{vo}\:-\:{P}_{hf}-0.42{p}^{{\prime\:}}$$ Where \(\:{\sigma\:}_{vo}\) is the total overburden stress, and \(\:{P}_{hf}\) is the hydrostatic pressure. The \(\:{P}_{hf}\) can be easily calculated at any depth assuming the density of seawater at 1,024 kg/m 3 . In the absence of density data for the accretionary prism, the \(\:{\sigma\:}_{vo}\) was calculated using densities estimated from \(\:{V}_{p}\) 4 5 : $$\:\rho\:=\:-0.6997+2.2302{V}_{p}-0.598{V}_{p}^{2}+0.07036{V}_{p}^{3}-0.0028311{V}_{p}^{4}$$ Lastly, the λ * is given by: $$\:{\lambda\:}^{*}=\:\frac{{\sigma\:}_{vo}-\:{P}_{hf}-0.42{p}^{{\prime\:}}}{{\sigma\:}_{vo}-\:{P}_{hf}}$$ The resulting λ * profile was compared with a seismic reflection profile along the same line (Fig. 2 ) to examine the pore pressure distribution and its possible causes. To examine the effects of high \(\:{\lambda\:}^{*}\) , a reference porosity was used to model the effective stress for a normal consolidation case. This was defined as an exponential function of depth (z) and was constrained by drilling data at Site 1173 36 $$\:\varphi\:=0.75{e}^{-0.0011z}$$ Declarations Author Contribution P.C.M.F. wrote the manuscript. S.K, K.S., G.F., Y.N., and R.A. provided supervision. K.S. G.F. Y.N., and R.A. conducted the full waveform inversion of the ocean-bottom seismograph data to create a high resolution P-wave velocity model. All authors read and approved the final manuscript. Acknowledgement This study was partly supported by the Japan Society for the Promotion of Science, Japan during the JSPS Postdoctoral Fellowship of P.C.M.F. We are also grateful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan for the management expense grants to the Research Institute for Marine Geodynamics at JAMSTEC and funding for the Research Project for Disaster Prevention on the Great Earthquakes along the Nankai Trough. 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The sea 4 , 53–58 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 06 Feb, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 10 Dec, 2025 Reviews received at journal 09 Dec, 2025 Reviews received at journal 21 Nov, 2025 Reviewers agreed at journal 16 Nov, 2025 Reviewers agreed at journal 16 Nov, 2025 Reviewers invited by journal 08 Nov, 2025 Editor assigned by journal 07 Nov, 2025 Editor invited by journal 06 Nov, 2025 Submission checks completed at journal 03 Nov, 2025 First submitted to journal 03 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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15:34:15","extension":"html","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":92105,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/2a5a4989bb30efd3c07b6beb.html"},{"id":97759208,"identity":"7346b875-7050-4f18-b181-68488605f24c","added_by":"auto","created_at":"2025-12-09 05:19:09","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":672813,"visible":true,"origin":"","legend":"\u003cp\u003eSlope map of the study area. The black line with black dots spaced every 10 km represents the location of the OBS wide-angle seismic survey line MROBS01\u003csup\u003e34\u003c/sup\u003e. Red circles show the location of the identified subducted seamounts\u003csup\u003e45–48\u003c/sup\u003e. Blue boxes show the Low Velocity Zones (LVZ)\u003csup\u003e32\u003c/sup\u003e. Purple rectangle shows the region of high pore pressure ratio determined from a dense network of seismic reflection data\u003csup\u003e29\u003c/sup\u003e. Black triangles show the location of Deep Sea Drilling Project/Ocean Drilling Program/Integrated Ocean Drilling Program drill sites. Green and yellow dots are epicenters of very low frequency earthquakes\u003csup\u003e12,13,15,19,49–51\u003c/sup\u003e and tremors\u003csup\u003e16,20,21,52\u003c/sup\u003e, respectively. Black rectangles are asperities determined from joint inversion of seismic waveforms and geodetic data\u003csup\u003e42\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/cfb882a76ade75c79b25ac72.jpeg"},{"id":97759212,"identity":"9ff0a3a5-c8cb-4910-a755-d8e87ce9d31a","added_by":"auto","created_at":"2025-12-09 05:19:09","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":493992,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The P-wave velocity model derived from full waveform inversion of MROBS01\u003csup\u003e34\u003c/sup\u003e. (b) Spatial velocity perturbation\u003csup\u003e34\u003c/sup\u003e. (c) Pre-stack depth migrated multi-channel seismic survey line SIN136 (Figure 1). (d) The number of slow earthquakes off Muroto (Figure 1) binned every 2 km along the MROBS01 line.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/61a36b4eb7eea963ada8b394.jpeg"},{"id":97759209,"identity":"21cf1f76-d543-4f3c-9069-6dc7da886ce8","added_by":"auto","created_at":"2025-12-09 05:19:09","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":953013,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The P-wave velocity model\u003csup\u003e34\u003c/sup\u003e. (b) The calculated pore pressure ratio. (c) The seismic reflection profile SIN136 superimposed with line interpretations\u003csup\u003e33\u003c/sup\u003e. (d) Close up view of the frontal region showing the negative polarity of the thrust faults (black triangles) and the decollement. (e) Enlarged section of the pore pressure ratio model and the seismic reflection profile.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/e583fbe6508ac158ec028586.jpeg"},{"id":97759217,"identity":"6abebe30-b956-40dd-b2e1-f2dc5c0ab589","added_by":"auto","created_at":"2025-12-09 05:19:09","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":358600,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Average P-wave velocity (red) in the subducted sediments delimited by the top of the oceanic crust and the decollement or roof thrust in Figure 2. The calculated porosity from the P-wave velocity is shown in blue. The reference porosity calculated as a function of depth\u003csup\u003e36\u003c/sup\u003e is shown in black. (b) The calculated pore fluid pressure (red curve), hydrostatic pressure (blue), and total overburden stress (green). Sediments without excess fluid pressure would plot on hydrostatic line (blue), while sediments with high pore pressure would plot near the lithostatic line (green). (c) The calculated effective mean stress from the P-wave velocity (red) and the reference porosity (black). Pore pressure ratio is shown by the blue line. Purple line marks the λ* = 0.4.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/5429dd15b63e53842ba55e39.jpeg"},{"id":102234780,"identity":"93c0965c-bff3-4c72-9b47-a521485c27ed","added_by":"auto","created_at":"2026-02-09 16:13:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3062560,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7966953/v1/d8fe94e4-13da-4b27-8e3d-2da7ea781053.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity","fulltext":[{"header":"Introduction ","content":"\u003cp\u003eFluids and tectonic processes have interconnected processes at subduction zones\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Pore fluid pressure is an important factor in the development of accretionary complexes\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and fault movements that can cause a large slip during megathrust earthquakes\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e or facilitate slow earthquakes\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. Tectonic processes drive pore fluid pressures and fluid geochemical evolution through loading and heating, and fluid escape is modulated by modifying permeability through faulting and deformation\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eSlow earthquakes are characterized by their seismic moment that is proportional to its source duration\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. In the Nankai Trough, high pore pressure has been commonly invoked to explain the occurrence of shallow (\u0026lt;\u0026thinsp;10 km) slow earthquakes\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Research on slow earthquakes has significantly improved our understanding of slip phenomena but remains a complex topic because slow earthquakes have been linked to both rupture initiation and termination\u003csup\u003e\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. The association of high pore fluid pressure and slow earthquake activity is also not a direct relationship because other factors have also been proposed to possibly explain the occurrence of slow earthquakes such as seamount subduction\u003csup\u003e\u003cspan additionalcitationids=\"CR12 CR13\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, plate coupling\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, sediment thickness and lithology\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, friction\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, dynamic stress changes from earthquakes\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, and tides\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eMany previous studies have estimated the pore pressure in Nankai Trough off Muroto using different methods such as drilling\u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24 CR25\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, taper angle\u003csup\u003e\u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, P-wave velocity\u003csup\u003e\u003cspan additionalcitationids=\"CR30\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, and S-wave velocity\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. The existence of fluid overpressure off Muroto has only been recently confirmed because of mud outflow during drilling at site C0023\u003csup\u003e26\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). However, the cause and extent of fluid overpressure is not yet clearly defined, and each method has its limitations. Flores et al.\u003csup\u003e29,33\u003c/sup\u003e proposed a conceptual model integrating multi-seamount subduction, sediment underplating, and lithologic heterogeneity to explain fluid overpressure and slow earthquake activity, but quantitative pore pressure estimates beyond ~\u0026thinsp;10 km from the frontal thrust were lacking. A recently published high-resolution P-wave velocity model along the Muroto transect that spans 100 km and extends to 20 km depth achieved vertical and horizontal resolutions of ~\u0026thinsp;0.5 and 1.0 km above the oceanic crust through full waveform inversion of wide-angle ocean-bottom seismograph data\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. A laterally continuous low-velocity band within subducted sediments was identified and interpreted as indicative of high pore pressures\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThis study quantified the pore pressure ratio (λ*), also called normalized pore pressure, from the P-wave velocity model of Shiraishi et al.\u003csup\u003e34\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) using empirical relationships between velocity, porosity, and effective mean stress\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. The resulting λ* model was then compared with structural and tectonic features to infer the processes controlling fluid overpressure and explore its spatial correlation with shallow slow earthquake activity.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003ePore pressure distribution\u003c/h2\u003e\u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\u003ch2\u003eUnderthrust sediments in the outer wedge (131 words)\u003c/h2\u003e\u003cp\u003eThe λ* in the underthrust sediments in the outer wedge is generally greater than 0.4 but there are patches of higher λ* (\u0026gt;\u0026thinsp;0.6) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). This is consistent with the results of Hirose et al.\u003csup\u003e26\u003c/sup\u003e where they inferred the presence of sub-kilometer scale overpressured aquifers beneath the Nankai Trough decollement off Muroto by modeling the transient borehole mud flow at Site C0023 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Our results indicate the possible widespread existence of these overpressured aquifers in Nankai Trough especially in Muroto where a low taper angle has been reported by previous studies\u003csup\u003e\u003cspan additionalcitationids=\"CR27 CR28\" citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. However, the cause of this patchy distribution is still unknown because there is no clear change in the seismic reflection signature of the underthrust sediments that coincides with the occurrence of the overpressured aquifers (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee).\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\n\u003ch3\u003eUnderplated sediments in the inner wedge\u003c/h3\u003e\n\u003cp\u003eA wide region of high λ* (\u0026gt;\u0026thinsp;0.6) coincides well with the location of the underplated sediments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The underplated sediments are generally composed of strong and discontinuous reflections while the underthrust sediments in the outer wedge are transparent. These 2 units are separated by the decollement step down (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). Our results contradict the earlier studies where they attributed the decollement step down to the increasing shear strength of the decollement because of drained conditions in the inner wedge\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Recent studies on the other hand using S-wave velocities showed a large velocity reduction (\u0026gt;\u0026thinsp;20%) in this region and a λ* of 0.75\u003csup\u003e32\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The presence of the high λ* can be explained by the deformation caused by multiple seamount subduction proposed by Flores et al.\u003csup\u003e29,33\u003c/sup\u003e. They interpreted that the underplated sediment is composed of fluid-rich trench-fill sediments that were dragged down or underplated by Seamount A or any older seamount (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Tectonic compression from newly-subducted seamounts then generated this high pore pressure region.\u003c/p\u003e\n\u003ch3\u003eThrust Faults \u003c/h3\u003e\n\u003cp\u003eVertical columns of high λ* can be observed in many of the thrust faults. A prominent region is observed between the 60 to 65 km mark which is interpreted as evidence of upward fluid flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This is supported by the presence of cold seeps near fault zones reported from deep-sea observations\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Some of the thrust faults in the outer wedge show a negative polarity and high λ* (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-d) but the dip angle of the high λ* and the thrust faults do not align well. The vertical orientations of the high λ* areas may be an artefact of the full waveform inversion, however, their co-location with the thrust faults can be indicative of their possible existence. Previous studies also reported a negative polarity decollement in Muroto\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e which can be explained with high pore pressure along the fault\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Thus, the high λ* in the thrust faults is also interpreted as evidence of fluid flow where faults act as permeable pathways.\u003c/p\u003e\u003cp\u003e\u003cb\u003eAssociation of pore pressure with slow earthquake activity\u003c/b\u003e (499 words)\u003c/p\u003e\u003cp\u003eThe reference porosity model decreases from ~\u0026thinsp;0.25 at the frontal thrust to 0.05 at ~\u0026thinsp;20 km landward, while the calculated porosity from P-wave velocity shows a modest decrease from ~\u0026thinsp;0.20 to 0.10 from the frontal thrust up to ~\u0026thinsp;50 km landward (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The pore pressure line nearly parallels the lithostatic line for most of the profile (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) and this indicates that the increasing lithostatic load associated with progressive underthrusting is borne by trapped pore fluids\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. The effect of fluid pressure is further emphasized by the significantly lower effective mean stress calculated from the empirical relation with P-wave velocity than the effective mean stress calculated from the reference porosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003eThe number of slow earthquakes increases with distance from the deformation and is maximized at around 50\u0026ndash;55 km (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This region corresponds to the underplated sediments discussed in the previous section where cold seeps have also been reported\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Fluid migration plays an important role in activating slow earthquakes\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. This implies that the underplated sediments can be a source region for fluids\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e and episodic fluid flow from the underplated sediments may also activate slow earthquakes. This episodic fluid flow may be modulated by the tectonic compression from the subducted seamounts\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eA recent study using borehole data in the Kumano region has documented that slow earthquakes can migrate seaward and breach the trench\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. The negative polarity decollement and negative polarity thrust faults near the trench showing high λ* (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) may indicate that slow earthquakes can also migrate and breach the trench in this region. Additionally, since drilling data in Kumano also indicates that the fault may experience both fast and slow slip at different times\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e, this brings up important questions if it is also possible for future megathrust slip to breach the trench in Muroto region considering that subducted seamounts (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) have been proposed to have functioned as a rupture barrier during the 1946 Nankai earthquake\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. Other studies on the other hand using joint inversion of seismic waveforms and geodetic data from the 1946 Nankai earthquake identified two asperities separated by Seamount A and maximum slip of 5.1 m occurring at a point south of Cape Muroto\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Hence, we infer that it may be possible for trench breaching slip to occur in the future. However, factors controlling whether the subducted seamount will function as a barrier or asperity is still unknown.\u003c/p\u003e\u003cp\u003eThe λ* from the deformation front up to ~\u0026thinsp;65 km landward is generally greater than 0.4 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), which agrees with previous observations that slow earthquakes appear to only occur in overpressured areas where λ* ranges from ~\u0026thinsp;0.4 to 0.9\u003csup\u003e43\u003c/sup\u003e. However, it is also important to note the heterogenous pore pressure distribution especially in the outer wedge where patches of overpressured aquifers were observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Overall, the generated pore pressure map using the high-resolution P-wave velocity model derived from full waveform inversion of ocean seismometer data\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e provided new insights into the fluid and tectonic processes in Nankai Trough, and supports previous studies on the existence of overpressured aquifers\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e and the conceptual models for creating a high pore pressure environment from multiple seamount subduction off Muroto\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eThe pore pressure ratio (λ\u003csup\u003e*\u003c/sup\u003e), also called normalized pore pressure, was calculated using empirical relationships between P-wave velocity \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left({V}_{p}\\right)\\)\u003c/span\u003e\u003c/span\u003e, porosity \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(\\varphi\\:\\right)\\)\u003c/span\u003e\u003c/span\u003e, and effective mean stress \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left({p}^{{\\prime\\:}}\\right)\\)\u003c/span\u003e\u003c/span\u003e \u003csup\u003e35\u003c/sup\u003e.This study used the latest \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{p}\\)\u003c/span\u003e\u003c/span\u003e model off Cape Muroto which has a vertical resolution of 0.5 km and a horizontal resolution of 1.0 km above the top of the oceanic crust\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe P-wave velocity is first converted to porosity\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{V}_{p}=A+B\\varphi\\:+\\:\\frac{0.305}{{(\\varphi\\:\\:+C)}^{2}\\:+\\:D}+E\\left({v}_{sh}-F\\right)[\\text{tanh}\\left(G\\left(\\varphi\\:-{\\varphi\\:}_{c}\\right)\\right)-\\left|\\text{tanh}\\left(G\\left(\\varphi\\:-{\\varphi\\:}_{c}\\right)\\right)\\right|]$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{sh}\\)\u003c/span\u003e\u003c/span\u003e is the shale volume equal to 1.057; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varphi\\:}_{c}\\)\u003c/span\u003e\u003c/span\u003e is the critical porosity equal to 0.295; A,B,C,D,E,F, and G are constants equal to 0.746, 0.532, 0.124, 0.0513, 0.61, and 1.123, respectively. The calculated \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varphi\\:\\)\u003c/span\u003e\u003c/span\u003e is then converted to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{p}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e using the equation:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\frac{\\varphi\\:}{1\\:-\\:\\varphi\\:}=0.79-\\:0.40\\text{log}{p}^{{\\prime\\:}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThen, the pore fluid pressure \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left({P}_{f}\\right)\\)\u003c/span\u003e\u003c/span\u003e is calculated using the equation:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{P}_{f}=\\:{\\sigma\\:}_{vo}\\:-\\:{P}_{hf}-0.42{p}^{{\\prime\\:}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{vo}\\)\u003c/span\u003e\u003c/span\u003e is the total overburden stress, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{hf}\\)\u003c/span\u003e\u003c/span\u003e is the hydrostatic pressure. The \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{hf}\\)\u003c/span\u003e\u003c/span\u003e can be easily calculated at any depth assuming the density of seawater at 1,024 kg/m\u003csup\u003e3\u003c/sup\u003e. In the absence of density data for the accretionary prism, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{vo}\\)\u003c/span\u003e\u003c/span\u003e was calculated using densities estimated from \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{p}\\)\u003c/span\u003e\u003c/span\u003e\u003csup\u003e4\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e:\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$\\:\\rho\\:=\\:-0.6997+2.2302{V}_{p}-0.598{V}_{p}^{2}+0.07036{V}_{p}^{3}-0.0028311{V}_{p}^{4}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eLastly, the λ\u003csup\u003e*\u003c/sup\u003e is given by:\u003cdiv id=\"Eque\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Eque\" name=\"EquationSource\"\u003e\n$$\\:{\\lambda\\:}^{*}=\\:\\frac{{\\sigma\\:}_{vo}-\\:{P}_{hf}-0.42{p}^{{\\prime\\:}}}{{\\sigma\\:}_{vo}-\\:{P}_{hf}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe resulting λ\u003csup\u003e*\u003c/sup\u003e profile was compared with a seismic reflection profile along the same line (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) to examine the pore pressure distribution and its possible causes. To examine the effects of high \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\lambda\\:}^{*}\\)\u003c/span\u003e\u003c/span\u003e, a reference porosity was used to model the effective stress for a normal consolidation case. This was defined as an exponential function of depth (z) and was constrained by drilling data at Site 1173\u003csup\u003e36\u003c/sup\u003e\u003cdiv id=\"Equf\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equf\" name=\"EquationSource\"\u003e\n$$\\:\\varphi\\:=0.75{e}^{-0.0011z}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eP.C.M.F. wrote the manuscript. S.K, K.S., G.F., Y.N., and R.A. provided supervision. K.S. G.F. Y.N., and R.A. conducted the full waveform inversion of the ocean-bottom seismograph data to create a high resolution P-wave velocity model. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis study was partly supported by the Japan Society for the Promotion of Science, Japan during the JSPS Postdoctoral Fellowship of P.C.M.F. We are also grateful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan for the management expense grants to the Research Institute for Marine Geodynamics at JAMSTEC and funding for the Research Project for Disaster Prevention on the Great Earthquakes along the Nankai Trough.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe seismic data in this study are available upon reasonable request from the JAMSTEC seismic survey database (https://doi.org/10.17596/0002069).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSaffer, D. M. \u0026amp; Tobin, H. J. Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure. \u003cem\u003eAnnu. Rev. Earth Planet. Sci.\u003c/em\u003e \u003cb\u003e39\u003c/b\u003e, 157\u0026ndash;186 (2011).\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eDavis, D., Suppe, J. \u0026amp; Dahlen, F. A. 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Seismic refraction. \u003cem\u003eThe sea\u003c/em\u003e \u003cb\u003e4\u003c/b\u003e, 53\u0026ndash;58 .\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":true,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7966953/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7966953/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"(192 words) High pore pressure ratio (λ*) has been associated with the occurrence of slow earthquakes. Many studies have estimated the λ* using taper angle, P-and S-wave velocities, and drilling but the extent is not yet clearly defined. This study utilized a recently published high-resolution P-wave velocity model off Muroto derived from a two-step tomographic inversion of ocean-bottom seismograph data to determine the λ* using empirical relationships between velocity, porosity, and effective mean stress. We determined an extensive zone of high λ* (\u003e0.4) from the frontal thrust up to ~65 km landward and to a depth of 8 km with three characteristic observations. First, the underthrust sediments in the outer wedge show patches of overpressured aquifers where λ* \u003e0.6, consistent with previous drilling results. Second, the high λ* (\u003e0.6) region in the inner wedge coincides with previously reported underplated sediments composed of fluid-rich trench-fill sediments dragged down by seamounts. The high λ* may be caused by tectonic compression from the newly subducted seamounts. Lastly, vertical columns of high λ* were observed in many of the thrust faults with some faults showing a negative polarity, which are interpreted to be evidence of fluid flow.","manuscriptTitle":"Extensive fluid overpressure in the shallow slow earthquake zone of Nankai Trough off Muroto mapped with high-resolution P-wave velocity","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-09 05:19:04","doi":"10.21203/rs.3.rs-7966953/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-10T05:58:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-10T03:09:19+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-21T22:52:25+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"311530716780574658260946432548364607136","date":"2025-11-17T00:47:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"247824871209965575019511752718272304695","date":"2025-11-16T22:56:18+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-09T04:29:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-08T03:21:57+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-11-06T13:02:07+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-04T02:25:05+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-11-04T02:14:58+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fa2faef3-a241-453d-8d9c-e20b0f39f2e4","owner":[],"postedDate":"December 9th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":58418109,"name":"Earth and environmental sciences/Natural hazards"},{"id":58418110,"name":"Earth and environmental sciences/Solid earth sciences"}],"tags":[],"updatedAt":"2026-02-09T16:09:46+00:00","versionOfRecord":{"articleIdentity":"rs-7966953","link":"https://doi.org/10.1038/s41598-026-38308-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-02-06 15:59:55","publishedOnDateReadable":"February 6th, 2026"},"versionCreatedAt":"2025-12-09 05:19:04","video":"","vorDoi":"10.1038/s41598-026-38308-x","vorDoiUrl":"https://doi.org/10.1038/s41598-026-38308-x","workflowStages":[]},"version":"v1","identity":"rs-7966953","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7966953","identity":"rs-7966953","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}