Subducting Philippine Sea Research Articles

Improved geometry of the subducting Philippine Sea plate beneath the Suruga Trough

The Philippine Sea (PHS) plate subducts beneath the Eurasian plate at the Suruga Trough where megathrust earthquakes have repeatedly occurred. We used data from permanent seismic stations on land and temporary seismic stations beneath Suruga Bay to resolve the seismic velocity structure from the surface to 40–60 km depth beneath the Tokai region. The Median Tectonic line runs through the Tokai region and forms a boundary between high-V and low-V zones. We constrained the configuration of the subducting PHS plate using results from seismic tomography, hypocentral distribution of microseismicity, and low-angle thrust-type earthquakes. The oceanic crust of the PHS plate is characterized by low-V and high-Vp/Vs values. Events with thrust-type focal mechanisms are expected at the plate boundary. We constructed the geometry of the surface of the subducting PHS plate using this information. This geometry is also consistent with the results of receiver functions. The surface of the PHS plate is slightly (approximately 6–10 km) shallower than estimated from previous studies, especially in the zone extending from the Suruga Trough to a depth of 20 km. The data combination of permanent land and temporary OBS station with rays crossing the plate boundary clarify the plate configuration at the shallow subduction zone.

Two-dimensional thermal modeling associated with subduction of the Philippine Sea plate in southern Kyushu, Japan

In Hyuga-nada, southern Kyushu in southwest Japan, afterslip events were found in association with the two large interplate earthquakes, which occurred on October 19 and December 3, 1996. In Kyushu, low-frequency earthquakes (LFEs) and tectonic tremors are not common, but a considerable concentration of tectonic tremors is observed beneath the Pacific coast of the Miyazaki prefecture. To investigate the generation mechanisms of these seismic events, we performed 2-D box-type time-dependent thermal modeling in southern Kyushu. As a result, the temperature range of the upper surface of the subducting Philippine Sea (PHS) plate, where the afterslip occurred, reached approximately 300 to 350°C. The temperatures where the tectonic tremors occurred ranged from 450 to 650°C in the mantle wedge corner. We also estimated the spatial distribution of water content within the subducting PHS plate, using phase diagrams of hydrous mid-ocean ridge basalt (MORB) and ultramafic rock. Then, we found that no characteristic phase transformations accompany the dehydration of the subducting PHS plate in the afterslip region, but phase transformation from lawsonite blueschist to lawsonite eclogite is expected within the oceanic crust of the PHS plate just below the active region of the tectonic tremors. Our estimated water content distribution is consistent with the VP/VS ratio calculated from the seismic tomography. Therefore, we conclude that the occurrence of the afterslip is controlled by the temperature condition at the plate boundary, and occurs near the down-dip limit of the seismogenic zone. On the other hand, determining the major factors leading to the occurrence of the tectonic tremors is difficult, we estimated the temperature in the mantle wedge is ranging from 450°C to 650°C, and dehydration of 1.0wt% would be expected from the subducting PHS plate near the active region of the tectonic tremors.

Spatiotemporal distribution of low-frequency earthquakes in Southwest Japan: Evidence for fluid migration and magmatic activity

Low-frequency earthquakes (LFEs) in the lower crust and uppermost mantle are widely observed in Southwest Japan, and they occur not only along the subducting Philippine Sea (PHS) slab interface but also beneath active arc volcanoes. The volcanic LFEs are still not well understood because of their limited quantities and less reliable hypocenter locations. In this work, seismic tomography is used to determine detailed three-dimensional (3-D) P- and S-wave velocity (Vp and Vs) models of the crust and upper mantle beneath Southwest Japan, and then the obtained 3-D Vp and Vs models are used to relocate the volcanic LFEs precisely. The results show that the volcanic LFEs can be classified into two types: pipe-like and swarm-like LFEs, and both of them are located in or around zones of low-velocity and high-Poisson’s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes. The pipe-like LFEs may be related to the fluid migration from the lower crust or the uppermost mantle, whereas the swarm-like LFEs may be related to local magmatic activities or small magma chambers. The number of LFEs sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration. The spatiotemporal distribution of the LFEs may indicate the track of migrating fluids. As compared with the tectonic LFEs along the PHS slab interface, the volcanic LFEs are more sensitive to fluid migration and local magmatic activities. High pore pressures play an important role in triggering both types of LFEs in Southwest Japan.

P and S wave attenuation tomography of the Japan subduction zone

AbstractWe determine the first high‐resolution P and S wave attenuation (Q) tomography beneath the entire Japan Islands using a large number of high‐quality data collected from P and S wave velocity spectra of 4222 local shallow and intermediate‐depth earthquakes. The suboceanic earthquakes used in this study are relocated precisely using sP depth phases. Significant landward dipping high‐Q zones are revealed clearly, which reflect the subducting Pacific slab beneath Hokkaido and Tohoku, and the subducting Philippine Sea (PHS) slab beneath SW Japan. Prominent low‐Q zones are visible in the crust and mantle wedge beneath the active arc volcanoes in Hokkaido, Tohoku, and Kyushu, which reflect source zones of arc magmatism caused by fluids from the slab dehydration and corner flow in the mantle wedge. Our results also show that nonvolcanic low‐frequency earthquakes (LFEs) in SW Japan mainly occur in the transition zone between a narrow low‐Q belt and its adjacent high‐Q zones right above the flat segment of the PHS slab. This feature suggests that the nonvolcanic LFEs are caused by not only fluid‐affected slab interface but also specific conditions such as high pore pressure which is influenced by the overriding plate.

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.

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

We conducted a detailed investigation of seismic activity from January 2011 to February 2013 along the Nankai trough off the Kii Peninsula, central Japan, by using data obtained from the DONET ocean-bottom observation network. The hypocenters are mostly within the subducting Philippine Sea (PHS) plate, although a few are along the plate boundary or in the sedimentary wedge below the Kumano forearc basin. The seismic activity can be separated into events above and below 20 km depth, which corresponds approximately to the Moho. The hypocenter distributions are distinctly different for these groups. The seismic activity in the oceanic crust can be further separated into three clusters. Most of the seismic activity recorded in our data represents aftershocks of the 2004 off the Kii Peninsula earthquakes (M JMA = 7.1, 7.4, and 6.5), which occurred in the PHS plate. The hypocenter distribution in the oceanic crust correlates well with the location of the Paleo-Zenisu ridge, which is formed by a chain of seamounts that is subducting beneath the forearc basin. The hypocenters in the uppermost mantle are aligned on a plane dipping to the southeast, consistent with the existence of a thrust fault cutting through the lithosphere of the oceanic plate. The focal mechanisms of the earthquakes show that the axis of compressive stress in the PHS plate is oriented N–S, almost perpendicular to the direction of plate convergence, indicating a complex tectonic regime in this region. These results suggest that intraplate shortening may be occurring in the subducting oceanic plate.

Waveguide effects within the Philippine Sea slab beneath southwest Japan inferred from guided SP converted waves

SUMMARY High-sensitivity, dense seismic observations and numerical simulations revealed a distinct guided SP converted wave of oceanic mantle earthquakes that occurred within the subducting Philippine Sea (PHS) plate and a waveguide effect of the PHS Plate beneath southwest Japan. Based on seismograms, we identify the oceanic mantle earthquakes of the double seismic zone and find a new type of later phase for these earthquakes. The primary features of these later phases are as follows: (1) these phases are observed at the northeastern inland stations of the convex slab axis, (2) the vertical and radial components are dominant and (3) the apparent velocity is approximately 8.0 km s –1 . To examine the origin of these later phases, we simulate a2 -DP-SV seismic wavefield considering the PHS slab geometry. Based on the numerical models, we conclude that the later phases are guided SP converted waves trapped within the oceanic crust, which acts as low-velocity layer. A series of numerical simulations indicates that two factors are necessary to reproduce the observed SP guided waves: (i) a low-velocity channel must exist between stations and sources and (ii) the oceanic crust must only be in contact with the lower crust near the stations.

Reappraisal of the Arc-Arc Collision in Taiwan

Al though it is evident that Tai wan has been formed by the collision of the west-facing Luzon arc with the Eurasian continental mar gin, there re main a lot of enigmas in this collision. The major ones are: (1) a trans form fault presently connecting the Manila and Ryukyu Trenches in the Philip pine Sea-Eurasia relative motion direction is missing, and in stead, the Ryukyu Trench ex tends near off shore E. Tai wan, (2) the western edge of the intermediate-depth seismicity associated with the Philip pine Sea plate subduction beneath NE Tai wan has a NNW trend, not NW, and (3) a large negative Bouguer gravity anomaly, with undulation of its amplitude in a wave length of 60-80 km, exists along the southernmost Ryukyu forearc. We propose a new model of the collision in Tai wan to re solve these enigmas, assuming that the southern Ryukyu forearc was migrating to the south west with respect to Eurasia for the past several m.y. and the Luzon arc has been colliding with this actively migrating Ryukyu forearc. The northern most Luzon arc is divided into two parts by the NNW line directing along the Philip pine Sea-Ryukyu forearc motion from its initial intersection point with the Ryukyu Trench; the part west of this line has been obducted on the Ryukyu forearc-Eurasian mar gin, producing the collision orogen in Taiwan, and the part east of it has been subducted beneath the Ryukyu forearc. This evolutionary scenario re solves enigmas (1) and (2) kinematically. This model also predicts that the South China Sea slab has to be torn by the west ward component of the motion of the subducting Philip pine Sea slab to Eurasia. This would have brought large lateral compression in the shallow portion of the Philip pine Sea slab at its western border, which might lead to buck ling of the slab causing the ob served undulated gravity anomaly.

Seismic anisotropy in the wedge above the Philippine Sea slab beneath Kanto and southwest Japan derived from shear wave splitting

We conduct shear wave splitting measurements on waveform data from the Hi-net and the broadband F-net seismic stations in Kanto and SW Japan generated by shallow and intermediate-depth earthquakes occurring in the subducting Philippine Sea and Pacific slabs. We obtain 1115 shear wave splitting parameter pairs. The results are divided into those from the shallow (depth < 50 km) and the deep (depth > 50 km) events. The deep events beneath Kanto are further divided into PHS1 and PHS2 (upper and lower planes of the double seismic zone in the Philippine Sea slab, respectively), PAC1 and PAC2 (western and eastern Pacific slab, respectively) events. The results from the shallow events represent the crustal anisotropy, and their fast directions are more or less aligned in the σ Hmax directions, implying that the anisotropy is produced by the alignment of the vertical cracks in the crust induced by the compressive stresses. In Kanto, Kii Peninsula and Kyushu regions, the results from the deep events suggest a contribution from the mantle wedge anisotropy. Events from all groups beneath Kanto show NW, NE and EW fast directions. This complex pattern seems to be produced by the corner flows induced by both the WNW PAC plate subduction and the oblique NNW PHS slab subduction with the associated olivine lattice-preferred orientations (LPOs), and the anisotropy frozen in the PHS slab. The deep events beneath Kii Peninsula show NE and NW fast directions and may be produced by the corner flow produced by the NNW PHS slab subduction with the associated olivine LPOs. The NE directions might also be produced by the segregated melts in the thin layers parallel to the PHS slab subduction. The deep events beneath N Kyushu show NNW fast directions, which may result from the southeastward flow in the upper mantle inferred from the stresses in the upper plate. Results from the deep events beneath middle-south Kyushu show dominantly E–W fast directions, in both the fore- and back-arcs. They may be produced by the corner flow of the westward PHS slab subduction with the olivine LPOs. Because the source regions with multiple fast directions are not resolved in this study, further detailed analyses of shear wave splitting are necessary for a better understanding of the stress state, the induced mantle flow, and the melt-segregation processes.

A tectonic interpretation of NW-SE strike-slip faulting during the 2004 off the Kii peninsula earthquakes, Japan: Probable tear of the Philippine Sea plate

Abstract The 2004 off the Kii peninsula earthquakes (M w 7.5 for the main shock) occurred within the subducting Philippine Sea (PHS) plate near its boundary, the Nankai trough, southwest Japan. The rupture mode of the foreshock-main shock-aftershock sequence was complicated, a combination of ENE-WSW striking (almost trough parallel) reverse faulting beneath the trough and NW-SE trending (almost trough normal) strike-slip faulting mostly on the landward side of the former. In this paper, we discuss the tectonic meaning of this NW-SE running strike-slip fault. We examined hypocenter distribution and focal mechanisms of slab earthquakes from October 1997 through September 2004 and confirmed a NW-SE striking tear of the PHS slab beneath the middle part of the Kii Peninsula pointed out by Miyoshi and Ishibashi (2004). According to the Earthquake Research Committee (2004) there is a NW-SE trending structural discontinuity in the PHS crust to the southeast of the main shock epicenter. Putting all features together, we interpret that there is a NW-SE striking fracture within the PHS plate continuously from the Nankai trough region to the slab beneath the Kii Peninsula, and that a partial rupture of this fracture occurred during the off the Kii peninsula earthquakes together with trough-parallel reverse faulting. It should be noted that two disastrous M 7-class slab earthquakes seem to have occurred along this tear beneath the peninsula in 1899 and 1952.

Time‐dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan

We estimate time variation of interplate coupling between the subducting Philippine Sea and the overriding continental plates along the western part of the Nankai trough, Japan, during the period between 1996 and 1999. After subtracting annual components and average annual velocities, deviation from steady crustal deformation in the Shikoku and Kyushu islands is examined. Around 15 GPS stations facing the Bungo channel detected the following three‐stage ground displacements of up to 3 cm in 1997, with a duration of ∼1 year: (1) increased displacement rate associated with the 1996 Hyuga‐nada earthquakes; (2) an increasingly rapid anomalous period of deformation from June 1997 to the end of 1997; and (3) a slowdown period and return to normal rates of deformation after the end of 1997. Linearized least squares locate the slip area of this slow event near the plate boundary beneath the Bungo channel. The estimated slip is opposite the motion of the Philippine Sea plate. These results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the Philippine Sea plate. Time‐dependent inversion analysis shows the following slip history. During the initial period of increased displacement rate, a visible slip area is resolved beneath the coastal area of southwestern Shikoku. In the next stage of accelerated displacements, the slip area expanded southwestward to the Bungo channel beginning around June 1997, when postseismic slip of the 1996 Hyuga‐nada earthquakes almost ended. After this accelerated period, the estimated thrust slip ends with time.

Crustal deformation and interplate coupling in the Shikoku district, Japan, as seen from continuous GPS observation

Data from the nationwide GPS continuous tracking network, which has been operated by the Geographical Survey Institute (GSI) of Japan since April 1996, were used to study crustal deformation in the Shikoku district, Japan. The crustal deformation data are inverted to estimate interplate coupling between the subducting Philippine Sea (PHS) plate and the overriding continental plate. For this purpose, we first extracted site coordinates from daily SINEX files for the period from 1 April 1996 to 31 March 1998, and estimated average site velocities by linear regression. Estimated velocities under ITRF96 are converted into a kinematic reference frame [K. Heki, J. Geophys. Res. 101 (1996) 3187–3198] to discuss the crustal deformation relative to the stable interior of the Eurasian (EUR) plate. To segregate the signal and noise in the thus obtained velocity vectors, we employed the least-squares prediction technique. Estimated signals (displacement vectors) were used to reconstruct the strains (dilatations, maximum shear strains, principal axes of strains) and to estimate the interplate coupling in the area. Results of crustal strains clearly portray tectonic straining of the Shikoku island: (1) dilatational strains show that the Shikoku island is under a compressive strain regime induced by the subducting PHS plate; (2) maximum shear is high in the island, which may be due to the oblique subduction; (3) the principal axes of the strains indicate that the Shikoku island is under the influence of the converging oceanic plate beneath the island. In contrast, the results of the inversion analysis show: (1) there is a high coupled region extending from 10 to 30 km deep; (2) the average back slip rate in this region reaches 43±5 mm/yr; and (3) the direction of the plate convergence at the Nankai trough is N42±6°W.

Interplate coupling and relative plate motion in the Tokai district, central Japan, deduced from geodetic data inversion using ABIC

SUMMARY The spatial distribution of the strength of interplate coupling between the subducting Philippine Sea and overlying continental plates in the Tokai district, central Japan, was investigated in detail through the inversion analysis of geodetic data using Akaike's Bayesian Information Criterion (ABIC). The geodetic data used for the analysis are annual rates of level changes (1972-1984) and horizontal length changes (1977-1988), which presumably represent average crustal movements during the interseismic period. The result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth. The total seismic moment accumulated in this area since the last event (the 1854 Ansei earthquake) is roughly estimated to be 5.5 × 1027 dyne cm, which corresponds to Ms= 7.8. The interplate coupling becomes weaker toward the shallower and deeper portions. This is consistent in general tendency with a rheological model inferred from petrological viewpoints. The strength of coupling also tends to decrease toward northeast over the west coast of Suruga Bay. The direction of plate convergence inferred from the inversion analysis is oriented N30°W. This is significantly different from the direction of relative plate motion between the Philippine Sea and Eurasian plates but concordant with that between the Philippine Sea and North American plates.

Three-dimensional Q structure beneath the Kanto-Tokai district, Japan

The Q structure of the upper mantle beneath the Kanto-Tokai district, central Japan, where the Eurasian, Pacific and Philippine Sea plates are converging, is investigated from seismic P- and S-wave spectral ratios by means of a three-dimensional (3-D) inversion method. It is found that the subducting Philippine Sea and Pacific plates are characterized by high- Q zones. A high- Q zone also exists between the Philippine Sea plate and the Pacific plate, to the east of the volcanic front. On the other hand, a low- Q zone in the uppermost mantle extends westward horizontally from the volcanic front under the Philippine Sea plate, and appears to exist over the wedge mantle beneath the western Tokai district. A very low Q value obtained here is roughly equal to the average Qvalue in the low-velocity layer derived from surface wave and free oscillation data. A low- Q region that suggests a magma chamber under active volcanoes is not detected in the crust.