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In southern Chile, the Nazca plate is subducting beneath the South American plate. This region was struck by megathrust earthquakes in 1960 and 2010 and is characterized by the existence of a volcanic chain. In this region, we modeled a three-dimensional thermal structure associated with the subduction of the Nazca plate by using numerical simulations. Based on the obtained temperature distribution, we determined the updip and downdip limit temperatures for the region ruptured by these two megathrust earthquakes. In addition, the distributions of water content and dehydration gradient were calculated by using appropriate phase diagrams and compared with the location of the volcanic chain. As a result, we infer that the coseismic slip of the 2010 Mw8.8 Maule earthquake occurred only at temperatures lower than and around the 350 °C isotherm that resembles the beginning of the brittle‒ductile transition. We also deduce that the rupture of the 1960 Mw9.5 Valdivia earthquake propagated up to the 450 °C isotherm because the magnitude was considerably large and the young hot plate subducted near the Chile Ridge. In addition, the hydrous minerals in the turbidites, MORB and ultramafic rocks released fluids via dehydration reactions, and dehydrated water migrated upward almost vertically, decreasing the melting point of the mantle wedge and contributing to the formation of the volcanic chain. [ABSTRACT FROM AUTHOR]

We used Global Navigation Satellite System (GNSS) time series data to estimate the spatiotemporal slip distribution for a long-term slow slip event (L-SSE) that occurred in the Tokai region, central Japan, from 2012 to 2016. Since all the used GNSS data were affected by the postseismic deformation associated with the 2011 Mw9.0 Tohoku-Oki earthquake, we removed such postseismic signal from the time series of three components at each of the stations. The minimal time window for an inversion analysis was set to 0.5 years (6 months), taking into account the signal-to-noise ratio of displacements for each time window. In the horizontal displacement fields, displacements were observed in the south‒southeast and southeast directions on the west and east sides of Lake Hamana, respectively, with temporal changes in their amounts and directions. In the vertical displacement fields, uplift was observed on the east side of Lake Hamana. From these data, we estimated the L-SSE initiated in approximately 2012.5 and ended by 2017.0, indicating the duration time is 4.5 years and the duration was much longer than that obtained in a previous study. Using these data, we performed the inversion analysis, in which three a priori information were assumed, i.e., the spatial distribution of slip is smooth, slip mainly occurs in the direction of plate convergence, and the temporal variation in the slip is smooth, to obtain the spatiotemporal slip distribution on a plate boundary with 3-D geometry. As a result, we identified that the L-SSE consisted of two subevents. The first subevent initiated on the southwest side of Lake Hamana and expanded during the period from 2013.0 to 2014.5. The maximum slip velocity during the period from 2012.5 to 2017.0 was estimated to be approximately 3.5 cm/year there for 2013.5–2014.0. The second subevent took place on the west side of Lake Hamana gradually from 2015.0 to 2015.5, continued, and expanded from 2015.5 to 2016.5. From the cumulative slip distribution, we found that its shape spread in the dip direction and obtained a maximum slip of approximately 10.6 cm, a moment release of 2.7 × 1019 Nm, and an equivalent moment magnitude of 6.9. Comparing our results with the L-SSE that occurred in the Tokai region between 2000 and 2005, we found that the slip initiation location was almost the same, but the subsequent slip location was more southerly for the 2012–2016 Tokai L-SSE. Additionally, the maximum slip velocity and moment magnitude were smaller for the 2012–2016 L-SSE. [ABSTRACT FROM AUTHOR]

The southern Chile subduction zone is a complex tectonic environment, where the Chile Ridge, the Nazca (NZ) and Antarctic (AN) plates subduct underneath the South American (SA) plate. The intersection between the NZ, AN and SA plates is referred to as the Chile Triple Junction (CTJ). In this region, a gap, often referred to as a slab window, has been formed between the NZ and AN slabs due to the divergence in their plate motion velocities, with volcanoes existing mainly above the subducted NZ and AN plates. In this study, we constructed a three-dimensional thermomechanical model associated with simultaneous subduction of the NZ and AN plates near the CTJ. The results show that the current temperature distributions on the upper surface of the slabs are higher closer to the Chile Ridge, and the AN plate has a distribution of elevated temperatures relative to the NZ plate at the same depth due to the northward migration of the CTJ and the slower convergence rate of the AN plate. Moreover, we calculated the water content and dehydration gradient from the temperature distribution near the upper surface of the slab and discussed their relationship to the distribution of volcanoes. In the northern part of the model domain, high dehydration gradients were obtained below the volcanic chain. Therefore, we suggest that the water released from the slab and the mantle wedge decreased the melting point of the mantle wedge just above the slab and produced melts, which may have contributed to form the overlying volcanoes. [ABSTRACT FROM AUTHOR]

In this study, we investigated the relationship between the strain accumulation before a slow slip event (SSE) and the strain release during the SSE for three recent SSEs along the Suruga Trough, Sagami Trough, and Nankai Trough, which are subduction zones in central to southwest Japan. The three analysed SSEs were the 2013–2016 Tokai long-term SSE (L-SSE), the 2018 Boso-Oki short-term SSE (S-SSE), and the 2019–2021 Central Shikoku L-SSE. We applied exponential and logarithmic functions to remove the postseismic deformations caused by the Mw 9.0 2011 Tohoku-Oki earthquake. We discovered a strong negative correlation between strain accumulation and strain release in the dilatation of all three SSEs and in the maximum shear strain of the 2018 Boso-Oki S-SSE. A comparison of the amount of strain accumulation with that of strain release revealed that approximately 30% of the strain was released in the 2013–2016 Tokai L-SSE, that 40% of the strain was released in the 2019–2021 Central Shikoku L-SSE, and that approximately 60% of the strain was released in the 2018 Boso-Oki S-SSE. This finding suggests that all of the accumulated strains are not necessarily released by the SSEs. [ABSTRACT FROM AUTHOR]

The transition from cold forearc to hot arc is considered fundamental to subduction dynamics. However, the range and depth of the cold nose changing significantly along the strike are difficult to constrain. By incorporating the three‐dimensional high‐resolution slab topography and MORVEL plate motion field to constrain thermomechanical modelling, we estimate the subduction thermal state and petrological dehydration in Izu‐Bonin. The multilayered hydrothermal regime gradually transitions from cold to less cold forearc, associated with a large‐scale subduction dewatering process varying along the strike. The dehydration embrittlement resulting from eclogitization and harzburgitization influences the occurrence of fast to slow earthquakes on megathrusts. Impressively, the cold nose is constrained to a depth of 60 km between the arc and Moho‐depth plate interface. The lowered temperature and delay of rock harzburgitization in the cold center of the subducted plate at depth contribute to the occurrence of deep earthquakes below the hot backarc interface. [ABSTRACT FROM AUTHOR]

The catastrophic 2004 Sumatra‐Andaman Mw9.1 earthquake associated with destructive tsunamis has characterized Sumatra as one of the most dangerous convergence zones. Nevertheless, the effects of the thermohydrous state on the strongly coupled megathrust of the incoming plate remain enigmatic. By using a three‐dimensional (3‐D) thermomechanical model to compute the temperature variation and the complicated phase transition process of the water‐bearing descending plate which generates unstable thrust slips, we find that Sumatran earthquakes at varying depths are likely under the control of the inter or intraplate hydrothermal regime, which occurs in or close to the petrological metamorphism transition area. The slab dehydration of the water‐rich mid‐ocean‐ridge basalts (MORB) and the ultramafic rocks in the oceanic lithosphere releases a large amount of fluid to the continental wedge and further facilitates arc magmatism. The fluids are prone to upwelling following the subduction channel along the plate interface and thus contribute to the clustering of earthquakes updip of the dehydration front beneath offshore Sumatra. Brittle failure and dehydration embrittlement at depth, along with the temperature differences caused by variant slab geometry, are conjectured to greatly influence the occurrence of fast and slow earthquakes in Sumatra. Key Points: Sumatran earthquakes are likely under control, and they occur in or close to the area transitioning from petrological metamorphismSlab dehydration of the water‐rich rocks in the oceanic lithosphere release a large amount of fluid to the continental wedgeBrittle failure and dehydration embrittlement have greatly influenced the occurrence of fast to slow earthquakes in Sumatra [ABSTRACT FROM AUTHOR]

Massive M>8 earthquakes have repeatedly struck the coastal Chilean subduction megathrust at intervals of nearly ten years, and such tsunamigenic earthquakes have caused recurrent damage. A widely accepted mature seismic gap along the strongly coupled plate interface in northern Chile was filled unexpectedly by the occurrence of the 2014 M w 8.2 Iquique earthquake, which occurred 130 years after the previous quake. This event revealed that our understanding of the state of seismogenesis along the Chilean margin is still poor. This study aims at investigating the temperature and water content distributions within the Nazca slab along a large portion of the Chilean margin. We employ 3-D time-dependent thermomechanical numerical models that incorporate up-to-date slab geometry and phase diagrams of hydrous minerals in the subducting plate to study the north-central Chilean (19 °S–33 °S) coupled megathrust, including the region where the slab is subhorizontal (28 °S–32 °S). Our modeling results show that most shallow off- and onshore interplate earthquakes in north-central Chile in the last decade have been mainly controlled by the phase transformation from lawsonite blueschist to amphibolite at shallow depths. On the other hand, some of the deep subduction seismicity is in good agreement with eclogitization within the oceanic crust and formation of harzburgite in the ultramafic rock in the lower slab. The M > 6 interplate earthquakes are estimated to occur where our models predict slab temperatures less than 400–600 °C and slab dehydration less than 0.03–0.05 wt%/km. Slow slip events tend to occur in regions where interplate temperatures are 400–600 °C (0.03–0.05 wt%/km). The activity level of regional interplate earthquakes and slow slip events is interpreted to be affected by the fluid release from and the temperature distribution in the Nazca slab. [ABSTRACT FROM AUTHOR]

We performed 2‐D high‐resolution thermomechanical numerical simulations associated with subduction of the Philippine Sea (PHS) plate in central Japan, where a well‐defined gap of low‐frequency earthquakes (LFEs) and tectonic tremors (TTs) is observed. We investigate the complex relationships among megathrust earthquakes, LFEs, TTs, and interplate temperature distributions and dehydrations caused by phase transformations of hydrous mid‐ocean ridge basalt (MORB) of the oceanic crust and ultramafic rock of the slab mantle. Our models predict a temperature range of 480–600 °C corresponding to both active LFEs and TTs regions and their gap region located on the upper surface of the PHS plate. A moderate amount of fluids (<0.8 wt%) is released due to phase transformations of the hydrous MORB near the thermally estimated seismogenic zone on the upper surface of the PHS plate. Hydrous MORB was found to transform from greenschist to epidote amphibole, releasing a small amount of fluids (>0.3 wt%) near the belt‐like form regions of LFEs and TTs. We found that the total amount of dehydrated water near the active region of LFEs and TTs is larger than that near the gap of LFEs and TTs. Our modeling results show that this difference stems from the presence of the phase transformation from epidote amphibole to eclogite in the former, and it may play a significant role for the occurrence of LFEs and TTs. The difference in the amount of released fluid between the active and gap regions of LFEs and TTs may be related to their generation mechanism. Key Points: We performed 2‐D thermal modeling to investigate relations among several seismic events, interplate temperature, and dehydration processIn our model, dehydration gradient near the active region of LFEs and TTs is different from that near the gap region [ABSTRACT FROM AUTHOR]

Abstract: The downdip limit of seismogenic interfaces inferred from the subduction thermal regime by thermal models has been suggested to relate to the faulting instability caused by the brittle failure regime in various plate convergent systems. However, the featured three‐dimensional thermal state, especially along the horizontal (trench‐parallel) direction of a subducted oceanic plate, remains poorly constrained. To robustly investigate and further map the horizontal (trench‐parallel) distribution of the subduction regime and subsequently induced slab dewatering in a descending plate beneath a convergent margin, we construct a regional thermal model that incorporates an up‐to‐date three‐dimensional slab geometry and the MORVEL plate velocity to simulate the plate subduction history in Hikurangi. Our calculations suggest an identified thrust zone featuring remarkable slab dehydration near the Taupo volcanic arc in the North Island distributed in the Kapiti, Manawatu, and Raukumara region. The calculated average subduction‐associated slab dehydration of 0.09 to 0.12 wt%/km is greater than the dehydration in other portions of the descending slab and possibly contributes to an along‐arc variation in the interplate pore fluid pressure. A large‐scale slab dehydration (>0.05 wt%/km) and a high thermal gradient (>4 °C/km) are also identified in the Kapiti, Manawatu, and Raukumara region and are associated with frequent deep slow slip events. An intraslab dehydration that exceeds 0.2 wt%/km beneath Manawatu near the source region of tectonic tremors suggests an unknown relationship in the genesis of slow earthquakes. [ABSTRACT FROM AUTHOR]

Giant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3-D time-dependent thermomechanical model that incorporates an up-to-date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3-D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid-rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500-700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3-D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia. [ABSTRACT FROM AUTHOR]

In this study, we performed two-dimensional numerical simulations of temperature distribution associated with subduction of the Philippine Sea (PHS) plate, and estimated the dehydration process of hydrous mid-ocean-ridge basalt (MORB) in the oceanic crust in the Tokai district, central Japan. We discuss the relationship among temperature, dehydration, and a seismogenic zone of an expected megathrust Tokai earthquake, and the occurrence of a slow slip event (SSE) and deep low-frequency earthquakes (LFEs). The depth range of the seismogenic zone for the megathrust earthquake was estimated to be 8–22 km, narrowing toward the east. The most suitable value of the pore pressure ratio on the plate interface was estimated to be 0.97, indicating minimal frictional heating there. The temperatures of the upper surface of the PHS plate, where the Tokai SSE occurred from 2000 to 2005, were estimated to be 350–450 °C. Therefore, the Tokai SSE is considered to have occurred at the transition zone between unstable and stable sliding. In addition, hydrous MORB was transformed from blueschist into greenschist near the region where the Tokai SSE occurred. The temperatures of the upper surface of the PHS plate, where LFEs occur, were estimated to be 450–500 °C. Therefore, LFEs are considered to occur near the down-dip limit of the transition zone. The amount of dehydration from the oceanic crust of the subducting PHS plate near the region where LFEs are distributed in a belt-like form decreases toward the east with deepening of the hypocenters of the LFEs. Thus, the mechanisms of generation of LFEs in the Tokai district might differ from those in southwest and central Japan where LFEs generally occur at depths between 27 and 35 km. In summary, we consider that the occurrence of LFEs is related to both temperature conditions and dehydration process. However, there is an exceptional area in the eastern part of the Tokai district where the occurrence of LFEs is restricted by temperature conditions rather than dehydration from the oceanic crust because the hypocenter depth is greater than that in southwest and central Japan. [ABSTRACT FROM AUTHOR]

Recent high-pressure and high-temperature experiments indicate that metastable olivine might persist in the cold core of a slab due to the low reaction rate of the olivine–wadsleyite phase transformation. Recent seismological observations detected a metastable olivine wedge that survives to a depth of 630 km in the Mariana slab. To consider the problem of non-equilibrium phase transformation, we developed a two-dimensional (2-D) Cartesian numerical code that incorporates the effects of kinetics into a thermal convection model. We consider the kinetics of the 410-km olivine–wadsleyite and the 660-km ringwoodite–Pv + Mw phase transformations, including the effects of water content at the 410-km phase boundary. The latent heat release of the 410-km non-equilibrium phase transformations inside the slab is also considered. The results show positive correlations between some of the controlling parameters and the length of the metastable olivine wedge: the faster the subducting velocity, and the lower the water content, the deeper is the metastable olivine wedge. With increasing depth of phase transformation, the effect of latent heat release is enhanced: heating of, at most, 100 °C occurs if olivine transforms into wadsleyite at a depth of approximately 570 km in our model setting. Temperature increase due to the latent heat released stimulates further phase transformation, resulting in further temperature increase, acting as a positive feedback effect. We also attempt to explain the seismological observations by calculating the temperature and phase structures in the Mariana slab. If we assume that the age of the Mariana slab is 150 Myr, the subduction velocity is 9.5 cm/yr, phase transformation occurs from the grain boundary of the parental phase, and the water content is 250 wt. ppm for a grain size of 1 mm, 300 wt. ppm for one of 5 mm, and 100 wt. ppm for intracrystalline transformation, then the metastable olivine wedge survives to a depth of 630 km, which is in good agreement with the seismological observations. This suggests that the deeper portion of the Mariana slab is relatively dry. Assuming that the depression of the 660-km discontinuity by ∼20–30 km within the Mariana slab, as is indicated by seismological observation, is explained by a combination of the depression caused by a negative Clapeyron slope in the cold slab and that due to the kinetics of the 660-km phase transformation, we obtain a gentle Clapeyron slope of −0.9 MPa/K for the phase transformation from ringwoodite to Pv + Mw. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we constructed a two-dimensional thermal convection model associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan to estimate the thermal state. To evaluate the validity of the calculated temperature distribution, we compared the calculated heat flows with observed heat flow data such as the High Sensitivity Seismograph Network Japan (Hi-net) borehole, land borehole, marine probe, and bottom-simulating reflectors. Hi-net heat flow data, which have never been used for thermal modeling, have spatially high resolutions on land and enable detailed estimation of the thermal state beneath the Japanese islands. We considered the spatio-temporal change in the age of the PHS plate, change in its plate motion direction at 3Ma, and the geometry of its upper surface. Calculated heat flows associated solely with subduction of the PHS plate, passing through central and eastern Shikoku and the Kii Peninsula, were not consistent with observations. They tended to be lower on the seaward side of the corner of the mantle wedge and to have much longer spatial wavelengths on the continental side. To explain heat flow data in southwest Japan, frictional heating at the plate interface on the seaward side of the corner of the mantle wedge and temperature changes due to surface erosion and sedimentation on land associated with crustal deformation during the Quaternary must be incorporated into the model. The most suitable pore pressure ratio to explain the heat flow data is estimated to be 0.95. Temperatures on the upper surface of the PHS plate obtained in this study are lower at shallow depths and higher at greater depths than those obtained by Hyndman et al. (1995), indicating that the seismogenic zone inferred from this study is narrower than that of their model. Microearthquakes within the subducting PHS plate occur at temperatures below 700–800°C. [Copyright &y& Elsevier]

Abstract: We estimated the slip-deficit rate distribution on the plate boundary between the subducting Philippine Sea plate and the continental Amurian plate along the Nankai Trough, southwest Japan. Horizontal and vertical displacement rates were calculated from land-based Global Positioning System (GPS) data during the 5-year period from 1 January 2005 to 31 December 2009. We employed an inversion analysis of geodetic data using Akaike's Bayesian information criterion (ABIC), including an indirect prior constraint that slip distribution is smooth to some extent and a direct prior constraint that slip is mainly oriented in the plate-convergent direction. The results show that a large slip deficit exists at depths ranging from 15 to 20km on the plate boundary in a belt-like form. The maximum slip-deficit rate was identified off Shikoku and reached 6cm/year. The slip-deficit rate differed by as much as 1cm/year when using a different geometric model of the subducting plate. On the basis of the spatial distribution of estimation errors and the resolution of the obtained slip-deficit rate on the plate boundary, we also found that the offshore slip-deficit rate cannot be estimated with sufficient accuracy using only land-based GPS data. Therefore, we tested the improvement in results when introducing hypothetical ocean-bottom GPS stations. The stations were arranged in four along-arc and across-arc spacings of 80km and 40km. The ocean-bottom data improved the estimation errors and resolutions, and successful results were obtained for a checkerboard with each square 75km×76km. Our results indicate that 40-km along-arc and across-arc two-dimensional spacing of ocean-bottom GPS stations is required to obtain reliable slip-deficit distributions near the trough axis, assuming the current estimation accuracy for ocean-bottom horizontal displacement rates. [Copyright &y& Elsevier]

Abstract: Oceanic plates subducted at trenches penetrate into the deep mantle, and encounter a structural boundary at a depth of 410km where olivine, the dominant element of mantle rocks, transforms into a higher density form wadsleyite. This transformation may be delayed within the coldest core of subducting plates (slabs) due to kinetic effects, and it has been suggested that metastable olivine may persist deeper than 410km. Using high density seismic array data in Japan, we show the direct image of the structure corresponding to this metastable olivine wedge (MOW) beneath southwest Japan. Numerical simulation of a subducting slab, including the kinetic effect of water (H2O) on the olivine–wadsleyite transformation, indicates that the presence of the imaged MOW requires an insignificant amount of water (less than 100wt. ppm) be present in the slab mantle, thus a deep dry cold slab. We infer that the transportation of water into the deep mantle occurs along the top surface of the subducting slab, but no significant amount within the slab itself. We also demonstrate that a numerical simulation including the kinetics of 660-km phase transformation can reconcile the observed deep depression of the 660-km discontinuity with a gentle Clapeyron slope. [Copyright &y& Elsevier]

Abstract: Global seismic tomography has recently revealed horizontally lying slabs near the upper and lower mantle boundary beneath the Northwestern Pacific region. Although physical mechanisms that could produce such slab stagnation have been proposed based on numerical simulations, there has been little research into what occurs after slab stagnation. We proposed trench advance and trench jumps as effective mechanisms related to the fall of stagnant slabs into the lower mantle, and our numerical simulations of temperature and fluid flow associated with slab subduction in a 2-D box model confirmed these mechanisms. Our results indicate that a supply of slab material associated with further slab subduction after slab stagnation plays an important role in differentiating further slab stagnation from the falling of slabs into the lower mantle. A shortage of material supply would produce extended slab stagnation near the 660-km boundary for ringwoodite to perovskite+magnesiowüstite phase transformation, whereas downward force due to further slab subduction on a stagnant slab would enhance its fall into the lower mantle. The behaviors of falling stagnant slabs were not affected by Clapeyron slope values associated with phase equilibrium transformation within the range from −3.0 to 0.0MPa/K. Compared with models of normal mantle viscosity, a high-viscosity lower mantle played a role in hindering the fall of slabs into the lower mantle, resulting in complicated shapes and slow falling velocities. Lower mantle viscosity structure also affected slab behavior. Slabs tended to stagnate when a low-viscosity zone (LVZ) existed just below a depth of 660km because friction between the slab and the LVZ was weak there. Slab stagnation around a depth of 660km also occurred when a high-viscosity zone existed below a depth of 1200km and acted as a resistive force against a slab, even if the slab existed in the lower part of the upper mantle. [ABSTRACT FROM AUTHOR]

Abstract: The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C. [Copyright &y& Elsevier]

Abstract: To investigate the influence of spatial change of viscosity on postseismic deformation associated with the interplate 1946 Nankai earthquake (M 8.0) at the Nankai Trough, southwest Japan, we newly constructed a realistic viscoelastic structure model, taking into account temperature- and depth-dependent viscosity of materials. For this purpose, we first compiled leveling and triangulation data during postseismic periods and clarified characteristics of the amount and spatial patterns of postseismic vertical displacement and principal strain fields. Then, we calculated the spatial distributions of viscosity from temperature and flow fields, which were obtained from 2D subduction models. By incorporating the obtained viscosity structure into 3D viscoelastic finite element models, we constructed a temperature- and depth-dependent viscosity structure model (MODEL P2). Based on MODEL P2, we constructed a viscoelastic structure model, taking into account Poisson''s ratio for the oceanic plate and low-velocity regions and the existence of low-viscosity materials beneath the Shikoku and Chugoku districts (MODEL P3), which were revealed from seismic tomography. We also constructed a conventional layered viscoelastic structure model (MODEL L1) and plate subduction model (MODEL P1) with constant viscosity for each region and evaluated the effects of different viscoelastic structures on postseismic surface deformations, using the coseismic slip distribution obtained by inversion analyses of geodetic data. We also compared the calculated surface deformations with the observed postseismic crustal deformations in and around Shikoku. The results show that postseismic surface deformation fields for the newly constructed MODEL P2 are rather different from those for MODELs L1 and P1. Landward horizontal displacements for MODEL P2 are smaller than those for MODELs L1 and P1, seaward horizontal displacements are negligible, and vertical displacement is characterized by small subsidence over Shikoku. The postseismic horizontal principal strain field for MODEL P2 is characterized by contractions in the N–S to NW–SE directions at amounts smaller than those for MODELs L1 and P1. Postseismic surface deformations for MODEL P3 are almost the same as those for MODEL P2. The observed postseismic vertical displacement and horizontal principal strain fields could not be explained by the viscoelastic response for the realistic viscoelastic structure models P2 and P3. This indicates that the effects of elastic and viscoelastic responses due to interplate coupling on the plate interface, after-slip at the extension of the coseismic slipped region, and poroelasticity should be taken into account to precisely estimate postseismic surface deformation. This also suggests that, in order to evaluate postseismic crustal deformations derived from a large interplate subduction zone earthquake, it is essential to use realistic temperature- and depth-dependent viscoelastic structure models. [Copyright &y& Elsevier]