Author's personal copy · Author's personal copy Imaging ofVp, Vs, and Poisson s ratio anomalies beneath Kyushu, southwest Japan: Implications for volcanism and forearc mantle wedge

Author's personal copy

Imaging of Vp, Vs, and Poisson’s ratio anomalies beneathKyushu, southwest Japan: Implications for volcanism

and forearc mantle wedge serpentinization

Mohamed K. Salah *, Tetsuzo Seno

Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan

Received 16 November 2006; received in revised form 10 July 2007; accepted 23 July 2007

Abstract

We determine detailed 3-D Vp and Vs structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan,using a large number of arrival times from local earthquakes. From the obtained Vp and Vs models, we further calculate Poisson’s ratioimages beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson’s ratio structuresbeneath Kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies. Our results show very clear lowVp and low Vs anomalies in the crust and uppermost mantle beneath the northern volcanoes, such as Abu, Kujyu and Unzen. Low-velocity anomalies are seen in the mantle beneath most other volcanoes. In contrast, there are no significant low-velocity anomaliesin the crust or in the upper mantle between Aso and Kirishima. The subducting Philippine Sea slab is imaged generally as a high-velocityanomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities. The Poisson’s ratio is almost normalbeneath most volcanoes. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the lowPoisson’s ratio zone. A high Poisson’s ratio region is found in the forearc crustal wedge above the slab in the junction area with Shikokuand Honshu; this high Poisson’s ratio could be caused by fluid-filled cracks induced by dehydration from the Philippine Sea slab. ThePoisson’s ratio is normal to low in the forearc mantle in middle-south Kyushu. This is consistent with the absence of low-frequency trem-ors, and may indicate that dehydration from the subducting crust is not vigorous in this region.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Kyushu; Southwest Japan; Shikoku; Philippine Sea; Seismic tomography; Poisson’s ratio; Serpentinized forearc mantle; Low-frequencytremor; Volcano

1. Introduction

At present, beneath the Japanese Islands, the PhilippineSea plate (PHS) is subducting beneath the Eurasian platealong the Nankai Trough and the Ryukyu Trench in a�N50�W direction at a rate of 4–5 cm/yr, while the Pacificplate is subducting from the east beneath the PHS and theOkhotsk plate at a rate of 6–8 cm/yr (Fig. 1, Seno et al.,1993, 1996). Beneath Kyushu, the subducting PHS formsintraslab seismic activity down to a depth of about

200 km (Fig. 2). Associated with this subduction, activevolcanoes form a volcanic front in the central part of theisland (Figs. 1 and 2). Quaternary volcanoes also existalong the coast of the Sea of Japan in Chugoku (Yokoy-ama et al., 1987). It was first believed that these Quaternaryvolcanoes are also associated with the subduction of thePHS, and that they form a continuous volcanic front fromKyushu (Sugimura, 1960). Later, petrological and geo-chemical studies indicated that the volcanoes in Chugokuand those in NW. Kyushu, including Unzen, have beenformed from more fertile magma sources deep in the man-tle (e.g., Nakamura et al., 1989; Iwamori, 1991; Nakadaand Kamata, 1991; Sumino et al., 2000). On the otherhand, Yoshida and Seno (1992) argued that the Aso

1367-9120/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2007.07.003

* Corresponding author. Present address: Geology Department, Facultyof Science, Tanta University, Tanta 31527, Egypt.

E-mail address: [email protected] (M.K. Salah).

www.elsevier.com/locate/jaes

Available online at www.sciencedirect.com

Journal of Asian Earth Sciences 31 (2008) 404–428


volcano is distinct from other volcanoes at the volcanicfront, such as Kirishima, because the seismic slab doesnot reach the depth beneath it, and, thus, might not berelated with the PHS subduction, along with other volca-noes in north (N.) Kyushu – Chugoku, such as Unzen,Kujyu, and Abu.

Seismic tomography results provide useful informationabout the production, transfer, and distribution of fluidsand/or melts in subduction zones in addition to otherdetails of the subduction process. Thus, many authors havestudied both the velocity and/or attenuation (Q) structuresbeneath Japan (e.g., Hirahara, 1981; Zhao et al., 1992a,2000a,b; Tsumura et al., 2000; Nakajima et al., 2001a;Hasegawa et al., 2005). All these studies confirmed the exis-tence of high-velocity/low-attenuation subducting slabsand low-velocity/high-attenuation anomalies landward,indicating an ascending flow from the deep mantle in thebackarc side to the crust beneath active volcanoes. How-ever, most of these studies were concentrated in the north-eastern (NE) Japan subduction zone or in wide areas.Therefore, seismic tomography studies in Kyushu wouldprovide useful information on the subduction process ofthe PHS, and on seismicity and volcanism.

Previous seismic tomography studies, revealing mostlylarge-scale 3-D seismic structures, had a low resolutionbeneath Kyushu (Hirahara, 1981; Zhao et al., 1994,2000a; Sadeghi et al., 2000; Honda and Nakanishi, 2002;Nakamura et al., 2002). Zhao et al. (2000a) showed a P-

wave velocity structure in Kyushu, using earthquake dataof the Japan University Network Earthquake Catalogue(JUNEC) during 1993. They showed low P-wave velocityanomalies in the mantle beneath the Kujyu volcano bothin the backarc and forearc sides. The number of events usedis, however, only 486, recorded by few seismic stations,which is barely adequate to reveal the detailed structure.Sadeghi et al. (2000) analyzed P-wave arrivals covering awide area from southwest (SW.) Japan to eastern China.They found no significant low-velocity anomalies justbeneath the volcanic front in Kyushu, and found themrather in the upper mantle west (W.) of Kyushu. The reso-lution of these tomographic studies, however, seems too lowto satisfactorily discuss the volcanism in Kyushu.

Recently, high Poisson’s ratio regions have been foundin the forearc mantle in Kanto – SW. Japan (Kamiyaand Kobayashi, 2000, 2006; Honda and Nakanishi,2003). These high Poisson’s ratio regions have been inter-preted as serpentinized mantle hydrated by water released

Fig. 2. Distribution of the 4224 events used in this study shown as circles.Event symbols vary in color according to focal depth and in size accordingto magnitude. Open and solid triangles denote Quaternary and activevolcanoes, respectively.

Fig. 1. Distribution of active and Quaternary volcanoes on the JapanIslands. Curved lines show the trenches, which represent the major plateboundaries around the Japanese region. Black rectangle shows the presentstudy area. Arrows and attached numerals show direction and rate ofmovement (mm/yr) of the Pacific (PA), and Philippine Sea (PHS) slabs.EU, Eurasian plate; OK, Okhotsk plate.

M.K. Salah, T. Seno / Journal of Asian Earth Sciences 31 (2008) 404–428 405


from the subducting PHS slab (Kamiya and Kobayashi,2000; Seno, 2005). On the other hand, low-frequency trem-ors have been found to occur just above the slab upper sur-face at 30–45 km depths in the SW. Japan forearc (Obara,2002; see also Katsumata and Kamaya, 2003). These low-frequency tremors are also believed to have been causedby dehydration of the PHS slab (Obara, 2002; Seno andYamasaki, 2003). Seno and Yamasaki (2003) pointed outthat the regions lacking the low-frequency tremors, east(E.) Shikoku and middle (M.)-south (S.) Kyushu in SW.Japan, and Kanto–N. Izu in central Japan, might be theplaces where dehydration from the subducting PHS crustdoes not occur. If this is true, it is expected that serpentini-zation of the forearc mantle might not be conspicuous inthese regions. Consistently, the results of Honda and Nak-anishi (2003) show that the Poisson’s ratio is low in E. Shi-koku and modest in M.-S. Kyushu.

The extent of the dehydration from the subducting crustmay also be inferred from the occurrence of intraslabearthquakes in the crustal part (Seno and Yamasaki,2003), if the dehydration embrittlement is accepted as amechanism for intraslab earthquakes (Kirby et al., 1996;Seno and Yamanaka, 1996; Peacock and Wang, 1999;Yamasaki and Seno, 2003). Recently, Okamoto et al.(2005) examined whether intraslab earthquakes in Kyushuare occurring in the crust or in the mantle by examininglater phases traveling within the crust. They found that inM. Kyushu, there are several intracrustal earthquakes,and in contrast, in S. Kyushu, there are no crustal earth-quakes and all events are occurring in the peridotite partof the slab. Therefore the presence of fluids in the forearccrust-mantle related to the occurrence or non-occurrenceof dehydration from the subducting PHS slab should befurther investigated in this region.

In this study, we select a large number of arrival timedata generated by local earthquakes recorded by denseseismic networks, and use them to determine detailedseismic velocity and Poisson’s ratio structures beneathKyushu and the junction area with Honshu and Shi-koku, SW. Japan. The obtained images down to a depthof 150 km, with their reliable resolution enable us tounderstand the magma generation processes, shallowcrustal seismicity, and serpentinization state in the fore-arc mantle wedge, which are associated with the subduc-tion of the PHS slab.

2. Data and method

2.1. Data

We use P- and S-wave arrival time data from shallowand intermediate-depth earthquakes that occurred inand around Kyushu Island, SW. Japan, between lati-tudes 30�–35�N, and longitudes 129�–133�E (Fig. 1) inthe depth range of 0–200 km. We have carefully selected4224 local earthquakes recorded by the High Sensitivity(Hi-net) and the JUNEC seismic networks. A total of

3712 events were selected from JUNEC data from theperiod from January, 1990 to December, 1998, and512 events were selected from Hi-net data from the per-iod from December, 1999 to October, 2000 (Fig. 2). TheJUNEC network has 46 seismic stations; while the Hi-net has 106 seismic stations falling within the study area(Fig. 3). The JUNEC stations are operated by eightnational universities in Japan and are equipped withshort-period and broadband seismographs (Tsuboiet al., 1989). The data of this seismic network are col-lected and published by the Earthquake Research Insti-tute (ERI), University of Tokyo. The Hi-netseismograph network consists of more than 600 seismicstations covering the whole of Japan (Obara, 2002), andare operated by the National Research Institute forEarth Sciences and Disaster Prevention (NIED). Com-bining these two different data sets together significantlyimproves the station density (Fig. 3) and the seismicitycoverage compared with many previous studies.

We try to select events that are very well located anduniformly distributed in the study area. For shallow earth-quakes (depth 6 20 km), the focal depth error does notexceed 2.5 km and all events are recorded by more than10 seismic stations. For deeper events (20 km <

Fig. 3. Distribution of Hi-net (solid squares), and JUNEC (solid triangles)seismic stations used in the present study.

406 M.K. Salah, T. Seno / Journal of Asian Earth Sciences 31 (2008) 404–428


depth 6 200 km), we select those whose maximum focaldepth error does not exceed 4 km and are recorded by atleast 8 seismic stations. This is because we need a largernumber of deeper events to accurately image the uppermantle velocity structure. The shallow earthquakes areuniformly distributed in the study area; while the deeperevents are associated with the subduction of the PHS slab.However, we notice that there is a cluster of shallow seis-micity in W. Kyushu around Unzen volcano (Fig. 2). Theselected events generate a total number of P- and S-wavearrival times of 71,100 and 44,990 respectively. The uncer-tainty of P-wave first arrival times is estimated to be about0.1 s; while the uncertainty for S-wave first arrivals isslightly greater, being about 0.1–0.15 s. To check the hor-izontal ray path coverage for P- and S-wave data sets, weplot each path between an epicenter and a station as onestraight line in Fig. 4. It is clear that both P- and S-wavedata have generally similar and comparable ray coveragein most parts of the study area.

2.2. Method

The seismic tomography method was first developedby Aki and Lee (1976). Subsequently, many researchershave improved this technique and applied it to variousregions (e.g., Hirahara, 1977; Thurber, 1983; Spakmanand Nolet, 1988; Zhou and Clayton, 1990; Zhao et al.,1992a, 1994; Nakajima et al., 2001a; Mishra et al.,2003). In this study, we use the tomographic methodof Zhao et al. (1992a). This method is preferred for sub-duction zone regions, where many seismic discontinuities,such as the Conrad, Moho, or the subducting slabboundary, exist. It uses an efficient 3-D ray tracingscheme to compute travel times and ray paths. We adopta grid spacing of 0.2� horizontally and 10–30 km indepth (Fig. 5). In our tomographic inversion, we taketwo discontinuities (the Conrad and the Moho) intoaccount in the velocity model. It is well known that thesediscontinuities are not simple flat planes, but have com-plicated geometries (Horiuchi et al., 1982a,b; Hasegawaet al., 1983; Zhao et al., 1992b; Nakajima et al., 2002;Salah and Zhao, 2004). For this reason, it is necessaryto consider these discontinuities in the initial velocitymodel. The Conrad and Moho depths determined byZhao et al. (1992b) are adopted in the present inversion.Table 1 shows the initial velocity model used in thetomographic inversion. It is generally similar to the mod-els used previously by other researchers in SW. Japan(e.g., Zhao and Negishi, 1998; Zhao et al., 2000a; Salahand Zhao, 2003a,b, 2004; Salah et al., 2005). Velocitiesat grid nodes are taken as unknown parameters andthe velocity at any point in the model is calculated bylinearly interpolating the velocities at the eight grid nodessurrounding that point. For more details about themethod, see Zhao et al. (1992a, 1994).

The interpretation of tomographic images is usuallynon-unique, because any given velocity anomaly can be

Fig. 4. Ray path coverage for P-wave data (a) and S-wave data (b). Everypath between an epicenter and a station is drawn as one straight line.



attributed to either a thermal or a chemical variation.For this reason, it is useful to consider some other phys-

ical parameters for a more reliable interpretation. Com-pared to the seismic velocities themselves, the Poisson’sratio (or Vp/Vs ratio) is a better indicator for the contentof fluids and/or magma (Zhao and Negishi, 1998; Kayalet al., 2002; Takei, 2002; Salah and Zhao, 2003a; Nakaj-ima et al., 2005; Sadeghi et al., 2006) or serpentinization(Christensen, 1996; Kamiya and Kobayashi, 2000).Therefore, after Vp and Vs models are calculated fromtravel time inversions, we compute the Poisson’s ratio(r) from Vp/Vs. To ensure having reliable Poisson’s ratioanomalies, we consider results only in areas having well-constrained Vp and Vs structures with nearly equal reso-lution (Widiyantoro et al., 1999; Gorbatov and Kennett,2003).

Fig. 5. Configuration of the grid net adopted for the present study in horizontal directions (a), and in depth direction (b). Straight lines in (a) show thelocations of cross sections AA 0, BB 0, CC 0, DD 0, EE 0, and FF 0 shown in Figs. 13–18. Numbers (1)–(10) represent the names of volcanoes as follow: (1) Abu;(2) Tsurumi; (3) Kujyu; (4) Aso; (5) Unzen; (6) Kirishima; (7) Sakura-jima; (8) Kaimon; (9) Kikai; (10) Kuchinoerabu-jima.

Table 1Initial velocity model adopted for the present study

Layer No. Depth (km) Vp (km/s) Vs (km/s)

1 5 5.90 3.452 15 6.00 3.503 25 6.70 3.804 40 7.70 4.355 55 7.75 4.376 72 7.80 4.407 90 7.86 4.428 120 7.95 4.479 150 8.05 4.51

10 180 8.14 4.56



3. Resolution and results

3.1. Resolution

Before describing main features of the results, weshow the results of the checkerboard resolution test(CRT) (Inoue et al., 1990; Zhao et al., 1992a, 1994) to

see the reliability of the obtained tomographic imagesboth in map views and in cross sections. The values ofassumed checkerboard-type perturbations assigned togrid nodes are ±3%. Synthetic arrival times are then cal-culated for the checkerboard model. Numbers of stationsand events with their exact locations in the syntheticdata are taken as the same as those in the real data

Fig. 6. Results of a checkerboard resolution test (CRT) of P-wave velocity (see text for details) for seven representative depth layers. The grid separation is20 km. Solid and open circles denote high- and low-velocities, respectively. The depth of each layer is shown at the lower right. The perturbation scale isshown at the bottom.



set. The Gaussian noise of 0.1 s is assigned to the arrivaltimes.

The CRT results for the P- and S-wave velocity struc-tures at some representative depth slices are shown in Figs.6 and 7, respectively. It is clear that the resolution is goodwhere many ray paths are included. At the crustal anduppermost mantle layers, most parts of the study area havea reliable resolution, as most of the input synthetic anom-alies are very well recovered, except for the northwestern

part of the area at 40 and 55 km depths. For deeper layers(depths 90, 120, and 150 km), the resolution is good wherethe intermediate-depth events that occur in the subductingPHS slab are located (Figs. 6 and 7). However, some partsof the backarc side and sea-side portions, especially in thedeep layers have no resolution because of the insufficientlengths and directions of the rays traveling in these layers.

Figs. 8 and 9 show the results of the CRT along sixcross sections (see Fig. 5 for the location of the cross

Fig. 6 (continued )



sections) for which P- and S-wave velocity, and Pois-son’s ratio structures are shown. A close inspection ofthese results reveals also that most of the obtainedanomalies have a good resolution except for the deepzones in the backarc side and the shallow and deepzones in the forearc. In general, both P- and S-wavevelocity structures have almost identical spatial resolu-tions. We plot the P- and S-wave velocity structures

both in the map views and in the cross sections onlywhere more than 25% of the synthetic anomalies arerecovered. We then plot the Poisson’s ratio only in areashaving a same resolution for both P- and S-waves.Therefore, we believe that the estimated Poisson’s ratioanomalies are reliable features and their estimationerrors are on the same level of P- and S-wave velocityperturbations.

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



3.2. Results

The final inversion results were obtained after three iter-ations. The P- and S-wave root-mean-square (RMS) traveltime residuals calculated after the three iterations were0.245 and 0.344 s, respectively. This is equivalent to a lar-ger than 30% reduction from the initial residuals. In the fol-lowing paragraphs, we describe the results of Vp, Vs, andPoisson’s ratio.

Figs. 10 and 11 show velocity perturbations of the P-and S-waves at depths of 5, 15, 40, 55, 90, 120, and150 km, respectively, together with the distribution of seis-micity within a 10–30 km thick slice around the studieddepth. Fig. 12 shows perturbations of the Poisson’s ratioat the same depth slices.

In the upper crust (5 and 15 km depths), the velocitystructure is generally heterogeneous having strong lateralvariations amounting to ±5%. Prominent low-velocity

Fig. 7 (continued )



anomalies are detected beneath the Abu, Unzen, Tsuru-mi, and Kujyu volcanoes, but not beneath the Aso, Kiri-shima, and Sakura-jima volcanoes (see Fig. 5 for thelocation of the volcanoes). The Poisson’s ratio is highin some parts of the forearc and near some volcanoesbut low to moderate in other areas. Shallow crustal seis-mic events occur in both low- and high-velocity zones.However, their frequency seems to decrease very nearand/or directly below the volcanic areas. In contrast,the shallow seismicity tends to occur in the low Poisson’sratio region.

At depths of 40 and 55 km in the uppermost mantle,seismicity is concentrated in the subducting PHS slab alongthe southeastern parts of the study area. A low-velocityregion is seen beneath the forearc mantle at a depth of55 km in the junction of Shikoku and Honshu and at adepth of 40 km in the forearc of M.-S. Kyushu. Moderateto high Poisson’s ratio anomalies are mapped near volcanicareas at a depth of 40 km. A high Poisson’s ratio zone isalso detected in the junction of Shikoku and Honshu at adepth of 40 km (Fig. 12). From the results in cross sectionsshown later, we see that this high Poisson’s ratio is mostlyin the crustal part of the wedge above the slab. We will dis-cuss the origin of this high Poisson’s ratio anomaly in alater section.

The subducting PHS slab is clearly seen as a 4% higher-thanaverage velocity zone at depths of 90, 120, and 150 km.These high-velocity anomalies reach the location beneaththe volcanic front at depths of 90 and 120 km in the south,but not in the north. This is concordant with the conclusionof Yoshida and Seno (1992) based on the slab seismicity.Intermediate-depth seismicity occurs within these high-velocity anomalies. The subducting PHS slab has mostlynormal to high Poisson’s ratio (Fig. 12).

Line AA 0 (Fig. 13) passes through the Quaternary vol-cano (Abu), and lines CC 0, DD 0, EE 0, and FF 0 (Figs. 15–18) run through or near four active volcanoes (Kujyu,Aso, Unzen, and Kirishima, respectively). Line BB 0,however, crosses between volcanic areas (Fig. 14). A linethrough Sakura-jima is not shown because the resolutionin this region is low. Low-velocity zones in the crust areseen beneath active volcanoes along lines AA 0, CC 0, andEE 0. No prominent low-velocity zone is seen in the crustbeneath DD 0 and FF 0 (Aso and Kirishima volcanoes,respectively). The Poisson’s ratio is slightly above theaverage beneath these volcanoes. In lines BB 0 and EE 0,volcanoes are absent at the location of the volcanic frontand there is no low-velocity zone in the crust, but thereis a slight indication of low-velocities in the uppermostmantle.

Fig. 8. Results of CRT of P-wave velocity structures along the six cross sections shown in Fig. 5. Solid and open circles denote high- and low-velocities,respectively. The perturbation scale is shown at the bottom.



The PHS slab has generally normal to higher than aver-age velocity at most depths. However, in cross sections CC 0

and FF 0, some parts of it have moderate- to low-velocities.This unusual feature has also been detected by other stud-ies (e.g., Yamane et al., 2000; Zhao et al., 2000a,b; Senoet al., 2001; Honda and Nakanishi, 2003). The forearcwedge above the subducting slab has a high Poisson’s ratiobeneath lines BB 0 and DD 0 in the junction area (Figs. 14and 16). It can be seen that this high Poisson’s ratio regionis mostly in the crustal part of the wedge (depth <30 km).The forearc mantle in other sections has a normal to lowPoisson’s ratio.

4. Discussion

4.1. Subduction of the PHS slab

The angle of the subducting PHS slab beneath Kyushuchanges from the Shikoku Basin to the Kyushu-PalauRidge around 32�N (Fig. 1). The Shikoku Basin wasformed in the Miocene (Okino et al., 1994) and the Kyu-shu-Palau Ridge dates back to the Eocene (Seno, 1988).The seismic PHS slab subducts with a small angle (�30�)to a depth of about 40–60 km beneath Shikoku, and witha large angle (more than 60�) to depths of about 100–

150 km beneath Kyushu (Fig. 2, see also Uyehira et al.,2001; Honda and Nakanishi, 2003), which may reflect theabove age difference. We detect the high-velocity anomaliesindicating the PHS slab, down to a depth of about 150 kmbeneath Kyushu, with a reasonable resolution. Somepatches of low-velocity anomalies are seen within the seis-mic PHS slab, and this may partly be due to low resolutionor real heterogeneous structures within the slab itself.There is no significant distinction in the magnitude of thehigh-velocity between the junction area (lines AA 0 andBB 0) and Kyushu (lines CC 0–FF 0).

4.2. Seismological evidence on magmatism in Kyushu

The low-velocity anomalies beneath some volcanoesamounting to 5% cannot be explained only by high temper-atures (e.g., Nakajima et al., 2005). For this reason, innorthern Honshu, such anomalies have been explained bythe existence of melt-filled pores and cracks (Nakajimaet al., 2001a,b, 2005). The similar magnitude of the low-velocity anomalies in the crust and upper mantle associatedwith the northern volcanoes, Abu, Kujyu, and Unzen(Figs. 13, 15, and 17) suggests that melting beneath thesevolcanoes occurs similarly. The high-velocity seismic slab,however, does not reach the depth below these volcanoes.

Fig. 9. The same as Fig. 8, but for S-wave velocity.



Because the melting of the wedge is related to the flow inthe mantle and the porous flow of the aqueous fluidsinduced by the subduction (e.g., Iwamori and Zhao,2000; Hasegawa and Nakajima, 2004), the absence of theslab directly beneath the volcanoes does not preclude apossibility of the volcanism to be induced by the PHS sub-duction. The low-velocity anomalies in the mantle beneath

some volcanoes inclined landward above the slab wouldprovide a more useful discriminator for the slab-inducedvolcanism (Hasegawa and Nakajima, 2004). The low-velocity anomaly beneath Kujyu is inclined landward (lineCC 0, Fig. 15); almost parallel to the subducting PHS slab.On the other hand, the low-velocity anomaly in the forearcmantle wedge pointed out by Zhao et al. (2000a) is not seen

Fig. 10. P-wave velocity structures (in %) at seven depth slices. Red and blue colors indicate low- and high-velocities, respectively. Open and solid trianglesshow Quaternary and active volcanoes, respectively. The depth to each layer is shown at the lower right. Numbers between brackets show the depth rangeof seismicity (crosses) plotted along with the velocity image. The color scale at the bottom varies from �5% to 5% at depths of 5 and 40 km; while it variesfrom �4% to 4% at depths of 55, 90, 120, and 150 km. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)



in this study. The low-velocity anomalies associated withAbu and Unzen are not inclined landward. Although thesevolcanoes have been argued to have a plume origin fromthe deep mantle (Iwamori, 1991; Nakada and Kamata,1991; Sadeghi et al., 2000; see also references in Seno,1999), it is difficult to discriminate their origin from theresults in this study, because of the lack of the resolutionin the backarc side and deeper portion of the study area.

Beneath Aso, Kirishima and Sakura-jima (Figs. 10, 11,16, and 18), and the area where volcanoes are absentbetween Aso and Kirishima, the crust shows high-velocitieswhile the upper mantle has an indication of low- to moder-ate-velocities. Previously, Sadeghi et al. (2000) pointed outthat there is no low-velocity anomaly beneath the volcanicfront in Kyushu. This does not seem correct except for lim-ited areas.

Fig. 10 (continued )



4.3. Dehydration of the subducting slab and serpentinized

forearc mantle

The serpentinization of the forearc mantle would haveimportant implications for tectonics of arcs and proto-arcs(Seno, 2005; Kirby et al., 2006). There are four ways toinfer the existence of the serpentinized mantle associatedwith the subduction of the altered basaltic crust of the sub-ducting slab (Seno, 2005); they are the downdip limit of int-

erplate thrust earthquakes, intraslab earthquakes withinthe crust, low-frequency tremors in the forearc, and seismictomography.

In Kyushu, interplate earthquakes are mostly occurringat the contact of the oceanic plate with the crust of theupper plate (Yagi and Kikuchi, 2003). Because the temper-ature structure of the plate interface is unknown in thisregion, the usefulness of the downdip limit of interplateearthquakes to infer the serpentinization (Hyndman

Fig. 11. The same as Fig. 10, but for S-wave velocity structures.



et al., 1997; Hyndman and Peacock, 2003; Seno, 2005) islimited in this region.

The low-frequency tremors along the SW. Japan forearc(Obara, 2002) are not seen in Kanto, N. Izu, E. Shikoku,and M.-S. Kyushu. Seno and Yamasaki (2003) inferredfrom this that the dehydration from the crust of the sub-ducting PHS is not occurring in these places, due to thesubduction of non-typical oceanic crust or island-arc crust.

The serpentinization of the forearc mantle was expected tobe weak in these regions, although a limited high Poisson’sratio region was found in the mantle wedge beneath Kantoaround depths of 30–40 km (Kamiya and Kobayashi, 2000;Matsubara et al., 2005). In other regions without low-fre-quency tremors, the Poisson’s ratio in the forearc mantleis low in E. Shikoku or modest in M.-S. Kyushu (Hondaand Nakanishi, 2003).




Our tomography results show that the Poisson’s ratio ishigh in the forearc crustal wedge in the northeastern part ofthe study area at the junction of Shikoku and Honshu(Figs. 14 and 16). Low-frequency tremors were detectedin this area (Obara, 2002). The high Poisson’s ratio regionin the crustal wedge may represent the effects of fluid-filledcracks induced by the dehydration of the PHS slab (seeTakei, 2002 for such effects). Previously, Ohkura (2000)detected intraslab earthquakes that occurred within thesubducting crust beneath W. Shikoku and N. Kyushu fromthe later phases traveling within the crust. If we accept thatintraslab earthquakes occur due to dehydration embrittle-ment, these intracrustal earthquakes represent dehydrationin the crust beneath W. Shikoku and N. Kyushu, which isconsistent with the above inference on the dehydration of

the PHS slab from the low-frequency tremors and the seis-mic tomography. It is, however, difficult to recognize theexistence of the high Poisson’s ratio zone in the forearcmantle (e.g., Fig. 14). The dehydration of the subductingslab thus might not be so persistent to produce serpenti-nized mantle in this region.

Our tomography results show that the Poisson’s ratio islow in the forearc mantle in M.-S. Kyushu (Fig. 12). Withthe absence of low-frequency tremors in this region (Obara,2002), this may imply that the dehydration from the crustof the subducting slab is not significant. The recent studyof intraslab earthquakes beneath Kyushu (Okamotoet al., 2005) made the situation more complex. They foundseveral intracrustal earthquakes in the slab in M. Kyushu,but not in S. Kyushu. One possible interpretation of this

Fig. 12. Distribution of Poisson’s ratio structures at seven depth slices. Red and blue colors denote high and low Poisson’s ratio, respectively. The range ofthe color scale to the right is 0.21–0.35 in the first two layers and 0.26–0.40 in the upper mantle layers. Other details are similar to those of Fig. 10. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



fact is to hypothesize that in M. Kyushu, dehydration isstrong enough to cause intracrustal earthquakes, but isnot yet in the stage to induce low-frequency tremors or ser-pentinization of the forearc mantle. A closer examinationof the dehydration of the subducting slab and the state ofserpentinization of the forearc mantle will be necessary inthis region.

5. Conclusions

We selected a large number of high-quality arrival timedata from JUNEC and Hi-net seismic networks and used

them to study P- and S-wave velocity structures down toa depth of 150 km beneath Kyushu, SW. Japan. Fromthe obtained velocity models, we further determined Pois-son’s ratio structures. Low-velocity anomalies are detectedbeneath the active and Quaternary volcanoes. However,there are no significant low-velocities in the crust beneathsouthern volcanoes. The subducting Philippine Sea slab isimaged clearly as a high-velocity zone down to a depthof 150 km, but with some portions of low- to intermedi-ate-velocity. It does not reach the depth beneath the volca-nic front in the northern region, but reaches it in thesouthern region. The seismicity in the crust of the upper




plate occurs in the low Poisson’s ratio region. The forearccrustal wedge in N. Kyushu at the junction of Shikoku andHonshu has a high Poisson’s ratio, suggesting that it isaffected by fluid-filled cracks due to dehydration of the

slab. The forearc mantle wedge in M.-S. Kyushu has alow Poisson’s ratio, suggesting that dehydration of the slabis not significant. This is consistent with the inference of thenon-existence of the low-frequency tremors in the latter

Fig. 13. Vertical cross sections of Vp, Vs, and Poisson’s ratio along line AA 0 (see Fig. 5 for location). Low-velocities and high Poisson’s ratio are shown inred; high-velocities and low Poisson’s ratio are shown in blue. Crosses show seismicity within 30 km wide-zone around the profile. Open triangle is Abuvolcano. The perturbation scale is shown at the bottom and for Poisson’s ratio; it varies from 0.21 to 0.35 in the top 20 km and from 0.26 to 0.40 at greaterdepths. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



Fig. 14. The same as Fig. 13, but along cross section BB 0.



Fig. 15. The same as Fig. 13, but along cross section CC 0 passing through Kujyu volcano (solid triangle).



region (Obara, 2002), although the occurrence of intraslabearthquakes in the crust in M. Kyushu (Okamoto et al.,2005) makes the situation more complex.

Future work is needed to get more fine seismic velocitystructures with higher spatial resolutions, because the

backarc side and the off-shore regions have not been accu-rately imaged owing to the paucity of rays passing beneaththem. Installation of Ocean Bottom Seismographs (OBS)off the coasts, and employment of the depth phases of off-shore earthquakes (Wang and Zhao, 2005) will help to

Fig. 16. The same as Fig. 13, but along cross section DD 0 passing through Aso volcano (solid triangle).



Fig. 17. The same as Fig. 13, but along cross section EE 0 passing near Unzen volcano (solid triangle).



Fig. 18. The same as Fig. 13, but along cross section FF 0 passing through Kirishima volcano (solid triangle).



solve this matter, and will enable us to study this area inmore detail.

Acknowledgements

We thank the Hi-net data center and JUNEC for pre-paring the arrival time data for academic research pur-poses. We also thank Dapeng Zhao for letting us use histomography source code. Comments by Masao Nakadaand the two anonymous reviewers greatly improved themanuscript. Most figures in this paper were produced usingGeneric Mapping Tools (GMT) software written by Wesseland Smith (1998). M.K. Salah thank JSPS for a Postdoc-toral Fellowship (2005/2007) to conduct research at theEarthquake Research Institute, University of Tokyo,where this work was done.

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Author's personal copy · Author's personal copy Imaging ofVp, Vs, and Poisson s ratio anomalies beneath Kyushu, southwest Japan: Implications for volcanism and forearc mantle wedge