Subducting Philippine Sea Slab Research Articles

Isotopic evolutionary track of water due to interaction with rocks and its use for tracing water cycle through the lithosphere

Hydrogen and oxygen isotopic compositions of natural spa waters from hot and/or mineral-rich springs (including flowing/pumping wells) potentially provide important clues about water cycle in Earth's interior, because some of the waters are deep-seated and non-meteoric. However, the isotopic compositions of these waters are typically altered by contamination with meteoric water and are thus poorly known. To reconstruct their original (i.e., before these waters mix with meteoric water) isotope compositions, here we develop an isotopic evolution model of the lithosphere-enclosed waters and derive the “ocean-origin lithospheric water curve” (OLWC; δ2H = 60 / (δ18O – 11)) as a typical evolutionary track starting from ocean water. Then we apply them to spa waters from central Japan, a globally unique geotectonic region where double subduction of slabs occurs. The OLWC explains well observed isotope data of subseafloor pore waters, submarine mud volcano pore waters, coastal oil-field brines, and subduction zone volcanic steam. Mixing analysis indicates that lithospheric water accounts for 19–95 % of each spa water we investigated. The nonlinear relationships of the reconstructed water δ18O and Cl or Li concentrations justify our hypothesis that the lithospheric waters are undergoing evolution from ocean water to magmatic water by rock-water isotopic exchange and that they are not simply mixtures of the two. A comparison of the reconstruction results with model predictions under slab subduction settings suggests that most spa waters in the southern to central part of the study region contain progressively evolving lithospheric waters released from the subducting Philippine Sea slab, and that a few at western edge of the region contain waters released from the Pacific slab. The others in the northern and eastern part of the region contain waters at the early-to-mid evolutionary stage, which are discharged from Tertiary marine sedimentary rocks (including tuffs). All these waters were enclosed in the lithosphere for more than 1–5 Ma. Our approach provides useful tools for tracing sources of non-meteoric water and highlights water cycle through lithosphere for more than one million years.

Temporal Variations in QP−1 ${Q}_{P}^{-1}$ and QS−1 ${Q}_{S}^{-1}$ Above a Megathrust Following Episodic Slow‐Slip Events

AbstractRecent observations beneath central Japan have shown that periodic fluid drainage occurs during slow‐slip events (SSEs) based on temporal variations in above the megathrust boundary of the subducting Philippine Sea slab. However, no previous studies have estimated associated . A comparison of and can provide clues to the mechanism of seismic attenuation because of different propagation characteristics of the two waves. We estimate temporal variations in and via spectral analyses of waveform data from November 2009 to August 2021 period. The results indicate that both and exhibit temporal variations at about 1‐year periodicity and there are systematic differences between and that have smaller values, less insignificant variation, and weaker correlations with SSEs. Furthermore, increases concurrent with SSEs. These differences suggest that attenuation is caused by the wave‐induced fluid flow. could be an important parameter for detecting the presence of fluid.

Slab Morphology Around the Philippine Sea: New Insights From P‐Wave Mantle Tomography

AbstractWe apply seismic tomography to 590,213 P‐wave arrival‐time data of 23,486 local earthquakes and 300,051 relative travel‐time residuals of 27,492 teleseismic events to study the 3‐D velocity structure of the crust and upper mantle beneath the entire Philippine Sea Plate (PSP) and its adjacent areas. Our results reveal significant high‐velocity anomalies beneath the Philippine Trench, the Japan‐Izu‐Bonin‐Mariana Trench, and the Nankai Trough, which reflect the subducted Philippine Sea slab, the Pacific slab, and the Shikoku Basin, respectively. On the basis of the tomographic images, we construct morphological models of these slabs. The slab morphologies become complex from the east to west. Taking into account previous results of geochemistry, paleomagnetism and numerical simulations, we deem that the complicated morphologies of the subducted slabs are related to the slab age, subduction rate, collision between different plates, clockwise rotation of the PSP, and retreat rate of trenches.

Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets

AbstractThe Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid‐rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network‐MT survey in addition to a conventional wideband‐MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer‐scale dipoles of the Network‐MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama‐Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower‐crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.

Structural control on the 2018 and 2019 Hualien earthquakes in Taiwan

The 2018 Mw 6.4 Hualien earthquake and the 2019 Mw 6.1 Hualien earthquake occurred beneath the eastern coast of Taiwan, at depths of <~20 km. The two earthquakes caused a total of 18 fatalities and widespread damage to infrastructures, especially in the Hualien City. To better understand the generating mechanism of the damaging earthquakes, we conducted seismic tomography to determine high-resolution 3-D models of P and S wave velocity (VP, VS) and Poisson's ratio (σ) in the source zone. We used 334,645 P- and 238,581 S-wave arrival times of 20,825 local earthquakes (1.5 ≤ ML ≤ 6.9) recorded at 47 seismic stations in Taiwan. Our results show that the hypocenters of the two Hualien mainshocks are located at a sharp vertical boundary where seismic velocity and Poisson's ratio change drastically in the horizontal direction, which corresponds to the location of the Longitudinal Valley Fault and its branches. Furthermore, the hypocenters are located right above a high-σ anomaly that is connected to the estimated upper boundary of the subducting Philippine Sea slab, which may reflect ascending fluids dehydrated from the slab. These results suggest that the 2018 and 2019 Hualien earthquakes were triggered by ascending fluids from the slab dehydration, invading into active faults that may be the northern extension of the Longitudinal Valley Fault.

Stress loading and the occurrence of normal-type earthquakes under Boso Peninsula, Japan

Boso Peninsula, Japan, was formed by the interaction of the Philippine Sea, Eurasian and Pacific plates around the trench–trench–trench Boso triple junction. Normal-type earthquakes are persistently observed in the subducting Philippine Sea slab under the peninsula at a depth of ~ 30 km, including a recent (2019) Mw 4.9 earthquake which caused shaking throughout the Kanto region (greater Tokyo). Such shallow intraplate earthquakes are potentially hazardous to this heavily populated region, yet their mechanism is poorly understood, especially in the context of a three-plate system. Here, we calculate stress rates in the Philippine Sea slab and the surrounding area, using a subduction model constructed in a previous study, to explain the generation of the regional stress field and its effect on earthquake occurrence. In general, the calculated stress rates under Boso Peninsula are horizontally extensional both above and below the Eurasian–Philippine Sea plate interface. We apply our calculated stress rates to the nodal planes of the observed earthquakes to calculate the Coulomb failure function (ΔCFF). These calculated ΔCFFs are generally positive on normal-type earthquakes under Boso. The ΔCFFs are also consistent with earthquakes in adjacent areas that are seismically active, for example, in the Philippine Sea plate to the south, in the collision zone around Izu Peninsula, and in the cluster in the Eurasian plate northeast of Boso Peninsula, which further supports our stress loading model. Calculation of the individual contributions of Philippine Sea plate and Pacific plate subduction shows that the development of the stress field around Boso is dependent upon contributions from both subducting plates. In contrast, the arc–arc collision at Izu Peninsula has little influence.

Triple Seismic Zone Revisited

Subduction off Miyagi and Fukushima prefectures, northern Honshu, has been recognized as having a triple-planed structure of seismicity at the deep thrust zone in the 40-60 km depth range. This triple seismic zone is composed of thrust-type earthquakes, down-dip compressional and down-dip tensional earthquakes from top to bottom. At the time of the 2011 Tohoku-oki earthquake, peculiar phenomena such as radiation of short-period seismic waves and pre- and after- slow slips within the asperities of M7 class earthquakes were observed over this thrust zone west of the 2011 main rupture zone. Further to the south, where the Philippine Sea plate is subducting beneath Kanto, a triple seismic zone has also been recognized particularly under southwest Ibaraki prefecture. The thrust-type earthquakes at the top of the triple seismic zone off northern Honshu and beneath Kanto have been believed to be interplate events representing the relative motion between the overriding and subducting plates. I conclude that the thrust-type earthquakes beneath southwest Ibaraki prefecture are in fact within the crust of the subducting Philippine Sea slab, not at the surface because their slip vectors are different from the relative motion between the subducting and overriding plates. Therefore, there would be an aseismic plate boundary above the seismicity. I also show that the dip angles of the westerly dipping fault planes of the thrust-type earthquakes off Miyagi prefecture are smaller by 6° in average than the dip of the slab surface in this region, except for the six years prior to the Tohoku-oki earthquake, i.e. prior to 2006. Furthermore, the slip vectors coincide with the relative motion between the overriding and subducting plates only during this period. I infer that the topmost earthquakes of the triple seismic zone off Miyagi prefecture prior to 2006 are thus likely to be within the crust of the subducting plate. The slow slips before and after the Tohoku-oki earthquake would have occurred not within the asperities but along the aseismic plate interface, and the short-period seismic waves would have been radiated due to fractures within the crust associated with the overshooting rupture at the time of the Tohoku-oki earthquake. Many of the so-called repeating earthquakes at the topmost surface of the subducting plate would be in fact intra-crustal events within the slab. M9 earthquakes would interact with the triple seismic zone, not only mechanically, but also through fluid migration, because earthquakes in the triple seismic zone involve dehydration reaction. The irregularity of the occurrence of M9 earthquakes might be due to the inhomogeneous distribution of hydrated minerals in the incoming plate. The subduction zones having M9 earthquakes or under Kanto have a collisional character. I propose to term subduction having both a collisional character and a triple seismic zone as “super-subduction”. The relative motion between the plates is accommodated by the deformations of the crust of the subducting slab as a “plate boundary zone”. The viewpoint of “super-subduction” is necessary to understand earthquakes in subduction zones with a collisional character and dehydration reactions in the slab.

An inhomogeneous across-slab conduit controlled by intraslab stress heterogeneity in the Nankai subduction zone

Nonvolcanic, deep low-frequency tremors and slow-slip events occur simultaneously in the transition zone from locked to continuously creeping fault in the down-dip portion of the Nankai Trough subduction zone, southwestern Japan. The occurrence of these slow earthquakes is discontinuous along the trench and attributed to the effect of high pore pressures at the plate boundary. Here, we show that spatial variations in intraslab stress may control fluid migration from the subducted Philippine Sea slab to the plate boundary. The triaxial normal faulting stress, detected by stress tensor inversion using focal mechanisms in the slab, controls anisotropically permeability that trends NNW–SSE subhorizontally from the subducted Philippine Sea slab to the plate boundary. The inhomogeneous permeability controlled by spatial stress heterogeneities in the subducted Philippine Sea slab controls the intraslab fluid pathway. This hypothesis is consistent with the spatial heterogeneity of slow earthquakes and 3He/4He ratio distributions.

Centroid Moment Tensor Inversion of Shallow Very Low Frequency Earthquakes Off the Kii Peninsula, Japan, Using a Three‐Dimensional Velocity Structure Model

AbstractWe conducted centroid moment tensor inversions of shallow very low frequency earthquakes (SVLFEs) that occurred around off the southeast Kii Peninsula, Japan. A set of Green's functions was evaluated using numerical simulations of seismic wave propagation in a three‐dimensional velocity structure model, including the subducting Philippine Sea slab and the accretionary prism. We used observed seismograms for periods of 20–50 s at an onshore broadband network, which enable us to investigate the long‐term activity of SVLFEs. Estimated centroid locations and focal mechanisms of SVLFEs, including SVLFEs that occurred prior to the deployment of seafloor observations, were very similar to those estimated using seafloor records. Low‐angle thrust faulting mechanisms were concentrated around the accretionary prism toe. SVLFE activity in April 2016 was concentrated further west in an area of previous activity. Our results imply seismic slip around the very shallow part of the Philippine Sea plate boundary.

Age of the Subducting Philippine Sea Slab and Mechanism of Low‐Frequency Earthquakes

AbstractNonvolcanic low‐frequency earthquakes (LFEs) usually occur in young and warm subduction zones under condition of near‐lithostatic pore fluid pressure. However, the relation between the LFEs and the subducting slab age has never been documented so far. Here we estimate the lithospheric age of the subducting Philippine Sea (PHS) slab beneath the Nankai arc by linking seismic tomography and a plate reconstruction model. Our results show that the LFEs in SW Japan take place in young parts (~17–26 Myr) of the PHS slab. However, no LFE occurs beneath the Kii channel where the PHS slab is very young (~15 Myr) and thin (~29 km), forming an LFE gap there. According to the present results and previous works, we think that the LFE gap at the Kii channel is caused by joint effects of several factors, including the youngest slab age, high temperature, low fluid content, high permeability of the overlying plate, a slab tear, and hot upwelling flow below the PHS slab.

Receiver function images of the distorted Philippine Sea slab contact with the continental crust: Implications for generation of the 1891 Nobi earthquake (Mj 8.0)

The 28 October 1891 Nobi earthquake was one of the largest intraplate earthquakes in Japan, with an estimated magnitude of 8.0 determined by the Japan Meteorological Agency. It occurred in central Japan on the Eurasian Plate, beneath which the young Philippine Sea Plate is subducting. The geometry of this plate is not smooth; rather, beneath western Japan it appears distorted over a relatively short lateral distance. Beneath the Kinki region of central Japan, the subduction angle is approximately horizontal, and the Nobi earthquake occurred just above the horizontal section of the Philippine Sea slab. Temporary seismic stations were deployed in this area to investigate local seismic structure and its possible relationship to the cause of the earthquake. Crustal structure was investigated using receiver function analysis with a spatially dense temporary seismic network. A clear image of the subducted Philippine Sea slab was resolved, with ridge-like structures beneath the epicentral area of the Nobi earthquake. The ridge-like structure is in contact with the continental crust, and the fault plane of the Nobi earthquake is located at the edge of one such structure; i.e., in the contact area. The unique structure of the area is inferred to cause stress concentration and strain accumulation on local faults, which resulted in the large size of the Nobi earthquake.

Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics

We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous–Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.

Rare Earth Elements of the Arima Spring Waters, Southwest Japan: Implications for Fluid Crust Interaction during Ascent of Deep Brine

Rare earth elements (REEs) of the eight Arima spring waters in southwest Japan, including Arima-type brine that represents a specific type of deep-seated brine of up to 6 wt.% NaCl in the non-volcanic fore-arc region, have been investigated in order to discuss their upwelling processes and origins. We found four distinct patterns of REE composition of the spring waters within the Arima area of ~1 km2 , based on which two sources for REEs and two aquifers are inferred in the modification of the original deep-seated brine composition. On the basis of the REEs and isotopic compositions of the original deep brine, one of the two sources is thought to be slab-derived fluid dehydrated from the subducted Philippine Sea slab beneath the Arima area, represented by the ‘Kinsen’ hot spring water [1]. The convex-down REE pattern of most Arima spring waters, except for ‘Kinsen’ and ‘Tansansen’, suggests the presence of an oxidizing aquifer deeper than 160 m that causes co-precipitation of REEs with oxyhydroxides. CO2 and He degassed from this aquifer flux the overlying shallow aquifer less than ~50 m in depth, producing highly carbonated water such as ‘Tansansen’ water that was originally derived from meteoric water. The carbonated water may dissolve a significant amount of REEs to the ‘Tansansen’ spring water from the host rocks, which are possibly silicic igneous rocks with Eu-negative anomalies. The four types of REE patterns with a wide concentration range, therefore, provide invaluable information concerning fluid–crust interaction during ascent of the deep brine.

Arima hot spring waters as a deep-seated brine from subducting slab

Abstract Non-volcanic hot springs are generally believed to originate through circulation of meteoric or buried sea water heated at depth. In this study, we report the geochemical characteristics of the Arima and Takarazuka hot spring waters, known as Arima-type deep brine, in a forearc region of southwestern Japan. We examine 14 water samples to determine the levels of 12 solute elements or components and the isotopic ratios of H, He, C, O, and Sr, and we perform correlation analysis of the data to deduce the source materials and origin of the deep brine. Moreover, we perform numerical modeling of oxygen and hydrogen isotopic fractionation along subducting slabs to examine the composition of slab-derived fluid as a possible candidate of the deep brine. The results suggest that the high salinity and solute concentrations with characteristic oxygen, hydrogen, carbon, and strontium isotope compositions, as well as high 3He/4He ratios, can be explained by a dehydrated component of the subducted Philippine Sea slab. Hence, this study may provide an invaluable understanding of geofluid processes over a significant depth range.

Non-volcanic seismic swarm and fluid transportation driven by subduction of the Philippine Sea slab beneath the Kii Peninsula, Japan

To understand the mechanism of an intensive non-volcanic seismic swarm in the Kii Peninsula, Japan, we used a dense seismic linear array to measure fine-scale variations of seismic velocities and converted teleseismic waves. A low-velocity anomaly confined to just beneath the seismic swarm area is clearly imaged, which correlates spatially with an uplifted surface area and a highly conductive and strong attenuative body. These results suggest that fluids such as partial melt or water are present beneath this non-volcanic seismic swarm area. It is notable that the island arc Moho below the seismic swarm area is at a depth of approximately 32 km in the northern part of the seismic swarm area and shallows to approximately 20 km towards the south, due to the raised structure of the serpentinized mantle wedge. In addition, we show that the hydrated oceanic crust of the subducting Philippine Sea slab is characterized by low velocities with a high Poisson's ratio at depths of less than 40 km. In contrast, dehydration conversion from oceanic basalt to eclogite takes place at depths greater than 50 km. Water released from the subducting oceanic crust could cause serpentinization of the mantle wedge and infiltration into the forearc base of the overlying plate. The interaction between dehydration of the subducting oceanic crust and hydration of the mantle wedge and overlying plate exerts an important role in driving the non-volcanic seismic swarm activity in the Kii Peninsula.

Rare Earth Elements and Sr-Nd-Pb Isotopic Analyses of the Arima Hot Spring Waters, Southwest Japan: Implications for Origin of the Arima-type Brine

Rare earth elements (REEs) and Sr-Nd-Pb isotopic compsotions of the Arima hot spring waters, a specific type of deep-seated brine (up to 6 wt.% NaCl) in the non-volcanic fore-arc region of southwest Japan, have been analyzed in order to discuss their source materials and origins. We have first examined the matrix effect associated with variable salinity (0 to 5 wt.% NaCl), and found that above 1 wt.% NaCl, the intensity of REE in ICP-MS measurement is drastically reduced due to the matrix effect. Accordingly, we have diluted the sample waters to contain ~0.6 wt.% NaCl, and then analyzed them by the standard addition method. The result shows that the abundance is appreciably high compared to near-surface waters, and exhibits almost a flat DMM-normalized pattern, which can be explained by mixing of a slab-derived fluid at relatively low temparature (400 to 500oC) and a near-surface water. The Sr-Nd- Pb isotopic compositions of the brineare consistent with the above interpretation on REEs, in that the deep brine is isotopically similar to a slab-derived fluid of the subducted Philippine Sea slab. These evidences suggest that the slabderived fluid is upwelling even in the non-volcanic region possibly through a fault zone along the tectonic line.

Complex slab structure and arc magmatism beneath the Japanese Islands

A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling.

P wave anisotropic tomography of the Nankai subduction zone in Southwest Japan

The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3‐D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using ∼540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high‐velocity (high‐V) anomaly which exhibits considerable lateral variations. Significant low‐velocity (low‐V) anomalies are revealed above and below the PHS slab. The low‐V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low‐V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench‐normal in the mantle wedge under the back‐arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench‐parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid‐ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.

Low-frequency earthquakes and tomography in western Japan: Insight into fluid and magmatic activity

Low-frequency (LF) microearthquakes are detected in the lower crust and uppermost mantle beneath western Japan, which may reflect fluid and magmatic activity in the subduction zone. In this work we combine seismic tomography and the LF earthquakes to study the crustal and upper-mantle structure and fluid and magmatic activity in western Japan. High-resolution tomographic images under this region are determined using high-quality arrival-time data. In Kyushu the subducting Philippine Sea slab is detected clearly as a high-velocity zone, and the arc-magma related low-velocity (low- V) anomalies under the active arc and back-arc volcanoes are imaged clearly. Prominent low- V zones are visible in the crust and uppermost mantle beneath the Quaternary volcanoes along the Japan Sea coast, and LF events occur actively under some of those volcanoes. LF events are also detected in the source areas of the 1995 Kobe earthquake (M 7.2) and the 2000 Tottori earthquake (M 7.3), supporting the hypothesis that the large crustal earthquakes were triggered by the crustal fluids. These results indicate that fluid and magmatic activities exist widely in western Japan, which are caused by dehydration of the subducting Philippine Sea slab and corner flow in the mantle wedge.

Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore‐arc mantle

Three‐dimensional tomographic images of the subducting Philippine Sea slab and the fore‐arc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in VP, VS, and VP/VS. Analysis of the trade‐off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data. The regularization parameters were also chosen to ensure suppression of artificial VP/VS anomalies. The subducting slab is characterized by high VP, high VS, and intermediate to low VP/VS. Notable in the mantle wedge is the high VP/VS anomalies that abut the surface of the subducting slab at depths of 30–80 km. The previously identified positive VP/VS channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert VS and VP/VS to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200°C–400°C relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches ∼15%, or 2% water content, at 50 km depth in the fore‐arc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt‐eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea Plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.