Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

remote sensing

Article Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

Huiyan Shi 1 , Tonglin Li 1,*, Rui Sun 2, Gongbo Zhang 3, Rongzhe Zhang 1 and Xinze Kang 1

1 College of Geo-Exploration Science and Technology, Jilin University, No.938 Xi Min Zhu Street, Changchun 130026, China; [email protected] (H.S.); [email protected] (R.Z.); [email protected] (X.K.) 2 CNOOC Research Institute Co., Ltd., Beijing 100028, China; [email protected] 3 State Key Laboratory of Geodesy and Earth’s Dynamics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China; [email protected] * Correspondence: [email protected]

Abstract: In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subducting high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates Citation: Shi, H.; Li, T.; Sun, R.; the cessation of the SCS spreading from the north to the south, which also implies that the northern Zhang, G.; Zhang, R.; Kang, X. part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate Insights from the P Wave Travel Time ◦ ◦ Tomography in the Upper Mantle is exhibited between 14 N and 9 N, with its subduction direction changing from westward to ◦ ◦ ◦ Beneath the Central Philippines. eastward near 13 N. In the range of 11 N–9 N, the subduction of the Sulu Sea (SS) lies on the Remote Sens. 2021, 13, 2449. https:// west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle ◦ ◦ doi.org/10.3390/rs13132449 transition zone (MTZ) between 14 N and 9 N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Academic Editor: José Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and Fernando Borges the tectonic evolution in SE Asia.

Received: 5 June 2021 Keywords: seismic tomography; upper mantle; slab tear; Philippine subduction; South China Sea; Accepted: 19 June 2021 proto-South China Sea Published: 23 June 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- iations. The Philippines are located in the collision and convergence region of the Eurasian plate, the PS plate and the Indo-Australian plate. The interaction of the three plates has created a complex tectonic environment in the area. The Philippines are surrounded by the bidirectional subduction of the PS plate and the SCS (Figure1). The Philippine subduction zone plays a critical role in the tectonic evolution of the SE Asia. The kinetic energy Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. generated by the strong convergence of the Eurasian and PS plates [1] is regulated and This article is an open access article absorbed by the bidirectional subduction system in this region [2]. distributed under the terms and The mechanism of the SCS opening dynamics and tectonic evolution are still unre- conditions of the Creative Commons solved issues. Many kinematic models have been put forward to explain the opening Attribution (CC BY) license (https:// process of the SCS [3–9]. However, there is no widely accepted model of the SCS spreading. creativecommons.org/licenses/by/ One of the major controversies in establishing the SCS opening model is which of the 4.0/). eastern and southwestern basins of the SCS opened ﬁrst. The Philippines lies on the eastern

Remote Sens. 2021, 13, 2449. https://doi.org/10.3390/rs13132449 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 17 Remote Sens. 2021, 13, 2449 2 of 17

eastern and southwestern basins of the SCS opened first. The Philippines lies on the east- boundaryern boundary of the of SCS. the SCS. The westwardThe westward subduction subduction of the of PS the plate PS beneathplate beneath the central the central Philip- pinePhilippine limited limited the eastward the eastward spreading spreading of the of SCS, the SCS, and theandSCS the SCS plate plate subducted subducted eastward east- beneathward beneath the Philippines the Philippines [10]. Previous[10]. Previous tomographic tomographic results results revealed revealed the the slab slab tear tear of theof SCSthe SCS slab slab [11,12 [11,12].]. However, However, there there were were few studiesfew studies on the on relationship the relationship between between subduction sub- andduction spreading and spreading of the SCS of the plate. SCS Therefore, plate. Therefor researche, research on deep on structuredeep structure of the of Philippine the Phil- subductionippine subduction zone will zone possibly will possibly contribute contribute to study to thestud openingy the opening of the SCS.of the SCS.

Figure 1. Tectonic framework of the central Philippine and surrounding regions. The topography Figure 1. Tectonic framework of the central Philippine and surrounding regions. The topography data are provided by GSHHG. The saw-toothed lines indicate the trench axes. The dashed lines dataindicate are providedthe collision by zone. GSHHG. The solid The saw-toothedlines indicate lines active indicate faults. The the trenchred triangles axes. Theindicate dashed the vol- lines indicatecanoes, which the collision are obtained zone. from The solidNCEI lines Volcano indicate Location active Database, faults. TheNOAA red National triangles Centers indicate for the volcanoes,Environmental which Information. are obtained from NCEI Volcano Location Database, NOAA National Centers for Environmental Information. Previous researchers have carried many studies to understand the geodynamic process Previousof the Philippine researchers subduction have carried zone many [13–16]. studies One to of understand the most important the geodynamic controversial process oftopics the Philippinein this region subduction is the PSCS. zone At [13 present,–16]. One there of thehas mostbeen importantan inconclusive controversial debate about topics inwhere this regionare the isPSCS the PSCS.slabs. Some At present, researchers there hasargued been that an the inconclusive PSCS once debate existed about in the where area arebetween the PSCS the southern slabs. Someboundary researchers of the SCS argued and Borneo that the [17,18]. PSCS Tang once et existed al. found in the500 areakm betweensoutheastward the southern high velocity boundary anomalies of the SCSbelow and northern Borneo Borneo, [17,18]. which Tang et were al. foundidentified 500 kmas southeastwardPSCS slabs [7]. Hall high et velocity al. argued anomalies that the below PSCS northernslabs were Borneo, at 800 km which between were East identiﬁed Borneo as PSCSand Southern slabs [7]. Philippines Hall et al. argued [19]. A that high-velocity the PSCS slabsanomaly were was at 800 discovered km between at 400 East km–700 Borneo andkm Southerndepth beneath Philippines the central [19]. APhilippine, high-velocity whic anomalyh was interpreted was discovered as the at PSCS 400 km–700 slab that km depthgenerated beneath by southward the central Philippine,subduction which [12]. wasHowever, interpreted some asresearchers the PSCS slabclaimed that generatedthat the by southward subduction [12]. However, some researchers claimed that the PSCS slab subducted northward to the present SCS [17,20]. According to the tomographic results,

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Shi et al. proposed that PSCS has subducted northward beneath Borneo and inferred that the Paleo-Tethys and the paleo-Pacific (PP) have never been connected by PSCS [21]. Lin et al. proposed a double-side subduction model to interpret the PSCS based on plate reconstruction [22]. To answer the above questions, a high-resolution tomographic model is needed. In this work, we selected 32,224 useful relative travel time residuals recorded by 87 receivers distributed in the central Philippines. Crustal correction was applied to remove the influence on the upper mantle. These data were adopted to image the detailed upper mantle structure in the Philippine subduction zone. Then, we present a 3-D high-resolution tomographic model in the upper mantle beneath the central Philippines. Our tomographic model clearly images the steeping and tearing of SCS slabs, which provide evidence to define the opening sequence of the SCS. The PS plate and SS slab are also imaged by the present model. The PSCS-PP slabs are also revealed by our tomographic result, which helps to determine the location of paleo-suture of PSCS and PP. The present study has significant implication for the establishment of the SCS opening mechanism and the study of the tectonic evolution of SE Asia.

2. Data and Methods 2.1. Data The range of study area is (8◦ N~18◦ N, 118◦ E~128◦ E). There are 87 receivers distributed in the study area (Figure2a). In this work, we picked travel time data recorded by these receivers from 1960 to 2020, which was primarily derived from International Seismological Center (ISC) [23]. The selection of teleseismic tomography data should satisfy the following conditions: (1) The magnitude of events is greater than 4.5; (2) The epicentral distance is 30◦–90◦, which reduces the inﬂuence of deep structures such as lower mantle; (3) Only events received by more than ﬁve receivers can be used for inversion calculation. The application of relative travel time residuals to seismic imaging is intended to eliminate the effects of teleseismic events localization errors and lateral homogeneities outside the study area. To obtain the relative travel time residuals, the mean value of each event over the whole receiver array is calculated and subtracted from the absolute travel time residuals for each receiver [24]. Finally, a total of 2921 teleseismic events and 32,224 relative travel time residuals were picked for tomographic computation (Figure2c). Figure2b shows the local seismic events occurred in the study area in recent 10 years.

2.2. Crust Correction According to the application effect of predecessors [25,26], the velocity imaging results will be affected by complex crustal structure when utilizing teleseismic tomography method to image the upper mantle structure. Some researchers have conducted different methods to solve this problem [27–29]. In this paper, the following method were utilized to eliminate the impact of crust. More details can be found in the reference [28]. (1) Selecting 1-D and 3-D crust velocity models. We selected ak135 [30] as 1D crust model, and crust 1.0 [31] as 3-D crust model, respectively. (2) Calculating travel time and travel time residuals in 1-D and 3-D crust models, respectively. In this paper, the average depth of the Moho surface of 30 km is taken as the thickness of the crust, the crust contains upper, middle and lower layers. The calculation formula is as follows: h h δT = T − T = ( l ) −( k ) (1) crust 3−D 1−D ∑ × 3−D ∑ × 1−D l cos θl Vl k cos θk Vk

where δTcrust represents the relative travel time residuals in the crust, T indicates the travel time in the crust, h indicates the thickness of each layer in the crust, θ represents the incident angle of rays at each interface and V denotes the velocity in each layer. The 3-D and 1-D subscripts indicate 3D crust model and 1-D model, respectively. Remote Sens. 2021, 13, 2449 4 of 17

(3) The real data used in the tomographic calculation are the measured travel time minus the theoretical travel time and then minus the travel time residuals in the crust. The expression of the formula is as follows:

t = (Tobs − Tcal) − δTcrust (2)

Remote Sens. 2021, 13, x FOR PEER REVIEWwhere t represents the relative travel time residuals, Tobs indicates the measured4 travelof 18

time and Tcal the theoretical travel time.

Figure 2. (a) Locations of receivers used in this work. The red invert triangles indicate receivers; (b) Locations of teleseismic Figure 2. (a) Locations of receivers used in this work. The red invert triangles indicate receivers; (b) Locations of teleseismic events picked for tomography inversion. The red star denotes the center point of the study area. The blue dots represent eventsteleseismic picked events; for tomography (c) Local seismic inversion. events The occurred red star in denotesthe study the area center in recent point 10 of years. the study Diffe area.rent colors The blue indicate dots different represent teleseismicdepths of seismic events; events. (c) Local seismic events occurred in the study area in recent 10 years. Different colors indicate different depths of seismic events. To obtain the relative travel time residuals, the mean value of each event over the whole receiver array is calculated and subtracted from the absolute travel time residuals for each receiver [24]. Finally, a total of 2921 teleseismic events and 32,224 relative travel time residuals were picked for tomographic computation (Figure 2c). Figure 2b shows the local seismic events occurred in the study area in recent 10 years.

2.2. Crust Correction According to the application effect of predecessors [25,26], the velocity imaging results will be affected by complex crustal structure when utilizing teleseismic tomography

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The method described above was applied to the ﬁltered selected teleseismic data. FigureThe3 presents method thedescribed mean valuesabove ofwas the applied picked to teleseismic the filtered data selected and the teleseismic crust-corrected data. relativeFigure 3 travel presents time the residuals mean values at each of receiver,the picked respectively. teleseismic data and the crust-corrected relative travel time residuals at each receiver, respectively.

Figure 3. Distribution map of mean value of relative travel time residuals for each receiver (a) before crustal correction (b) Figure 3. Distribution map of mean value of relative travel time residuals for each receiver (a) before crustal correction (b) after crustal correction. after crustal correction.

It can be seen that the the numerical numerical range range of of mean mean relative relative travel travel time time residuals residuals at at each each receiverreceiver isis fromfrom −22 s s to to 2 2 s. s. Whether Whether crustal crustal correction correction was was applied applied or or not, not, most most values values of of thethe meanmean relative travel time time residuals residuals in in th thee study study area area are are positive. positive. This This phenomenon phenomenon is is due to the existence of multiple subduction subduction plates plates beneath beneath the the study study area. area. When When the the rays rays passpass through the high-velocity body, body, especially especially a a subducted subducted slab, slab, the the travel travel time time of of rays rays willwill becomebecome muchmuch shorter,shorter, soso thethe relative relative travel travel time time residuals residuals at at these these regions regions are are positive. posi- Thetive. negativeThe negative values values on mean on mean relative relative travel travel time residualstime residuals in the in study the study area are area mainly are locatedmainly located in the east in the of east the subductionof the subduction zone ofzone the of SCS. the SCS. This This phenomenon phenomenon is probably is probably due todue the to subduction the subduction in the in Philippines the Philippines and asthenosphereand asthenosphere upwelling. upwelling. Thus, Thus, the travelthe travel time oftime the of rays the passingrays passing through through this region this region is longer, is longer, which which results results the negative the negative values values on the meanon the relative mean relative travel timetravel residuals. time residuals.

2.3.2.3. Methods In thisthis paper,paper, we we conducted conducted a teleseismica teleseismi tomographyc tomography routine routine to imageto image the the upper upper man- tlemantle structure structure beneath beneath the centralthe cent Philippines,ral Philippines, which which is so-calledis so-called fast fast marching marching teleseismic teleseis- tomographymic tomography (FMTT) (FMTT) [32,33 [32,33].]. This routineThis routine is composed is composed of the of forward the forward and inversion and inversion step. In thestep. forward In the forward step, the step, calculation the calculation of the travel of the time travel from time the teleseismicfrom the teleseismic event to the event bottom to ofthe the bottom local of model the local is based model on isthe based ak135 on the (Figure ak1354). (Figure FMM is4). performed FMM is performed to calculate to cal- the travelculate timethe travel of rays time from of therays bottom from the to thebotto receiversm to the in receivers the local in model the local [34,35 model]. In inversion [34,35]. step,In inversion a kind of step, subspace a kind method, of subspace is conducted method, tois conducted calculate the tovelocity calculate structure the velocity beneath struc- the receiverture beneath array. the The receiver relative array. travel The time relative residuals travel are time adopted residuals to determineare adopted the to 3-Ddetermine velocity structurethe 3-D velocity beneath structure the station beneath arrays. the Thestation initial arrays. models The forinitial inversion models are for parameterizedinversion are byparameterized a grid of nodes by a with grid tricubicof nodes B-spline with tricubic interpolation. B-spline Theinterpolation. calculation The of calculation this method of is this method is performed in spherical coordinates [32]. The Moho depth of the initial

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performedmodel was in modified spherical to coordinates 30 km, which [32]. Theaims Moho to satisfy depth the of thecondition initial modelfor using was the modiﬁed tomo- tographic 30 km, routine which [36]. aims to satisfy the condition for using the tomographic routine [36].

FigureFigure 4.4. InitialInitial 1-D1-D velocityvelocity modelmodel usedused inin computation:computation: ak135ak135 [[30].30].

3.3. Resolution Tests andand ResultsResults InIn thisthis section,section, the the checkerboard checkerboard resolution resolution tests tests are are first first conducted conducted to findto find the the optimal opti- inversionmal inversion grid intervals.grid intervals. Then, Then, the optimal the optimal grid intervals grid intervals were applied were applied in the followingin the follow- real computationing real computation and the and final the tomographic final tomographic model weremodel obtained. were obtained. Finally, Finally, we propose we propose a 3-D tomographica 3-D tomographic model model obtained obtained from the from inversion. the inversion. 3.1. Resolution Tests 3.1. Resolution Tests To qualify the resolving ability and quality of our travel time data, we completed To qualify the resolving ability and quality of our travel time data, we completed resolution tests by using checkerboard tests method. We applied a series of different grid resolution tests by using checkerboard tests method. We applied a series of different grid intervals to find the optimal inversion parameterization. The input model of checkerboard intervals to find the optimal inversion parameterization. The input model of checkerboard tests is consisting of +6% or −6% positive and negative perturbations based on the initial tests is consisting of +6% or −6% positive and negative perturbations based on the initial model generated by the ak135 model. Then, the input checkerboard model is used to model generated by the ak135 model. Then, the input checkerboard model is used to com- compute the synthetic travel time. The synthetic relative travel time residuals are used to pute the synthetic travel time. The synthetic relative travel time residuals are used to in- invert the recover checkerboard model. The spatial distribution of the receivers and events vert the recover checkerboard model. The spatial distribution of the receivers and events used in the theoretical tests are the same as that in the real observations. The optimal lateral used in the theoretical tests are the same as that in the real observations. The optimal lat- grid spaces are 0.65◦ × 0.65◦, and the optimal vertical grid interval is 40 km. Checkerboard eral grid spaces are 0.65° × 0.65°, and the optimal vertical grid interval is 40 km. Checker- resolution tests results show good recovery effect for the velocity perturbations (Figure5), whichboard demonstratesresolution tests the results observing show system good composedrecovery effect of receivers for the andvelocity teleseismic perturbations events can(Figure recover 5), which the velocity demonstrates anomaly the patterns observing very system well. Itcomposed can be seen of receivers that when and the teleseis- depth ismic less events than can 300 recover km, the the velocity velocity perturbations anomaly patterns beneath very the well. station It can arrays be seen can that be when well- resolved.the depth However, is less than the 300 velocity km, the perturbations velocity perturbations on the edge beneath of the study the station area cannot arrays be can well be recoveredwell-resolved. due toHowever, the poor the crossing velocity of perturbations ray paths. When on the edge depth of is the greater study than area 300 cannot km, thebe well velocity recovered perturbations due to onthe thepoor edge crossing of the studyof ray regionpaths. areWhen well-resolved the depth is because greater of than the enhancement300 km, the velocity of ray pathsperturbati crossing.ons on The the last edge subplot of the ofstudy Figure region5 shows are well-resolved the distribution be- ofcause rays of in the horizontal enhancement direction. of ra Ity showspaths thatcrossing. the observation The last subplot system of used Figure in this5 shows study the is effectivedistribution for studyingof rays in upper horizontal mantle direction. structures. It shows that the observation system used in this study is effective for studying upper mantle structures.

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FigureFigure 5. CheckerboardCheckerboard resolution resolution test re resultssults in horizontal direction.

3.2.3.2. Results Results AccordingAccording to to the the result result of checkerboard resolution test, an optimal grid was utilized toto parameterize parameterize the the initial initial model. model. After After several several inversion inversion calculations, calculations, we we obtained obtained a 3-D tomographictomographic model model with with a a lateral lateral resolution resolution of of 0.65° 0.65 and◦ and 0.65° 0.65 and◦ and vertical vertical resolution resolution of 40of km 40 kmbeneath beneath the central the central Philippines. Philippines. The nume Therical numerical range of range relative of relative travel time travel residu- time alsresiduals is signi isﬁcantly signiﬁcantly reduced reduced from ( from−4.5 s, (− +4.54.5 s,s) +4.5to (− s)1.4 to s, ( −+1.41.4 s) s, after +1.4 s)tomographic after tomographic inver- sioninversion (Figure (Figure 6). Both6). Bothbefore before and after and afterthe invers the inversion,ion, the values the values of the of relative the relative travel travel time residualstime residuals show showa normal a normal distribution. distribution. The present tomographic model is obtained by using the aforementioned approaches. Since the crust correction removes the influence of the complex crustal structure on the upper mantle structure, the velocity structure of the crust is not shown in our tomographic model. Figure 7 shows the velocity anomalies relative to background in different depths. Figures 8 and 9 show the slices along the different latitude.

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Remote Sens. 2021, 13, 2449 8 of 17 our tomographic model. Figure 7 shows the velocity anomalies relative to background in different depths. Figures 8 and 9 show the slices along the different latitude.

(a) (b)

Figure 6. Relative travel time residuals (a) before tomographic computation (b) after tomographic computation.

The present tomographic model is obtained by using the aforementioned approaches. Since the crust correction removes the influence of the complex crustal structure on the upper mantle structure, the velocity structure of the crust is not shown in our tomographic Figure 6. Relative travelmodel. time residuals Figure (7a )shows before thetomographic velocity anomaliescomputation relative (b) after to tomographic background computation. in different depths. Figures8 and9 show the slices along the different latitude. TheThe present tomographictomographic model model identifies identifies some some prominent prominent features. features. There areThere obvious are obviouscontinuous continuous high velocity high velocity anomalies anomalies beneath beneath the Manila the Manila trench tren andch Luzon and Luzon Island Island (white (whitedotted dotted line circled line circled area in area Figure in7 Figurea–i), which 7a–i), have which also have been also revealed been inrevealed the previous in the previousstudies [studies12,16]. [12,16]. These high These velocities high velocities are interpreted are interpreted as the as subduction the subduction of the of SCS the SCS slab slabbeneath beneath the Philippines,the Philippines, which which is consistent is consistent with with previous previous views. views. It can It can be seenbe seen that that the thesubduction subduction of the of SCSthe SCS plate plate is parallel is parallel to the to Manila the Manila trench andtrench distributed and distributed in a NS directionin a NS direction(Figure7 a–h).(Figure With 7a increasing–h). With depth,increasing the horizontal depth, the position horizontal of the position SCS subduction of the slabSCS subductionmoves from slab west moves to east, from reflecting west to that east, the reflecting SCS is subducted that the SCS from is westsubducted to east from below west the toPhilippines. east below Thethe relationshipPhilippines. betweenThe relation subductionship between plate subduction and spreading plate of and the SCSspreading will be ofdiscussed the SCS will in the be following discussed chapters. in the following chapters. TheThe present present tomographic tomographic model model reveals reveals a a prominent prominent high high velocity velocity body body at at depth depth rangerange from from 420 420 km km to to 720 720 km, km, which which is is circled circled by by red red dotted dotted lines lines (Figure (Figure 77gg–l).–l). The high velocityvelocity anomalies areare distributed distributed in in a NEa NE direction, direction, and itsand area its increasesarea increases with the with increase the increaseof depth. of The depth. present The studypresent suggests study thatsuggests these that high these velocity high anomalies velocity anomalies are the subduction are the subductionof the PSCS of slab the andPSCS the slab PP and plate. the it PP will plate. be discussed it will be in discussed the next section.in the next section.

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Figure 7. Map view of the tomographic model in the different depth. Fast anomalies are denoted by blue color, slow Figure 7. Map view of the tomographic model in the different depth. Fast anomalies are denoted by blue color, slow anomalies are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes. anomalies are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes.

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Figure 8. Nine velocity cross-sections along different latitudes. Fast anomalies are denoted by blue color, slow anomalies Figure 8. Nine velocity cross-sections along different latitudes. Fast anomalies are denoted by blue color, slow anomalies are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes. The two are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes. The two black horizontal dotted lines indicate the interfaces at 410 km and 660 km, respectively. White dots, gray dot and black blackdots horizontal indicate the dotted location lines of indicate source thefor interfacesthe earthquakes at 410 occu km andrred 660 within km, 0.25° respectively. on both Whitesides of dots, the grayprofile, dot respectively. and black dots ◦ indicateThese data the locationwere obtained of source from for the the ISC earthquakes seismic catalog occurred [23]. within 0.25 on both sides of the proﬁle, respectively. These data were obtained from the ISC seismic catalog [23].

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FigureFigure 9. 9.Nine Nine velocity velocity cross-sections cross-sections alongalong differentdifferent latitudes.latitudes. FastFast anomalies are are denoted denoted by by blue blue color, color, slow slow anomalies anomalies are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes. The two are denoted by red color. The color bar of the anomaly is located at the bottom. Red triangles indicate volcanoes. The two black horizontal dotted lines indicate the interfaces at 410 km and 660 km, respectively. White dots, gray dot and black black horizontal dotted lines indicate the interfaces at 410 km and 660 km, respectively. White dots, gray dot and black dots dots indicate the location of source for the earthquakes occurred within◦ 0.25° on both sides of the profile, respectively. indicateThese data the locationwere obtained of source from for the the ISC earthquakes seismic catalog occurred [23]. within 0.25 on both sides of the proﬁle, respectively. These data were obtained from the ISC seismic catalog [23].

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Another two features identiﬁed in the present model are PS plate and SS slab. The high velocity anomalies indicating the subduction of the SS slab and the PS plate are circled by the purple (Figure7a–f) and black (Figure7a–i) dotted lines, respectively. According to the results, it can be seen that the area of the SS slab gradually becomes larger as the depth increases. Combining the characteristics of the horizontal and vertical velocity anomalies, the deepest part of the SS slab is above the MTZ. On the contrary, the area of the high velocity anomalies representing the PS plate is gradually smaller with the increase of depth, and its position is also moved from the west side of the Philippine Trench to the east side.

4. Discussion 4.1. Subduction of South China Sea The phenomenon of slab tearing was first discovered in the study of the interaction between seafloor spreading ridges and trenches [37,38]. Dickson and Snyder proposed the concept of slab window, if the mid-ocean ridge continues to expand during the subduction to the trench, and a continuous expansion gap is formed between the two sides of the plate. This gap is called the slab window, and the phenomenon is widely distributed in the circum-Pacific subduction zone [37]. Later, it was found that the hotter, younger plates are susceptible to stretching and tearing during subduction to form plate tear zones [39–44]. The present tomographic model clearly reveals a steeping subduction of SCS slab. The SCS slab shows a near-vertical morphology along the east-west spreading profile north of 16◦ N (Figure8a–d). Especially, it shows a near-vertical subduction angle on a slice along 17.5 ◦ N (Figure8a). However, in the south of 16 ◦ N, the subduction angle of SCS slab gradually flattens out as its position moves from north to south (Figures8e–i and9a–d ). The subduction angle is only 45◦ in the southernmost slice, which is along 11.5◦ N(Figure9d ). The above phenomenon reflects the tearing of the subduction slab in the SCS, which have been also revealed by others [11,44]. A decrease of the subduction angle along an island-arc was also observed in Central America [45,46], which is important as the subduction angle has an impact on the stress field and island-arc and the earthquake distribution. According to the model of this study, the slab window is located near 16◦ N. It can be seen that a seismic blank zone is formed near the slab window (Figure2b), which is probably due to the existence of the slab window that prevents stress concentration in this region [44]. A series of volcanoes distributed along the island-arc are formed on both sides of the plate window. High velocity anomalies indicating the SCS slab are also found between 13◦ N and 11◦ N in our tomographic model, while the previous results indicated the SCS slab only existed north of 13◦ N[12]. Previous researchers have suggested that the steepening of the SCS slab was mainly associated with the collision between the Palawan microcontent block and Philippines [12]. The above factors do influence the subduction angle of SCS slab, while we argue that the more important cause of this phenomenon is the difference in the timing of the eastward subduction of the SCS. The opening of the SCS was blocked by the PS plate, which resulted in the subduction of the SCS beneath the Philippines [10]. Different scholars hold different views on the opening sequence of the SCS. Some scholars believed that the eastern sub-basin opened earlier than the southwestern sub-basin [3,8,47], while others held the opposite opinion [48,49]. On the slice along 17.5◦ N (Figure8a), the subduction depth is even down to 800 km. The depth of the SCS slab is only 450 km along a 12◦ N slice (Figure9d). It demonstrates that the subduction depth of SCS slab changes from deep to shallow from north to south. Our tomographic model indicates that the subduction of the SCS was developing from north to south. It is derived that the northern part of the SCS subducted earlier than the southern part. Furthermore, it can also be inferred that the northern part of the SCS opened earlier than the southern part. This provides evidence of deep tectonics to determine the opening sequence of the SCS. We have drawn a cartoon diagrams to illustrate the slab tearing of the SCS plate (Figure 10). The location of the slab tear is below Luzon Island, which is close to the mid-ocean ridge in the eastern sub-basin of the SCS. As the slab tearing forms a slab window, it provides a channel for the upwelling of the Remote Sens. 2021, 13, 2449 13 of 17

Remote Sens. 2021, 13, x FOR PEER REVIEWhigh temperature asthenosphere material [44]. It may also be one of the reasons13 for of the17

development of volcanic activity in the region.

FigureFigure 10. 10.Interpretation Interpretation cartoon cartoon presentingpresenting thethe slabslab tearingtearing ofof thethe South China Sea in the the Philippine Philippine subduction subduction zone. zone.

4.2.4.2. SubductionSubduction ofof Proto-SouthProto-South China Sea PreviousPrevious researchersresearchers have conducted a a large large number number of of studies studies on on the the PSCS PSCS and and formedformed differentdifferent understandingsunderstandings [[11,12,17,19,11,12,17,19,21].21]. Some Some scholars scholars believe believe that that the the PSCS PSCS onceonce existedexisted inin thethe areaarea between the southern boundary boundary of of the the SCS SCS and and Borneo Borneo [17,18]. [17,18]. TangTang etet al.al. interpreted interpreted a a 500 500 km km southeastward southeastward high high velocity velocity anomaly anomaly below below Borneo Borneo as asthe the slab slab remnant remnant of PSCS of PSCS [7]. Zahirovic [7]. Zahirovic et al. suggested et al. suggested slabs found slabs beneath found beneathnorth Borneo north Borneoand the andSCS theat a SCS depth at less a depth than less1000 than km were 1000 the km remnant were the of remnant the PSCS of [50]. the Hall PSCS et [al.50]. Hallclaimed et al. that claimed the PSCS that slabs the PSCSwere at slabs 800 werekm between at 800 km East between Borneo Eastand South Borneo Philippines and South Philippines[19]. Palawan [19 was]. Palawan classified was as a classiﬁed PP subduction as a PP accretion subduction zone, accretion which was zone, different which from was differentprevious from results previous [8]. Zhou results et al. [8 determined]. Zhou et al. the determined location of the the location paleo-suture of the of paleo-suture PSCS and ofthe PSCS PP subduction and the PP accretion subduction zone accretion based on zone the basedstudy onof the the paleogeographic study of the paleogeographic evolution of evolutionthe Late Mesozoic of the Late rocks Mesozoic in SE Asia rocks [51]. in SE Therefore, Asia [51 ].the Therefore, PSCS is basically the PSCS widely is basically recognized widely recognizedsubducted below subducted Borneo. below The Borneo. more controversial The more controversial point is whether point the is PSCS whether extended the PSCS to extendedthe east. to the east. TheThe presentpresent tomographictomographic model reveals prominent high high velocity velocity anomalies anomalies in in the the MTZMTZ (Figures(Figures8 8h–ih–i and and9 a–i).9a–i). Yumul Yumul et et al. al. have have ever ever suggestedsuggested thatthat therethere preservedpreserved anan ancientancient subductedsubducted slabslab beneathbeneath thethe central PhilippinesPhilippines [52]. [52]. The The ophiolite ophiolite belt belt in in the the Sunda Sunda continentcontinent isis consideredconsidered asas evidenceevidence ofof the extinctionextinction of the PSCS [53]. [53]. Fan Fan et et al. al. interpreted interpreted thethe highhigh velocityvelocity anomaliesanomalies revealedrevealed in the central Philippines as as the the PSCS PSCS slabs slabs [12]. [12]. The The aboveabove evidenceevidence suggestsuggest that the high velocity anomalies anomalies imaged imaged in in MTZ MTZ is is the the PSCS PSCS slab. slab. TheThe morphology morphology of of the the remnant remnant of of the the PSCS PSCS slabs slabs identiﬁed identified by by our our model model differs differs somewhat some- fromwhat that from of that predecessors of predecessors in the in study the study area [ 12area]. The [12]. high The velocityhigh velocity anomalies anomalies representing representing the PSCS slabs found in this study contain two parts, one part is between 410 km– 800 km in depth beneath the Philippine Trench in the range of 14° N–8° N (Figures 8h–i and 9a–i). This part high velocity anomalies are basically consistent with the previous

Remote Sens. 2021, 13, 2449 14 of 17

the PSCS slabs found in this study contain two parts, one part is between 410 km–800 km in depth beneath the Philippine Trench in the range of 14◦ N–8◦ N (Figures8h–i and9a–i). This part high velocity anomalies are basically consistent with the previous studies [12]. The other part is between the MTZ beneath Palawan Island (Figure9a–e), which have been partially destroyed by lava activity. Sibuet et al. have ever deﬁned the location of the paleo-suture of PSCS in south of Palawan and north Borneo [8]. We speculate that these two parts were connected at the beginning. According to the present tomographic model and other evidence, we sense that the PSCS subduction extended eastward to the Philippines. Combining the present results with previous study [8,21,51], we propose that the high velocity anomalies identiﬁed in MTZ is the eastward extension of the PSCS slabs and the PP plate. Zhao et al. suggested that on the time scale, the superposition of the subduction of PP plate and the subduction of the PSCS might play an important role for the opening of the SCS [54], which also supports the rationality of our interpretation. This view helps to determine the location of paleo-suture of PSCS-PP from Borneo to the Philippines. Lin et al. also mentioned the possibility that the PSCS extended to the Philippines [22].

4.3. Subduction of Philippine Sea and Sulu Sea Our model reveals two high-velocity anomalies subducted along the Philippine Trench (black and purple dotted line circles in Figure7a–i). One of them subducted eastward in the range of 11◦ N to 8◦ N (Figure9e–i). Based on its horizontal distribution and previous studies [12,13], we interpret this high velocity anomaly as subducted slab of SS. The other high velocity anomalies are in the range of 14◦ N–8◦ N, which are interpreted as subduction of the PS plate. It is not obviously imaged in the range of 13◦ N–11◦ N, which might be result by magma upwelling and the melting of the subduction slab. The PS plate subducted westward south of 11◦ N, while north of 11◦ N it turns to subducted eastward. Its subduction depth is much greater in the southern part than in the northern part, with the maximum subduction depth up to 600 km. This indicates that the subduction of the PS plate along the Philippine trench occurs from the south to the north. The subducting slab of both the SS slab and the PS plate are above the PSCS slab, which demonstrates that the subduction of the PSCS predates the SS and the PS plate.

5. Conclusions In this study, by applying FMTT program based on teleseismic travel time data derived from ISC receivers, we present a high-resolution 3-D tomographic model, which imaged detailed upper mantle structure beneath the central Philippines. Considering the results of teleseismic tomography, the main scientific problems in the study area are studied in detail. The present tomographic model identifies high velocity anomalies beneath the Manila trench, which is interpreted as the subduction of SCS slab. The steeping and tearing of SCS slab are clearly imaged by our tomographic model. The subduction angle of the SCS slab changes from steep to gentle from north to south, and the depth of subduction decreases from the deepest 800 km to about 450 km. This indicates that the subduction at the eastern boundary of the SCS developed from the north to the south, and it can also be inferred that the SCS opened earlier in the north than in the south. Our tomographic model images high velocity anomalies representing the SCS slab between 13◦ N and 11◦ N, which is different from previous study. The most prominent high velocity anomalies revealed by the present tomographic model are in the MTZ between 14◦ N–8◦ N, which is interpreted as the PSCS slab and PP slab. It extends the location of the paleo-suture of PSCS-PP eastward from Borneo to the Philippines, which should be considered in the study of mechanism of the SCS and tectonic evolution in SE Asia. The PS plate and SS slab are also identified in the present tomographic model. The PS plate subducted at 14◦ N–8◦ N, which subducted westward to the north of 13◦ N and eastward to the south of 11◦ N. The subduction depth in the south is greater than that in the north. The deepest point is about 650 km below the MTZ. The disappearance of the PS subduction plate between 13◦ N and 11◦ N might be due to mantle melting and the SS slab subducted at 11◦ N–8◦ N. Remote Sens. 2021, 13, 2449 15 of 17

Author Contributions: Conceptualization, T.L.; methodology, H.S.; software, H.S.; validation, H.S. and T.L.; formal analysis, R.Z. and X.K.; investigation, H.S. and R.Z.; resources, R.S.; data curation, H.S.; writing—original draft preparation, H.S.; writing—review and editing, T.L.; visualization, H.S.; supervision, G.Z. and T.L.; project administration, R.S. and T.L.; funding acquisition, T.L. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the Key Technologies Research and Development Program [grant number 2017YFC0601600]; National Major Science and Technology Projects of China [grant number 2016ZX05026-007-001]; and the Graduate Innovation Research Project of Jilin University [grant number 101832020CX227]; China Postdoctoral Science Foundation [grant number 2020TQ0114]. Data Availability Statement: The data presented in this study are openly available in International Seismological Centre, On-line Bulletin at https://doi.org/10.31905/D808B830 (accessed on 1 May 2021), reference number [23]. Acknowledgments: We thank Rawlinson for allowing us to use his FMTT program in this work. We also thank the ISC for providing data used in this work [23,55,56]. Most of the figures in the paper were generated by the GMT software package distributed by Wessel and Smith [57]. Conflicts of Interest: The authors declare no conflict of interest.

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