Nankai Forearc Research Articles

Frontal Thrust Ramp‐Up and Slow Earthquakes Due To Underthrusting of Basement High in the Nankai Trough

AbstractRecently, integrated geophysical‐geological surveys in the Nankai subduction zone in Japan have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes propagated around the frontal thrust of the accretionary wedge. The frontal thrust ramps up from the basal décollement and slips over trench‐filling sediment along the landward edge of the Nankai trough floor. Here, the Paleo‐Zenisu ridge has been subducted beneath the inner‐outer slope border. In addition, ocean floor topography and geologic structure revealed by seismic reflection surveys completed before 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust has a seamount and a horst‐like basement high. The northern edge of the basement high is located at the ramp‐up position of the frontal thrust. The 2020–2021 clustered slow earthquakes started at the Paleo‐Zenisu ridge and propagated to the topographic highs beneath the deformation front. Considering that the relative plate convergence between the upper Amurian Plate of the Nankai forearc and the subducting Philippine Sea Plate is ∼6.0 cm/year, the basement high at the deformation front has uplifted the frontal crest of the wedge at an average rate of 2.7–5.7 mm/year for several tens to hundred thousand years. These rates are among some of the highest rock uplift rates measured in the world. The slow earthquakes in the off‐Kumano Nankai Trough in 2020–2021 are a snapshot of a “living” Nankai frontal thrust during the megathrust interseismic period.

Nankai Forearc Structural and Seismogenic Segmentation Caused by a Magmatic Intrusion off the Kii Peninsula

AbstractThe causes for forearc basin and megathrust rupture zone segmentation are controversial. The Nankai forearc, Japan, is separated into five domains based on topography: Enshu, Kumano, Muroto, Tosa, and Hyuga. The boundaries of these domains correspond to the rupture limits of large earthquakes. We examined the geologic structure of the boundary region between the Kumano and Muroto domains off the Kii Peninsula using multichannel seismic reflection data to evaluate the role of upper plate composition in controlling segmentation. The results suggest that thick cover sediments and underlying accretionary prism are obliquely thrust landward over the igneous basement complex rock in the region of offshore of Cape Shionomisaki and separate the forearc basin. The igneous basement complex rocks directly overlying the plate interface in the hypocentral regions of 1944 Tonankai and 1946 Nankai earthquakes. The 1944 earthquake originated at the base of the complex, and the rupture extent slipped past its basement boundary, whereas the 1946 event nucleated at the updip boundary of the basement complex. The dense igneous rocks might have worked as a heavily loaded barrier on the seismogenic megathrust and separated the rupture area of both the earthquakes. Upper plate geology may be an important factor in controlling seismogenesis in the Nankai Trough and may serve as an example for understanding the controls on megathrust slip in other subduction zones.

Bathymetric Signatures of Submarine Forearc Deformation: A Case Study in the Nankai Accretionary Prism

AbstractLarge earthquakes and tsunamis in subduction zone forearcs occur via slip on the shallow plate boundary and upper plate faults, but the locations, geometries, and slip histories of these faults can be difficult to constrain in regions with minimal subsurface geophysical and stratigraphic data. Here, we test a new approach to quantify the submarine seafloor geomorphic response to forearc deformation in order to identify structures that contribute to active deformation, to interpret their geometry and kinematics, and to evaluate their relative rates, magnitudes, and timing of deformation. We develop a workflow that uses filtered bathymetric digital elevation models, where long wavelength topography has been removed, to isolate the slope, relief, curvature, ridgelines, and trough lines associated with faults, fault‐related folds, and slope failures. We apply these methods to the Kumano region of the Nankai accretionary prism, southeastern Japan, where existing constraints on fault geometry, kinematics, and deformation history allow us to both evaluate the efficacy of our approach and to identify the lateral continuity of deformation processes. Our bathymetric analyses yield a high‐resolution tectono‐geomorphic map of active structures and reveal along strike variations in strain accumulation and out‐of‐sequence deformation. These metrics also demonstrate the importance of a strike‐slip fault system at the seaward edge of the Kumano Basin as a primary structure that accommodates deformation and partitions strain in the Nankai forearc. These results show the utility of using a submarine tectono‐geomorphic approach to evaluate active deformation in forearcs, particularly in regions with limited geophysical and core data.

Rejuvenated extension of the Philippine Sea plate and its effect on subduction dynamics in the Nankai Trough

AbstractThe Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough and the causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku Island, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa‐Bae seamount and other high‐relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst‐and‐graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, were rejuvenated locally at ~6 Ma and more regionally at ~3.3 Ma and have remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab‐pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.

Structural heterogeneity and megathrust earthquakes in Southwest Japan

Large interplate megathrust earthquakes occurred in the Nankai subduction zone off Southwest Japan with an interval of ~100 to 150 years. Because of a fast convergence rate (4–6 cm/yr) and a young buoyant lithosphere, the subducting Philippine Sea plate and the overriding Eurasian plate are strongly coupled in the forearc region along the upper boundary of the subducting plate. However, the causal mechanism of megathrust earthquakes in the Nankai forearc domain is still not very clear. In this study, we present 3-D high-resolution P and S wave velocity and Poisson's ratio images of the Nankai subduction zone, which are determined using a large number of high-quality arrival-time data of well-located onshore and offshore local earthquakes. The obtained seismic images, as well as residual topography and gravity anomalies, show similar structures in both the upper and lower plates above and below the rupture zones of the 1944 Tonankai earthquake (M 8.1), the 1946 Nankai earthquake (M 8.3), and the 1968 Hyuganada earthquake (M 7.5). These results indicate that the nucleation and rupture processes of these megathrust earthquakes were controlled by both the upper and lower plates, similar to those in the Tohoku forearc. However, the physical properties and seismogenesis of the megathrust zone in the Nankai forearc appear more complicated than those in the Tohoku forearc.

Punctuated growth of an accretionary prism and the onset of a seismogenic megathrust in the Nankai Trough

Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan.

Sediment flow routing during formation of forearc basins: Constraints from integrated analysis of detrital pyroxenes and stratigraphy in the Kumano Basin, Japan

The evolution of sediment flow routing during complete evolution of the Kumano forearc basin is determined through integration of stratigraphic and sediment provenance analyses in the upper Nankai forearc. A new approach uses the compositional variability of detrital clinopyroxenes and orthopyroxenes collected at eight major rivers in Japan and three drill sites in the basin and nearby slope environment, including the first drill cuttings retrieved by the Integrated Ocean Drilling Program (IODP). Joint interpretation of these datasets reveals that the sedimentation history of the basin is characterised by three main phases separated by newly-recognised time-transgressive boundaries. We show that the Kumano Basin initiated as a trench-slope basin in the early Quaternary (∼1.93 Ma) and that it progressively evolved towards an upper slope environment with increased turbidite confinement and influence from climatic forcing. Basin initiation was broadly synchronous with development of the Nankai megasplay fault, suggesting a causal relationship with construction of the Nankai accretionary prism. Unlike preceding studies documenting long-distance longitudinal transport of clastic material along the lower Nankai forearc, only limited longitudinal transport is documented by detrital pyroxenes in the upper forearc. These results suggest that transverse canyons are a major control on the sediment flow routing during maturation of forearc basins and that long-distance longitudinal flows along convergent margins are principally restricted to near-trench environments, even in the presence of large forearc basins.

Origins of lithium in submarine mud volcano fluid in the Nankai accretionary wedge

To investigate fluid regimes in the Nankai accretionary wedge, we estimated the temperatures of lithium (Li) reservoirs from the Li isotope ratio (7Li/6Li) in fluids recovered from Kumano mud volcano No. 5 (KMV#5) in the Nankai forearc basin. The 7Li/6Li ratios in the KMV#5 fluids are among the lightest reported from submarine mud volcanoes. The estimated Li reservoir temperatures (310 °C at maximum) were significantly higher than the smectite-to-illite conversion temperature (60–150 °C), whereas the temperature of fluid from the deeper Nankai décollement was estimated to be 150 °C at maximum. Data obtained from previous conductivity surveys across the trench suggest that there are two fluid reservoirs associated with distinct dehydration processes in the Nankai subduction zone. From these results, we inferred that the discontinuous mud diapirism in the Kumano forearc basin results from the periodic injection of deep-seated fluid accumulated in the corner of the serpentinized forearc mantle wedge that travels upward via a thrust fault. The intermittent upwelling of deep-seated fluid from the Nankai accretionary wedge due to periodic breaches of a low-permeability barrier above the fluid reservoir may be associated with seismogenic fault activity.

Mid-Quaternary decoupling of sediment routing in the Nankai Forearc revealed by provenance analysis of turbiditic sands

Coring during Integrated Ocean Drilling Program Expeditions 315, 316, and 333 recovered turbiditic sands from the forearc Kumano Basin (Site C0002), a Quaternary slope basin (Site C0018), and uplifted trench wedge (Site C0006) along the Kumano Transect of the Nankai Trough accretionary wedge offshore of southwest Japan. The compositions of the submarine turbiditic sands here are investigated in terms of bulk and heavy mineral modal compositions to identify their provenance and dispersal mechanisms, as they may reflect changes in regional tectonics during the past ca. 1.5 Myrs. The results show a marked change in the detrital signature and heavy mineral composition in the forearc and slope basin facies around 1 Ma. This sudden change is interpreted to reflect a major change in the sand provenance, rather than heavy mineral dissolution and/or diagenetic effects, in response to changing tectonics and sedimentation patterns. In the trench-slope basin, the sands older than 1 Ma were probably eroded from the exposed Cretaceous–Tertiary accretionary complex of the Shimanto Belt and transported via the former course of the Tenryu submarine canyon system, which today enters the Nankai Trough northeast of the study area. In contrast, the high abundance of volcanic lithics and volcanic heavy mineral suites of the sands younger than 1 Ma points to a strong volcanic component of sediment derived from the Izu-Honshu collision zones and probably funnelled to this site through the Suruga Canyon. However, sands in the forearc basin show persistent presence of blue sodic amphiboles across the 1 Ma boundary, indicating continuous flux of sediments from the Kumano/Kinokawa River. This implies that the sands in the older turbidites were transported by transverse flow down the slope. The slope basin facies then switched to reflect longitudinal flow around 1 Ma, when the turbiditic sand tapped a volcanic provenance in the Izu-Honshu collision zone, while the sediments transported transversely became confined in the Kumano Basin. Therefore, the change in the depositional systems around 1 Ma is a manifestation of the decoupling of the sediment routing pattern from transverse to long-distance axial flow in response to forearc high uplift along the megasplay fault.

Seismic imaging of the Southwest Japan arc from the Nankai trough to the Japan Sea

Detailed three-dimensional P- and S-wave velocity (Vp and Vs) models of the entire Southwest Japan arc from the Nankai trough to the Japan Sea are determined for the first time using a large number of high-quality arrival-time data from local earthquakes. The suboceanic earthquakes used in the tomographic inversion were relocated precisely using sP depth phase data. Our results show that strong lateral heterogeneities exist in the interplate megathrust zone under the Nankai forearc. Large interplate earthquakes mainly occurred in or around high-velocity (high-V) patches in the megathrust zone. These high-V patches may represent asperities formed by the subducted oceanic ridges and seamounts. Low-velocity (low-V) zones in the megathrust zone may contain sediments and fluids associated with slab dehydration and so become weakly coupled areas. Our results also show that the coseismic slip distributions of some megathrust earthquakes are not limited in the high-V patches (asperities) where the ruptures initiated. Because of the weak interplate coupling in the low-V areas, the rupture of an interplate earthquake could unimpededly pass through the low-V anomalies and so result in a great megathrust earthquake.

Slumping and mass transport deposition in the Nankai fore arc: Evidence from IODP drilling and 3‐D reflection seismic data

Multiple lines of evidence exist for a range of sediment mass movement processes within the shallow megasplay fault zone (MSFZ) area and the adjacent slope basin in the outer fore arc of the Nankai subduction zone, Japan. Diagnostic features observed in three‐dimensional reflection seismic data and in cores of the Integrated Ocean Drilling Program (IODP) document a multifarious mass movement history spanning ∼2.87 million years. Various modes and scales of sediment remobilization can be related to the different morphotectonic settings in which they occurred. From this evidence, we decipher the tectonic control on slumping and mass transport deposition in the Nankai fore arc. Three periods of intensified mass wasting coincided with pulses of enhanced activity on the splay fault: (1) an initial phase of juvenile out‐of‐sequence thrusting ∼1.95 to 1.7 Ma, (2) a reactivation phase between ∼1.55 and 1.24 Ma, and (3) at about 1 Ma, during a phase of uplift of the fore‐arc high and motion along the MSFZ. We suggest that slope oversteepening, extensional stress regimes, and lateral transmission of fluid overpressures may have preconditioned the slope sediments to fail. Individual mass‐wasting events may have been triggered by dynamic loading from earthquake waves and/or transient pulses of pore pressure along the splay fault. Overall, our results provide insights into the complicated interplay between tectonic and submarine mass movement processes. We demonstrate that detailed knowledge about the spatial and temporal distribution of submarine mass movements can be integrated into a holistic reconstruction of tectonostratigraphic evolution of accretionary margins.

Present-day deformation across the southwest Japan arc: Oblique subduction of the Philippine Sea plate and lateral slip of the Nankai forearc

We estimate long-term/permanent crustal deformation in the Nankai forearc, southwest Japan, that accumulated over one earthquake deformation cycle in the Nankai subduction zone. A short-term deformation due to an oblique subduction of the Philippine Sea plate is modeled based on coseismic fault slips associated with interplate thrust earthquakes at the Nankai Trough, and subtracted from the interseismic crustal velocity field observed by GPS. The long-term/permanent deformation left in the data shows arc-parallel movement of the forearc at a rate of 5— 10 mm/yr. We interpret that the forearc movement, driven by the oblique plate convergence, is accommodated by steady aseismic slip on the deep portion of the fault plane of the Median Tectonic Line (MTL) on the rear boundary of the Nankai forearc. The rate of the forearc movement is consistent with the geological slip rate of the MTL in the late Quaternary, and also with that expected from deflection of the slip vector of the 1946 Nankai earthquake from the current plate convergence vector.

Stress–strain ‘paradox’, plate coupling, and forearc seismicity at the Cascadia and Nankai subduction zones

In the continental forearcs of the Cascadia and Nankai subduction zones, geodetic strain measurements indicate crustal contraction in the direction of plate convergence (nearly ‘margin-normal’), but earthquake focal mechanisms and other stress indicators show maximum compression along strike (‘margin-parallel’). It is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles, not the absolute stresses. The absolute margin-normal stress is not only much less than the margin-parallel stress but also no greater than the lithostatic value, indicating a very weak subduction thrust fault. Weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults. Great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime. Because these small elastic stress perturbations are relatively fast, they give large strain rates that are detected by geodetic measurements. The present nearly margin-normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence. With the margin-parallel compression being dominant, the small increase in margin-normal stress may change the forearc seismicity in Cascadia from a mixture of thrust and strike–slip types into mainly thrust before the next great subduction earthquake. In Nankai, the change should be from strike–slip to quiescence, as observed in the Nankai forearc prior to the 1944–1946 great subduction earthquakes.

四国山地の隆起と城山層・竜王層の堆積環境

四国山地の隆起と城山層・竜王層の堆積環境

Sediment deformation and fluid activity in the Nankai, Izu-Bonin and Japan forearc slopes and trenches

A variety of sediment and crustal deformation, associated with fluid activity, has been observed in the present forearc slopes, trenches and ancient onland outcrops in the Japanese island arcs. The Nankai forearc represents a typical elastic-dominated accretionary prism where the expulsion of pore fluids from sediments seems to have occurred intermittently, through both channelized and diffusive mechanisms, some of which appears to be pulsed. Mud diapirs occur within the trench, near the toe and upper slope of the accretionary prism. The widespread development of a gas-hydrate phase boundary (bottom simulating reflector (BSR)) may indicate pervasive fluid advection throughout the prism. The Japan Trench forearc is characterized by tectono-gravity slope instability and collapse, resulting in zones of fluid escape along normal faults. No BSR is observed in the Japan forearc despite the presence of organic-rich sediments. The Izu-Bonin forearc, apparently unlike the Japan and Nankai Trench forearcs, contains recently discovered serpentinite diapirs in which there are blocks of basic-ultrabasic rocks, together with blueschist. Fossil Calyptogena beds in the Pliocene basins of the Izu Collision Zone provide good examples of fluid activity associated with venting in an ancient accretionary prism now exposed onland. Contrasting tectonic processes in the three forearcs may be explained by differences in the plumbing systems and hydrogeologic characteristics of the forearc basements: dewatering sediments in the Nankai Trough prism; impermeable and consolidated sediments in the Japan Trench prism, and the very permeable pillow-lava-volcaniclastic complex of the Izu-Bonin Trench forearc.