Eastern Nankai Trough Research Articles

Empirical evaluation of the strength and deformation characteristics of natural and synthetic gas hydrate-bearing sediments with different ranges of porosity, hydrate saturation, effective stress, and strain rate

Evaluating the mechanical properties of gas (primarily methane) hydrate-bearing sediments is essential for commercial production as a next-generation resource and understanding the global carbon cycle. Triaxial and uniaxial compression tests have been conducted on synthetic gas hydrate and natural core samples recovered from deep-sea beds using pressure coring techniques. The results show that four factors are vital in establishing the strength of hydrate-bearing sediments: hydrate saturation, effective confining stress, porosity, and strain rate. However, no study has evaluated these factors in a unified and quantitative manner, and even if the physical properties of the reservoir are known in detail from logging, predicting the strength has been challenging. In this study, pressure cores were drilled and recovered from the Eastern Nankai Trough in April 2018 after Japan’s second offshore production test, and triaxial or uniaxial compression tests were performed on 12 pressure core samples brought back to the laboratory. The mechanical properties of the hydrate-bearing sediments were classified with previous obtained results from 53 pressure cores and 223 synthetic cores, and empirical equations for triaxial compressive strength and deformation modulus were proposed as functions of gas hydrate saturation, effective confining pressure, porosity, and strain rate. The obtained equations were found to correlate well with the experimental data and can predict the strength and deformation modulus from logging data.

Archaeal lipid biomarkers in near-surface sediments at a giant colony of the bivalve Calyptogena: Molecular records of a massive methane release event associated with methane hydrate dissociation

To investigate the possibility that dissociation of subsurface methane hydrate (MH) in the eastern Nankai Trough, offshore of Japan, led to the formation of a giant colony of the bivalve Calyptogena (currently mostly dead), the carbon isotope ratios (δ13C) of archaeal lipids and methane were measured in near-surface core sediments at Daini-Tenryu Knoll. The irregular variation of porewater methane δ13C with depth (from −75 ‰ to −26 ‰) suggested that originally low δ13C microbial methane was degraded in different proportions by anaerobic methane oxidation. Consistent with this inference, biomarkers of anaerobic methanotrophic archaea (ANME), namely, crocetane (2,6,11,15-tetramethylhexadecane), PMI (2,6,10,15,19-pentamethylicosane), and diethers (archaeol and hydroxyarchaeols), were detected in lipid extracts. The low diether δ13C values (−121 ‰ to −104 ‰) were characteristic of ANME, but less variable than the methane δ13C values, and the relationships between diethers and methane δ13C values deviated from regression lines derived using worldwide data from modern methane seep sites. In contrast, δ13C values of the ANME source methane predicted from those regression lines and the diether δ13C values agreed well with methane δ13C values in MH samples obtained by nearby deep drilling. This result strongly suggests that most of the diethers were produced by ANME that proliferated during a past massive methane release event associated with MH dissociation. The crocetane δ13C value, measured in a mixture with phytane and estimated from the correlation of the δ13C of the mixture with the mole fraction of crocetane, was about −127 ‰. More than half of the PMI δ13C values were greater than −100 ‰, suggesting the background presence of fossil PMI from methanogens.

Simulation of gas production from hydrate reservoirs (AT1) of Eastern Nankai Trough, Japan

Gas hydrates are regarded as one of the most promising alternative sources of energy, which have the potential to address the energy demand of a contemporary society. Based on the field explorations in the Eastern Nankai Trough (Japan), a multilayered hydrate reservoir model has been conceptualised and its behaviours during depressurisation production are simulated. This model incorporates the effects of the initial reservoir temperature and permeability on the mechanism of hydrate dissociation, which in turn affects the gas production. It is shown that the dissociation process is largely affected by the initial temperature distribution within the reservoir layers, and the ‘warmer’ reservoirs show (consistently) higher production potential. Furthermore, the gas production could be improved significantly, by increasing the permeability of the wellbore region, which can be achieved through the fracturing process. The close match between the simulation results and the field tests is noteworthy. The proposed multilayered model would be quite useful for analysing the efficacy of the ‘production strategy’, in most real-life situations.

Enhancing gas recovery from natural gas hydrate reservoirs in the eastern Nankai Trough: Deep depressurization and underburden sealing

Depressurization is recognized as the most effective way to recover natural gas hydrate. However, the bottomhole pressure is usually controlled above the quadruple point and few studies focus on reducing the pressure below that one. Even though some laboratory-scale studies have been performed, the application is not validated in the reservoir-scale production. Therefore, this study constructs a prediction model based on the field survey data in Japan to test the feasibility of deep depressurization (i.e., reducing the bottomhole pressure below the quadruple point) in the actual reservoir. The effect of ice formation on gas production is discussed in detail. Moreover, a method of deep depressurization combined with underburden sealing is innovatively proposed to further stimulate productivity and the different sealing effects are quantitatively analyzed. The results suggest that deep depressurization can effectively enhance gas production by forming ice and increasing driving force for hydrate dissociation, and the flowing channel blockage may be negligible. Underbuden sealing can further improve productivity by increasing pressure drop transfer under deep depressurization. A relatively higher sealing ratio corresponds to a higher gas recovery and a higher gas-to-water ratio, and the total gas production can increase by about 41.9% when the sealing ratio reaches 50.

Occurrence mode of Holocene tsunami overwash controlled by the geomorphic development along the eastern Nankai Trough, central Japan

Re-analysis of the lithological, paleontological, and geochemical data of three lagoonal sediment cores reveal the tsunami overwash history from 8000 to 6500 cal BP along the northern Suruga Bay coast, facing the eastern Nankai Trough. The coring sites are located in a transcect 0.8, 1.5, and 1.7 km inland, respectively, from the coastline at the time the overwash events occurred. The washover sand beds are distinguished from the lagoonal muds by the presence of quartz and mica, which are abundant on the outer (marine) side of the lagoon, as well as marine shell fossils. The increased C/S ratio often associated with sand beds, reflects input of open seawater to the reductive lagoon, and supports this interpretation. A maximum of eight tsunami deposits were identified between ∼7800 and 6500 cal BP, many of which can be correlated between the three cores. The magnitude of tsunami overwash implied by the thickness and grain size of tsunami deposits, and increase of C/S ratio following their deposition is relatively large at ∼7500-7200 cal BP during the post-glacial sea level rise and decreases with time during the subsequent high sea level stand. This occurrence mode (timing and scale of formation) of tsunami washover is explained by temporal changes in coastal geomorphology, rather than differences in the magnitude of individual tsunamis. Around 7500-7200 cal BP, sea level rise equilibrated with or exceeded the growth of the bay mouth barrier, and tsunamis could easily cross the small barrier and enter the lagoon, leaving behind thick and coarse-grained deposits. As the barrier becomes larger during the subsequent sea-level highstand, only the highest-amplitude waves exceeded the height of the barrier. By ∼6500 years ago, the barrier was high enough to prevent tsunamis from entering the lagoon. Similarly, the occurrence mode of tsunami deposits reported along the coast of Suruga Bay and Enshu-nada over the past 6000 years can be explained in relation to the growth of coastal barriers.

Comparative Analysis on the Evolution of Seepage Parameters in Methane Hydrate Production under Depressurization of Clayey Silt Reservoir and Sandy Reservoir

Gas hydrates are likely to become an important strategic resource with commercial development prospects. It is therefore of great significance to realize the long-term and efficient production of methane hydrate reservoirs. Previous studies have shown that the lithological characteristics of hydrate reservoirs have a significant impact on reservoir productivity by influencing the evolution of seepage parameters in the process of hydrate production. The porosity (Φ) and initial hydrate saturation (SH) affect the amount of hydrate decomposition and pressure transfer, and also indirectly affect the reservoir temperature field. The permeability (k) directly affects the rate of pressure-drop transmission and methane gas discharge. Due to the differences in seepage parameters caused by different reservoir lithology, a sandy hydrate reservoir (SHR) in Japan and a clayey silt hydrate reservoir (CHR) in China were found to have different gas production rates and the spatial evolution characteristics of the temperature and pressure fields varied in gas hydrate production tests. Therefore, to ensure the long-term and efficient production of the CHR in China, two models were established for a comparative analysis based on a numerical simulation. The two models were depressurizing models of the CHR of the W11 drilling site in the Shenhu Sea area of the South China Sea and the SHR of the AT1 drilling site in the Eastern Nankai Trough of Japan. Both models considered the heterogeneity of seepage parameters, and the TOUGH+HYDARATE (T+H) code was used in subsequent calculations. Four key results were obtained: (a) The order of the significance levels of the lithological parameters on productivity was k > SH > Φ in the CHR and SH > k > Φ in the SHR. (b) The heat conduction and heat convection in the CHR were weaker than in the SHR, which made it difficult to recover the low-temperature area caused by hydrate decomposition. (c) The exploitation of a high k hydrate reservoir should be given priority when the other initial conditions were the same in both the CHR and SHR. (d) The exploitation of both the CHR and SHR should not only rely on the hydrate content or seepage capacity to determine the reservoir exploitation potential, but the combined effect of the two parameters should be fully considered.

Risk and preventive strategies of hydrate reformation in offshore gas hydrate production trials: A case study in the Eastern Nankai Trough

Hydrate reformation in offshore gas hydrate production trials is an important issue that can readily cause flow blockage accidents, resulting in the termination of production trials. Previous studies on hydrate flow obstacle were mostly focused on conventional oil and gas production or transportation. However, there is a knowledge gap on prediction and management of hydrate reformation risk in gas hydrate production trials. In this study, a fully coupled thermal and multiphase flow model was developed to evaluate the risk of hydrate reformation in gas hydrate production wells, considering its specially designed well configuration and depressurization strategy. Model predictions were in good agreement with field monitoring data. A thorough analysis on the issues of hydrate reformation in twice offshore production trials of the Eastern Nankai Trough was conducted for the first time. The results indicate that in AT1-P and AT1-P3 wells, there is a greater risk of hydrate reformation in the water line (WL) and no risk exists in the gas line (GL); however, the hydrate reformation risk is also encountered in the GL of AT1-P2 well because wellhead backpressure and lower depressurization amplitude were implemented. Moreover, a series of sensitivity analyses were conducted with different well design and production parameters. It reveals that the hydrate reformation in the WL is inevitable unless complete downhole gas-liquid separation is achieved, but that in the GL can be completely prevented by raising the depressurization amplitude to above 6 MPa. Additionally, a new hydrate management strategy by rationally configuring wellbore pressure and improving thermal utilization efficiency was proposed. It has proven that new method enables to fully eliminate the risk of hydrate reformation in the production well without the used of the hydrate inhibitors. This work provides valuable references and practical implications for future short-term or long-term offshore gas hydrate production trials.

A Thermodynamic Method for the Estimation of Free Gas Proportion in Depressurization Production of Natural Gas Hydrate

Free gas saturation is one of the key factors that affect the overall production behaviors of hydrate reservoirs. For example, different free gas contents could alter the thermal response of hydrate reservoirs to the artificial stimulation and hence change the gas production performance. To investigate whether and how much the hydrate reservoir contains free gas, we proposed a thermodynamic method to calculate the total heat consumption of hydrate dissociation throughout gas production and assess the free gas proportion. Based on the monitoring data of the first offshore hydrate production in Japan, we calculated the total heat consumption and analyzed the contributions of heat convection, heat conduction, and sensible heat during the entire test. The calculation results showed that there is likely to be a certain amount of free gas in the hydrate reservoir in the Eastern Nankai Trough. In addition, the analysis of different heat sources revealed the critical thermodynamic phenomenon in which the reservoir sensible heat was the main source for enthalpy of hydrate dissociation, which consistently contributed more than 95% of the total heat supply during the 6-day production test. The results of this work may help upgrade the production strategy for natural gas hydrates.

Cultivation and biogeochemical analyses reveal insights into methanogenesis in deep subseafloor sediment at a biogenic gas hydrate site

Gas hydrates deposited in subseafloor sediments are considered to primarily consist of biogenic methane. However, little evidence for the occurrence of living methanogens in subseafloor sediments has been provided. This study investigated viable methanogen diversity, population, physiology and potential activity in hydrate-bearing sediments (1–307 m below the seafloor) from the eastern Nankai Trough. Radiotracer experiments, the quantification of coenzyme F430 and molecular sequencing analysis indicated the occurrence of potential methanogenic activity and living methanogens in the sediments and the predominance of hydrogenotrophic methanogens followed by methylotrophic methanogens. Ten isolates and nine representative culture clones of hydrogenotrophic, methylotrophic and acetoclastic methanogens were obtained from the batch incubation of sediments and accounted for 0.5–76% of the total methanogenic sequences directly recovered from each sediment. The hydrogenotrophic methanogen isolates of Methanocalculus and Methanoculleus that dominated the sediment methanogen communities produced methane at temperatures from 4 to 55 °C, with an abrupt decline in the methane production rate at temperatures above 40 °C, which is consistent with the depth profiles of potential methanogenic activity in the Nankai Trough sediments in this and previous studies. Our results reveal the previously overlooked phylogenetic and metabolic diversity of living methanogens, including methylotrophic methanogenesis.

Numerical History-Matching of Modeling and Actual Gas Production Behavior and Causes of the Discrepancy of the Nankai Trough Gas-Hydrate Production Test Cases

Comprehensive data sets were obtained during the gas production tests from a gas-hydrate reservoir in Eastern Nankai Trough (2013 and 2017), including the gas and water production rates, downhole pressure, temperature-monitoring data, log- and core-derived reservoir property information, and seismic volume. Numerical simulations using a compositional and thermal-coupling numerical model (MH21-HYDRES) had been conducted earlier and after each production test for production behavior prediction purposes and history-matching to reevaluate the reservoir property information. The actual production behavior demonstrated some deviations from the prediction by the numerical model. For example, the model predicted a gradual increase in the gas production rates under the constant pressure drawdown but real gas rates were almost constant or slightly decreasing, some inconsistencies in the temperature data and derived vertical and temporal production profiles were observed, and differences in the production behavior exist among the wells that were drilled within a 60 m radius area. To reduce the gap between the model and actual results, understand the nature of the gas-hydrate reservoir, and understand the physical processes during the gas-hydrate dissociation, comparison of the modeling results and obtained data was repeated, along with reexamination of the geological and geophysical data. The study results revealed two features, namely, vertical and horizontal heterogeneities of the formation and complexly disturbed situation in the near-wellbore region due to the unconsolidated nature of the sediments, which were determined as the main causes for the discrepancy in the model and actual production behavior.

The importance of including density in multiparameter asymptotic linearized direct waveform inversion: a case study from the Eastern Nankai Trough

SUMMARY Iterative least-squares reverse time migration (LSRTM) is the state-of-the-art linearized waveform inversion method to obtain quantitative subsurface parameters. The main drawback of such an iterative imaging scheme is the significant computational expense of many modelling/adjoint cycles through iterations. In the context of the extended domain, an interesting alternative to LSRTM is the asymptotic linearized direct waveform inversion, providing quantitative results with only a single iteration. This approach was first proposed for constant-density acoustics and recently extended to the variable-density case. The former is based on the application of the asymptotic inverse Born operator, whereas the latter has two more extra steps: building an angle-dependent response of the asymptotic inverse Born operator and then solving a weighted least-squares approach for simultaneous inversion of two acoustic parameters. To examine the importance of accounting for density variations, we compare constant- and variable-density linearized direct waveform inversion techniques applied to a marine real data set from the Eastern Nankai Trough, offshore Japan. The inversion results confirm the efficiency of the asymptotic linearized direct waveform inversion in estimating quantitative parameters within a single iteration. The variable-density direct inversion yields subsurface images that (1) exhibit a superior resolution and (2) better reconstruct the field data than does the constant-density approach, even if the data set does not contain large enough surface offset to fully decompose velocity and density perturbations.

Mitigating the ill-posedness of first-arrival traveltime tomography using slopes: application to the eastern Nankai Trough (Japan) OBS data set

SUMMARYFirst-arrival traveltime tomography is one of the most used velocity model building techniques especially in sparse wide-angle acquisitions for deep crustal seismic imaging cases. Relying on the inversion of a picked attribute, the absolute traveltimes, the approach is ill-posed in terms of non-uniqueness of the solution. The latter is remedied by proper regularization or the introduction of prior information. Indeed, since traveltime kernels are vulnerable to the velocity–depth ambiguity, the inversion is stabilized by the introduction of complementary data like reflections and explicit reflectors in the velocity models. Here, we propose to supplement first-arrival traveltimes by their slopes, in other words the horizontal component of the slowness vectors at the sources and/or receivers. Slopes are a crucial attribute in state of the art scattering-based or reflection-based tomographic methods like slope tomography or wavefront tomography where the differential information is needed in order to locate the scattering events position or to parametrize the wavefront. The optional but valuable injection of slopes as an objective measure in first-arrival traveltime tomography stabilizes the problem by constraining the emergence angle or in turn implicitly the turning point depth of the rays. We explain why slopes have a tremendous added value in such a tomographic problem and highlight its remedial effect in cases where the medium is unevenly illuminated. We also show that the contribution of slopes become even more significant when the acquisition is sparse as it is generally the case with ocean-bottom seismometer surveys. The inferred models from such an extended time-attributes tomography will be used as initial guesses in a full-waveform inversion workflow context. The proposed strategy is benchmarked in 2-D media against a dip section of the SEG/EAGE overthrust model and then followed by a revisit of ocean bottom seismometers data from the eastern-Nankai subduction margin as a real deep crustal case study.

Influence of a Subducted Oceanic Ridge on the Distribution of Shallow VLFEs in the Nankai Trough as Revealed by Moment Tensor Inversion and Cluster Analysis

AbstractThe eastern Nankai Trough is a unique site where many shallow, very low frequency earthquakes (sVLFEs) are recorded by nearby broadband ocean bottom seismometers. Here, we estimated the locations and seismic moment tensors (MTs) of sVLFEs based on the low‐frequency (<0.07 Hz) components of the records. Although some sVLFEs exhibited long‐duration signals (>100 s), indicating a degree of source complexity, the MT inversions were limited to events with impulsive and short durations (20–30 s). Nevertheless, cluster analysis confirmed that the MTs of impulsive sVLFEs reasonably represented the overall deformation for an event group including many longer‐duration events. The distribution of MTs indicates that deformations associated with sVLFEs are influenced by a subducted oceanic ridge in this region, which produces an along‐the‐trough variation in a strain accumulation/release pattern and probably controls the spatial patterns of tsunami generation.

Stress Orientations in the Nankai Trough Constrained Using Seismic and Aseismic Slip

AbstractThe strength of subduction thrust faults is key to understanding seismogenesis at the provenance of Earth's largest earthquakes. Earthquake focal mechanisms are routinely inverted to constrain the stress state at seismogenic depths. However, on some megathrusts, deformation is accommodated by both earthquakes and types of slow fault slip. We employ focal mechanisms of short‐term slow slip events (SSEs), a type of slow fault slip, and earthquakes in a regional stress inversion to investigate the stress state of the Nankai Trough megathrust and interpret the results in the context of regional tectonics Previous studies using earthquake‐only stress inversions found principal stress orientations in this region that are incompatible with thrust faulting on the megathrust. When both SSEs and earthquakes are considered, the stress state of the central and eastern Nankai Trough megathrust is well oriented for thrust faulting. Our results suggest that slow fault slip source regions may appear to have misoriented stress fields if slow fault slip constitutes a substantial proportion of fault slip and the stress field is not well constrained by earthquakes. In the SSE region, we find that faults are well oriented for failure, suggesting they have strengths similar to their surroundings. Combined with low Vp/Vs ratios and sensitivity to small stress changes, our results imply that the megathrust and surroundings operate at low deviatoric stresses in the SSE source region. Further, we show that the coefficient of friction for areas hosting SSEs is low (μ=0.19–0.50), implying frictionally weak materials in the SSE source region.

Numerical investigation of the natural gas hydrate production tests in the Nankai Trough by incorporating sand migration

A large number of data on gas production and reservoir characteristics were released after the 2013 and 2017 offshore natural gas hydrate production tests in the Eastern Nankai Trough of Japan. However, the systematic comparison of the two production tests is still limited. Therefore, in this study, we first reviewed the gas and water production processes of the three production boreholes in the two production tests. Some problems were found by comparing the gas and water production rates of each production well and with the existing numerical simulation results. For instance, the gas production rate of well AT1-P3 was extremely low and, distinguished with the previous simulation results, the gas production rates of wells AT1-P and AT1-P3 didn’t increase with time. Considering the severe sand production in these two wells, we speculated that the plugging of sand control device (SCD) caused by solid particles deposition was the cause of the above problems. Thus, for the first time, numerical simulation considering the sand production and SCD plugging was performed to evaluate the gas and water production performance of the three production wells. In addition, a preliminary analysis of the production efficiency of each well was carried out. The simulation results showed that a serious plugging occurred at well AT1-P3, which is responsible for its low gas production. In contrast, well AT1-P had a slight plugging, and well AT1-P2 avoided severe sand production owing to the reduced pressure drop, and obtained much higher gas production rate than well AT1-P3 with a lower pressure drop. Furthermore, the simulation results indicated that economically viable gas production is difficult to achieve with current production strategies even if the problems of sand production and SCD plugging are solved. This requires the use of some emerging production technologies in the future.

Upscaling techniques for fully coupled THM simulation and application to hydrate gas production tests

Abstract Numerical simulation is important for practical and efficient assessment of hydrate reservoir formation stability and gas production potential. The history-matching simulation of hydrate gas production tests is complex due to coupled THM (thermo-hydro-mechanical) phenomena. The well log data from the Eastern Nankai Trough methane gas production site suggest vertically heterogeneous field properties. In the previous numerical simulation research of this site, the heterogeneous geological data were homogenized by adopting the standard mean-field theory, which can potentially lead to inaccurate simulation results due to the mesh size effect. By introducing new upscaling techniques for the permeability profiles and mechanical responses, a revised homogenization approach is proposed to improve the coupled THM simulation accuracy. In this study, seven gas production simulations of a hypothetical reservoir, six simulations of the Eastern Nankai Trough gas production test, and four simulations of the randomly generated site formation production test with different mesh sizes and different homogenization approaches were carried out to demonstrate that the proposed upscaling techniques can improve the accuracy of the simulation results with a coarse mesh model. This work, in turn, provides researchers and field engineers a much quicker way to assess the complex geomechanical behaviors of hydrate gas production site.

Effects of far-field boundary conditions on the simulation of hydrate production

The world’s first offshore methane hydrate extraction was conducted in 2013 at the Daini-Atsumi Knoll located in the eastern Nankai Trough. The results indicated that the gas production rate of a single vertical well cannot meet the requirement for commercialisation of methane hydrate production. Therefore, a trial multi-well system has been considered for future hydrate production. Understanding the ‘influence distance’ of extraction wells within methane hydrate turbidite reservoir formations, such as that of the Nankai Trough, is significant to the design of the proposed multi-well system. In this study, the boundary effects are examined for cases involving different values of permeability and initial hydrate saturation for the modelled turbidite formations. A relationship is proposed to predict the longest simulation time for a given set of lengths and parameters. The two concepts of critical production time (CPT) and turbidity/homogeneity coefficient ratio (THCR) are introduced to analyse the interaction between the effects of model size and heterogeneities. It is found that the CPT can be normalised by the radius of the model; the THCR is affected by the relative thickness of the sand and clay layers.

Water Permeability Evolution With Faulting for Unconsolidated Turbidite Sand in a Gas‐Hydrate Reservoir in the Eastern Nankai Trough Area of Japan

AbstractGas hydrates in sediments are forecast to become an unconventional natural gas resource. In 2013 and 2017, an offshore natural gas production test in subseabed gas hydrate‐bearing sand layers was carried out in the eastern Nankai Trough area of Japan. However, permeability evolution with progressive shearing of these faulted reservoirs has remained enigmatic. It is necessary to understand fault behavior under shear, as well as how normal stress affects permeability evolution. Here, we examine the relationship between fault development and permeability reduction in unconsolidated turbidite sandy sediment from Japan's eastern Nankai Trough area using a laboratory ring‐shear experiments. At each stress condition, the permeability decreased significantly following shear displacements of ~0.2–0.5 m, gradually decreasing further with shear displacements up to 10 m. Permeability after shearing was reduced by 3 to 4 orders of magnitude compared to that before shearing. Grain crushing and compaction within the shear zone and surrounding host sediment are dependent on stress and mineralogy. The findings of the role of shear zone development in modifying fault permeability for turbidite sand are expected to contribute directly to rational evaluation of gas‐hydrate reservoir, production simulations, and development strategies in future studies.

Tsunami deposits refine great earthquake rupture extent and recurrence over the past 1300 years along the Nankai and Tokai fault segments of the Nankai Trough, Japan

Study of prehistoric to medieval-age tsunami deposits along a riverbank site near the eastern Nankai Trough, central Japan, show that, not only did Tokai earthquakes occur with a higher frequency than previously thought, but that contemporaneous ruptures of the Tokai and Nankai fault segments were also more common. The site revealed a ∼1-km long coast-normal cross section of the strand plain and exposed four sandy tsunami deposits, each of which indicates inundation over 2 km inland of the coast. Radiocarbon dating of previously studied and newly discovered deposits in the region indicates a shorter recurrence time for Tokai earthquakes and clarifies their linkage with Nankai earthquakes. We attribute the younger two tsunami deposits to the 1498 and 1096 CE Tokai earthquakes. The older two deposits confirm the occurrence of the Tokai earthquakes in 887 CE and in the latest 7th century. These events are not reliably recorded in historical documents in the Tokai region but were noted in the Nankai area. The 887 CE earthquake likely represents a full-length rupture of the Tokai and Nankai segments, as was the case for the 1707 CE earthquake. Integrated with the previous studies, these new results show that nine Tokai earthquakes occurred over the last 1300 years, the oldest in the latest 7th century, and in 887, 1096, 1361, 1498, 1614, 1707, 1854 and 1944 CE. Recalculated recurrence intervals range from 90 to 265 years. Except for the 1498 Meio Tokai earthquake, the Tokai earthquakes occurred simultaneously with Nankai earthquakes.

Modeling the mechanical response of gas hydrate reservoirs in triaxial stress space

Exploitation of gas from deep-sea methane hydrate-bearing layers might lead to some geological disasters, including marine landslides and excessive settlement of marine ground. The first offshore gas production tests for methane hydrate-bearing sediments were carried out in eastern Nankai Trough. However, knowledge on mechanical behavior of gas hydrate reservoirs with similar gradation and minerology component to the marine sediment is still insufficient. Consequently, proper modeling of geomechanical properties of methane hydrate-bearing sediments is crucial for reservoir simulation and deep ocean ground stability analysis for long-term gas production in the future. This study conducted a series of triaxial shear tests to examine the shear response of methane hydrate-bearing sediments with a similar grading curve and minerology components to the hydrate-rich sediments in Nankai Trough. The test results demonstrated that the presence of hydrate mass between sand grains altered the stress-strain pattern from strain-hardening to postpeak strain-softening. A simple constitutive model based on several empirical relationships of granular materials is proposed to describe the stress-strain relationship of methane hydrate-bearing sediments under triaxial stress condition. This model can reproduce the enhancement of shear strength, initial stiffness, and dilation behavior of methane hydrate-bearing sediments containing different amounts of fines content with a rise in the methane hydrate saturation at a wide range of effective confining pressures. The numerical results indicate that the parameter A associated with initial stiffness of stress-strain curve and the parameter α related with dilation properties are jointly governed by the confining pressure, fines content, and hydrate saturation.