Japan Subduction Zone Research Articles

Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave traveltimes

AbstractWe present 3‐D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high‐quality P and S wave arrival time data of local earthquakes and teleseismic events. A tomographic method for P wave velocity azimuthal anisotropy is modified and extended to invert S wave traveltimes for 3‐D S wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3‐D azimuthal anisotropy of the Japan subduction zone. Our results show that the subducting Pacific and Philippine Sea (PHS) slabs exhibit mainly trench‐parallel fast‐velocity directions (FVDs), which may reflect frozen‐in lattice‐preferred orientation of aligned anisotropic minerals formed at the mid‐ocean ridge as well as shape‐preferred orientation such as normal faults produced at the outer‐rise area near the trench axis. Trench‐normal FVDs are generally revealed in the mantle wedge, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench‐normal FVDs are also visible in the subslab mantle, which may reflect the subducting asthenosphere underlying the slabs. Our results also reveal toroidal mantle flows in and around a window (hole) in the PHS slab beneath SW Japan, suggesting that the occurrence of the PHS slab window may have caused a complex flow pattern in the mantle wedge above the Pacific slab.

Numerical modelling of the relationship between the present tectonic stress field and the earthquakes in the Western Pacific Subduction Zone

AbstractThe tectonic stress field of the Western Pacific Subduction Zone (WPSZ) has been affected by the interaction among the Euro‐Asian, Philippine and Pacific Plates, and the effect is manifested by the occurrence of huge earthquakes in this region. Here, using numerical simulation of the finite element method, attempts were made to evaluate the NW‐directed subduction of the Western Pacific Plate on the three secondary subduction zones, namely the Japan Subduction Zone, the Izu‐Bonin Subduction Zone and the Mariana Subduction Zone. In these zones, the structural geometry and viscoelastic property of the model were determined, as well as two main megathrust fault planes identified, by means of a comprehensive structural analysis of tomography and Crustal 2.0 data. This study shows that the tectonic stress field and earthquakes in the three secondary subduction zones have different distribution features under continuous NW‐directed subduction of the Western Pacific Plate; i.e. (1) the strong coupling area in the Japan Subduction Zone reflected subducted seamounts, ocean ridges or other topographic highs that control the frequent occurrence of historical large earthquakes and stress concentration; (2) in Izu‐Bonin Subduction Zone, there were fewer earthquakes compared with the Japan Subduction Zone, the reason for this kind of distribution is that the contact area and properties have been changed between the subducting Pacific Oceanic Plate and the overlying Philippine Oceanic Plate; and (3) earthquakes that happened in the Mariana Subduction Zone have complicated types, including thrust‐type, normal‐type and strike‐slip‐type earthquakes, which are triggered by the subducting and retreating of the Pacific Oceanic Plate, and an angular difference between the subducting direction of the Pacific Oceanic Plate and the strike of the Mariana Trench. In summary, formation of differences among the tectonic stress field and earthquake distribution of the three subduction zones are correlated not only to the geometrical structures of the major faults but also to the subducted seamounts and the strike directions of the trenches. Copyright © 2016 John Wiley & Sons, Ltd.

Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku‐oki earthquake: A comprehensive search using template events

AbstractWe developed a new method to classify the faulting types of small earthquakes using interevent waveform similarity and applied it to earthquakes in the northeast Japan subduction zone. In the method, we used separate time windows for P and S waves and established a relationship between waveform similarity and differences in focal mechanisms from event pairs whose focal mechanisms we know. Then we applied the relationship to many pairs of such focal‐mechanism‐known events (5607 from the moment tensor catalogue and 3623 events from the interplate repeating earthquakes catalogue) and focal‐mechanism‐unknown events for the period from 1984 to 2013. As a result, 8984 earthquakes were newly classified into interplate (N = 5401), noninterplate thrust (N = 631), normal (N = 1070), and strike‐slip faulting earthquakes (N = 165). From the new data set, which doubles the number of mechanism types, we confirmed that there have been almost no interplate earthquakes in the area of large coseismic slip of the Tohoku‐oki earthquake since that event. We also saw that this trend continued until at least the end of 2013, suggesting a nearly complete stress release and slow interplate stress recovery. The abundant interplate aftershocks also indicate the precise spatial extent of postsesimic slip, which is usually difficult to obtain from land‐based geodetic data. The postseismic slip also suggests stress concentration at the asperities of the 1968 Tokachi‐oki (M7.9) and 1994 Sanriku‐oki (M7.6) earthquakes. The present‐day faulting types offshore Tohoku correlated well with the static‐stress change from the Tohoku‐oki earthquake, suggesting a stress state change during the earthquake cycle of megathrust earthquakes.

Seismological detection of low‐velocity anomalies surrounding the mantle transition zone in Japan subduction zone

AbstractIn the Japan subduction zone, a locally depressed 660 discontinuity has been observed beneath northeast Asia, suggesting downwelling of materials from the mantle transition zone (MTZ). Vertical transport of water‐rich MTZ materials across the major mineral phase changes could lead to water release and to partial melting in surrounding mantle regions, causing seismic low‐velocity anomalies. Melt layers implied by low‐velocity zones (LVZs) above the 410 discontinuity have been detected in many regions, but seismic evidence for partial melting below the 660 discontinuity has been limited. High‐frequency migrated Ps receiver functions indicate LVZs below the depressed 660 discontinuity and above the 410 discontinuity in the deep Japan subduction zone, suggesting dehydration melting induced by water transport out of the MTZ. Our results provide insights into water circulation associated with dynamic interactions between the subducted slab and surrounding mantle.

P and S wave tomography of Japan subduction zone from joint inversions of local and teleseismic travel times and surface-wave data

We determined P and S wave velocity tomography of the Japan subduction zone down to a depth of 700km by conducting joint inversions of a large number of high-quality arrival-time data of local earthquakes and teleseismic events which are newly collected for this study. We also determined 2-D phase-velocity images of fundamental mode Rayleigh waves at periods of 20–150s beneath Japan and the surrounding oceanic regions using amplitude and phase data of teleseismic Rayleigh waves. A detailed 3-D S-wave tomography of the study region is obtained by jointly inverting S-wave arrival times of local and teleseismic events and the Rayleigh-wave phase-velocity data. Our inversion results reveal the subducting Pacific and Philippine Sea slabs clearly as dipping high-velocity zones from a 1-D starting velocity model. Prominent low-velocity (low-V) anomalies are revealed in the mantle wedge above the slabs and in the mantle below the Pacific slab. The distinct velocity contrasts between the subducting slabs and the surrounding mantle reflect significant lateral variations in temperature as well as water content and/or the degree of partial melting. The low-V anomalies in the mantle wedge are attributed to slab dehydration and corner flows in the mantle wedge. A sheet-like low-V zone is revealed under the Pacific slab beneath NE Japan, which may reflect hot upwelling from the deeper mantle and subduction of a plume-fed asthenosphere as well. Our present results indicate that joint inversions of different seismic data are very effective and important for obtaining robust tomographic images of the crust and mantle.

Combining GPS and repeating earthquakes for a high resolution analysis of subduction zone coupling

Increasingly complex spatiotemporal patterns of subduction zone coupling are being revealed by geodetic and seismic observations. Understanding the mechanisms which control it is useful for improving seismic hazard assessments. GPS and characteristically repeating earthquakes (CREs) are complementary datasets for monitoring aseismic slip. Here, both of them are combined to estimate the rate and distribution of creep on the northeast Japan subduction zone between 21 March 1996 and 24 September 2003. We find that the majority of the upper part at 0–30km depth remains locked. There are three regions creeping at 7–8cm/yr distributed along-strike at 40–70km depth. We observe that these creeping regions occur in areas of low effective pressure and reduced porosity, which are inferred from Vp and Vs velocities. Moreover, an area of high clay content and high effective pressure coincides with the rupture area of the Tohoku-oki earthquake. We discuss these results in the context of potential mechanisms governing creep in northeast Japan. Our results highlight the benefits of combining GPS and CREs for advancing our understanding of the seismic cycle in subduction zones.

Teleseismic shear wave tomography of the Japan subduction zone

Teleseismic shear wave tomography of the Japan subduction zone

Intraplate volcanism influenced by distal subduction tectonics at Jeju Island, Republic of Korea

The drivers behind the inception of, and the variable, pulsatory eruption rates at distributed intraplate volcanic fields are not well understood. Such broad areas of monogenetic volcanism cover vast areas of the world and are often heavily populated. Reliable models to unravel their behaviour require robust spatio-temporal frameworks within the fields, but an analysis of the potential proximal and distal regional volcano-tectonic processes is also needed. Jeju Island (Republic of Korea) is a volcanic field that has been extensively drilled and dated. It is also located near one of the world’s best-studied tectonic plate boundaries: the subduction zone in southwestern Japan, which generates the Ryukyu and SW Japan arcs. A new set of 40Ar/39Ar ages collected from cores penetrating the entire Jeju eruptive pile, along with geochemical information, is used to construct a temporal and volumetric model for the volcano’s growth. The overall pattern indicates inception of volcanism at ~1.7 Ma, with an initial 1.2 Myr of low-rate activity, followed by over an order of magnitude rise over the last 0.5 Myr. The magma flux at Jeju correlates well with increased extension rates in the arc/backarc region. In particular, we infer that the increased trenchward mantle flow, caused by the greater rollback of the Philippine Sea Plate, activated pre-existing shear weaknesses in the mantle beneath Jeju, resulting in mantle upwelling and decompression melting that caused a change in compositions and an increase in eruption rates at Jeju. Thus, the volcanic activity of an intraplate field system can be modulated by regional subduction processes occurring more than 650 km away. This model may explain the frequent observation of pulsatory behaviour seen in many monogenetic volcanic fields worldwide that lie within 1,000 km of subduction zones.

Postseismic response of repeating earthquakes around the 2011 Tohoku‐oki earthquake: Moment increases due to the fast loading rate

AbstractWe examined the temporal variation of the size of repeating earthquakes related to the 2011 Tohoku‐oki earthquake (M9.0) in the northeastern Japan subduction zone for the period from July 1984 to the end of 2011. The repeaters (M2.5–6.1) show postseismic magnitude increases for most sequences located in the area of large postseismic slip at the downdip extension of the M9 source region. The magnitudes of the first events after the M9 earthquake increased by an average of about 0.3 for sequences having three or more earthquakes over the 9 months following it. We also examined the slip area in detail for Kamaishi repeaters whose magnitudes had been M4.9 ± 0.2 but which increased by about 1 after the M9 earthquake. Waveform modeling shows that the slip area for the post‐M9 Kamaishi earthquakes overlaps with that before the Tohoku‐oki earthquake but enlarged by about 6 times. Until the occurrence time of the last event (September 2011) in the analysis period, the rupture area remained larger than before but appeared to shrink over time. The enlargement of the rupture area suggests that an aseismic‐to‐seismic transition occurred in the region surrounding the pre‐M9 repeaters and is most likely related to fast loading of the repeaters due to rapid postseismic slip estimated to have occurred in the area. The existence of conditionally stable regions around the repeating earthquakes and/or patches slightly larger than the earthquake nucleation sizes may explain such behavior. The temporal change of loading rate is an important factor in determining earthquake size in this case.

Structural control on the nucleation of megathrust earthquakes in the Nankai subduction zone

AbstractTo clarify the causal mechanisms of megathrust earthquakes, we studied the detailed three‐dimensional P and S wave velocities (V), attenuation (Q), and Poisson's ratio (σ) structures of the Nankai subduction zone in southwest Japan, using a large number of high‐quality arrival time and t* data measured precisely from seismograms of local earthquakes. The suboceanic earthquakes used are relocated precisely using sP depth phase and ocean bottom seismometer data. Our results show the existence of two prominent high‐V, high‐Q, and low‐σ patches separated by low‐V, low‐Q, and high‐σ anomalies in the Nankai megathrust zone. Megathrust earthquakes during 1900 to 2013 nucleated in or around the high‐V, high‐Q, and low‐σ patches, which may represent strongly coupled areas (i.e., asperities) in the megathrust zone. Our results indicate that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of the Nankai megathrust earthquakes.

Kinematic Barrier Constraints on the Magnitudes of Additional Great Earthquakes Off the East Coast of Japan

Online Material: Figures showing potential areas and magnitudes of earthquakes rupturing partially coupled regions of the subduction zones. In the wake of the 2011 great Tohoku‐Oki, Japan, earthquake, questions were asked about the extent to which this earthquake ruptured a part of the subduction zone that had been strongly coupled in the decades prior to the event (Avouac, 2011; Lay and Kanamori, 2011; Loveless and Meade, 2011). The primary means for investigating this question is the analysis of interseismic geodetic data. These data record both tectonic motions and the accumulation of elastic strain the upper crust. Combined with elastic dislocation and block models, Global Navigation Satellite System (GNSS) data from the GNSS Earth Observation Network System (GEONET) array had been used to estimate the spatial variation in interseismic coupling (defined as the fraction of relative motion between tectonic plates that contributes to elastic strain accumulation) along the Japan trench subduction interface prior to the occurrence of the 2011 event (Nishimura et al. , 2004; Suwa et al. , 2006; Hashimoto et al. , 2009; Loveless and Meade, 2010). Each of these estimated interseismic coupling distributions identified a partially to strongly coupled region, extending for ∼400 km between 36° and 40° north latitude along the Japan trench, approximately bounding the lateral and down‐dip extents of the coseismic rupture (Loveless and Meade, 2011). However, none of these estimates show a perfect spatial correspondence between the coseismic rupture extent and complete preseismic coupling. Rather, the coseismic rupture was laterally bounded by regions of the Japan subduction zone with a coupling fraction c of ∼0.3, in which 30% of the relative plate motion contributes to elastic strain accumulation during the interseismic regime (Loveless and Meade, 2011). This spatial relationship could result from insufficient elastic strain accumulation around portions of the interface characterized by …

Triggering of tremor and inferred slow slip by small earthquakes at the Nankai subduction zone in southwest Japan

AbstractThe correlation of earthquakes with tremor and slow slip has not been clearly quantified. We investigate 12 year earthquake and tremor catalogs for southwest Japan and find that nearby small intraslab earthquakes are weakly correlated with tremor. In particular, the intraslab earthquakes with magnitudes ≥2.7 tend to be followed by tremor more often than expected at random by a factor of 2 to 6. The excess number of tremor before earthquakes is not as significant, although marginally more than expected. The underlying physical mechanism of the observed triggering of tremor and inferred slow slip by earthquakes is most likely to be the dynamic stress changes (several to several tens of kilopascals) rather than the much smaller static stress changes. The rate of triggering of tremor by earthquakes is similar to, although somewhat lower than, rates observed for similar amplitude stress changes due to the lower‐frequency teleseismic surface waves and tidal stressing.

Decoupling of Pacific subduction zone guided waves beneath central Japan: Evidence for thin slab

AbstractThe fine‐scale seismic structure of the northeast Japan subduction zone is studied based on waveform analyses of moderate‐sized (M4.5–6), deep‐focus earthquakes (h >350 km) and the finite difference method (FDM) simulation of high‐frequency (up to 8 Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source‐receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high‐frequency coda can be captured with the two‐dimensional (2‐D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high‐frequency S wave due to low‐Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high‐frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered‐Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high‐frequency slab‐guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.

Stress reversal recorded in calcite vein cuttings from the Nankai accretionary prism, southwest Japan

The Nankai Trough subduction zone in southwest Japan is a typical convergent margin where the Philippine Sea plate subducts in the northwest direction beneath the Eurasian plate, and devastating earthquakes have repeatedly occurred in this region in the past. In order to investigate the evolution of the stress state in the subduction zone, we analyzed deformation microstructures and the preferred orientation of calcite grains in two cuttings of calcite veins from Hole C0002F that was drilled through the inner wedge of the Nankai accretionary prism during the Integrated Ocean Discovery Program (IODP) Expedition 338 in 2012. For both samples collected at depths of 1,085.5 and 1,885.5 meters below the sea floor (mbsf), the c-axes of calcite grains are preferentially oriented perpendicular to the vein wall, which is indicative of competitive growth of calcite during the vein opening caused by a vein normal extension. Also, mechanical e-twins were developed in both samples, and these are inferred to have been developed under the same stress field as that responsible for the formation of calcite veins based on the paleostress analyses in grains with e-twins. For the calcite vein retrieved at the depth of 1,885.5 mbsf, kink bands were also developed by the compression in the direction perpendicular to the vein wall, which is indicative of stress reversal after the formation of mechanical e-twins. Although we could not reach a definite conclusion for the cause of the stress reversal, it could have occurred during either fold development or seismic cycles in the Nankai accretionary prism.

Seismic versus aseismic slip: Probing mechanical properties of the northeast Japan subduction zone

Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku Mw 9.0 earthquake ruptured with >50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17–8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M>8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space.

Late Quaternary uplift rate inferred from marine terraces, Shimokita Peninsula, northeastern Japan: A preliminary investigation of the buried shoreline angle

After estimating tectonic uplift rates along the northern part of the northeast Japan forearc (the overriding plate in the northeast Japan subduction zone) by mapping the elevation of the inner edges of marine terrace surfaces, we refined this estimate through elevation measurements of the buried shoreline angle beneath well-dated marine terrace surfaces, from which we could derive more accurate paleo-sea levels. The uplift rate initially inferred from the inner edge of marine terrace T4, correlated with marine isotope stage MIS 5e by tephrochronology, increases eastward from 0.11–0.22mky−1 around the backarc volcanic front to 0.17–0.32mky−1 in the forearc on the peninsula of Shiriyazaki. We refined the uplift rates for T4, on the basis of the shoreline angle elevation, from the reconstructed profile of the paleo-sea cliff and wave-cut platform on a rocky coast and the reconstructed profile of the swash zone sediments and terrace deposits on a sandy coast. The refined uplift rates were 0.14–0.25mky−1 on the rocky coast and 0.14–0.23mky−1 on the sandy coast, slightly slower than the rates we inferred from the height of T4 and about one-half to three-fourths of previously reported rates. By extrapolation from the example of the sandy coast, the refined uplift rate around the volcanic front was 0.09–0.18mky−1. The vertical deformation across the forearc of the Shimokita Peninsula since MIS 5e is possibly associated with regional isostatic uplift of 0.09–0.18mky−1 and anticlinal deformation by an offshore fault, interpreted from acoustic profiles, of 0.05–0.07mky−1.

Slow slip and aseismic deformation episodes associated with the subducting Pacific plate offshore Japan, revealed by changes in seismicity

Aseismic phenomena, including slow slip, can alter the surrounding seismicity. We here investigate how seismicity can be used in order to reveal episodes of aseismic deformation. An objective method is proposed that accounts for both earthquake interactions and transient loading. Applying it to the 1990–2011 (pre‐Tohoku) seismicity of the Japan subduction zone, we find several significant instances of aseismic transients. Small‐scale and short‐duration transients are favored updip of the subducting plate. Large‐scale transients are mostly observed offshore Ibaraki prefecture, in a partly decoupled zone that extends downdip. The four most intense of such transients have occurred periodically every 5.9years and are likely due to slow‐slip episodes. Other aseismic phenomena, including possible fluid intrusion in the outer rise, are also detected. Finally, the seismicity in January and February 2011, close to the epicenter of the megathrust Tohoku earthquake, is found to be due to aseismic loading, confirming previous studies, although this transient is only one among others and is not the most intense nor the most significant for the 21year long period studied here.

Aseismic deep subduction of the Philippine Sea plate and slab window

We have made great efforts to collect and combine a large number of high-quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both South Korea and West Japan. This is the first time that a large number of Korean and Japanese seismic data sets are analyzed jointly. As a result, a high-resolution 3-D P-wave velocity model down to 700-km depth is determined, which clearly shows that the Philippine Sea (PHS) plate has subducted aseismically down to ∼460km depth under the Japan Sea, Tsushima Strait and East China Sea. The aseismic PHS slab is visible in two areas: one is under the Japan Sea off western Honshu, and the other is under East China Sea off western Kyushu. However, the aseismic PHS slab is not visible between the two areas, where a slab window has formed. The slab window is located beneath the center of the present study region where many teleseismic rays crisscross. Detailed synthetic tests were conducted, which indicate that both the aseismic PHS slab and the slab window are robust features. Using the teleseismic data recorded by the Japanese stations alone, the aseismic PHS slab and the slab window were also revealed (Zhao et al., 2012), though the ray paths in the Japanese data set crisscross less well offshore. The slab window may be caused by the subducted Kyushu-Palau Ridge and Kinan Seamount Chain where the PHS slab may be segmented. Hot mantle upwelling is revealed in the big mantle wedge above the Pacific slab under the present study region, which may have facilitated the formation of the PHS slab window. These novel findings may shed new light on the subduction history of the PHS plate and the dynamic evolution of the Japan subduction zone.

Along‐arc variation in water distribution in the uppermost mantle beneath Kyushu, Japan, as derived from receiver function analyses

AbstractWe investigated the seismic velocity discontinuities in the uppermost mantle of Kyushu, a subduction zone in Japan, using receiver function analyses developed especially for discontinuities with high dipping angles. We elucidated the regional variation of discontinuities, as being the continental Moho, oceanic Moho, and upper boundary of the Philippine Sea slab. From the geometry of these discontinuities and contrast in S wave velocities, we discovered that the oceanic crust of the Philippine Sea slab has a low velocity and is possibly hydrated down to 70 km beneath south Kyushu, 80–90 km beneath central Kyushu, and less than 50 km beneath north Kyushu. The fore‐arc mantle wedge beneath central Kyushu has a low‐velocity zone and possibly contains hydrated materials and free fluid but such a low‐velocity zone does not exist in the fore‐arc mantle wedge beneath north Kyushu and south Kyushu. Beneath south Kyushu, water dehydrated from the slab could move to the back‐arc side and cause arc volcanism. Beneath central Kyushu, water dehydrated from the slab could move to the fore‐arc side and cause a gap in volcanism and hydration of fore‐arc mantle materials. Beneath north Kyushu, the oceanic crust does not appear to convey water abundantly in the mantle wedge. The low‐velocity hydrated fore‐arc mantle extends landward beneath the northern part of central Kyushu and could produce volcanic rocks partially contaminated by slab‐derived fluid.

Evidence for mechanical decoupling of the upper plate at the Nankai subduction zone: Constraints from core‐scale faults at NantroSEIZE Sites C0001 and C0002

Detailed analyses of cores from two drill sites landward of the Nankai trough provide constraints on the evolution of stress across the SW Japan subduction zone. Core‐scale faults at Sites C0001 and C0002 suggest changing stress geometries that shed light on mechanical coupling across this plate boundary. Normal faults dominate the record of brittle strain in both the sedimentary cover and the upper accretionary prism, and these cut older shear zones and vein structures. Based on cross‐cutting relations normal faults postdate thrust faults with the latter interpreted to be inactive. The normal faults, in contrast, are consistent with contemporary stress proxies such as borehole breakout and anelastic strain recovery data. Stress inversion for faults that are restored to original orientations using paleomagnetic data reveal that thrusting has accommodated trench subperpendicular shortening associated with interplate coupling. This phase of deformation is overprinted by normal faulting attributed to mechanical decoupling of the upper plate from the lower plate in response to the underplating of anomalously weak sediments between an out‐of‐sequence thrust and the décollement. These observations suggest that core‐scale faults record a stress history that is, to date, not apparent from in situ stress proxies such as borehole breakout data or anelastic strain recovery data. We conclude that the brittle faults provide a unique record of changing stress conditions that can be recognized only through coring.