Tectonophysics 633 (2014) 176–192

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Tectonophysics

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P- and S-wave tomography of the Hainan and surrounding regions:Insight into the Hainan plume

Jinli Huang a,b,⁎a Key Laboratory of Geo-detection (China University of Geosciences, Beijing), Ministry of Education, 100083, Chinab School of Geophysics and Information Technology, China University of Geosciences, Beijing, 100083, China

⁎ Tel.: +86 10 82321251.E-mail address: huangjl@cugb.edu.cn.

http://dx.doi.org/10.1016/j.tecto.2014.07.0070040-1951/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 November 2013Received in revised form 17 June 2014Accepted 2 July 2014Available online 12 July 2014

Keywords:Hainan plumeTeleseismic tomographyP- and S-wave velocitiesUpper mantle and transition zonesDynamics

High-resolution 3-D P- and S-wave velocity structures beneath Hainan and surrounding regions down to a depthof 700 km are determined using teleseismic tomographic inversion of 10,873 P and 9147 S relative arrival timeresiduals from 165 teleseismic events recorded by 88 permanent seismic stations belonging to three regionalseismic networks of Hainan, Guangdong and Guangxi. The present tomographic models show that a low-velocity (low-V) anomaly extends below the transition zone (TZ) in and around the Hainan hotspot. The low-V zone does not exhibit a simple vertical columnar shape but a complex and deflected image toward the north-east. At depths of 13 to 170 km, themain low-V anomaly exists right beneath theHainan hotspot, but in thedepthrange of 170 to 700 km, the low-V anomaly gradually deflects to the coastal area of Guangdong, which is to thenortheast of the Hainan hotspot, an area with a diameter of approximately 200 km. I think that this area may bethe extent of the Hainan plume from the deep upper mantle to the bottom of the TZ. On the basis of the presenttomographic models, combined with the receiver function and geochemical results, I believe that the Hainanplume may originate from the low mantle, and is characterized by a high-temperature anomaly.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hainan and adjacent regions are located in the intersection area ofthe Eurasian, Indo-Australian and Pacific–Philippine Sea plates (Fig. 1).The collision of the Indian andEurasianplates, thewestward subductionof the Philippine Sea and Pacific plates and the northwardmovement ofthe Indo-Australian plate have significantly impacted the tectonic evo-lution of this region. The Hainan hotspot is situated at Hainan Island,which is in the southernmost portion of South China Fold Belt (SCFB)and is separated from mainland China by the Qiongzhou strait. Hainanvolcanic activity began in the early Tertiary and continued to the Holo-cene. During these periods, the Hainan volcano erupted 59 times in 10phases in northeast Hainan Island and Leizhou Peninsula, and it formed177 craters (Liu, 2000). The basaltic lava flows were widely distributedand occupied an area of 7000 km2 (Ho et al., 2000; Liu, 1999, 2000).

The petrology findings showed a large quantity ofwidespread Ceno-zoic alkali basalts in this area, which display the geochemical character-istics of ocean island basalt (OIB) and contain high-magnesian olivinephenocrysts. These features suggest that their origin is deep in theman-tle (Flower et al., 1992; Ho et al., 2000; Niu and O' Hara, 2003; Tu et al.,1991; Xu et al., 2012; Yan and Shi, 2007). Maruyama (1994) deduced

that the plume might exist under the lithosphere of the southernSouth China Sea. Gong and Li (1997) noted that local convection causedby the plume and large-area stretching resulted in frictionwith the bot-tom of the rock layer, whichmay explain the geological phenomenon ofthose basins' extension in different directions and different structuralparts during the early Tertiary. The S-wave velocity model of SoutheastAsia (Lebedev and Nolet, 2003) revealed that a low-V anomaly extendsto a depth of 600 km in the coastal area of Guangdong. Lebedev et al.(2000) first proposed the existence of the Hainan plume based on thetomographic model.

In recent years, multiscale seismic tomographic models have shownimages of several mantle plumes at different depth ranges (Allen et al.,2002; Antolik et al., 2003; Bijwaard and Spakman, 1999; Hung et al.,2004; Lei et al., 2009; Nataf, 2000; Ritsema et al., 1999; Yang et al.,2006; Zhao, 2001). Early models mainly showed the Hawaii, Iceland,South Pacific and East Africa plumes. With the improvement of tomo-graphic resolution, more plumes were detected. The finite-frequencyglobal tomographic models revealed 35 hotspots worldwide, includingthe Hainan plume (Montelli et al., 2004, 2006). These P- and S-wave ve-locity models showed that a broad low-V anomaly in and around theHainan hotspot extends to 1900 kmdepth. A high-resolutionmantle to-mographic model beneath China and surrounding regions also showeda broad low-V anomaly extending to a depth of more than 1000 km inand around the Hainan hotspot (Huang and Zhao, 2006). However, for

Fig. 1. Location of the Hainan hotspot (large red triangle) and tectonic background of East Asia. The color base map shows topography of East Asia. The small red triangles denote othervolcanoes in mainland China: Wudalianchi (WDLC), Changbai (CB), Tengchong (TC), Datong (DT), Tianshan (TS), and Kunlun (KL). The red rectangle indicates the present study region.White dots denote the earthquakes with magnitude (M) equal or greater than 5.0 relocated by Engdahl et al. (1998). Thick black lines denote the plate boundaries. Thin traces representactive faults in China. The gray curved lines show tectonic units boundaries.

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the other volcanoes in Eastern Asia, such as Wudalianchi, Changbai,Datong and Tengchong, this model revealed that low-V anomaliesonly extend to 200–300 km depth.

Newmodels also showed that theHainan volcano has different deepstructural features from the other volcanoes in East Asia (Wei et al.,2012; Zhao et al., 2013). Magnetotelluric surveying, deep seismicsounding and Pn tomography revealed the existence of an obviouslow-resistance or low-V anomaly in the crust and uppermost mantlein and around the Hainan hotspot (Hu et al., 2007; Jia et al., 2006;Liang et al., 2004). Lei et al. (2009) applied local and teleseismic data re-corded by 9 stations from the Hainan Seismic Network to determine ahigh-resolution P-wave velocity model under Hainan Island and adja-cent region, which showed a low-V anomaly under the Hainan hotspotextending to a depth of 250 km. However, the deeper structure was un-certain because the depth range for this model is limited to 300 km dueto the smaller study area.

Due to the limited study region, depth range and spatial resolution,many issues concerning the Hainan plume are still under debate, suchas its extent, origin depth and temperature anomaly. Considering thecomplex geodynamic setting and geological tectonic evolution of Hai-nan and adjacent areas, it is important to study the deep structure ofthe Hainan plume to understand the formation and evolution of SouthChina Sea and its petroleum resources. In this study, 3-D P- and S-wave velocity models beneath Hainan and surrounding regions to adepth of 700 km are determined by applying an updated teleseismic to-mography to a large number of P- and S relative arrival time residualsfrom teleseismic events recorded by 88 broad-band regional seismicstations. The present results cast new light on the deep structure andgeodynamics of the Hainan plume.

2. Data and methods

Eighty-eight seismic stationswere used in this study. They belong tothree regional seismic networks of Hainan (22 stations), Guangdong(44 stations), and Guangxi (22 stations), respectively (Fig. 2a). Thethree regional seismic networkswere reconstructed in 2007, and all sta-tions are equipped with three-component broadband seismographs.Since July 2007, the seismic data of the regional seismic networks hasbeen transmitted to the Data Management Centre of the China NationalSeismic Network (CNSN) and can be obtained there. The Hainan volca-no is located in the northeast part of theHainan Island, which is situatedin the southernmost portion of the SCFB (Fig. 1). This study region in-cludes all of Hainan Island, Guangdong and Guangxi provinces, andthe northern portion of the South China Sea. Because seismographscan only be deployed on land, the seismic stations used in the studyare unevenly distributed in the northern portion of the study region.

From 529 teleseismic events recorded between August 2007 andMarch 2010, 165 events are selected. Fig. 2b shows the epicenter distri-butions of 165 earthquakes relocated by Engdahl et al. (1998) used inthis study. The magnitudes of these events are restricted to M 5.4 orgreater. All events are located between 30° and 95° from the center ofthe study area. The distribution of the events is not uneven. Only theclear teleseismic waveforms are analyzed by means of the Human–Computer Interaction (HCI) Software and manually picked up the firstP arrivals from the vertical component seismogram (Fig. 3a). Althoughthe S-wave is a later arrival phasewith some interference on the originalseismogram, the S waveform arrivals can be seen clearly by simulatingseismograms and then are picked up in the horizontal component ofthe simulating seismogram (Fig. 3b). In this way, later S arrivals can

Fig. 2. (a) Distribution of 88 seismic stations (blue triangles) used in this study.White dotdenotes station QIZ on the Hainan Island. The black curved lines show province bound-aries, and the letters on the map show three provinces in the study region: Hainan,Guangdong and Guangxi. (b) Epicentral locations of the 165 teleseismic events (bluedots) used in this study. The concentric circles denote the epicentral distances of 30°,60°, 90° and 100° from the center of the present study area (red triangle).

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be picked up accurately. Finally, 10,873 P and 9147 S-wave arrivals arepicked up. Each event has at least 27 P arrivals and 21 S arrivals. Forall teleseismic events used in this study, the average recording numbersof P and S arrivals are 66 and 55, and their picking accuracies are esti-mated to be 0.1 s and 0.2 s, respectively.

On basis of the IASP91 1-D velocity model (Kennett and Engdahl,1991), theoretical travel times were calculated considering the Earth'sellipticity (Dziewonski and Gilbert, 1976) and the station elevation.When calculating theoretical travel times, the teleseismic ray paths be-tween the hypocenter and receiver were determined and the intersec-tion between the ray path and the boundary plane of the modelingspace was found. Then, the ray path between the intersection and sta-tion was determined by using the 3-D ray tracer of Zhao et al. (1992).

In this study, only teleseismic data are used to determine 3-D veloc-ity structure of the uppermantle under the region.However, teleseismic

rays arrive at stations nearly vertically and rarely crisscross in shallowdepths. Raw teleseismic travel time residuals are incapable of imagingcrustal structures, but crustal structure has an impact on themantle ve-locity structure (Lei and Zhao, 2007). Therefore, to better image themantle structure, crustal correction needs to be made (Hung et al.,2004; Lei and Zhao, 2007; Zhao et al, 2006), and the correction valuefor each ray is equal to the difference between the theoretical traveltime based on the IASP91 1-D model and the combined 3-D crustaland IASP91 1-D mantle models.

The crustal correction is made by using the 3-D crustal velocitymodels of CRUST2.0 (http://mahi.ucsd.edu/Gabi/rem.dir/crust/crust2.html), which is an updated version of CRUST5.1 (Mooney et al., 1998)and is specified on a 2° × 2° grid for the lateral variations of the crustvelocity and Moho topography. The calculated results show that thecrustal correction values for P and S arrivals are 0.06–0.36 s and0.11–0.54 s, respectively. It is noted that all of the crustal correctionvalues are positive because the study region is located in the coastalarea, and the real crustal thickness is thinner than the theoretical crustalthickness in the IASP91 1-D model.

In order to minimize errors introduced by the hypocentral misloca-tions, origin times and path effect outside the modeling space, relativeteleseismic travel time residuals are adopted in the tomographic inver-sion. From travel time residual tij between the j− th event to the i− thstation obtained after above-mentioned crustal correction, relative re-siduals tijREL can be expressed as

tRELij ¼ tij−t j: ð1Þ

The mean residual is given by

t j ¼1mj

Xmj

i¼1

tij; ð2Þ

where mj is the number of observed arrivals for the j − th event.Because teleseismic rays arrive at stations sub-vertically, the general

trend of positive or negative relative travel-time residuals primarily re-flects the low-V or high-V structure of the crust and uppermantle underthe seismic stations. Fig. 4a shows the distribution of the P-wave aver-age relative travel-time residuals at each station from all teleseismicevents. It is a well-regulated pattern; the positive residuals (delayed ar-rivals) appear at all stations on Hainan Island and the coastal area ofGuangdong, while negative residuals (early arrivals) are visible at sta-tions located in the northern portion of the study area. This pattern sim-ply suggests that low-V anomalies may exist under Hainan Island andthe coastal area of Guangdong, but high-V anomalies should existunder the northern portion of the study area. The azimuthally depen-dent mean relative residuals (Fig. 4b–e) exhibit systematic changes inresidual pattern and reflect strong spatial changes of velocity structurein the upper mantle. For the mean residuals from 0 to 90° and 90 to180° azimuthal events (Fig. 4(c), (e)), delayed arrivals appear at all sta-tions onHainan Island, but change to early arrivals for those from180 to270° and 270 to 360° azimuthal events (Fig. 4(d), (b)); this pattern in-dicates that high-V anomalies may exist in the western part to HainanIsland. For all quadrants (see Fig. 4(b)–(e)), the mean residuals at allstations located in the Guangdong coastal area are positive (delayed ar-rivals), which suggests that strong low-V anomalies may exist in theupper mantle under the Guangdong coastal area.

Fig. 5 shows the distribution of the S-wavemean relative residuals ateach station. This pattern is very similar to the corresponding P-wave,but for the S-wave, the mean relative residual at each station is muchlarger than that of the P-wave and thedifference of themean relative re-siduals from four quadrants is also more obvious than the P-wave. Forthe S-wave, the maximum value of the positive residuals (delayed ar-rivals) amounted to +2.5 s at the station near the Hainan hotspot(Fig. 5c), which is almost 2.5 times the P-wave residual maximum.

Fig. 3. Example of P-wave shown in vertical-component seismograms (left panel) and S-wave shown in horizontal-component seismograms (right panel) recorded by the 15 seismic sta-tions from a teleseismic event (mb6.8) that occurred in South of Fiji islands onDecember 9, 2007. The vertical thin bars show the P and S arrivals thatwere picked. The letters and numberson the map denote the corresponding name of seismic stations, components, seismographies and arrival times.

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The results may indicate that the S-wave is more sensitive to possiblehigh-temperature anomalies like plumes.

The tomographic method of Zhao et al. (1994) was applied to therelative travel time residuals for determining the 3-D velocity structurebeneath the study region. A 3-D gridwas set up in themodel, and veloc-ity perturbations at the grid nodes were taken as unknown parameters.The velocity perturbation at any point in the model can be obtained byinterpolating the velocity perturbations at the eight grid nodes sur-rounding that point. In this study, after performing many resolutiontests with different grid intervals, the grid spacing of the model wasset to 0.7° × 0.7° in the horizontal direction and 13–100 km in the ver-tical direction. A conjugate gradient algorithm (Paige and Saunders,1982) with damping and first-order smoothing regularizations(Huang and Zhao, 2006; Zhao, 2001) was used to resolve the largeand sparse system of observational equations.

Using the above grid spacing, tomographic inversion is conducted;Fig. A1 and Fig. A2 show the distributions of the hit counts (the number

of rays passing through each grid node) inmap view for P- and S-waves,respectively. In the depth range of 13 to 170 km, the hit counts have anuneven distribution (Fig. A1a–d, Fig. A2a–d), which is relevant to thedistribution of seismic stations and azimuths of the teleseismic events.The higher hit counts are distributed in Hainan Island and the coastalarea of Guangdong. In the depth range of 220 to 700 km, the hit countsbecome more and more uniform with increasing depth throughout thestudy region (Fig. A1e–l, Fig. A2e–l). Overall, the S-wave hit counts areslightly less than the P-wave hit counts, but in either case the hit countsfor most of the grid nodes are greater than 100. To ensure the reliabilityof the inversion result, only the velocity perturbation was resolved atgrid nodes with hit counts greater than 10.

3. Results and resolution analysis

Many tomographic inversionswere conducted using different valuesof the damping parameter and plotted trade-off curves (Fig. A3) for the

Fig. 4.Distribution of P-wave travel-time residuals per source quadrant for the teleseismic events, calculated with respect to the network average. Quadrants are as follows: (a) all quad-rants in 0–360°. (b) northwest: 270–360°, (c) northeast: 0–90°, (d) southwest: 180–270° and (e) southeast: 90–180°. Blue crosses andwhite solid circles denote delayed and early arrivals,respectively. The scale for the size of the residuals is shown on the right of (a).

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variance of the velocity perturbations and root-mean-square (RMS)travel time residuals that can measure the misfit of each solutionmodel to the data (Boschi, 2006; Eberhart-Phillips, 1986; Ekstromet al., 1997; Hansen, 1992). An optimal damping parameter is usuallythought of as that corresponding to the maximum curvature of thetrade-off curve (e.g., Hansen, 1992; Huang and Zhao, 2004). Followingthis criterion, the optimal values of the damping parameter werefound to be 8.0 and 10.0 for P-wave tomography and S-wave tomogra-phy, respectively. After many tomographic inversions with differentsmooth values, the optimum smooth parameters were determined tobe 0.15 and 0.08 for P- and S-wave tomography, respectively.

Figs. 6 and 7 showmap views of P- and S-wave tomography, relativeto IASPEI91 1-Dmodel. At first sight, P- and S-wave velocity images dis-play a rather clear correlation, and in bothmodels distinct low-V anom-alies in and around the Hainan hotspot extend from shallow to deep,which may be related to the Hainan plume. Two prominent low-Vanomaly zones appear at depths of 13 to 120 km. The main anomaly isunder the Hainan hotspot, and the other is located in the coastal areaof Guangdong, which is to the northeast of the Hainan hotspot (Fig.6a–c, Fig. 7a–c). Previous studies have indicated that the two regionsare prominent geothermal areas in mainland China (Wang et al.,2001). The stable South China Fold belt (SCFB) and the western portion

to Hainan Island exhibit obvious high-V anomalies, as was also revealedby ambient noise tomography (Zhou et al., 2012). The study region hastwo groups of active faults oriented in theNE andNWdirections (Fig. 6).Most of the large earthquakes (M≥ 6.0) occurred in the faults orientedin the NE direction or intersection areas of the two groups of activefaults,which are also boundary zones between low-V and high-V anom-alies (Fig. 6a, Fig. 7a). At depths of 170 to 420 km, two low-V anomalyzones are connected, and the main parts move gradually to the coastalareas of Guangdong with increasing depth (Fig. 6d–i, Fig. 7d–i). Atdepths of 500 to 700 km, the scope of the low-V anomaly gradually de-creases (Fig. 6j–l, Fig. 7j–l). In the depth range of 270 to 700 km, themain low-V anomaly is located in the coastal area of Guangdong,which is to the NE of the Hainan hotspot.

Although S-wave and P-wave images are similar, the amplitude ofS-wave velocity anomaly is twice that of the P-wave. In general, mantleplumes consist of high-temperature or even partial-melt materials.HammondandHumphreys (2000) investigated the seismicwave veloc-ity reduction resulting from the presence of partial melt in the uppermantle. Their results showed that P- and S-wave velocity reductionare at least 3.6% and 7.9%, respectively, per percent of partial melt. Theresults indicate that S-wave velocity is more sensitive than P-wavevelocity to the effect of partial melt. According to the amplitudes of

Fig. 5. The same as Fig. 4 but for the S-wave travel-time residuals.

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P- and S-wave velocity anomalies obtained, it can be deduced that par-tial melting may be present beneath Hainan and surrounding region.The other reasons that caused the reduction of the velocities will bediscussed in Section 4.3.

To clearly show the feature of the Hainan plume from shallow todeep, Fig. 8 delineates P- and S-wave tomographic images along threevertical cross-sections passed through theHainanhotspot. Themain fea-tures of the P andS images are essentially similar. The vital feature is thata low-V anomaly extends from the surface to the deep mantle in andaround the Hainan hotspot, and the low-V anomaly zone is not a simplevertical columnar shape but a complex and deflected image. For otherplumes, a similar deflected featurewas also revealed by tomographic re-sults, and this feature was generally interpreted as mantle flow (Leiet al., 2009; Yang et al., 2006; Zhao, 2001). In the WE cross-section, thelow-V anomaly deflects slightly toward the east (Fig. 8a–b), while theSN cross-section shows another low-V anomaly zone in the north from220 to 410 km depth, except for the main low-V anomaly zone underthe Hainan hotspots (Fig. 8e–f). In both the WE and SN cross-sections,the low-V anomaly does not pass through the 660 km discontinuity.However, in the cross-section SW–NE, the low-V anomaly clearly de-flects to the NE of the Hainan hotspot with increasing depth and passesthrough 660 km discontinuity (Fig. 8c–d).

In order to confirm the reliability of the main features in the tomo-graphic images, two kinds of resolution tests to assess the adequacy of

the ray coverage and evaluate the resolution are conducted. Checker-board resolution tests (Zhao et al., 1994) were used to evaluate the spa-tial resolution of tomographic images in the entire study area (Figs. 9, 10and 11), while synthetic tests (Huang and Zhao, 2006; Zhao, 2001)wereused to determine whether the assumed plume-like feature can indeedbe resolved (Fig. 12). The only difference between them is in the inputmodels.

A checkerboard input model was created by assigning alternatelypositive and negative velocity anomalies to the 3-D grid nodes in themodeling space. Synthetic travel times were calculated for the checker-board inputmodel with the same numbers of earthquakes, stations, andray paths as those in the real data set. Then, random noise with a zeromean and a standard deviation of 0.1 s was added to the syntheticdata to account for data errors that are usually present in a real dataset. These synthetic data were then inverted by using the same algo-rithm as that used to invert the real data. The images of the checker-board test results are straightforward and easy to distinguish. Thespatial resolution is considered to be good for the areawhere the check-erboard pattern is well recovered. Leveque et al. (1993) showed that insome cases small-sized structures in a checkerboard test can be re-trieved effectively while larger structures are poorly retrieved. Inorder to knowwhether there is such a problem in our tomographic im-ages, three checkerboard tests with different grid spacings were con-ducted, which are 0.5° × 0.5°, 0.7° × 0.7° and 1° × 1° grid spacings,

Fig. 6.Map view of P-wave velocity perturbations (in %) from the 1-D IASP91 Earth model with the crustal correction (see text for details). The depth of each layer is shown at the lower-right corner of each map. Red and blue colors denote low and high velocities, respectively. The velocity perturbation scale is shown at the bottom. White circles show earthquakes withmagnitude equal to or greater than M 6.0. The black triangles denote Hainan hotspot. White thin lines show major active faults.

182 J. Huang / Tectonophysics 633 (2014) 176–192

respectively. Fig. 9 and Fig. 10 show the results of the P- and S-wavecheckerboard resolution tests with a grid spacing of 0.7° × 0.7°, respec-tively. At depths of 13 to 170 km, the velocity perturbations in the seaareas are not recovered due to the lack of seismic rays (Figs. 9a–d and10a–d; Figs. A1a–d and A2a–d), but the checkerboard pattern is basical-ly recovered in the land areas and the recoverable scope gradually in-creases with depth (Fig. 9a–d and Fig. 10a–d). At depths of 220 to600 km, the resolution is good in the entire study area (Fig. 9e–k andFig. 10e–k). The resolution becomes slightly poorer at 700 km depth,but the checkerboard's positive and negative pattern is basically recov-ered (Fig. 9l and Fig. 10l). From these resolution tests it can be said thatthe tomographic images obtained in this study have a spatial resolutionof 0.7° × 0.7° in the horizontal direction, and large-scale structures canalso be resolved.

Fig. 11 shows the checkerboard resolution test results along threevertical cross-sections in the star–cross way (Huang and Zhao, 2009;Lei and Zhao, 2005). Stars and crosses denote the grid nodes where

the checkerboard pattern was inverted correctly and wrongly, respec-tively. The size of the star and cross symbols indicates the ratio of theinverted amplitude of the velocity anomalies to that in the inputmodel. The star–cross way is more obvious and straightforward tojudge where the resolution is good and where it is poor, particularlywhen a cross-section does not right pass through the grid nodes. For de-tails, see Lei and Zhao (2005).

The resolution is good along theWE cross-section, except for the seaarea in the depth range of 0–170 km (Fig. 11a and b). The checkerboardpattern is recovered except for the north portion in the depth range of0–220 km along the SN cross-section (Fig. 11e and f), and the resolutionis very good along the SW–NE cross-section (Fig. 11c and d). In thisstudy, my main objective was to investigate the deep structure of theHainan plume, and I can see that the resolution is good under theHainan hotspots and adjacent area (Figs. 9–11).

The P- and S-wave tomographic results show clearly that a low-Vanomaly extends to the bottom of the TZ in and around the Hainan

Fig. 7. The same as Fig. 6 but for the S-wave velocity perturbations.

183J. Huang / Tectonophysics 633 (2014) 176–192

hotspot (Figs. 6–7). The low-V zone does not exhibit a simple verticalcolumnar shape but a complex and deflected image toward the north-east. This image may represent the Hainan plume feature. To furtherconfirm this special structure, several synthetic tests were carried outby assuming different geometries of low-V anomalies in the inputmodels. The first three input models have a low-V anomaly of up to−3% right beneath the Hainan hotspot, from the surface to depths of220, 420 and 700 km (Fig. 12a, b, g, h, m and n), which represent theHainan plume originating from different depths. In the fourth inputmodel (Fig. 12s), from the surface to 200 km, the low-V anomaly isright beneath the Hainan hotspot, while from 200 km to 700 kmdepth, the low-V anomaly deflects toward the northeast of the Hainanhotspot, similar to those in the final image (Fig. 8c and d). In these syn-thetic tests, the purpose of the first three tests is to confirm whetherplume-like low-V anomalies right beneath the Hainan hotspot with dif-ferent origin depths can be recovered (Fig. 12a–r). The objective of thefourth test is to affirm whether the geometry of the northeastwarddeflected plume in the deep is a real feature (Fig. 12s–u). The resultsof these synthetic tests show that these features are nicely recovered

after the inversion of the synthetic data (Fig. 12). Therefore, I believethat the origin depth revealed by the present models is reliable, andthe Hainan plume deflected toward northeast in the deep has alsobeen imaged reliably with the data set and inversion technique.

4. Discussion

In this study, the obvious low-V anomalies beneath Hainan andsurrounding region are detected by using teleseismic tomographyto a large number of relative travel time residuals. However, thecomputation of relative arrival-time residuals removes the backgroundmean delay time for the region, so the present tomographic resultsshow seismic velocity variations with respect to the regional mean,not the global average (Bastow, 2012; Foulger et al., 2013). Globaland large-scale seismic tomographic studies which use large datasets of absolute travel times (Huang and Zhao, 2006; Li andVan der Hilst, 2010; Montelli et al., 2006; Zhao et al., 2013) all revealslow P- and S-wave velocity anomalies in the upper-mantle, implyingthat the mean velocity structure in this region is genuinely slow

Fig. 8.Vertical cross sections of P-wave (left panels) and S-wave (right panels) velocity perturbations. The surface topography along each profile is shown on the top of each cross section.Black open circles show the earthquakes that occurredwithin a 25-kmwidth from each profile. The red triangles denote Hainan hotspot. The velocity perturbation scales of P- and S-waveare shown at the bottom. The black curved lines denote the Moho, 410-km and 660-km discontinuities. Locations of the vertical cross-sections shown in the inset map.

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compared to the global average. To further confirm this case, I alsoanalyzed 1895 P and 1061 S travel time data from bulletins of theInternational Seismological Center (ISC) and Annual Bulletin ofChinese Earthquake (ABCE) for the period 2000–2002 for permanentstation QIZ (see Fig. 2a) in the Hainan Island. The analysis resultsshow that the mean absolute delay times with respect to theIASP91 travel time tables for P and S waves arriving at QIZ are 1.17s and 1.68 s, respectively; if the effects of thin crust producing earlyarrivals of ∼0.36 s and 0.54 s for P-wave and S-wave are accounted,respectively (see Section 2), the mean delay times should be 1.53 s

and 2.22 s for P-wave and S-wave relative to the IASP91 travel timetables and the mantle beneath QIZ should be really slower with re-spect to the global average. In this section, I use the present modelsto discuss some vital issues about the Hainan plume.

4.1. The extent of the Hainan plume

The present tomographic models show an obvious low-V anomalyextending below the TZ in and around the Hainan hotspot, and thelow-V zone does not exhibit a simple vertical columnar shape but a

Fig. 9. Results of the checkerboard resolution test for the P-wave velocity structure in map view. The depth of the layer is shown at the lower-right corner of each map. The red triangledenotes Hainan hotspot. Open and solid circles denote slow and fast velocity anomalies, respectively. The velocity perturbation scale is shown at the bottom.

185J. Huang / Tectonophysics 633 (2014) 176–192

complex and deflected image toward the northeast. Previous studiesalso revealed that a broad low-V anomaly exists beneath the Hainanhotspot and surrounding regions (Huang and Zhao, 2006; Lebedevand Nolet, 2003; Li and Van der Hilst, 2010; Montelli et al., 2004,2006; Wei et al., 2012; Zhao et al., 2013). These results imply the exis-tence of the Hainan plume, but its extent in the deep remains uncertain.The finite frequency global P- and S-wave tomographic models(Montelli et al., 2004, 2006) revealed that the low-V anomaly extendsto a depth of 1900 km beneath the Hainan hotspot and surrounding re-gion. At the 300 km depth, the low-V anomaly is widely distributed inthe coastal area of Guangdong and the Southeast Asian nations. At650 km depth, the low-V anomaly separates into two parts: one is inthe coastal area of Guangdong, and the other is in the Southeast Asiannations. At and below 1000 km, the main low-V anomaly is distributedin the Southeast Asian nations, but there is still a small-scale low-V

anomaly in the coastal area of Guangdong. The S-wave velocity modelobtained by Lebedev and Nolet (2003) also showed a distinct low-Vzone concentrated in the upper mantle in the coastal area of Guang-dong. Due to limited resolution, the other global and large regionalmodels showed that a low-V anomaly in and around theHainan hotspotis rather broad (Huang and Zhao, 2006; Li and Van der Hilst, 2010; Weiet al., 2012; Zhao et al., 2013). The local tomographic model showed alow-V anomaly extending beneath the Hainan hotspot and thendeflecting slightly toward the southeast to a depth of 250 km, but thedeeper velocity structure was uncertain or unknown (Lei et al., 2009).Although the range of the low-V anomaly revealed by the abovemodelsis not entirely consistent, one commonality is that these models allshowed a low-V anomaly in the deep upper mantle in the coastal areaof Guangdong. In particular, the new high-resolution P- and S-wave ve-locity models exhibit rather clear correlation, and both models show a

Fig. 10. The same as Fig. 9 but for the results of the checkerboard resolution test for S-wave velocity structure.

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low-V anomaly in the coastal area of Guangdong from the uppermantleto the bottom of the TZ.

Because the changes of the discontinuity depth at 410 km and660 km (hereafter referred as “410” and “660”) and the transitionzone thickness (TZT) are very sensitive to the high-temperaturemantleplume feature, the receiver function becomes another important seis-mologic means of studying the plume (Li et al., 2000; Shen et al.,1998; Suetsugu et al., 2009). Wang and Huang (2012) obtained the410 and 660 depths and TZT in Hainan and surrounding regions byusing the receiver function to analyze the teleseismic waveform record-ed by the same 88 stations used in this tomography study. To compre-hensively analyze the tomographic and receiver function results todetermine the existence scope of the Hainan plume, the P- andS-wave tomographic images at several typical layers and the resultsfrom the receiver function are placed in the same figure (Fig. 13). Thearea of the thinning TZ with a diameter of about 200 km is delineatedwith a black line (Fig. 13m), and the black line is marked on eachpanel of Fig. 13. It can be found that the main reason for the thinning

TZ (Fig. 13m) is the 410 depression (Fig. 13k and m), while there is aslight uplift on 660 (Fig. 13l). Comparing these results, it can be seenthat at the depth range of 370 to 700 km, the low-V anomaly zone(Fig. 11a–j) is basically consistentwith the area of the thinning TZ delin-eated by the black line (Fig. 11m).

Overall, through a comprehensive analysis of existing tomographicmodels, especially the present regional high-resolution models, com-bined with the results from the receiver function, I think the Hainanplume exists right beneath the Hainan hotspot in the shallow uppermantle and deflects to the northeast of Hainan hotspot from the deepupper mantle to the bottom of the TZ, an area with a diameter ofabout 200 km.

4.2. The origin of the Hainan plume

Ringwood (1989) suggested that the subducting plate accumulatedat the interface at the 660 km depth is heated and may form a buoyantthermal column, which may cause the mid-oceanic intraplate hotspots.

Fig. 11.Vertical cross sections of the checkerboard resolution test for P-wave (left panels) and S-wave (right panels) in star–crossway. Stars denote the grid nodeswhere the pattern of theinput velocity anomalies is retrieved correctly after the inversion, that is, fast velocity anomalies in the inputmodel are recovered to be fast, and slow ones in the inputmodel are recoveredto be slow after the inversion, while crosses denote the grid nodes where the pattern of the input velocity anomalies is wrongly recovered after the inversion. The size of star and crosssymbols denotes the ratio of the inverted amplitude of the velocity anomaly to that in the inputmodel. The stars with values of 100% show the grid nodes where the checkerboard patternis recovered perfectly. The scale for the degree of recovery (in %) is shown at the bottom. Locations of the vertical cross-sections are shown in (g).

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Anderson (1995) pointed out that this plume originates from the bot-tom of the convective systems, which are magma sources for oceanislands. Zhou and Song (1998) proposed that the plumemight originatefrom 400 km, 670 km or the core–mantle boundary (CMB) on the basisof the global multilayer convection model. Courtillot et al. (2003) sum-marized the five criteria of hotspots by analyzing 49 hotspots aroundtheworld and divided the origin depth of hotspots into three categorieson the basis of these criteria: (1) The upper mantle above 410 km. Theorigin of suchhotspotsmay be related to the asthenospheric convection,structure, stress and fracture of the lithosphere (Anderson, 2000);

(2) The bottom of the TZ (660 km). The super-mantle upwells to thebottom of the TZ and promotes continued upwelling of the plume atthe bottom of the TZ; (3) The bottom of the lower mantle (CMB). Theunstable small-scale convection in the D″ layer above the CMB cancause plume upwelling (Olson et al., 1987).

From the present tomographic models and the receiver function re-sults (Wang and Huang, 2012), it can be confirmed that the Hainanhotspot certainly does not originate from the upper mantle, so theHainan hotspots may originate from the bottom of the TZ, the lowman-tle or the CMB. Numerical simulations and experimental studies show

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that if theplumeoriginates from the bottomof the TZ, the horizontal ex-tent of the high-temperature anomaly zone in the thermal boundarylayer needs to be much larger than the diameter of plume (Olsonet al., 1988). For a hot anomaly of the Hainan plume with a diameterof approximately 200 km to upwell from a thermal boundary layer atthe bottom of the TZ, it needs a high-temperature thermal boundarylayer with a thickness of 50–100 km and a horizontal range of 1000–2000 km. That is, if the Hainan plume originates from the thermalboundary layer at the bottom of the TZ (660 km), there should be ahigh-temperature thermal boundary layer with a horizontal range of1000–2000 km. However, the present tomographic results show thatthe scope of the P- and S-wave low-V anomalies is much smaller atthe bottom of the TZ (Figs. 8, 13a–j), and the thinning TZ also only ap-pears in an area with 200 km in diameter (Wang and Huang, 2012).Thus, these results mean that it is impossible for the Hainan plume tooriginate from the bottom of the TZ.

Fig. 12. Four synthetic tests. Left (a, b, g, h,m, n, s) panels show inputmodel,middle (c, d, i, j, o, pcircles and crosses denote slow and fast velocity anomalies. The velocity perturbation scale isMoho, 410-km and 660 km discontinuities. (v) Locations of the cross sections.

Geochemical studies in the Leiqiong area also show that the Hainanvolcano has deep-source characteristics (Ho et al., 2000; Li et al., 2005;Tu et al., 1991; Yan and Shi, 2007, 2008). The Nd–Sr–Pb isotopic compo-sition of basalt in the Leiqiong area exhibits ocean island basalt (OIB)characteristics, which are similar to those in Iceland, Hawaii and theGalapagos Islands. It is generally agreed that the source of OIB is thedeep mantle. Thus, to summarize the results of this study, I believethat the Hainan volcano originates from the low mantle.

4.3. The temperature anomalies of the Hainan plume

Seismic velocity changes are affected by many factors such as tem-perature, composition, and partial melt, but researchers still have no ef-fective methods to discriminate between these contributions. Previouswork has shown that the effect of thermal variations on seismic veloci-ties in the deep mantle is most likely considerably larger than the effect

, t) and right (e, f, k, l, q, r, u) panels show P- and S-wave outputmodels, respectively. Openshown at the bottom. Black triangles denote the Hainan hotspot. Dashed lines denote the

Fig. 12 (continued).

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of reasonable composition variations (Jackson, 1998; Jordan, 1979),which are thought to be b1% if there are no strongly depleted Mg-richharzburgites present (Sobolev et al., 1996). Synthetic velocity calcula-tions for 1-D thermal models based on experimentally determined pa-rameters predict a 0.5 to 2% decrease in P-wave velocity for a 100 Kincrease in temperature. For S-waves the decrease in velocity wouldbe between 0.7 and 4.5% for a 100 K increase in temperature (Goeset al., 2000). While the effects of partial melt are also expected to bevery strong, they cannot be quantified with much confidence. Mantlepotential temperature is a key parameter for describing the thermalstate of the upper mantle. If assuming that temperature is the mainfactor which influences the seismic velocities in the upper mantle, theexcess temperatures of the plumes can be estimated by the method ofGoes et al.(2000). An and Shi (2007) estimated upper-mantle tempera-ture structure of the Chinese continent by using the method to 3-DS-velocity model from surface tomography (Huang et al., 2003). Onthe basis of the present P and S velocitymodels, a temperature anomalyis estimated approximately to be higher than 150 K beneath the Hainanplumeusing themethod and an elasticitymodelQ1 of Goes et al.(2000).On the other hand, receiver function results showed that the TZT is ap-proximately 25 km of thinning within an area with a diameter of about

200 km, which is equal to a high temperature anomaly of 180 K (Wangand Huang, 2012).

On the basis of olivine-melt equilibrium on Hainan Cenozoic alkalibasalts and the method of Yan and Shi (2007), it can be inferred thatthe potential temperature under the Hainan Island and surrounding re-gions is significantly higher than that of the normal mantle; the averagetemperature anomaly is approximately 200 K which is similar to that ofthe Eifel plume and the Massif Central plume in Europe (Granet et al.,1995; Ritter et al., 2001). This value is between those of Hawaii andIceland volcanoes, where the average temperature anomalies are ap-proximately 213 K and 162–184 K, respectively (Putirka, 2005). Recentstudies show that the excess temperature for East Africa is only ~140 °C(Julia andNyblade, 2013; Rooney et al., 2012), which is lower than someestimates for Hainan. A higher temperature anomaly alsowas estimatedbyWang et al. (2012)who identifiedmagnesian olivine (Fo90.7) pheno-crysts in late Cenozoic Hainan basalts; the high magnesian olivinepoints to an unerupted picritic melt (~16%–18% MgO), generated by ahot mantle with a temperature anomaly of about 260 K.

Overall, both geophysics and geochemistry have been used to studythe temperature anomaly of the Hainan plume, the relative high poten-tial temperatures for the deep mantle under the Hainan hotspot and

Fig. 13. P-wave (left panels, a–e) and S-wave (middle panels, f–j) tomographic images inmapviewat several representative depths; the depth of each layer is noted in themiddle betweenleft panels and middle panels. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scales are shown at the bottom. The results from receiver func-tions (RFs) are shown on the right panels: depth distribution of the 410 km(k) and 660 km(m) and the transition zone thickness (TZT) (n) (Wang andHuang, 2012); the scales are shownat the bottom of each map; the black triangle denotes the Hainan hotspot. The black line marks the area of the thinning TZT.

190 J. Huang / Tectonophysics 633 (2014) 176–192

adjacent area are obtained by different method, but the temperatureanomaly values still have some uncertainties.

5. Conclusions

A teleseismic tomographic method was applied to a large numberof high-quality, hand-picked arrival times to determine detailed 3-DP- and S-wave velocity models down to a depth of 700 km beneathHainan and surrounding regions. The 3-D crustal velocity model ofCRUST2.0 was employed to make crustal correction, and as a resultthe uppermantle velocity structure was improved. Comparedwith pre-vious global and large regional models, the present models have ahigher resolution, and they have a deeper velocity structure comparedwith the local model. In particular, for the first time a regional high-resolution S-wave velocitymodel from bodywave tomographywas ob-tained. The P- and S-wave models display a rather clear correlation, sothe P-wave model, combined with the S-wave model which is moresensitive to plume anomaly, casts new light on the deep structure anddynamic features of the Hainan plume.

The present tomographic models show that the low-V anomaly ex-tends down below the TZ in and around the Hainan hotspot. The low-V zone does not show a simple vertical columnar shape, but complexand deflected images toward the northeast. The low-V anomaly valuesfor the P-wave and S-wave are as large as−3% and−6%, respectively.In the depth ranges of 13 to 170 km, themain low-V anomaly is locatedbeneath the Hainan hotspot, but from 170 km to 700 km depth, it

gradually deflects toward the northeast to the coastal area of Guang-dong. The low-V zone is an area with a diameter of approximately200 km and is basically consistent with the zone of the thinning TZfrom the receiver function. On the basis of the present P- and S-wave ve-locitymodels, combinedwith the receiver function and geochemical re-sults, I believe that the Hainan plume may originate from the lowmantle and has a high-temperature anomaly.

Acknowledgments

I thank the Editor Prof. Hans Thybo and three anonymous refereesfor providing many constructive comments and suggestions which im-proved themanuscript. I am grateful to Prof. Dapeng Zhao for providinghis tomographic code, his early comments and help. This research issupported by the National Science Foundation of China (91114205,41074061, 40774040). I also thank the Data Management Centre ofthe China National Seismic Network at the Institute of Geophysics,China Earthquake Administration for providing the waveform dataused in this study. The figures were made by using GMT (Wessel andSmith, 1995).

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2014.07.007.

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