Philippine Sea Plate Research Articles

Relationships among temperature, dehydration of the subducting Philippine Sea plate, and the occurrence of a megathrust earthquake, low-frequency earthquakes, and a slow slip event in the Tokai district, central Japan

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 8–22km, 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 35km. 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.

Evolution of the Sea of Japan back-arc and some unsolved issues

The Sea of Japan back-arc system provides an exceptional opportunity to study a virtually intact continent-ocean back-arc system in which recent tectonic inversion has exposed entire sequences of back-arc structure on land. Moreover, Japan's dense seismic/geodetic-monitoring networks, deployed country-wide, as well as moveable pools of ocean bottom seismometers, provide a rich data set through which to investigate deep back-arc structure. Earlier investigations have produced an initial understanding of back-arc opening: timing, structural evolution, temporal/spatial patterns of magmatic activity. Many questions remain, among them, the mechanism of back-arc opening (pull-apart or trench-rollback), the dynamics of interacting plates (location of the Philippine Sea plate with time), the origin of the anomalously thick Japan Sea ocean crust, and possible influences of far-field forces (India-Asia collision). Given existing high-resolution geophysical data sets and extensive on-land exposures of back-arc sequences and structures, the Sea of Japan back-arc is a promising context in which to address both local and more universal questions of how back-arc systems evolve. Here we review the tectonic setting and geological evolution of the Sea of Japan, based our own and others' work, and briefly discuss outstanding questions that invite further investigation.

Tomographic imaging of a seismic cluster in northern Taiwan and its implications for crustal fluid migration

After the occurrence of the 1999 magnitude 7.3 Chi–Chi earthquake, a cluster of NE-SW trending earthquakes, almost distributed along the surface trace of the Lishan fault, has been detected in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan Fault cluster (LFC) can quantify the influence of crustal fluids on the occurrence of seismicity after the 1999 Chi–Chi earthquake. In order to investigate the possible causes of the seismicity clusters and their relationship to the movement of fluids through part of the collision/subduction system, high-resolution 3-D tomographic images of the crust are determined under the entire region of northern Taiwan by inverting a large number of arrival time data of P-waves and S-waves. The results of seismic tomographic inversion indicate that the LFC extends down to about 10km depth and seems to be distributed in or around the low Vp, high Vp/Vs zones. This temporal expansion of the focal area during the first week may be attributed to fluid diffusion. The b-value of the LFC is about 0.98 close to the average value, 1.0 of the entire Taiwan region. Our tomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems to form a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a role of an active fluid conduit, transferring additional fluids of the Philippines Sea plate along the east-dipping fault zone into the upper crust.

Introduction to special section: The South China Sea deep-water: Stratigraphy, sedimentology, and resources

As the largest marginal sea in the western Pacific region, the South China Sea (SCS) occupies a unique location at the conjunction of modern Eurasia, Indo-Australian, and Philippine Sea plates, and between the largest continent, Eurasia, and the largest ocean, the Pacific. The marginal sea is

Backarc spreading and mantle wedge flow beneath the Japan Sea: insight from Rayleigh-wave anisotropic tomography

We present the first high-resolution Rayleigh-wave phase-velocity azimuthal anisotropy tomography of the Japan subduction zone at periods of 20–150s, which is determined using a large number of high-quality amplitude and phase data of teleseismic fundamental-mode Rayleigh waves. The obtained 2-D anisotropic phase-velocity models are then inverted for a 3-D shear-wave velocity azimuthal anisotropy tomography down to a depth of ∼300km beneath Japan. The subducting Pacific slab is imaged as a dipping high-velocity zone with trench-parallel fast-velocity directions (FVDs) which may indicate the anisotropy arising from the normal faults produced at the outer-rise area near the Japan trench axis, overprinting the slab fossil fabric, whereas the mantle wedge generally exhibits lower velocities with trench-normal FVDs which reflect subduction-driven corner flow and anisotropy. Depth variations of azimuthal anisotropy are revealed in the big mantle wedge beneath the Japan Sea, which may reflect past deformations in the Eurasian lithosphere related to backarc spreading during 21 to 15 Ma and complex current convection in the asthenosphere induced by active subductions of both the Pacific and Philippine Sea plates.

Active interseismic shallow deformation of the Pingting terraces (Longitudinal Valley – Eastern Taiwan) from UAV high-resolution topographic data combined with InSAR time series

ABSTRACTWe focus herein on the location, characterization and the quantification of the most active structural feature of Taiwan: the Longitudinal Valley Fault that corresponds to the suture in between the Philippine and Eurasian Plates. In order to determine and monitor its present inter-seismic deformation, we focus on the Pingting Terraces area, situated in the South Longitudinal Valley (Eastern Taiwan). We first determine the structural geometry issued from both photo-interpretation deduced from new unmanned aerial vehicle (UAV) high-resolution Digital Terrain Model data that we acquired (34.78 km2 with 7.73 cm ground sampling distance), combined with geological field work. In order to characterize and quantify the present deformational patterns over the Pingting terraces, we used an InSAR time series Interferometry algorithm (MT-InSAR) applied to nine L-band SAR images from ALOS satellite acquired over the period 2007–2010. The unprecedented density of measurements (about 120 points per km2 for a total of 6,400 points) gives a continuous overview of the inter-seismic shallow deformation. The structural geometry combined with the mean velocity map (MT-InSAR) reveals two clear active faults situated above the scarps of the Pingting terraces and responsible for up to 7 and 20 mm/yr velocity offset along the radar line of sight. A temporal analysis of the deformation is performed with one measurement at each SAR acquisition date, giving major improvements in the characterization and quantification of the Longitudinal Valley active Fault trace.

Transition of the Taiwan-Ryukyu collision-subduction process as revealed by ocean-bottom seismometer observations

Located at the arc-continental collision region between the Eurasian (EP) and Philippine Sea Plates (PSP), Taiwan is usually considered to have a complex tectonic environment, particularly along the eastern coast of the island. To gain a better understanding of the geological evolution of the east Taiwan area, the data from 8 Ocean Bottom Seismometers (OBS) acquired during the Across Taiwan Strait Explosion Experiment in 2012 and 14 inland seismic stations were used to determine a more detailed and accurate distribution of marine earthquakes. Based on the 333 relocated earthquakes and available geophysical data, we suggest two main tectonic boundaries for eastern Taiwan. South of 23.25°N, the homogeneous distribution of earthquakes in the crustal portion for both the inland and offshore areas suggests an ongoing collisional process. North of this location, between approximately 23.25°N and 23.8°N, the abrupt increasing of seismicity depth infers that the underthrusted arc/fore-arc material is deforming due to the collisional compression at depth. In this segment, the subsidence of the arc/fore-arc area determines the transition from collision to subduction. North of 23.8°N, the northwestern dipping PSP is well illustrated by the seismicity both onshore and offshore, indicating a dominant subduction process.

Increased rates of large-magnitude explosive eruptions in Japan in the late Neogene and Quaternary.

Tephra layers in marine sediment cores from scientific ocean drilling largely record high‐magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global data set suggests that the majority of the visible tephra layers used in this study are the products of caldera‐forming eruptions with magnitude (M) > 6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ∼8 Ma, 6–4 Ma, and further increase after ∼2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike‐slip‐dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ∼8 Ma to present period led to (1) increased arc‐normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favorable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera‐forming eruptions.

The influence of Ryukyu subduction on magma genesis in the Northern Taiwan Volcanic Zone and Middle Okinawa Trough — Evidence from boron isotopes

Boron (B) is an excellent geochemical tracer for investigating crustal recycling processes at convergent margins, due to its high fluid mobility under high P–T conditions, distinct elemental abundances and isotopic compositions in the mantle wedge and subducting slabs. The Northern Taiwan Volcanic Zone (NTVZ), wherein the nature of magma genesis has long been a topic of debate, is located at the rear side of the Okinawa Trough (OT), an atypical back-arc rift in the Ryukyu subduction system. In this study, B and B isotopes (δ11B) were measured in 19 volcanic rocks collected from the NTVZ and the middle Okinawa Trough (MOT) to assess the influence of the Ryukyu subduction system on magma genesis.The B concentrations in the MOT and NTVZ volcanic rocks are 5.8 to 13.6mg/L and 2.2 to 48.6mg/L, respectively. The large B abundances variation in the NTVZ was caused mainly by variable degrees of partial melting. The Nb/B and δ11B in the MOT have small ranges of 0.5 to 0.6 and −2.7‰ to 0.2‰, respectively, whereas they range widely from 0.4 to 2.5 and from −8.6‰ to 2.4‰, respectively in the NTVZ. These Nb/B values suggest that the magma contains a smaller subduction component than that normally observed in arcs, although this component is still more substantial than in a typical back-arc setting.The δ11B results indicate insignificant influence of the subducting Philippine Sea Plate at 2.6Ma, but it becomes more substantial later in the NTVZ. The mixing proportions of sediment derived fluids in onshore volcanoes in the NTVZ imply a rather heterogeneous mantle wedge near the plate boundary, most likely due to either a heterogeneous source of slab derived fluids or more complicated mantle flow. A substantial B flux from the subducting slab in the incipient back-arc rifting in the MOT and NTVZ may reflect characteristics of a cold, steep and fast subducting slab, which may be capable of carrying volatiles efficiently into greater depth in subduction zones.The δ11B data also provide insight into the nature of the different subduction components. A three-end-member mixing model suggests that MOT and NTVZ magmas may have incorporated subduction fluids derived from altered oceanic crust or serpentinite, whereas the onshore volcanoes in the NTVZ, and in particular in the Tsaolingshan, may have involved a higher proportion of sediment component.

Origin of transform faults in back‐arc basins: examples from Western Pacific marginal seas

AbstractTransform faults in back‐arc basins are the key to revealing the evolution of marginal seas. Four marginal basins in the Western Pacific, i.e. the South China Sea (SCS), Okinawa Trough (OT), West Philippine Basin (WPB) and Shikoku‐Parece Vela Basin (SPVB), were studied to redefine the strikes and spatial distribution of transform faults or fracture zones. Based on high‐resolution tectonomorphology, gravity and magnetic anomalies, pattern of magnetic lineations, seismic profiles, geometry of basins and palaeomagnetic data, together with analyses of regional geological setting, plate reconstruction and geodynamic analysis, this paper suggests that all the transform faults in the four marginal basins are in general NNE‐trending. Moreover, by comparing with the contemporary structural framework of the East Asian Continental Margin, we propose new models concerning marginal seas spreading and have revised the previous Cenozoic plate reconstruction models related to the East Asian Continental Margin and the Western Pacific marginal seas. There are three possible origins of these NNE‐trending transform faults. 1. Inheriting the orientation of the strike‐slip faults at the rifting continental margin (e.g. the SCS and OT). The real strike of transform faults should not be NW but NNE. The large‐scale NNE‐trending dextral strike‐slip faults distributed in the continental shelf of the SCS control a series of pull‐apart basins of the SCS. Due to a higher degree of pull‐apart, oceanic crust began to open. Then they evolved into the NNE‐trending transform faults in the SCS and could also be regarded as a natural extension of the NNE‐trending strike‐slip faults in the South China Block (SCB). The geodynamic mechanism of the OT is similar to that of the SCS. Consequently, transform faults of the OT should also be NNE‐trending, which is not perpendicular to the spreading axis but instead displays oblique spreading. 2. Izu‐Bonin‐Mariana (IBM) Trench retreat to the NNE and NE. Subduction rollback to the NE and NNE produced the NE‐ and NNE‐striking horizontal tensile stress, resulting in the rifting of the Kyushu‐Palau Ridge (KPR), controlling the spreading of the SPVB and forming the NE‐ and NNE‐trending transform faults. This also involves oblique spreading. 3. The later overall rotation of the Philippine Sea Plate (PSP). Since 25 Ma, the WPB has rotated clockwise about 40°. Therefore the NW‐ and NNW‐trending transform faults that formed at the later spreading stage have rotated to be the near‐N–S‐ or NNE‐striking faults. These transform faults are almost perpendicular to the spreading axis. Copyright © 2016 John Wiley & Sons, Ltd.

Three‐dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the Philippine Sea plate, southwest Japan

AbstractWe investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three‐dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3‐D simulation. Using the model, we estimated a realistic and high‐resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long‐term slow slip events (L‐SSEs), and nonvolcanic low‐frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3‐D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L‐SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350–500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid‐ocean ridge basalt of the subducting PHS plate.

Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods

AbstractWe reconstructed Philippine Sea and East Asian plate tectonics since 52 Ma from 28 slabs mapped in 3‐D from global tomography, with a subducted area of ~25% of present‐day global oceanic lithosphere. Slab constraints include subducted parts of existing Pacific, Indian, and Philippine Sea oceans, plus wholly subducted proto‐South China Sea and newly discovered “East Asian Sea.” Mapped slabs were unfolded and restored to the Earth surface using three methodologies and input to globally consistent plate reconstructions. Important constraints include the following: (1) the Ryukyu slab is ~1000 km N‐S, too short to account for ~20° Philippine Sea northward motion from paleolatitudes; (2) the Marianas‐Pacific subduction zone was at its present location (±200 km) since 48 ± 10 Ma based on a >1000 km deep slab wall; (3) the 8000 × 2500 km East Asian Sea existed between the Pacific and Indian Oceans at 52 Ma based on lower mantle flat slabs; (4) the Caroline back‐arc basin moved with the Pacific, based on the overlapping, coeval Caroline hot spot track. These new constraints allow two classes of Philippine Sea plate models, which we compared to paleomagnetic and geologic data. Our preferred model involves Philippine Sea nucleation above the Manus plume (0°/150°E) near the Pacific‐East Asian Sea plate boundary. Large Philippine Sea westward motion and post‐40 Ma maximum 80° clockwise rotation accompanied late Eocene‐Oligocene collision with the Caroline/Pacific plate. The Philippine Sea moved northward post‐25 Ma over the northern East Asian Sea, forming a northern Philippine Sea arc that collided with the SW Japan‐Ryukyu margin in the Miocene (~20–14 Ma).

Similarity and differentiation between the East China Sea Shelf Basin and Cenozoic basins in the northeast South China Sea

The East China Sea Shelf Basin (ECSSB), the Pearl River Mouth Basin (PRMB) and the Taixinan Basin (TXNB) in the northern continental margin of the South China Sea (SCS) are important oil‐ and gas‐bearing basins on the Western Pacific Continental Margin. During the Paleocene to Late Miocene, their strata can be compared, and the lithofacies were continuous from the TXNB, via the Taixi Basin (TXB) to the ECSSB. The lithologies mainly consist of interbedded shale, sandstone and mudstone layers. In addition, these basins have similar tectonic and sedimentation features. The three basins had similar marine‐terrigenous facies in the Paleocene and marine–lacustrine–fluvial facies during the Eocene and Late Miocene. The basins experienced several coeval tectonic movements and episodes, and they developed a series of NE‐ and NNE‐trending faults during the Paleocene and Eocene, which controlled the structure of the basins. During the Early Oligocene and Middle Miocene, they developed a series of NW‐ and WNW‐trending strike‐slip faults, reverse folds and flower‐like strike‐slip faults. However, there were obvious differences in sedimentary and tectonic evolution since the Late Miocene. The TXNB and TXB developed marine facies after the Late Miocene, while in the Quaternary, open marine facies replaced the Pliocene terrigenous facies in the ECSSB. Since the Late Miocene, the south of the ECSSB developed into a subsidence stage, and fault activity stopped. The TXB and TXNB developed some inverse structures and then developed into a thermal subsidence episode after the Dongsha Movement. Thus, this paper proposes that the ECSSB and the Cenozoic basins in the SCS were originally a unified basin and then subsequently separated into two basins as a result of the indentation of the Philippine Sea Plate (PSP) and the arc–continent collision between the Luzon Arc and the Eurasian Plate. Copyright © 2016 John Wiley & Sons, Ltd.

Philippine Sea Plate inception, evolution, and consumption with special emphasis on the early stages of Izu-Bonin-Mariana subduction

We compiled the most relevant data acquired throughout the Philippine Sea Plate (PSP) from the early expeditions to the most recent. We also analyzed the various explanatory models in light of this updated dataset. The following main conclusions are discussed in this study. (1) The Izanagi slab detachment beneath the East Asia margin around 60–55 Ma likely triggered the Oki-Daito plume occurrence, Mesozoic proto-PSP splitting, shortening and then failure across the paleo-transform boundary between the proto-PSP and the Pacific Plate, Izu-Bonin-Mariana subduction initiation and ultimately PSP inception. (2) The initial splitting phase of the composite proto-PSP under the plume influence at ∼54–48 Ma led to the formation of the long-lived West Philippine Basin and short-lived oceanic basins, part of whose crust has been ambiguously called “fore-arc basalts” (FABs). (3) Shortening across the paleo-transform boundary evolved into thrusting within the Pacific Plate at ∼52–50 Ma, allowing it to subduct beneath the newly formed PSP, which was composed of an alternance of thick Mesozoic terranes and thin oceanic lithosphere. (4) The first magmas rising from the shallow mantle corner, after being hydrated by the subducting Pacific crust beneath the young oceanic crust near the upper plate spreading centers at ∼49–48 Ma were boninites. Both the so-called FABs and the boninites formed at a significant distance from the incipient trench, not in a fore-arc position as previously claimed. The magmas erupted for 15 m.y. in some places, probably near the intersections between back-arc spreading centers and the arc. (5) As the Pacific crust reached greater depths and the oceanic basins cooled and thickened at ∼44–45 Ma, the composition of the lavas evolved into high-Mg andesites and then arc tholeiites and calc-alkaline andesites. (6) Tectonic erosion processes removed about 150–200 km of frontal margin during the Neogene, consuming most or all of the Pacific ophiolite initially accreted to the PSP. The result was exposure of the FABs, boninites, and early volcanics that are near the trench today. (7) Serpentinite mud volcanoes observed in the Mariana fore-arc may have formed above the remnants of the paleo-transform boundary between the proto-PSP and the Pacific Plate.

Lineages of Dissent: Popular and Artistic Resistance to Imperialism and Extraction on 'The Beautiful Island'

Lineages of Dissent: Popular and Artistic Resistance to Imperialism and Extraction on 'The Beautiful Island'

Simulation of tectonic evolution of the Kanto Basin of Japan since 1 Ma due to subduction of the Pacific and Philippine Sea plates and the collision of the Izu-Bonin arc

The Kanto Basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled the tectonic evolution of the Kanto Basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the latest collision of the Izu Peninsula block has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0–0.5Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following the shift in plate motion. Observed changes in the uplift/subsidence pattern are better explained by scenario (2), suggesting that recent (<1My) deformation in the Kanto Basin shows a lag in crustal response to the plate motion shift. We also calculated stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions.

Thermal modeling of the SW Ryukyu forearc (Taiwan): Implications for the seismogenic zone and the age of the subducting Philippine Sea Plate (Huatung Basin)

Subduction mega-thrust earthquakes in the SW Ryukyu trench pose a seismic and tsunami hazard. One of the objectives of this study is to estimate the downdip width of the seismogenic zone using numerical modeling to determine the temperature distribution along the plate interface. However, this approach depends strongly on the thermal parameters of the subducting slab. While the Philippine Sea plate (PSP) subducting beneath the central and eastern Ryukyu arc is of Eocene age (35–50Ma), its age west of the Gagua Ridge is uncertain, with proposed ages ranging from Lower Cretaceous (140Ma) to Upper Eocene (35Ma). Since the sparse available heat flow data are insufficient to resolve this debate, both end-member hypotheses are tested as input parameters. We examined two transects at 122.5°E and 123.5°E on either side of the N-S trending, 4-km high, Gagua Ridge. The shallow forearc geometry is obtained from wide-angle seismic data. The deep slab geometry was obtained from hypocenter distribution and tomography. For an Eocene slab age, we obtain a 100km and 110km wide seismogenic zone (between the 150°C and 350°C isotherms) west and east of Gagua Ridge, respectively. This is in good agreement with the observed distribution of hypocenters. Using a Cretaceous slab west of Gagua Ridge predicts a deep seismogenic zone (25km–60km depth), inconsistent with observed thrust earthquakes. Tomographic images at 122.5°E and 123.5°E show a similar slab thickness of 70–80km suggesting that the oceanic lithosphere has a young (Eocene) thermal age. The westernmost PSP (Huatung Basin) may have been thermally rejuvenated by mantle convection near the slab corner. The tectonic history since 6Ma (transition from subduction to collision beneath Taiwan) may have also perturbed the thermal structure.

Dynamics of the opposite‐verging subduction zones in the Taiwan region: Insights from numerical models

AbstractThe Taiwan mountain belt between the Eurasian and the Philippine Sea plate is a rare example for an orogen bracketed by two opposite‐verging subduction zones. The influences of the double subduction zones on regional dynamics have long remained unknown. In this study lithospheric deformation and mantle circulation in the Taiwan region are calculated with double subduction‐collision models. The results show that the limitedly deformed subducted Eurasian plate separates the highly deforming orogen above from the highly deforming mantle below. The edge flow driven by the rollback of the Philippine Sea slab dominates in the asthenosphere primarily as a result of the longer slab if the gap between the two slabs beneath Taiwan is sufficiently wide. The induced toroidal current gives rise to a pattern of seismic anisotropy compatible with that measured with teleseismic phases and coincidentally accordant with the strike of the orogen. The additional presence of a frequently hypothesized lithospheric fragment or slab tear disrupts the toroidal circulation and mars the model predictions for seismic anisotropy. We found that the rollback of the Eurasian slab deflects the plate downward and neutralizes the uplift, posing difficulty to models with Eurasian slab extending too far north. These results bolster the view that the Taiwan mountain belt is a subduction‐dominated orogen and both subduction zones play a key role on regional dynamics. Conceptual models specifically developed for large‐scale continent‐continent collision zones that have been commonly applied to the Taiwan region are inadequate for this ocean‐continent, opposite‐verging subduction‐collision system.

Crust and uppermost mantle structure of the Kyushu-Palau Ridge, remnant arc on the Philippine Sea plate

We acquired 27 wide-angle seismic profiles to investigate variation in crustal structure along the Kyushu-Palau Ridge (KPR), a 2600-km-long remnant island arc in the center of the Philippine Sea plate; 26 lines were shot across the strike of the KPR at 13°–31°N, and one was shot along the northernmost KPR. The derived P-wave velocity (Vp) models show that the KPR has a crustal thickness of 8–23 km, which is thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and the Shikoku and Parece Vela Basins to the east. While the KPR crust consists mainly of lower crusts with a Vp of 6.8–7.2 km/s, the thicker crust contains a thick middle crust with Vp of 6.0–6.8 km/s. In general, the KPR crust is thicker in the north than in the south. The uppermost mantle velocities just below the KPR bathymetric highs are lower than 8.0 km/s and are commonly associated with a slightly high Vp of 7.2 km/s at the base of the crust. Large amplitude reflection signals are sometimes observed at far offsets on several lines suggesting the existence of several reflectors at depths of 23–40 km in the mantle beneath the KPR. The characteristics of these reflections are similar to these observed beneath the Izu–Ogasawara (Bonin) island arc, the tectonically conjugate arc of the KPR before backarc basin spreading. Very thin crust and high Pn velocities characterize the transition between the KPR and the eastern basins, which is probably a relic of the initial stage of the rifting. West of the KPR, the crust varies in structure from north to south as a result of the different tectonic settings in which it evolved.

The source model and recurrence interval of Genroku-type Kanto earthquakes estimated from paleo-shoreline data

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.