Crust and upper mantle structure beneath the Yellow Sea, the East China Sea, the Japan Sea, and the Philippine Sea

ABSTRACT A new 3D S-velocity model for the crust and upper mantle beneath the Yellow Sea, the East China Sea, the Japan Sea, and the Philippine Sea is determined by means of Rayleigh-wave analysis for depths ranging from 0 to 400 km, and the most conspicuous features of the earth structure in this region are revealed from this model. In the depth range from 5 to 30 km, the S-velocity is principally affected by the thickness of the crust. In the areas with thin crust (oceanic crust), the highest S-velocity values are determined, while the lower S-velocity values are shown for the areas with a transitional crust. For the Japan Sea, the decrease observed in crustal thickness towards the north can be the result of the back-arc spreading that formed this sea from 32 to 10 Ma. Thus, from the four models proposed to explain the oceanic structure of this sea, the model supported by the results determined in the present study is the back-arc spreading model. For the Philippine Sea, the difference in the type of crust determined for the western part and the eastern-northeastern part is consistent with the two different theories, proposed to explain the origins of both parts of this sea. In the depth range from 30 to 60 km, the western part of the Philippine Sea shows higher S-velocity values than the eastern part, because the age of western part of this sea is greater than that of eastern part. The S-velocity difference of 0.2 km/s determined between both parts of this sea implies that the temperature difference within the lithosphere may reach ~370°C. For the western part of this sea, the controversy between the previous different lithospheric-thickness determinations is solved in the present study determining a lithosphere thickness of 90 km. The Japan Sea, the East China Sea, and Okinawa Trough are characterized by thin lithosphere, thick asthenosphere, and low S-velocities. These results and other evidence suggest that from the two models proposed to explain the formation of East Asian rifting system, the model of the back-arc spreading is the most realistic model. The asthenosphere beneath all study area has been precisely located and mapped in S-velocity, for the first time. The Pacific and Philippine Sea slabs and their corresponding mantle wedges above the slabs are also mapped with S-velocities.


Introduction
The sea area between East Asia and the western Pacific, the Yellow Sea, the East China Sea, the Japan Sea, and the Philippine Sea, is a very interesting area in the framework of Asian-Pacific tectonics because the formation and geologic evolution of this sea area are closely related to the tectonics of the Eurasian and Pacific plates. This area consists of different tectonic units made by the strong interaction among plates, which include continental and transitional tectonic units and oceanic basins (Okabe et al. 2004;Huang and Zhao 2006;Li et al. 2014;Wang et al. 2017). Obviously, a strong lateral heterogeneity of the crust and upper-mantle structure is expected for this complex area. This interesting area has been the subject of many seismic velocity studies of the crust and upper mantle structure, from which the analysis of surface waves have been performed in the more recent studies (Zheng et al. 2000;Zhu et al. 2002aZhu et al. , 2002bOkabe et al. 2004;Huang et al. 2009;Bourova et al. 2010;Cho et al. 2011;Huang and Xu 2011;Li et al. 2013;Schaeffer and Lebedev 2013;Tang and Zheng 2013;Legendre et al. 2014Legendre et al. , 2015Pandey et al. 2014;Pasyanos et al. 2014). However, the crust and upper mantle structure determined in these studies is not sufficient yet because most of these studies are global and large-scale tomographies, in which the grid spacing is too large to show fine details for the relatively small area considered in the present study. Other local and smallscale studies (which have a good resolution, i.e., a grid spacing relatively short to give fine details of the Earth's structure) are limited to the crust (even shallow crust) or very shallow upper mantle. Thus, this paper presents a necessary new 3D S-velocity model with the maximum resolution and depth possible (from 0 to 400 km-depth), which will be achieved considering the path-coverage of Rayleigh waves within (and near) the study area and the widest period range possible (from 5 to 200 s). This new model joint to those determined for the South China Sea (Corchete 2021) and the Okhotsk Sea, Kuril basin and the Kuril Islands basin (Corchete 2022), will be very important tools that can help to understand the tectonics, the formation, the geologic evolution, and the geodynamics of the giant block of Neozoic marginal basins called Cenozoic marginal sea basin (with ~8000 km-length and 2500-4000 km-width) that comprises from the Sea of Japan, North China -South China and South China Sea to Philippine Sea, which is fundamental to know that for the Eurasian and Pacific plates. Both models jointly will consolidate the knowledge of the elastic structure of the earth for this block, through the S-wave velocity determinations.

Geological setting
The Japan Sea (JS), The East China Sea (ECS), and the Okinawa Trough (OT) are part of a major area of Neozoic marginal basins (Figure 1), which is a block of the Eurasian marginal seas called the Cenozoic marginal sea basin (Zhu et al. 2002b;Zhu 2007). This block is formed by the Cenozoic basins (Zhu 2007): Sea of Okhotsk, Kuril Islands basin, JS, ECS, OT, South China Sea, Sulu Sea, and Celebes Sea; and is characterized by thin lithosphere (50-80 km-thick) and thick asthenosphere (160-200 km-thick). This block has a giant lowvelocity belt in the upper mantle with ~8000 km-length and 2500-4000 km-width from the Sea of Okhotsk, through the Kuril basin, JS, South China, and South China Sea to the Philippine Sea (PS, Figure 1, Zhu et al. 2002b). This low-velocity belt is associated with a giant rifting system called the East Asian rifting system, which is evidenced by many geological and geophysical characteristics (e.g. Zhu et al. 2002aZhu et al. , 2002bZhu 2007;Li et al. 2014). The formation of this rifting system can be explained by two different models or theories (Zhu et al. 2002b): (a) the Pacific plate subducted beneath East Asia and caused extension to form the rifts, basins, trenches, and arc system (this model is called back-arc extension assumption); (b) the Indian plate collided with Eurasia causing that the East Asia was dispersed towards East to form the rifts, marginal seas, and basins (this model is called extrusion assumption). From these two models, the back-arc extension model (a) has received more emphasis than the extrusion model (b), because the acceptance of model (a) means that the eastern part of East Asia was broken up and with asthenosphere upwelling the lithosphere was thinning and extending, to form the continental rift system (Zhu et al. 2002b). This hypothesis is supported by the giant upper mantle low-velocity zone and the lithosphere thinning found in East Asia and the West Pacific marginal seas (Zhu et al. 2002b;Zhu 2007).
The JS comprises three major basins (Yoon et al. 2014;Legendre et al. 2016): the Japan Basin in the northeast, the Yamato Basin in the southeast, and the Ulleung Basin in the southwest. The crustal thickness changes drastically for these three basins (Zhu 2007). The JS is one of the most interesting marginal seas to study because its oceanic structure has not been satisfactorily explained up to now. To explain this oceanic structure, four different models (or theories) have been proposed: (a) the oceanization of continental structure, (b) the loss of the upper low-density layer of continental crust by erosion and uplift (Gorai 1968;Minato and Hunahashi 1970); (c) back-arc spreading and creation of oceanic crust (Murauchi and Den 1966;Karig 1970Karig , 1971aKarig , 1971b; and (d) a trapped ocean (Shor 1964;Vasilkovsky et al. 1971;Ben Avraham and Uyeda 1973;Scholl et al. 1975). From these four models, the back-arc spreading (c) and the trapped ocean (d) models have received more emphasis than the oceanization (a) and erosion (b) models, by intercomparison of geological and geophysical features with many other island arc and marginal basin systems (Watanabe et al. 1977;Tanaka et al. 2004). The OT is a spreading belt with fracture, strong seismic and volcanic activities (Zheng et al. 2000). The OT is the youngest back arc pull-apart basin existing in the western Pacific Ocean (PO, Figure 1). This basin is of great importance to study the geological processes of formation of the marginal seas. The heat flow for this region is higher than the surrounding areas (Watanabe et al. 1970(Watanabe et al. , 1977, which could be associated with its transitional crustal structure, i.e., a thickened crust between oceanic and continental crust (Zheng et al. 2000). The transitional crust is also a feature of the Yellow Sea (YS, Figure 1) and the ECS, as is well known (Zhu et al. 2002a). For the ECS, the heat flow map (Lin et al. 2005) and the crustal thickness calculated from magnetic data Yang et al. 2016;Zhu et al. 2019) confirm its transitional crustal structure (Suo et al. 2015). Hao et al. (2006) determined the Moho depth between 27 and 28 km for the YS and the ECS regions, and ~20 km for the OT.
The PS is completely bounded by island arcs and trench systems (Kirillova 1993;Pubellier and Morley 2014). The complex nature of the tectonics in this region is evidenced by the complexity of the heat flow distribution determined by Watanabe et al. (1970). However and in spite of this complex tectonics, the Rayleigh-wave dispersion analysis performed in this study can provide crucial information about this interesting region (e.g., the type of crust of the PS and its structure), to understand the origin of the PS and the role of this region in the global tectonics of the Eurasian and Pacific areas. With respect to the oceanic structure of the PS, different and conflicting models (or theories) have been proposed to explain this structure, but the seismic wave studies suggest that two theories (models) are the most probable or reasonable (Louden 1980): (a) an older oceanic crust trapped by the initiation or relative motion of a subducting plate boundary, (b) a new crust formed in an extensional regime behind the island arc, which may be caused by the subduction of the oceanic plate. The former has been used to explain the origins of the older western part of the PS (Uyeda and Ben-Avraham 1972), and the latter has been used for the younger eastern part of this sea (Karig 1971a). Other information different from the seismic data, remnament magnetic inclinations of Deep Sea Drilling Project (DSDP) cores, heat flow measurements, and magnetic anomaly identifications (Watanabe et al. 1970;Louden 1980;Wu et al. 2016), have also provided valuable help to characterize and understand this interesting structure.

Data, methodology, and results
The seismic data used in this study were provided by the Incorporated Research Institutions for Seismology (IRIS): 101 events within (and nearby) the study area were selected from the earthquake catalogue, which were registered by 36 seismic stations. The traces corresponding to these events were analysed, as described by Corchete (2022), to calculate the regionalization and inversion of the Rayleigh-wave dispersion of the fundamental-mode group velocities for 324 source-station paths (Supplements 1a, 1b, and 1c), with periods ranging from 5 to 200 s (Supplement 1d). Figure 2 and Supplement 2 show the output of this analysis: the 3D S-velocity model (from 0 to 400 km depth) as 2D S-velocity maps versus depth. The 1-sigma errors and the resolution, corresponding to these S-velocities, are shown in Supplements 3 and 4, respectively. Figure 3 shows the principal discontinuities in depth determined beneath the study area, from the 3D S-velocity model shown in Figure 2 and Supplement 2. In Figure 3, the 0km depth (i.e., the reference surface) is considered as the bottom of the sea for representation purposes (Corchete 2018).

Depth range: 0-5 km
In Supplement 2 and for this depth range, the S-velocity values draw a picture of the sedimentary deposits present in the study area, with the lower S-velocity values (~2.2 km/s) associated with the thicker sedimentary deposits . The thickness of sediment-fill in the trenches is different among trenches depending on the proximity of the sediment source, e.g., for the Izu-Bonin Trench, the sediments are derived from the near island arc (Kirillova 1993). The S-velocity shows lower values for the trenches in which thicker sedimentary deposits are present, e.g., the low S-velocity values (~2.2 km/s) shown for the Mariana island arc are associated with the presence of a sedimentary cover of 3 km thick (Kirillova 1993).

Depth range: 5-30 km
The S-velocity (Supplement 2) is strongly affected by thickness of the crust, as is expected. The S-velocity values shown at the northeast of the JS, in the Japan Basin (Yoon et al. 2014;Xu et al. 2016), and at the southwest of the PS are higher than those shown for the other regions of the study area, because of the thin crust (oceanic crust) that exists in these areas (Figure 3(a)). For the JS (Figure 1), Figure 3(a) shows that the crust is thicker in the south of JS (~26 km-thick) than the north of JS (~16-18 km-thick): the Japan Basin (Yoon et al. 2014). This decreasing in crustal thickness towards the north can be the result of the tectonic history of the JS, i. e., it could be the result of the back-arc spreading that formed the JS from 32 to 10 Ma (Tamaki 1992;Pouclet et al. 1994;Choi et al. 2013;Zahirovic et al. 2013Zahirovic et al. , 2014Yoon et al. 2014). Thus, from the four models proposed to explain the oceanic structure of the JS, the model supported by the results determined in the present study is the back-arc spreading model. The Moho map of Figure 3(a) shows, in general, a good agreement with that calculated by others authors (Teng et al. 2002;Gao and Jin 2003;Li et al. 2014;Gozzard et al. 2018), even with the crustal thickness calculated by Sato et al. (2004) for the Japan Basin and by Kurashimo et al. (1996) for the Yamato Rise. Also, there is a good agreement with the crustal thickness calculated by Lin et al. (2005) and Yang et al. (2008) for the ECS continental shelf, and with the Moho depth determined by Hao et al. (2006) for the YS, the ECS and the OT. In Figure 3(a), the Moho depth for the western ECS is similar to that determined by Xuan et al. (2020) for the nearby China continent, which suggests that this marginal sea is resulted from extension of the Eurasian continent (Xu et al. 2014;Suo et al. 2015). Respect to the shallower Moho shown in Figure 3 the OT (22 km-depth), it implies that its crustal structure may be considered as a crustal rifting structure (Xuan et al. 2020). This Moho in the OT, clearly shallower than in the western ECS, confirms the difference in rifting history for both regions (Xu et al. 2014). The OT is considered a recent rifting structure, as it has been revealed in geological and geodetic studies (Kimura 1985;Yu and Chow 1997;Kato et al. 1998;Sella et al. 2002). The lower S-velocity values shown for this region in Figure 2 are expected for a transitional crust (i. e. a type crust between oceanic and continental crust), where the heat flow is higher than its surroundings areas (Watanabe et al. 1970(Watanabe et al. , 1977. The YS and the ECS appear also imaged with low S-velocities in Figure 2, because these regions are also characterized by a transitional crustal structure. However, higher S-velocities are determined for the south of the YS, compared with those determined for the northern part of the YS and for the ECS continental shelf (at the west of the ECS), which may be associated to basement uplift regions Wang et al. 2017). Legendre et al. (2014) determined higher Rayleigh-wave phase velocities in the same area, for periods of 20 and 30s. For the PS, the S-velocities (Figure 2) are higher in the western part than those in the eastern part, and the crust is thinner in the western part (Figure 3(a)), i. e., the western part of the PS shows a normal oceanic structure. The eastern and northeastern part of the PS show a thickened crust (Figure 3(a)), particularly in the ridges and their surrounding area. This thickened crust suggests a continental type of crust for this region, i. e., that the crust of the PS can be classified as an intermediate or transitional crust (a type of crust between oceanic and continental crust). The difference in the type of crust determined in the present study, for both regions of the PS (the western part and the eastern and northeastern parts), is consistent with the two different theories (or models) proposed to explain the origins of the older western part and the younger eastern part of the PS (Louden 1980). The low S-velocities ( Figure 2) determined in the present study for the eastern and northeastern part of the PS are also observed by Huang and Zhao (2006).

Depth range: 30-60 km
The western part of the PS shows higher S-velocity values than the eastern part ( Figure 2) because the PS was formed by back-arc spreading. Then, the age of western part of the PS is greater than that of eastern part (Zheng et al. 2000). The high S-velocity determined for the western part of the PS (Figure 2) is characteristic  Figure 3(c) to show the high S-velocity zone associated to slabs, in which no low-velocity channel (asthenosphere) can exist and no ABB can be defined.
of an older oceanic basin (Huang et al. 2009;Huang and Xu 2011). This region is a Cenozoic sea basin formed during the Eocene-Early Oligocene (Huang et al. 2009), and it can be considered as a relatively old and stable sea basin, in which the lithosphere is cooled and thickened. The heat flow determined by Watanabe et al. (1970) for the PS also tallies with this feature. This heat flow measured in the eastern part of the PS is higher than in the western part because the older lithosphere became cooler than another younger one. A higher heat flow can be associated with a partially melted mantle (asthenosphere), which involves lower seismic velocities. The S-velocity is affected by temperature-pressure condition in mantle, as well as its chemical composition. If an S-velocity difference of 0.2 km/s is determined between the eastern part and the western part of the PS (Figure 2), considering a Vp/Vs ratio of 1.83 for the upper mantle (Zhu et al. 2002b), a P-velocity difference of 0.366 km/s is calculated for the PS. This means that the temperature difference within the lithosphere may reach ~370°C because a temperature increase of 100°C causes a P-velocity decrease of 0.1 km/s according to laboratory experiments (Zhu et al. 2002b). On the other hand, the PS plate shows the highest S-velocity values of the study area (Figure 2), which indicates that the lithosphere characteristics of this oceanic plate are remarkably different from the other marginal seas, such as the JS, the ECS, and the YS.

Depth range: 60-260 km
The asthenosphere is found as a low S-velocity channel extended for all study area between the surfaces: lithosphere-asthenosphere boundary (LAB, Figure 3(b)) and asthenosphere base boundary (ABB, Figure 3(c)). The asthenosphere cannot be defined (or determined) for areas in which are present slabs, because this lowvelocity channel cannot defined in the high S-velocity area that characterizes a slab (e.g. Figure 4(a)). The JS, ECS and OT are characterized by thin lithosphere (Figure 3(b)), thick asthenosphere (Figure 3(b) and 3(c)), and low S-velocities ( Figure 2); as expected for Cenozoic basins (Zhu et al. 2002b;Zhu 2007). These results and the evidences from geology, petrology and geochemistry; suggest that from the two models proposed to explain the formation of East Asian rifting system (Zhu et al. 2002b), the model of the back-arc spreading (back-arc extension assumption) is the most realistic model. This giant rifting system is the result of the lithospheric deformation with extension since mid and late Mesozoic (Zhu et al. 2002b), and the present eastern part of East Asia and West Pacific marginal seas, the system of trench and the island arcs are the results of subduction of Pacific plate towards Eurasian plate. For the ECS, a prominent low S-velocity zone determined from 60 to 220 km-depth (Supplement 2 and 4a), evidences the lithosphere extension and mantle activity occurred in Mesozoic: rifting, volcanism, magmatism, mantle upwelling, and lithosphere thinning of the eastern China (Ren et al. 2002;Xu et al. 2008). In the present, the mantle activity beneath the ECS basin is affected by the westward subduction of the PS plate (Figure 4(a)). Legendre et al. (2014) determined very slow Rayleighwave velocities beneath the southern part of the ECS, which they related to the presence of fluids coming from the dehydration of the subducting slab (Figure 4(a)). The lower part of the ECS lithosphere is probably eroded by the thermal-chemical effect due to this subduction (Huang et al. 2009), which is probably the cause of the ECS lithosphere thinning towards the OT (Figure 3(b) and 4(a)). This conspicuous feature tallies with the lowvelocity zone (between 50 and 100 km-depth) determined by Cho et al. (2011), for the region between Korea and Taiwan. For the OT, Huang and Zhao (2006) determined a strong low-velocity zone down to ~200 km-depth. This conspicuous low-velocity is also observed in the present S-velocities but down to 220 km (Figure 2 and 4(a)), which provide the geophysical evidence of the active back-arc spreading of the OT (Kimura 1985;Huang et al. 2009). These low S-velocities are also associated to the high heat flow determined for this region, the active seismic, the volcanic activities and the high conductivity measured in the upper mantle for the OT (Kaneko and Honura 1987;Yamano et al. 1989;Hao et al. 2004). For the, Zheng et al. (2000) and Huang et al. (2009) determined a lithosphere thickness of 80 km, which coincides with that determined in the present study (Figure 3(b)). For the JS, a thick low S-velocity channel (the asthenosphere) is determined in the upper mantle from 60 to 260 km-depth ( Figure 2, 3(b) and 3(c)), which tallies with that determined by Zheng et al. (2000). This low S-velocity layer is determined in the mantle wedge above the Pacific and PS slabs (Figure 4(b) and 4(c)), which can reflect dehydrated fluids ascending from both slabs (Zhao 2017;Ma et al. 2019), indicating also that large-scale and high-temperature meltedmantle material may exist in the upper mantle. This conclusion is supported by Zhu et al. (2002b) in their temperature-pressure curves as a function with depth, in which the melting point for the mantle materials is reached at 60 km-depth in East Asia and West Pacific marginal seas. For the PS, the asthenosphere is determined from 70-90 to 240 km-depth ( Figure 2, 3(b) and 3 (c)). Louden (1980) determined a lithospheric thickness in the range of 40-60 km for the Parece Vela and West Philippine basins, from heat flow measurements. This lithospheric thickness is in discrepancy with that determined by Seekins and Teng (1977), from Rayleigh-wave analysis, which is in the range of 25-85 km thick. This discrepancy between thermal and seismic lithospheric thickness in the PS plate can be solved by taking into account an average lithospheric thickness of 50-60 km, which could be compatible with both studies. However, Huang et al. (2009) determined a lithosphere thickness of ~100 km, for the western part of the PS. This apparent controversy is solved in the present study, in which a lithosphere thickness of 90 km is determined for this region (Figure 3(b)), in agreement with that determined by Pandey et al. (2014), who found a value less than 100 km thick.

Depth range: 260-400 km
The S-velocity values increase notably with respect to those shown in the previous depth range, reaching the highest values determined for all study areas (Figure 2). The most conspicuous features are the high S-velocity areas associated with the subducting slabs (Pacific and PS). The Pacific plate is subducting beneath the Eurasian plate along the largest trench-arc-back-arc system on Earth; and in the eastern part of the present study area, the 3-D upper-mantle structure of this western Pacific subduction has been mapped in S-velocities (Figure 2) along the Japan trench, the Izu-Bonin trench, the Mariana Trench (Figure 1). The dipping angle of this Pacific slab becomes gradually greater southwards from the Japan trench to the Mariana Trench (Huang and Zhao 2006;Wu et al. 2016): 28°-31° for the Japan trench, 31°-50° for the Izu-Bonin trench and ~90° for the Mariana Trench, and the changes of this angle are associated with the different eastward extension of the high S-velocity area (corresponding to the slab) with the increasing depth, the areas with smaller angle (e.g. the Japan trench area) show a greater eastward S-velocity gradient versus depth (Figure 2, Supplement 2).

Conclusions
The 3D S-velocity model determined in the present study through Rayleigh-wave analysis for the YS, the ECS, the JS, and the PS reflects the structure of the lithosphere-asthenosphere system from 0 to 400 kmdepth. The most conspicuous features revealed from this model will be summarized as follows. 1) In the depth range from 0 to 5 km, the S-velocity map draws a picture in low values of the thicker sedimentary deposits present in the study area.
2) In the depth range from 5 to 30 km, the S-velocity is strongly affected by the thickness of the crust. In the areas with thin crust (oceanic crust), the northeast of the JS (the Japan Basin) and the southwest of the PS, the highest S-velocity values are determined. The lower S-velocity values are shown for the areas with a transitional crust: the YS, the ECS, the OT, and the eastern and northeastern parts of the PS. However, higher S-velocities are determined for the south of the YS, compared with those determined for the northern part of the YS and for the ECS continental shelf, which may be associated with basement uplift regions. For the JS, the decrease observed in crustal thickness towards the north can be the result of the back-arc spreading that formed the JS from 32 to 10 Ma. Thus, from the four models proposed to explain the oceanic structure of the JS, the model supported by the results determined in the present study is the back-arc spreading model. For the ECS, the similarity between the Moho depth determined for the western ECS and that determined for the nearby China continent, suggests that the ECS is resulted from extension of the Eurasian continent. For the OT, its shallower Moho implies that its crustal structure may be considered as a crustal rifting structure. For the PS, the difference in the type of crust determined for both regions of the PS (the western part and the eastern-northeastern part), is consistent with the two different theories (or models) proposed to explain the origins of both parts of the PS.
3) In the depth range from 30 to 60 km, the western part of the PS shows higher S-velocity values than the eastern part, because the PS was formed by back-arc spreading, i.e., the age of the western part of the PS is greater than that of the eastern part. The high S-velocity determined for the western part of the PS is a characteristic of a relatively old and stable sea basin, in which the lithosphere is cooled and thickened. The S-velocity difference of 0.2 km/s determined between the eastern part and the western part of the PS implies that the temperature difference within the lithosphere may reach ~370°C. On the other hand, the PS plate shows the highest S-velocity values of the study area, which indicates that the lithosphere characteristics of this oceanic plate are remarkably different to the other marginal seas, such as the JS, the ECS, and the YS. 4) In the depth range from 60 to 260 km, the asthenosphere is found to be a low S-velocity channel extended for all study areas between the LAB and the ABB surfaces. The JS, ECS, and OT are characterized by thin lithosphere, thick asthenosphere, and low S-velocities. These results and other evidence suggest that from the two models proposed to explain the formation of East Asian rifting system, the model of the back-arc spreading is the most realistic model. For the ECS, a prominent low S-velocity zone, determined from 60 to 220 km-depth, evidences the lithosphere extension and mantle activity occurred in Mesozoic. The ECS lithosphere thinning towards the OT may be associated with the erosion of the lower part of the ECS, by the thermal-chemical effect due to the westward subduction of the PS plate. For the OT, the low S-velocities provide the geophysical evidence of the active back-arc spreading of the OT, which is also associated with the high heat flow determined for this region, the active seismic, the volcanic activities, and the high conductivity measured in the upper mantle. For the JS, a thick low S-velocity layer is determined in the mantle wedge above the Pacific and PS slabs, which can reflect dehydrated fluids ascending from both slabs, indicating also that large-scale and high-temperature melted-mantle materials may exist in the upper mantle. For the western part of the PS, the controversy between the previous different lithosphericthickness determinations is solved in the present study determining a lithosphere thickness of 90 km. 5) In the depth range from 260 to 400 km, the most conspicuous features are the high S-velocity areas associated with the subducting slabs (Pacific and PS). For the Pacific slab, their associated high S-velocity areas extend eastwards with the increasing depth, and the areas with smaller dipping angle show a greater eastward S-velocity gradient versus depth.