Subducting Philippine Sea Plate Research Articles

Tomographic imaging of lithospheric structures under Taiwan

Tomographic images of the crustal and mantle velocity structures under Taiwan are obtained by simultaneous inversion of local earthquake P-wave arrival times for hypocenters and P-wave velocity structures. In northern Taiwan, a high-velocity zone, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. The imaged zone dips toward the north at an angle of 40° from a depth range of 20–55 to 100–130 km. An upper-mantle low-velocity wedge, ranging from 40 to 80 km in depth, exists above the subducted slab. Above this wedge is the Ilan Plain of northern Taiwan which lies at the west end of the Okinawa Trough, a well recognized back-arc basin; the crustal velocities under the Plain are also relatively low. The well-defined high-velocity zone and the low-velocity wedge provide some constraints on the thermal structures of the subduction system under northern Taiwan. In tomographic images across the central section of Taiwan, thickening of the crust and up-arching of the lower-crustal materials under the Central Range are commonly observed; the crust under the Western Foothills region is clearly thinner and the near-surface low-velocity layers are well developed. The structures under the Central Range show that although the Taiwan orogeny is quite young, a root, deeper in the north and shallowing to the south, has formed. The results of our tomography show that a significant portion of the lithosphere is involved in the Taiwan orogeny.

A kinematic subduction model for the genesis of back-arc low-K volcanoes at a two-overlapping subduction zone, central Japan: another volcanic front originated from the Philippine Sea plate subduction

Systematic geochemical and volumetric variations have been documented across many island arcs. Volcanism in the Shinetsu highland, the northeastern part of central Japan, shows various anomalies in such variations, though located in a subduction zone. In this highland, the distribution density of volcanoes and total volume of erupted materials decrease from the volcanic front toward the back-arc side. However, they increase again in the more distant back-arc area. Lavas from this back-arc area characterized by a low-K nature, are equivalent to those from the trench side of northeastern Japan. This volcanism occurred since 1.7 Ma, taking the place of the usual back-arc-type high-K volcanism. Furthermore, in the fore-arc region of this highland, the volcanic front had retreated about 20 km toward the back-arc side, during 4 – 3 Ma. Presently, the depth to the Wadati-Benioff zone beneath the volcanic front is 20–30 km deeper than that beneath the main part of northeastern Japan. To explain these features, a kinematic subduction model is proposed here which assumes the northward subduction of the Philippine Sea (PHS) plate over the previously subducting Pacific (PAC) plate. In this model, the retreat of the Pliocene volcanic front in the fore-arc region is considered to have resulted from the migration of a high-temperature region of the wedge mantle (> 1400 °C) toward the back-arc side by the sweeping effect of the subducting PHS plate at about 4-3 Ma. Generation of the low-K volcanism in the back-arc side of this highland is interpreted to have been caused by the increase of the supply rate of H 2O related to the dehydration of amphibole and chlorite dragged by subduction of the PHS plate which occurred slightly before 1.7 Ma.

日本列島周辺の海底地質 その３． 南海舟状海盆（トラフ）及びその付近

The Nankai Trough is regarded as a trench from the structural point of view, in spite of its shallowness and flatness. The topography of the sea floor of the Nankai Trough and its vicinity shows typical features consisting of the continental shelf, the upper slope (shelf slope), the deep-sea terrace, the lower slope (landward wall of the trough), the trough floor, and the ocean basin. The upper slope is comparatively monotonic. The deep-sea terrace is a large structural basin lying at the foot of the upper slope, behind the outer ridge rising on the continental slope. The lower slope from the outer margin of the deep-sea terrace to the trough floor consists of accretions which form ridges and troughs. This is divided into smaller scale units by the structure of faults and folds. Sediments on the sea bed of the Nankai Trough have been deformed in conformity with the formation of the hollow of the trough. The Nankai Trough and its related topographies, namely the upper slope, the outer ridge shifting landward on the whole, and the ridge-and-trough zone, were formed by the subduction of the Philippine Sea plate. Topographic trends of the Nankai Trough area are deviated largely at the boundary, which can be estimated by distribution of hypocenters, between sub-slabs in the subducting Philippine Sea plate. The age of formation of the major topographies of the Nankai Trough and its vicinity falls within the post-Pliocene.

人工地震による日本列島の地殻構造

The Research Group for Explosion Seismology of Japan, founded in 1950, have continuously conducted many explosion seismic observations in various regions of the Japanese Islands. Active investigations of the Group have provided important data about the crustal structure beneath the Japanese Islands which is quite useful as the most basic information for various researches of geosciences. After ages of the cradle in 1950's, the Group experienced large scale investigations under the International Upper Mantle Project and the International Geodynamics Project in 1960's and 1970's, and general features of crustal structure beneath the Japanese Islands became clear through the investigations in this period. Since 1979, the Group has conducted the investigations under the Japanese Earthquake Prediction Project in order to accumulate basic data for earthquake prediction researches. The series of these investigations have been conducted through highly dense observations, and the obtained data have revealed quite complex structure of the crust beneath the Japanese Islands. Application of some of data processing techniques in reflection survey to those data also revealed clear images of the subducted Philippine Sea plate beneath the Japanese Islands.

Difference in N2/Ar ratio of magmatic gases from northeast and southwest Japan: New evidence for different states of plate subduction

Two different plates are subductiog beneath the Japanese island arcs, the Pacific plate in the north and the Philippine Sea plate in the south. Geophysical evidence indicates that the Philippine Sea plate subducts without extending beneath many of the volcanoes in the southwest Japan arc. The N 2 /Ar ratio (424) of a common magmatic gas from geothermal and volcanic areas in southwestern Japan is an order of magnitude lower than the value (4250) in northeastern Japan. Because the high N 2 /Ar values reflect input from subducted sediments, the large difference in the N 2 /Ar ratios of magmatic gases indicates that the contribution of subducted sediment to the source of magma in southwestern Japan is much lower than that in northeastern Japan. The N 2 /Ar ratios off magmatic gases from island arcs elsewhere may also yield information regarding the degree of contribution of subducted sediments to arc magma sources and differences in the states of plate subduction.

日本における大地震 (<i>M</i>≧6.9) の余震数 (<i>M</i>≧5), およびその本震の震源スペクトルに対する関係

We have studied the number of aftershocks with JMA magnitude (MJ)≥5 occurring within the next 30 days after large shallow earthquakes (MJ≥6.9) in Japan. As a whole, the relationship between the number of aftershocks (N), and the moment magnitude (MW) of the main shock is similar to that obtained by SINGH and SUÁREZ (1988) for the events along the entire circum-Pacific subduction zones. For the same MW, N is generally smaller for the interplate earthquakes along the Nankai trough, where the Philippine Sea plate subducts, than for the interplate earthquakes along the Kurile and Japan trenches where the Pacific plate subducts. This relative lack and excess of N correlates with the supposedly strong and weak coupling of the two plate boundaries, respectively. On the other hand, N for the intraplate earthquakes on the Japan Sea side is similar to that for the interplate earthquakes along the Kurile and Japan trenches. Thus, it is difficult to explain the relative lack and excess of N in different tectonic settings in terms of interplate and intraplate events.It is reasonable to expect that N should correlate with the source spectrum of a main shock since they can both be affected by the degree of heterogeneity on and around the fault plane. We examined the correlation between N and the source type of a main shock that is determined by comparing the size of felt area with MJ. The source type is regarded as reflecting the enrichment of the source spectral amplitude at higher frequencies (1-5Hz) relative to that at lower frequencies (around 0.3Hz). Contrary to expectation, no clear correlation was found between them. It is concluded that the relationship between the aftershock activity and the source spectrum of a main shock is so complicated that it cannot simply be characterized in terms of such parameters as the number of aftershocks and the source type adopted in this study.

地殻内地震の発震機構から推定される関東平野の地殻応力の特徴

The crustal stress in the Kanto plain and its vicinity is analyzed by using directional distribution of P- and T-axis of focal mechanisms of earthquakes within the earth's crust. The area is divided into six tectonic stress provinces which have different directions of P- and T-axis. In most cases, geological tectonic lines or active faults exist in the tectonic stress boundary zone which divides the tectonic stress provinces. This fact indicates that the stress distribution has close conection with the geological structure. The six tectonic provinces in due to the stress change at the weak zone of the crust. The stress change has its origin in the heterogeneity in the crustal strength. Average direction of P-axis in northern part of the Kanto area differs from that in the southern part, i. e., direction of the P-axis lie within NW-SE and N-S in the southern part and E-W in the northern part, which corresponds to the movements of Philippine Sea plate and Pacific plate, respectively. On the other hand, very complicated directional distribution of P-axis was seen in the northern part of Tokyo bay and the southwestern part of the Kanto plain. This stress state is interpreted in terms of increase of dip angle of subducting Philippine Sea plate just below the area.

Body-wave waveform constraints on the source parameters of the Yangjiang, China, earthquake of July 25, 1969: A devastating earthquake in a stable continental region

On July 25, 1969, anM s 5.9 earthquake shook the Kwangtung Province near Yangjiang, China. Casualties and extensive damage were reported in the epicentral area. The Yangjiang earthquake occurred within the passive margin located along China's southeast coast. This stable continental setting is seismically one of the most quiet regions of China; historic records indicate this earthquake to have been the first devastating one in the area. A remarkable feature of this earthquake sequence is that its foreshocks and aftershocks are relatively small in terms of number and magnitude despite the relatively large main shock. Waveforms of long-and short-period teleseismicP and long-period teleseismicSH waves have been modeled to estimate the source parameters for this earthquake. The focal mechanism determined is predominately strike-slip with a small normal component (strike=263°, dip=79°, rake=194°) and is in good agreement with observations in the epicentral area regarding dislocation direction of fissures, distribution of aftershocks, and isoseismals. The average seismic moment is 5.15×1024 dyne-cm and the focal depth is estimated to be 9 km. The earthquake is characterized by a rather short source-time function and a high stress drop of approximately 380 bars. The observed pattern of the foreshock-main shock-aftershock sequence is interpreted in terms of the large stress drop associated with the main shock and the material properties at and near the source. TheP-axis orientation lies roughly NW-SE and is consistent with the maximum compressive stress observed along the coast of southeast China. This stress orientation is consistent with the NW subducting Philippine Sea plate and the SE ward push from central China due to the convergence between Indian and Eurasian subcontinents.

Collision and fan‐shaped compressional stress pattern in the Izu Block at the northern edge of the Philippine Sea Plate

The Izu block in central Japan, which is surrounded by the Nankai‐Suruga trough and the Sagami trough, is considered to be colliding with the Eurasian (EUR) plate at the northern edge of the Philippine Sea (PHS) plate. The stress state in the Izu block is investigated from focal mechanism data mainly obtained by the seismic network of the National Research Institute for Earth Science and Disaster Prevention. The predominant focal mechanisms in the central to eastern part of the Izu block indicate strike‐slip faulting with P axes oriented N–S to NW–SE and T axes oriented E–W to NE–SW, whereas focal mechanisms in the western part are strike‐slip and reverse faulting with P axes oriented N–S to NE–SW, indicating that the compressional axes remain horizontal throughout the region. The compressional stress trajectories estimated from the P axes show a fan‐shaped pattern, radiating from the northern edge of the Izu block where the collision is considered to be occurring. The fan‐shaped pattern of compressional stress trajectories is interpreted to be the result of the collision of the Izu block with EUR plate. Plane stress analysis by the finite element method supports this conclusion. The stable distribution of the T axis directions in the central to eastern part is basically interpreted as resulting from the slab pull force by the subducting PHS plate along the Sagami trough. Furthermore this model can explain the seismic quiescence in the northern part of the Suruga Bay region.

島根県隠岐，島後火山岩類のストロンチウム同位体組成の時間的変化

Fifty-three volcanic rocks from Dogo island, Oki, Shimane Prefecture, southwestern Japa were analyzed for Sr isotopic compositions with two basement rocks. The rock samples consit of talc-alkali rock suite, Nagaoda shoshonite-banakite suite, Oki trachyte-rhyolite suite, Dogo mugearite suite, Hei trachyte and Tsuzurao rhyolite series, and Daimanjiyama, Ohmine, Kuroshima, Shiroshimazaki, Saigo, and Misaki alkali basalt groups in the order of probable eruption sequence. The volcanic rocks of calc-alkali suite and shoshonite-banakite suite were produced before Japan Sea opening (ca. 15 Ma), and both have 87Sr/86Sr ratios higher than 0.7068. Long after Japan Sea opening Oki trachyte-rhyolite suite was erupted (ca. 6.6 Ma); they have rather low 87Sr/86Sr ratios (0.7066-0.7081). Mugearites followed and have similar Sr isotopic composition, whereas 4.6 Ma old Daimanjiyama basalts have clearly low 87Sr/86Sr ratios (0.7050-0.7051). The rocks erupted 3-4 Ma seem to have the lowest 87Sr/86Sr ratios: they are Ohmine, Kuroshima, Shirosimazaki alkali basalt suites (0.7044-0.7048). The 87Sr/86Sr ratios of the Saigo basalts erupted 0.84 Ma are higher than those erupted 3-4 Ma. The latest volcanic products in Dogo island, Misaki basalt suite has even higher 87Sr/86Sr ratios (0.7054-0.7057) than the Saigo basalt suite. Thus, temporal and systematic variation of Sr isotopic compositions of the volcanic rocks from Dogo can be recognized. The 87Sr/86Sr ratios of the rocks were once as high as 0.7066 or even higher than 0.708, but they started decreasing down to ca. 0.7044-0.7048 4-3 Ma ago. Since then the ratios rebounded to 0.7049-0.7055. The Hei trachyte and Tsuzurao rhyolite series are not included in this temporal and systematic change. The mantle diapir associated with Japan Sea spreading might have caused the decrease in the ratios, and either Pacific Ocean plate or Philippine Sea plate subduction may be responsible for this rebound.

The upper boundary of the Philippine sea plate beneath the western Kanto region estimated from S-P-converted wave

In the Kanto-Tokai district of central Japan, the configuration of the subducting Philippine Sea plate has been estimated using three-dimensional velocity inversions of the seismic wave velocity structure and the distribution of microearthquakes. However, it is difficult to show the configuration of the Philippine Sea plate from the distribution of microearthquakes beneath the western Kanto region, i.e. the Yamanashi and Nagano prefectures, because seismic activity is quite low there. A clear X-phase between P- and S-phases on seismograms from an earthquake is observed at temporary seismological stations located in this area. This X-phase is identified as the S to P converted wave at the upper boundary of the subducting Philippine Sea plate. Thus we have been able to deduce the shape of the Philippine Sea plate and to show that it dips northwest.

南部フォッサマグナ・フィリッピン海プレート北縁の地震活動の特徴

Recently, an idea was proposed that the eastern Japan Sea deformation zone represents a nascent convergent zone between the North American and Eurasian plates. This supposed convergent plate boundary extends southwards through the Fossa Magna, a colliding sector, to the Suruga Bay forming a triple junction near Mt. Fuji or the Kofu Basin where the three convergent boundaries among Eurasian, North American and Philippine Sea plates meet. In order to evaluate this idea, the seismic activity including focal mechanisms of large and small earthquakes in the South Fossa Magna and the northern margin of Philippine Sea plate are examined.The subducting Philippine Sea plate, descending northwestwards along the Suruga and Sagami troughs was well defined by using the hypocenters and focal mechanisms of high resolution and accuracy. However, the plate boundary passing through the inland region, the South Fossa Magna, between the Sagami and the Suruga troughs was not defined due to poor seismic activity. On the basis of available seismic data, the newly proposed convergent plate boundary passing through the South Fossa Magna could not be found.

フォッサマグナ地域の磁気的構造とキュリー点深度

Extensive magnetic surveys, which have been undertaken in the Islands of Japan and surrounding regions, have revealed marked magnetic anomalies in various regions including the Fossa Magna region. Some of the anomalies are likely to be closely related to tectonics. Magnetic data also provide valuable information on the temperature distribution in the crust through the determination of an isotherm corresponding to the so-called Curie point ; crustal rocks lose their magnetization at temperatures higher than the Curie point. In this paper, magnetic anomalies are presented with special reference to tectonics in the Fossa Magna region as well as the Curie point depth distribution there.In the Southern Fossa Magna region, a regional magnetic anomaly extends as shown in Fig. 1; a positive anomaly is located near the northern-most edge of the Philippine Sea plate and a negative anomaly extends behind the Sagami and the Suruga troughs. This anomaly can be interpreted as indicating the subduction of the oceanic Philippine Sea plate, the top of which is likely to be highly magnetized. Then the configuration of subducting plate is inferred from block models which can account for the observed magnetic anomalies, as clearly shown in Fig. 2 for some profiles.The depth to the subducting plate thus inferred seems to be somewhat shallower than that derived from the distribution of hypocenters of microearthquakes. Recently, a complicated configuration including fracture and flexure has been proposed and hence a threedimensional interpretation seems to be required to cope in a realistic way with such a complicated structure, whereas in block models a rather two-dimensional structure is implicitly assumed. In view of this defect, the discrepancy is not meaningful and we may conclude that the block models largely represent the depth to the subducting Philippine Sea plate.The Itoigawa-Shizuoka tectonic line turned out to be located at a steep gradient of Curie point depth, which is deeper in the west (see Fig. 4). This fact implies that the Itoigawa-Shizuoka line is in fact a tectonic boundary. Some tectonophysicists claim that the eastern margin of the Japan Sea is a plate boundary. As seen in Fig. 3, the Curie point depth is generally shallow there, whereas it is fairly deep at the Japan trench where the Pacific plate sinks. However, this fact would not contradict with the new hypothesis if we take into account the age of seafloor of the Japan Sea (about 20 Ma).The Curie point depth data also provide useful information on the magnitude of a possible largest earthquake. On the one hand, earthquakes are not generated at high temperature because of ductile behavior of rocks. On the other hand, the earthquake magnitude is determined by the area of rupture zone. Hence the magnitude should not be large in areas where the Curie point depth is shallow. This relation can be quantified and then the zoning of the magnitude for the largest earthquake was derived as shown in Fig. 5. The magnitude of 7.7 for the earthquake which occurred at the eastern margin of the Japan Sea exceeds the upper bound if the region is not supposed to be a plate boundary. This difficulty is removed if the relation for interplate earthquakes is applied in this region, supporting the new hypothesis that the eastern margin of the Japan Sea is a plate boundary.The electrical conductivity is also expected to provide important information on tectonic processes. Although high conductivity zones have been found in the crust beneath the Fossa Magna region, their relation to tectonics is not clear at present. In particular, the high conductivity zone beneath the Northern Fossa Magna (see Fig. 6) extends in a direction rather perpendicular to the Itoigawa-Shizuoka line. No clear conclusion seems to be derived until a systematic study is undertaken in the entire Fossa Magna region.

Crustal deformations associated with the 1986 fissure eruption of Izu-Oshima volcano, Japan, and their tectonic significance

Abstract We have investigated crustal deformations associated with the 1986 eruption of Izu-Oshima volcano, Japan, which was accompanied by an intensive fissure eruption. Two fissure crater chains, with NW-SE trend were created in the northern part of the caldera and on its northwestern flank. Their trend is consistent with the direction of compressive stress in this region. Depression of > 30 cm in the central zone including the caldera, and in the northwestern and southeastern parts in the island, was detected by precise leveling. On the other hand, uplifts up to 20 cm in the northeastern and southwestern parts were observed. Tide observations revealed that the Okada tide station, the leveling datum in Izu-Oshima, may have subsided by 5 cm after the eruption. An ∼1 m opening of fissure craters was detected by distance measurements of the baselines which cross fissure craters. Horizontal displacements obtained by reoccupation of control points showed a symmetrical pattern which was consistent with the opening of fissure craters. Anomalous strain changes were also observed in the surrounding regions—contractions were observed in the Boso and the Miura peninsula, northeast of Izu-Oshima, and extensions in the Izu peninsula. To interpret these crustal deformations, a model which consists of a nearly vertical tensile fault and a deflation source is presented. The tensile fault lies parallel to the fissures and is divided into two parts according to depth. The deeper part of the tensile fault is 12 km long, 10 km wide, and has 2 km burial depth and 2.7 m opening displacement. The shallower part, which may represent the fissure craters, is 4 km long, 2 km wide, and the amount of opening is estimated to be 1 m. However, the deflation source may be located at a depth of 10 km beneath the northwestern flank of the caldera and depression just above the source is estimated to be 30 cm. A deflation source is required to explain the subsidence at the Okada tide station and the extension in the Izu peninsula. This model suggests that the eruption might have released tensile stresses in and around the Izu region which result from bending of the subducting Philippine Sea plate.

A quantitative interpretation on possible correlations between intraplate seismic activity and interplate great earthquakes along the Nankai trough

The Tonankai earthquake occurred in 1944 along the Nankai trough between the continental Eurasian plate and the subducting Philippine Sea plate. We investigated spatial and temporal correlations between the occurrence of this event and intraplate seismic activities in the inland region of southwest Japan. In order to elucidate the possible relationship and dynamic coupling between the 1944 Tonankai and the 1946 Nankaido earthquakes, we calculated stress changes due to the Tonankai earthquake by applying a three-dimensional finite element method and compared them with post-seismic activity. Agreement between them appears quite well at a depth of ∼ 10 km, indicating that stress release in an E-W direction associated with the Tonankai earthquake may have reduced the seismic activity in the inland region. Particularly, the observed drastic decrease of seismicity in the Wakayama region just after this event can be explained by significant decrease of compressive stresses in the E-W direction. Extensive seismicity quiescence preceding the 1946 Nankaido earthquake, which occurred adjacent to the source region of the Tonankai event, may be related to the enhancement of N-S oriented compressive stress due to accelerating northward movement of the Philippine Sea plate. Therefore, in addition to the decrease of stresses in the E-W direction, the increase of the N-S oriented compressive stresses might play an important role to yield low seismic activity just after the Tonankai event. The calculated results show that the post-seismic shear stress increased and the normal stress decreased near the hypocentral area of the 1946 Nankaido earthquakes, while the shear stress decreased and normal stress increased in its surrounding area on the fault. This indicates that the Tonankai earthquake made it easier for the rupture of the Nankaido earthquake to originate at the observed hypocenter. This triggering mechanism may explain the synchronous pattern of historical great interplate earthquakes along the Nankai trough; a great earthquake occurs in the western region within a few years after the first breakage in the eastern block.

Confined ductile deformation in the Japan arc inferred from paleomagnetic studies

Miocene rocks collected from the Kanazawa and Daishoji areas in the western vicinity of the Itoigawa-Shizuoka tectonic line (ISTL) were investigated using paleomagnetic and biostratigraphic methods in order to clarify the deformation mode of a continuous granitic sliver in the southern part of the Japan arc which was bounded on east side by the ISTL. Seven sites in the Kanazawa area and nine sites in the Daishoji area were found to preserve stable characteristic remanent magnetization (ChRM) through progressive demagnetization by thermal and alternating field methods. Temporal and spatial changes in ChRM directions suggest that the areas were subjected to intra-arc tectonic deformation linked with the middle Miocene collision of the Izu-Bonin arc which succeeded to coherent clockwise rotation of the southern part of the Japan arc associated with the back-arc opening around 15 Ma. The data in this study, together with data previously obtained, indicate that the angle of relative counterclockwise rotation increases gradually toward the east, which was caused by the middle Miocene arc-arc collision. This result and the absence of identifiable faults which could compensate differential rotations, imply that the arc was ductilely deforming during the collision event. It is also suggested that significant rotation occurred in the eastern, confined, portion of the uniform sliver of continental crust. The confined ductile deformation of the crust observed in the Japan arc can be achieved by the irregular compressive force on the fore-arc side generated by landmasses on the subducting Philippine Sea plate and the plausible resisting force induced on the back-arc side.

Displacement fields due to the 1946 Nankaido earthquake in a laterally inhomogeneous structure with the subducting Philippine Sea plate—a three-dimensional finite element approach

The displacement fields associated with the 1946 Nankaido earthquake have been investigated in detail, taking into account the configuration of the Philippine Sea plate subducting beneath southwest Japan. We used a three-dimensional finite element method for the elastic problem to examine the effects of three-dimensionally inhomogeneous structure. Some differences in the displacement patterns between homogeneous and inhomogeneous media have been identified. The amounts of displacements are large in low-rigidity portions such as in the crust and mantle wedge, and are small in a high-rigidity portion composed of the subducting plate. The inhomogeneity affects horizontal displacements at the Earth's surface more than vertical displacements. If the rigidity and the Poisson ratio given here to southwest Japan are appropriate, the fault parameters of the earthquake, which have been estimated from simple dislocation models in a homogeneous semi-infinite medium, need to be modified somewhat. The present computations indicate that larger dip angles and smaller fault slips are more appropriate fault parameters when structural inhomogeneity is taken into account. This suggests that the discrepancy in the obtained total seismic moment between the geodetic and seismic models of the Nankaido earthquake could be reduced.

41B. Philippine sea plate subduction model for the genesis of the backarc low-K2O Quaternary volcanoes in Kenashi area, northern Shinetsu highland, central Japan.(Abstracts of Papers Presented at the Fall Meeting of the Society)

41B. Philippine sea plate subduction model for the genesis of the backarc low-K₂O Quaternary volcanoes in Kenashi area, northern Shinetsu highland, central Japan.(Abstracts of Papers Presented at the Fall Meeting of the Society)

Three‐dimensional fine velocity structure and hypocentral distribution of earthquakes beneath the Kanto‐Tokai District, Japan

We determine the three‐dimensional P wave velocity structure beneath the Kanto‐Tokai district, central Japan, and simultaneously relocated by inverting travel time data for 91 events that were observed at 61 stations operated by the National Research Center for Disaster Prevention. The variance of the travel time data is reduced by 75%, as compared to the variance for the routinely used laterally homogeneous model. In general, our velocity model is consistent with tectonic models of the Kanto‐Tokai district. Our velocity model is also in good agreement with seismic explosion data and Bouguer gravity anomalies. The uppermost layer (from 0 to 16 km depth) is characterized by lateral velocity differences of more than 20%. We find low‐velocity zones beneath Tokyo Bay, the middle of the Boso Peninsula, and Omae‐zaki, where there are thick sedimentary layers. A marked 30‐km‐thick high‐velocity zone extends to the northeast and the west of the Izu Peninsula. This zone dips gradually from the surface to a depth of 70 km in the eastern region and to a depth of 40 km in the western region and is interpreted as the Philippine Sea (PHS) plate, which is being subducted at the Sagami and the Suruga troughs. The data unquestionably require a high‐velocity zone beneath Mount Tsukuba that cannot be explained in terms of present tectonic models. We use our three‐dimensional velocity model to relocate about 23,000 earthquakes that took place from 1983 through 1985. The accuracy of the hypocentral parameters improved considerably. As a result, the double seismic zone within the subducting Pacific plate is more clearly delineated than in previous studies. The inclined seismic zone for the subducting PHS plate coincides fairly well with the high‐velocity zone of our three‐dimensional velocity model.

Seismic activity in the subducting Philippine Sea plate along the Suruga trough revealed by OBS observation.

A seismicity observation with 10 pop-up-type ocean bottom seismometers were conducted along the Suruga trough, south off central Japan, for about one month in 1984. This observation detected more than 300 earthquakes, among which about 60 earthquakes were located around the Suruga trough. In this observational period, seismically the most active area is the middle part of the Suruga trough. Applying station corrections, we precisely located the earthquakes around the Suruga trough. The hypocenters show a thin seismic zone dipping westward from the trough axis. The focal depths at the trough axis, 0-10 km, are shallower by 5 to 10 km than the principal focal depths of hypocenters obtained by the land-based seismic network. The hypocentral distribution by the present survey is interpreted as the seismic activity in the uppermost part of the Philippine Sea plate subducting from the Suruga trough axis. In contrast to the high seismicity at the middle part of the trough, only a small number of earthquakes were located in the northern and southern parts. The microearthquakes along the Suruga trough observed in the present survey tended to occur simultaneously in the segment over about 50 km length, synchronizing with the activation of the small swarm around the northeastern edge of the Nankai trough.