Nankai Trough Seismogenic Zone Experiment Research Articles

Direct assessment of the hydraulic structure of the plate boundary at the toe of the Nankai accretionary prism

SUMMARY The Nankai Trough is a locus of slow slip, low-frequency earthquakes, and large magnitude (Mw > 8) earthquakes. It is usually assumed that high pore pressure contributes substantially to earthquake dynamics. Hence, a thorough understanding of the hydraulic regime of the Nankai accretionary prism is needed to understand this diversity of behaviours. We focus on the toe of the accretionary prism by studying data from Hole C0024A, part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project, that intersected the décollement at 813 m below seafloor (mbsf) about 3 km away from the trench. We contribute to this understanding by innovatively integrating drilling and logging data to derive high-resolution hydraulic profiles along the borehole. A quantitative re-analysis of the variation in the downhole annular pressure monitored during drilling show localized fluid flow from the formation to the borehole in excess of 0.05 m3 s−1, especially in the damage zones at the footwall of the décollement. To validate the fluid flow profile, pore pressure was estimated independently from empirical relationships between pore pressure, porosity and P-wave velocity, obtained from consolidation experiments and Eaton-type methods based on drilling or sonic velocity data. The formation fluids are becoming significantly overpressurized with depth in the few hundred metres above décollement. The hydraulic profile suggests that the core of the décollement acts as a barrier inhibiting upward fluid convection, whereas the damage zone acts as an efficient longitudinal channel able to diffuse high pressure from the deeper part of the subduction.

Lowering of the kinetic barrier of clay-mineral reactions along a seismogenic fault: Example from the Nankai subduction zone, Japan

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S–I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is ∼15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20–30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura–Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20–30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite–illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones.

Thermal Conductivity Profile in the Nankai Accretionary Prism at IODP NanTroSEIZE Site C0002: Estimations From High‐Pressure Experiments Using Input Site Sediments

AbstractDepth profiles of sediment thermal conductivity are required for understanding the thermal structure in active seismogenic zones. During the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), a scientific drilling project of the International Ocean Discovery Program (IODP), a borehole was penetrated to a depth of 3,262.5 m below seafloor (mbsf) at Site C0002. Because core samples obtained from below ~1,100 mbsf in an accretionary prism are limited, a thermal conductivity profile over such depths usually determined by laboratory measurements using core samples is not available. To obtain the thermal conductivity profile at Site C0002, we used core samples collected from sediments that overlay the incoming subducting oceanic basement at Nankai Trough Seismogenic Zone Experiment Site C0012, which can be considered to have the same mineral composition as the accretional prism at Site C0002. The thermal conductivity of the C0012 core samples was measured at high pressure to simulate subduction by reducing the sample porosity. We measured the thermal conductivity of six core samples from 144–518 mbsf at Site C0012 up to a maximum effective pressure of ~50 MPa, corresponding to depths greater than ~4 km below seafloor. We obtained an empirical relation between thermal conductivity λBulk in Wm‐1K‐1 and fractional porosity ϕ for the Nankai Trough accretionary prism as λBulk = exp(−1.09ϕ + 0.977). Based on porosity data measured using core/cuttings samples and data derived from P wave velocity logs, we estimate two consistent and complete thermal conductivity profiles down to ~3 km below seafloor in the Nankai Trough accretionary prism. These profiles are consistent with the existing thermal conductivity data measured using limited core samples.

Linking Faults, Fractures, and Clay Mineral Occurrence With Fluid Transport in the Accretionary Prism of the Nankai Trough, Japan

AbstractThe interpretation of logging while drilling resistivity oriented images and geophysical logs from Hole C0002 drilled during the International Ocean Discovery Program (IODP) Expeditions 338 and 348 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project revealed a complex structure of the accretionary prism characterized by steep bedding and abundant folds and fault zones, indicative of specific areas of deformation within the clay‐dominated prism sediments. Local faults were undoubtedly interpreted on images from Hole C0002P (2,162.5–3,058.4 m below seafloor), whereas folds were identified in both Holes C0002P and C0002F (860–2,005.5 m below seafloor). The clay mineral analysis of rock samples (cuttings) shows that clay mineralization occurs within the range of the faulted and fractured zones. This linkage between local deformation and clay mineralization can have implications for enhanced fluid flow and localized fluid‐rock interaction within the highly fractured areas of the accretionary prism. We conclude that dissolution and neomineralization of illitic and smectitic phases within fractures and faults is a process that probably continues during active creep and fault reactivation and plays a key role in influencing weak fault and creep behavior within the Nankai Trough, and likely in shallow fault systems elsewhere.

Observed stress state for the IODP Site C0002 and implication to the stress field of the Nankai Trough subduction zone

Conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions. Stress magnitude is especially essential to comprehend stress heterogeneity, significant structural anisotropy, and/or pre-existing fracture systems. Stress state is influenced by rock strength, structural properties, and near-field principal stresses, which impact borehole integrity during drilling. Factors that we can measure through Logging While Drilling (LWD) and Measure while Drilling (MWD) to better understand borehole conditions, include mud weight, overburden weight and velocity. The LWD resistivity images in Holes C0002A and C0002F of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) indicate that most of the drillings in stable environments. However, the bottom hole assembly became stuck around 3 km depth in Hole C0002P and did not reach the mega-thrust fault. We evaluated the stress state for the Site C0002 by the geomechanical model. With the breakout width and rock strength estimation, we constrain the stress profiles in the drilled depth. Our analysis inferred that the instability of Hole C0002P had been caused by an overpressure drilling fluid state and low horizontal principal stress. Furthermore, we developed an optimally oriented plane (OOP) model to exam the stress state in the Site C0002 from Nankai accretionary prism to the Nankai Trough subduction zone. The normal faulting stress regime modeled from OOP is consistent with the geomechanical model in Site C0002. The horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone.

Seismogenic Zone Structures Revealed by Improved 3‐D Seismic Images in the Nankai Trough off Kumano

AbstractTo reveal the detailed deformation structures related to plate subduction in the Nankai Trough, we applied up‐to‐date technologies to improve our 3‐D seismic images. This region is dominated by a megasplay fault system that consists of a coseismic out‐of‐sequence thrust branching from the plate interface and separating the inner and outer accretionary prism. The 3‐D seismic volume was acquired off Kumano in 2006 as a preliminary site survey as part of the Nankai Trough Seismogenic Zone Experiment. The preprocessed data quality was improved by recovering the broadband responses and by better attenuating multiple reflections and noise. New reflection images were then produced using prestack time migration and prestack depth migration with updated velocity models. The new velocity model suggests the possible existence of a high‐velocity zone just above the megasplay fault, which might indicate petrophysical alteration in the seismogenic zone. The fault geometry with spatial dip angle variation and the overburden velocities are important factors for further estimating the force distribution along the coseismic fault. Deformation structures newly imaged beneath the Kumano Basin and dipping layers above the megasplay fault imply complex thrusting and possible fluid flow paths within the inner prism. Fine‐scale deformation features are clarified in the shallow areas from the outer prism to the transition zone, which are useful for reconstructing the accretionary prism development. A low‐reflectivity zone, including an isolated layered block, may be originally underthrusted sediments that have been remobilized during later strike‐slip faults along the edge of Kumano Basin.

Paleo-stress orientations and magnitudes from triaxial testing and stress inversion analysis in Nankai accretionary prism sediments

Combined knowledge on the magnitude as well as the orientation of paleo-stress is usually not given. Based on drilling investigations and laboratory experiments, we are able to provide both, constraints on paleo-stress magnitude as well as orientation in the shallow part of the Nankai accretionary wedge at Sites C0001 and C0002 of the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). Slip data of core-scale faults indicate multiple paleo-stress geometries, including normal, strike-slip, and reverse faulting regimes. Combining new and previously obtained results from triaxial compressional tests that constrain internal friction angles at critical state with the paleo-stress inversion results, we examine paleo-stress magnitudes for each stress state at each site. We observe that the paleo-stress magnitude for the normal faulting regime is smaller than that for the reverse faulting regime, although the paleo-stress magnitude for Site C0002 is not well-constrained. The change in stress magnitude may reflect changing horizontal tectonic stresses transmitted to the accretionary wedge from changing friction conditions along the subduction plate interface and in turn may have implications for the evolution of the accretionary wedge and/or the seismic cycle.

IODP workshop: Core-Log Seismic Investigation at Sea – Integrating legacy data to address outstanding research questions in the Nankai Trough Seismogenic Zone Experiment

Abstract. The first International Ocean Discovery Program (IODP) Core-Log-Seismic Integration at Sea (CLSI@Sea) workshop, held in January–February 2018, brought together an international, multidisciplinary team of 14 early-career scientists and a group of scientific mentors specialized in subduction zone processes at the Nankai Trough, one of the Earth's most active plate-subduction zones located off the southwestern coast of Japan. The goal of the workshop was to leverage existing core, log, and seismic data previously acquired during the IODP's Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), to address the role of the deformation front of the Nankai accretionary prism in tsunamigenic earthquakes and slow slip in the shallow portion of the subduction interface. The CLSI@Sea workshop was organized onboard the D/V Chikyu concurrently with IODP Expedition 380, allowing workshop participants to interact with expedition scientists installing a long-term borehole monitoring system (LTBMS) at a site where the workshop's research was focused. Sedimentary cores from across the deformation front were brought onboard Chikyu, where they were made available for new description, sampling, and analysis. Logging data, drilling parameters, and seismic data were also available for investigation by workshop participants, who were granted access to Chikyu laboratory facilities and software to perform analyses at sea. Multi-thematic presentations facilitated knowledge transfer between the participants across field areas, and highlighted the value of multi-disciplinary collaboration that integrates processes across different spatiotemporal scales. The workshop resulted in the synthesis of existing geophysical, geologic, and geochemical data spanning IODP Sites C0006, C0007, C0011 and C0012 in the NanTroSEIZE area, the identification of key outstanding research questions in the field of shallow subduction zone seismogenesis, and fostered collaborative and individual research plans integrating new data analysis techniques and multidisciplinary approaches.

Changes in Physical Properties of the Nankai Trough Megasplay Fault Induced by Earthquakes, Detected by Continuous Pressure Monitoring

AbstractOne primary objective of Integrated Ocean Drilling Program Expedition 365, conducted as part of the Nankai Trough Seismogenic Zone Experiment, was to recover a temporary observatory emplaced to monitor formation pore fluid pressure and temperature within a splay fault in the Nankai subduction zone offshore SW Honshu, Japan. Here we use a 5.3 year time series of formation pore fluid pressure, and in particular the response to ocean tidal loading, to evaluate changes in pore pressure and formation and fluid elastic properties induced by earthquakes. Our analysis reveals 31 earthquake‐induced perturbations. These are dominantly characterized by small transient increases in pressure (28 events) and decreases in ocean tidal loading efficiency (14 events) that reflect changes to formation or fluid compressibility. The observed perturbations follow a magnitude‐distance threshold similar to that reported for earthquake‐driven hydrological effects in other settings. To explore the mechanisms that cause these changes, we evaluate the expected static and dynamic strains from each earthquake. The expected static strains are too small to explain the observed pressure changes. In contrast, estimated dynamic strains correlate with the magnitude of changes in both pressure and loading efficiency. We propose potential mechanism for the changes and subsequent recovery, which is exsolution of dissolved gas in interstitial fluids in response to shaking.

A new perspective of the subduction zone derived from the Ocean Drilling Program for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)

A new perspective of the subduction zone derived from the Ocean Drilling Program for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)

Paleothermal structure of the Nankai inner accretionary wedge estimated from vitrinite reflectance of cuttings

AbstractThe paleothermal structure and tectonic evolution of an accretionary prism is basic information for understanding subduction zone seismogenesis. To evaluate the entire paleotemperature profile of the Integrated Ocean Drilling Program (IODP) Site C0002 located in the off‐Kumano region of the Nankai Trough and penetrate the inner accretionary wedge down to 3058.5 m below the seafloor (mbsf), we performed a vitrinite reflectance analysis for cuttings and core samples during IODP expeditions 338 and 348: Nankai Trough seismogenic zone experiment. Although vitrinite reflectance values (Ro) tend to increase with depth, two reversals of these values suggested the existence of thrust fault zones with sufficient displacements to offset the paleothermal structure. The estimated maximum paleotemperatures are 42–70°C at 1200–1300 mbsf, 44–100°C at 1600–2400 mbsf, and 56–115°C at 2600–3000 mbsf, respectively. These temperatures roughly coincide with estimated modern temperatures; however, at a smaller scale, the reconstructed partial paleogeothermal gradient (∼60–150°C/km) recorded at the hanging‐ and footwall of the presumed thrust fault zone is higher than the modern geothermal gradient (∼30–40°C/km). This high paleogeothermal gradient was possibly obtained prior to subduction, reflecting the large heat flow of the young Philippine Sea Plate.

Distribution of stress state in the Nankai subduction zone, southwest Japan and a comparison with Japan Trench

To better understand the distribution of three dimensional stress states in the Nankai subduction zone, southwest Japan, we review various stress-related investigations carried out in the first and second stage expeditions of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) by the Integrated Ocean Drilling Program (IODP) and compile the stress data. Overall, the maximum principal stress σ1 in the shallower levels (<~1km) is vertical from near the center of forearc basin to near the trench and; the maximum horizontal stress SHmax (interpreted to be the intermediate principal stress σ2) is generally parallel to the plate convergence vector. The exception to this generalization occurs along the shelf edge of the Nankai margin where SHmax is along strike rather than parallel to the plate convergence vector. Reorientation of the principal stresses at deeper levels (e.g., >~1km below seafloor or in underlying accretionary prism) with σ1 becoming horizontal is also suggested at all deeper drilling sites. We also make a comparison of the stress state in the hanging wall of the frontal plate-interface between Site C0006 in the Nankai and Site C0019 in the Japan Trench subduction zone drilled after the 2011 Mw 9.0 Tohoku-Oki earthquake. In the Japan Trench, a comparison between stress state before and after the 2011 mega-earthquake shows that the stress changed from compression before the earthquake to extension after the earthquake. As a result of the comparison between the Nankai Trough and Japan Trench, a similar current stress state with trench parallel extension was recognized at both C0006 and C0019 sites. Hypothetically, this may indicate that in Nankai Trough it is still in an early stage of the interseismic cycle of a great earthquake which occurs on the décollement and propagates to the toe (around site C0006).

Real-time drilling mud gas monitoring for qualitative evaluation of hydrocarbon gas composition during deep sea drilling in the Nankai Trough Kumano Basin.

BackgroundIntegrated Ocean Drilling Program Expedition 338 was the second scientific expedition with D/V Chikyu during which riser drilling was conducted as part of the Nankai Trough Seismogenic Zone Experiment. Riser drilling enabled sampling and real-time monitoring of drilling mud gas with an onboard scientific drilling mud gas monitoring system (“SciGas”). A second, independent system was provided by Geoservices, a commercial mud logging service. Both systems allowed the determination of (non-) hydrocarbon gas, while the SciGas system also monitored the methane carbon isotope ratio (δ13CCH4). The hydrocarbon gas composition was predominated by methane (> 1%), while ethane and propane were up to two orders of magnitude lower. δ13CCH4 values suggested an onset of thermogenic gas not earlier than 1600 meter below seafloor. This study aims on evaluating the onboard data and subsequent geological interpretations by conducting shorebased analyses of drilling mud gas samples.ResultsDuring shipboard monitoring of drilling mud gas the SciGas and Geoservices systems recorded up to 8.64% and 16.4% methane, respectively. Ethane and propane concentrations reached up to 0.03 and 0.013%, respectively, in the SciGas system, but 0.09% and 0.23% in the Geoservices data. Shorebased analyses of discrete samples by gas chromatography showed a gas composition with ~0.01 to 1.04% methane, 2 – 18 ppmv ethane, and 2 – 4 ppmv propane. Quadruple mass spectrometry yielded similar results for methane (0.04 to 4.98%). With δD values between -171‰ and -164‰, the stable hydrogen isotopic composition of methane showed little downhole variability.ConclusionsAlthough the two independent mud gas monitoring systems and shorebased analysis of discrete gas sample yielded different absolute concentrations they all agree well with respect to downhole variations of hydrocarbon gases. The data point to predominantly biogenic methane sources but suggest some contribution from thermogenic sources at depth, probably due to mixing. In situ thermogenic gas production at depths shallower 2000 mbsf is unlikely based on in situ temperature estimations between 81°C and 85°C and a cumulative time-temperature index of 0.23. In conclusion, the onboard SciGas data acquisition helps to provide a preliminary, qualitative evaluation of the gas composition, the in situ temperature and the possibility of gas migration.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-014-0015-8) contains supplementary material, which is available to authorized users.

Method for estimating ductile horizontal strain from magnetic fabrics in poorly consolidated clay-rich sediments

We propose a simple method based on the normalized eigenvalues of the tensor of anisotropy of magnetic susceptibility (AMS) to estimate horizontal ductile strain in detrital clay-rich sedimentary rocks subjected to layer parallel shortening (LPS). This method is mainly based on two assumptions: (1) The magnetic carriers are contacting clay-mineral platelets, and (2) The platelets respond to LPS by crenulation and formation of a zone axis of unchanged magnetic susceptibility perpendicular to the plane of deformation. We further simplify the problem by considering, as the exact distribution of particles orientations is unknown, that the AMS tensor arises from a single equivalent clay platelet, of which the angle with the bedding plane directly reflects the amount of horizontal strain. The proposed method is applied to data from samples retrieved during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project. Sample-wise finite strain values assist AMS data interpretation, which is traditionally based on the AMS ellipsoid shape parameter, and allow comparison with strain estimates obtained by other means. In the Nankai Accretionary Prism, AMS-derived strain values range from 0 to 25% and suggest substantial mechanical compartmentalization with inter-unit strain contrasts larger than 5 percentage points, including the penetrated shallow portion of a megasplay fault. At a local scale, AMS strain also appears to be affected by thrusting, what can be interpreted as a record of a ductile strain component away for the brittle zone or as an ‘interseismic’ ductile strain within otherwise brecciated material.

Consolidation state of incoming sediments to the Nankai Trough subduction zone: Implications for sediment deformation and properties

AbstractThe hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2–3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.

Long-distance fluid and heat transport in the oceanic crust entering the Nankai subduction zone, NanTroSEIZE transect

I examine the potential causes of anomalous seafloor heat flux on the oceanic plate in the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) transect offshore southern Japan. The most prominent anomaly is a ∼50 mWm−2 change in heat flux between Integrated Ocean Drilling Program Sites C0011 and C0012 over a distance of <10 km. I develop thermal models to investigate the effects of hydrothermal circulation in the basaltic basement of the oceanic crust and variations in heat input from the mantle. The <10 km wide transition in surface heat flux reflects a process in the shallow subsurface; variations in heat input from ⩾5 km depth would generate a >30 km wide transition at the seafloor. The observed surface heat flux pattern is indicative of hydrothermal circulation in the basement aquifer and advection of heat from the subducted crust into the aquifer on the incoming plate. For a 600 m thick aquifer, the permeability is likely ⩾7×10−11 m2, and hydrothermal circulation transports at least 300 times more heat than conduction alone. The heat flux from the subduction zone seaward to the incoming plate is consistent with hydrothermal circulation in the subducting crust persisting to ∼100 km landward of the deformation front. Vigorous fluid circulation in the basaltic basement is consistent with both the seafloor thermal anomalies and geochemical anomalies near the sediment–basement interface.

IODP Expedition 338: NanTroSEIZE Stage 3: NanTroSEIZE plate boundary deep riser 2

Abstract. The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is designed to investigate fault mechanics and seismogenesis along a subduction megathrust, with objectives that include characterizing fault slip, strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout an active plate boundary system. Integrated Ocean Drilling Program (IODP) Expedition 338 was planned to extend and case riser Hole C0002F from 856 to 3600 meters below the seafloor (m b.s.f.). Riser operations extended the hole to 2005.5 m b.s.f., collecting logging-while-drilling (LWD) and measurement-while-drilling, mud gas, and cuttings data. Results reveal two lithologic units within the inner wedge of the accretionary prism that are separated by a prominent fault zone at ~ 1640 m b.s.f. Due to damage to the riser during unfavorable winds and strong currents, riser operations were suspended, and Hole C0002F left for re-entry during future riser drilling operations. Contingency riserless operations included coring at the forearc basin site (C0002) and at two slope basin sites (C0021 and C0022), and LWD at one input site (C0012) and at three slope basin sites (C0018, C0021 and C0022). Cores and logs from these sites comprehensively characterize the alteration stage of the oceanic basement input to the subduction zone, the early stage of Kumano Basin evolution, gas hydrates in the forearc basin, and recent activity of the shallow megasplay fault zone system and associated submarine landslides.

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

Subduction earthquakes on plate-boundary megathrusts accommodate most of the global seismic moment release, frequently resulting in devastating damage by ground shaking and tsunamis. As many earthquakes occur in deep-sea regions, the dynamics of earthquake faulting in subduction zones is poorly understood. However, the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) and fault rock studies in accretionary prisms exhumed from source depths of subduction earthquakes have greatly improved our understanding of earthquake faulting in subduction zones. Here, we review key advances that have been made over the last decade in the studies of fault rocks and in laboratory experiments using fault zone materials, with a particular focus on the Nankai Trough subduction zone and its on-land analog, the Shimanto accretionary complex in Japan. New insights into earthquake faulting in subduction zones are summarized in terms of the following: (1) the occurrence of seismic slip along velocity-strengthening materials both at shallow and deep depths; (2) dynamic weakening of faults by melt lubrication and fluidization, and possible factors controlling coseismic deformation mechanisms; (3) fluid-rock interactions and mineralogical and geochemical changes during earthquakes; and (4) geological and experimental aspects of slow earthquakes.

Stress field observation and modeling from the NanTroSEIZE scientific drillings in the Nankai Trough system, SW Japan

Abstract Many studies have investigated the stress state in terms of drilling projects in the Nankai Trough region, the potential area for next devastating earthquakes. To understand the stress state and the geologic properties in Nankai, several drilling projects have been conducted over time. Among these projects, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) included studies regarding stress orientations and magnitude estimations by logging data from the Nankai Trough. Due to limitations in technology of drilling and the data processing, the actual stress orientation and the magnitude of the Nankai Trough at depth remains controversial. In this study, we discussed the stress state in each borehole and evaluated the stress state using various models. Our models evaluated stress states contributed by a slip deficit model in the Nankai Trough system and the coseismic dislocations of the two September 5th, 2004 earthquakes. Compared to the stress estimation from logging data, the simulated stress orientation could more or less explain the logging data at the NanTroSEIZE drilling boreholes. Based on these models, the stress tensor in Nankai can be extended from the boreholes to more dimensional area, even the locations where no drilling has been reached.

Fluid pressure and temperature transients detected at the Nankai Trough Megasplay Fault: Results from the SmartPlug borehole observatory

Abstract The SmartPlug is the first borehole observatory in the IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). It comprises a retrievable bridge plug with an autonomous instrument for pressure (P) and temperature (T) monitoring. The borehole observatory was installed at Site C0010 of the Integrated Ocean Drilling Program (IODP) Kumano transect crossing the Nankai Trough, SE offshore Japan, to obtain fluid pressure and temperature data from where the borehole penetrates one of the shallow branches of the subduction megasplay fault at 410 mbsf. In this manuscript, a 15 month-long P–T record collected by the SmartPlug is evaluated. Despite the 1 min sampling interval, pressure variations related to local storms and to tsunami and seismic waves from regional and distant earthquakes are observed. Seismic waves of one regional earthquake appear to have led to a drop in formation pressure that may be the consequence of a seismic-wave-induced increase in permeability. Pressure variations related to Rayleigh waves are typically larger in the formation than in the inadvertently sealed casing above, whereas seafloor loading signals imposed by tides, tsunamis, and storm-generated waves are larger in the casing than in the formation. This difference presumably reflects the different response to strain generated by formation deformation vs. strain caused by loading at the seafloor. No seismogenic or aseismic deformation event at the megasplay fault or within the accretionary prism was observed during this initial 15-month-long recording period.