Crustal deformation in Kyushu derived from GEONET data

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jb004690, 2007 Crustal deformation in Kyushu derived from GEONET data Hiromi Takayama 1 and Akio Yoshida 1 Received 11 August 2006; revised 12 February 2007; accepted 22 March 2007; published 21 June [1] An investigation was conducted on the horizontal crustal deformation in Kyushu using the coordinate data from GPS Earth Observation Network (GEONET) for the 4yearsfrom1998to2002,forwhichtheannualvariationsarecorrected.Weexaminedthe spatial differences in the displacement rate relative to the Amurian plate by subtracting the velocity of the Amurian plate at each of the Global Positioning System (GPS) stations. Models for interplate coupling in Hyuganada and the dilatational source beneath Sakurajima are obtained. The crustal deformation in eastern Kyushu is well explained by the model of interplate coupling in Hyuganada, which is strong in the north and weak in the south. When the effects of the interplate coupling and the dilatational source are removed, a southward extensional field is recognized in and to the south of the Beppu-Shimabara graben in western Kyushu. In southern Kyushu, to the south of around latitude 32 N, a southeastward movement is conspicuous. It must be noted that the displacement field is extensional, indicating that southern Kyushu is being dragged at the base by a flow in the mantle, not moving like a raft on the flow. It is probable that the flow in the mantle is driven by the rising of hot materials in the Okinawa Trough and a retreat of the trench where the Philippine Sea plate is subducting. A gap in the magnitude of the eastward displacement rate around latitude 32 N implies that the velocity of the flow in the mantle decreases substantially to the north of the line. The Beppu-Shimabara graben may be a continuation of the Okinawa Trough, but the significant southwestward displacement rate observed in the eastern part suggests that the deformation in the graben, at least to the east of Aso volcano, is affected by the westward movement of the forearc sliver produced by the oblique subduction of the Philippine Sea plate on the Nankai Trough off Shikoku Island. We think that the strong coupling in the Bungo Channel in our model is probably not real, but might be a result of our assumption that all the westward displacements in northeastern Kyushu, a part of which are actually due to the movement of the forearc sliver, are produced by interplate coupling. Citation: Takayama, H., and A. Yoshida (2007), Crustal deformation in Kyushu derived from GEONET data, J. Geophys. Res., 112,, doi: /2006jb Introduction [2] A number of studies concerning the crustal deformation on Kyushu Island in southwestern Japan have been conducted on the basis of the data from geodetic measurements and the Global Positioning System (GPS) network as well as on geological and seismic observations [e.g., Tada, 1984, 1985; Kimura, 1983; Kato et al., 1998]. In addition, various ideas have been proposed to interpret the features of these observations [e.g., Seno, 1999; Watanabe and Tabei, 2004; Kubo and Fukuyama, 2003; Nishimura et al., 1999]. However, there does not seem to be a consensus of opinion on how Kyushu Island has deformed on the whole and what causes the regional differences. The reason may be attributed to the vagueness in the principal tectonic sources that 1 Meteorological Research Institute, Tsukuba, Japan. Copyright 2007 by the American Geophysical Union /07/2006JB004690$09.00 are supposed to be producing a large part of the deformation, i.e., (1) the interplate coupling with the subducting Philippine Sea plate beneath Hyuganada, (2) the spreading of the Okinawa Trough, and (3) the motion of the forearc sliver on the southern side of the Median Tectonic Line (MTL). It is certain that interplate coupling exists in the sea region to the east of Kyushu, since magnitude 7-class earthquakes with a thrust-type mechanism have occurred repetitively at an interval of about 30 years [Utsu,1974],but the magnitude is small compared to that of earthquakes that have occurred along the Nankai Trough to the south of Shikoku (the largest earthquake that ever occurred in Hyuganada was M7.5). Furthermore, no large earthquake with a thrust-type mechanism is known to have occurred in the southern part of Hyuganada, so it is assumed that the interplate coupling is very weak or does not exist in the sea region to the east of the Osumi Peninsula [Earthquake Research Committee, 2004].Ahypothesishasbeenproposed that the Beppu-Shimabara graben, where the Unzen, Aso, Kuju, and Tsurumi volcanoes are aligned from west to 1of12

2 Figure 1. Map showing tectonic setting of Kyushu Island. B.S.G., Beppu-Shimabara graben; M.T.L., Median Tectonic Line; S.S.Z., Setouchi Shear Zone; B.C., Bungo Channel; A.V., Aso volcano; S.V., Sakurajima volcano; S.P., Shimabara Peninsula; O.P., Osumi Peninsula; K.B., Kagoshima Bay; T.I., Tanegashima Island; AM, Amurian plate; PH, Philippine Sea plate. The thin lines in the sea area indicate an isobathic line of 500 m. east, corresponds to the northeastern continuation of the Okinawa Trough, and Kyushu Island is split into north and south at that zone [Kimura, 1983;Tada,1984,1985].However, it is also argued that the principal deformation form in the graben is a right-lateral shear similar to that in the Setouchi shear zone [Tsukuda,1993;Kamata, 1992]. It is produced by the westward motion of the forearc sliver on the southern side of the MTL on Shikoku Island due to the oblique subduction of the Philippine Sea plate [Tabei et al., 2002]. Figure 1 illustrates the tectonic setting of Kyushu Island. [3] The objectives of this paper are to investigate the present characteristics of the crustal deformation in Kyushu by analyzing the GPS data from recent years in detail and to clarify how the above mentioned tectonic sources are involved in producing the deformation. 2. Data Processing [4] Our analyses are based on GPS Earth Observation Network (GEONET) coordinate data [Hatanaka et al., 2003] collected from 1 April 1998 through 1 April Since the data contain an annual periodic change [Murakami and Miyazaki,2001],wefirstderiveabestfitlinecomposed of a linear trend and an annual trigonometric curve representing the periodic component from the daily data using the least squares method. The average linear displacement rate per year at each GPS station during the 4 years is then calculated from the estimated trend component. The amplitude of the annual periodic change obtained by the processing was about 3 mm at most. We excluded data from two of the stations, Chikushino and Kagoshima1, because their displacement rates notably differ from those of the surrounding stations. There were no major earthquakes in the Kyushu district during the analyzed period to produce any coseismic changes that might affect the trend component, so no data were omitted to avoid the effect of earthquakes. A slow slip occurred in the Bungo Channel in 1997 [Hirose et al., 1999],butthis happened before the analyzed period. In the Chugoku and Shikoku districts, however, data from some of the stations near the foci of the 2000 Western Tottori Prefecture earthquake of M7.3 and the 2001 Geiyo earthquake of M7.0 were excluded because of noticeable coseismic steps at the times of those earthquakes. Kato et al. [1998] analyzed the crustal deformation in the Japanese islands, including Kyushu, on the basis of GPS data. We think that our results, based on a 4-year collection of data from which periodic annual variations have been subtracted, are more accurate and reliable than theirs because they used only 1 year of data collected from April 1996 to March [5] To calculate the velocities at the GPS stations in Kyushu Island and its surrounding areas relative to the Amurian plate, we used the same method as that taken in the work of Miyazaki and Heki [2001], which is based on the ITRF96 system [Sillard et al., 1998]and the Euler vector between Amurian and Eurasian plates obtained by Heki et al., [1999]. Since the GPS data provided by the Geographical Survey Institute are in the ITRF2000 system [Altamimi et al., 2002], we confirmed that the velocity at each station does not 2of12

3 and north-south components of the vector field representing the horizontal displacement rates seen in Figure 2. We then calculate the strain rates by performing spatial differentiation on the curved planes. Figure 3 shows the obtained twodimensional areal strain rate. A positive dilatation around Kagoshima Bay is notable, which is considered to be produced by an expansive source beneath Sakurajima volcano. [7] A number of research works have been conducted on the dilatational deformation around Sakurajima volcano [e.g., Iguchi and Ishihara, 1999; Tanaka et al., 2003]. In these studies, the location and magnitude of an expansive source beneath Sakurajima volcano were estimated, and the relationship between the changes in the magnitude of the source and the activity of Sakurajima volcano was examined. We now model the dilatational source using the 4-year GEONET data. [8] As is seen in Figure 2, the dilatational deformation around Sakurajima volcano overlaps the displacement velocity field oriented to the southeast. Noting that the prevailing southeastward displacement is larger on the eastern side than that on the western side and larger on Figure 2. Horizontal displacement rate per year calculated from trend component in GEONET coordinate data for 1April1998through1April2002. change within the limit of 0.01mm/year by the transformation between the ITRF96 system and the ITRF2000 system using the program by Tobita [2002]. Figure 2 shows the relative velocities that were obtained. In eastern Kyushu, movements toward the west-northwest are noticeable in the northern part, but the vectors change direction gradually to the west and to the south-southeast in the middle and southern parts. Owing to this change in the direction of the velocity field, the eastern side of Kyushu seems to be rotating anticlockwise. On the other hand, in the western side of Kyushu, conspicuous southeastward velocity is notable in the southernmost part, while small vectors pointing toward the southwest are discerned in the middle part, and almost no velocity is seen in the northern part. The southeastward movement of southern Kyushu has been a well-known feature, but the origin of the motion is still a matter of intense argument [Seno, 1999;Nishimura et al., 1999;Kubo and Fukuyama, 2003; Watanabe and Tabei, 2004]. 3. Modeling Dilatational Source Beneath Sakurajima and Interplate Coupling 3.1. Dilatational Source Beneath Sakurajima Volcano [6] Using the GMT [Wessel and Smith,1995],wefita curved surface with a minimum curvature to the east-west Figure 3. Areal dilatation rate calculated from spatial distribution of displacement rate shown in Figure 2. The intervals of the contours are strain/year. 3of12

4 the southern side than on the northern side, we try to express the spatial change in the displacement rates of the background field in the trapezoidal region delineated by broken lines in Figure 4a as a linear function of the location with axes in the east-west and north-south directions, using the least squares method. Figure 4b presents the obtained background displacement rates. It must be noted that the background velocity field is extensional. The direction of the largest extension rate calculated from the most appropriate linear function of the displacement rates is N147 E, and its magnitude is strain/year. The direction and magnitude of the smallest extensional strain rate are N57 E and strain/year. From Figure 4c, where the difference between the observed displacement rates and the estimated background velocity field are shown, the radial expansion centered at Sakurajima is clearly seen. Considering that the radial displacements are caused by expansive pressure in the magma reservoir, we estimated the depth and magnitude of the source on the basis of the Mogi s model [Mogi, 1958]. Using the results of previous studies [Iguchi and Ishihara, 1999;Tanaka et al., 2003], we assumed that the horizontal location of the source is on the northern coast of Sakurajima (31.64 N, E, the solid star mark in Figure 5a). In Figure 5b, the radial displacement rate at each GPS station is plotted against the distance between the source and the observation point. By applying the Mogi s model to the variation in the radial displacement rate with distance, we estimated the depth of the source to be about 8.9 km and the average volume increase rate over the 4 year period to be m 3 /year. The distribution of the vertical displacement rate around Sakurajima and the model fitting are shown in Figures 5c and 5d. The constraint imposed by the vertical displacements is not strong, but the change in their magnitude with the distance from the source is consistent with the curve calculated by the above model, which is obtained from the change in the radial displacement rate. [9] In the following analyses of the crustal deformation in Kyushu, we use displacement-rate data in which the effect of the volumetric expansion caused by the magma source beneath Sakurajima has been subtracted Evaluation of Interplate Coupling [10] To evaluate the interplate coupling, we assume that back slips occur in the direction of the relative motion between the Amurian and Philippine Sea plates. We adopt the estimations made by Miyazaki and Heki [2001] for the pole of rotation and the angular velocity between the two plates, according to which the direction and magnitude of the relative velocity in the Kyushu region are about N55 W and 7 cm/year. The difference in the direction between the 4of12 Figure 4. (a) Distribution of observed horizontal displacement rates around Sakurajima volcano. (b) Estimated displacement rates in background field. (c) Their differences. The displacement rates in the background field are assumed to change the magnitude as a linear function of the location. The most appropriate function representing the displacement rates is obtained by the least squares method using the data in the trapezoidal region indicated by the broken lines.

5 Figure 5. (a) Distribution of displacement rates around Sakurajima volcano, with estimated background displacement rates subtracted. (b) Fitting of Mogi s model to data for radial displacement rate. The abscissa represents the distance from the source, and the solid line represents the radial displacement rate on the ground surface calculated from the optimum model. (c) Distribution of vertical displacement rate around Sakurajima. (d) Fitting of vertical displacement rate of optimum source model to observational data. northern and southern parts of Hyuganada is as small as 0.5, and therefore we regard the direction as N55 W to estimate the back slips on the plate interface along the Nankai Trough to the east of Kyushu. Figure 6 depicts the segmentation of the coupling areas on the plate interface we use in this study to evaluate the interplate coupling. In the region along the Nankai Trough to the south of Shikoku and the Kii Peninsula, the segmentation is the same as that used by Miyazaki and Heki, which was first employed by Sagiya and Thatcher [1999]. In contrast to northern Kyushu, where the northwestward displacements associated with the interplate coupling are seen, opposite southeastward displacements are observed in southern Kyushu (Figure 2). Furthermore, thrust-type interplate earthquakes are not known to have occurred in the southern part of Hyuganada. These facts suggest that the interplate coupling is very weak or does not exist east of the Osumi Peninsula [Earthquake Research Committee, 2004].Consequently,wedonotplaceacoupling segment south of the focal regions of the two 1996 Hyuganada earthquakes, which showed thrust-type mechanism solutions. [11] We divide the coupled area into three zones, assuming that the strength of the interplate coupling changes spatially Figure 6. Segmentation of coupling areas and obtained optimum back slips. 5of12

6 Figure 7. Displacement rates (a) observed and (b) calculated in N55 W direction, (c) difference between them, and (d) calculated displacement rates in N35 E direction. 6of12

7 Figure 8. Displacement field in Kyushu with effects of interplate coupling as well as expansive source beneath Sakurajima volcano removed. from north to south in the area east of Kyushu. Furthermore, taking into consideration the difference in coupling strengths between the shallower (10 to 30 km) and deeper (30 to 60 km) areas, we divide the middle and southern zones into two parts each, ending up with five segments altogether in Hyuganada, that are presented in Figure 6. The configuration of the subducted slab in Hyuganada is based on the equidepth line model of the Philippine Sea plate from the Earthquake Research Committee [2004], originally taken from the works of Ichikawa [1997] and Uehira et al. [2001]. The inclination angle of the slab model is 16 for the shallower parts and 34 for the deeper parts. [12] Figures 7a 7c show the observed displacement rates in the direction of the relative plate velocity (N55 W), the displacement rates in the same direction obtained from the optimum model, and the difference in displacement rates between the observation and the model, respectively. Surface ground displacements produced by back slips are calculated using codes that were given by Okada [1992]. Figure 7c shows that the optimum model clearly explains the observed displacement rates. From Figure 7d, where the displacement rates in the direction perpendicular to the plate velocity (N35 E) are shown, we see that the interplate coupling produces negligible displacements in the direction perpendicular to the back slips. This feature validates the results of our analysis in which back slips on the segments are evaluated on the basis of the displacement rates in the direction of the relative velocity. [13] Some remarks should be made here regarding the optimum model for the distribution of the back slip rate. In performing the inversion analysis, we tried to minimize the differences between the observed and calculated displacement rates in the direction of the relative plate velocity (N55 W) for the GPS stations in the eastern part of Kyushu (32 34 N and E) using the least squares method. We first set the constraint that the magnitude of a back slip rate cannot exceed the relative velocity between the plates, and we assumed that the coupling coefficient in the deeper parts is half that in the shallower parts, following the analysis made by Miyazaki and Heki [2001]. However, the model obtained under these conditions failed to explain the displacements observed in western Shikoku and northeastern Kyushu. Consequently, we dropped the second assumption in our revised analysis and found that the coupling coefficient in the deeper segments in western Shikoku and in the northernmost segment of Hyuganada should be 1.0 to explain the observed displacement field. In the middle and southern zones of Hyuganada, the coupling coefficient of the optimum model in the deeper segments is half that in the shallower segments, for which the coupling coefficient is 100% for the middle and 15% for the southern zones. [14] Our inversion analysis indicates that the interplate coupling in the deeper segments in western Shikoku and in the northernmost part of Hyuganada is as strong as that in the shallower segments. We will discuss the implications of the result later. Before proceeding to the arguments, we examine the characteristics of the crustal deformation in Kyushu on the basis of the spatial distribution of the displacement rates presented in Figure 8, where the effects of the interplate coupling in Hyuganada as well as the effect of the expansive source beneath Sakurajima are eliminated. 4. Regional Characteristics of the Crustal Deformation [15] Some notable features of the crustal deformation seen in Figure 8 are the southeastward movement in the southern part and the southwestward movements at the stations in the Beppu-Shimabara graben. The southward movements are also discerned in the central part of Kyushu. In order to more clearly and minutely examine the spatial differences of these features, we made several maps showing the changes in the displacement rates in the specified directions in some of the selected zones. [16] Figure 9 shows the change in the displacement rate in the N120 W direction at the GPS stations in the Beppu- Shimabara graben. We consider the displacement in that direction to represent a continuation of the movement observed in the Setouchi shear zone on the northern side of the Median Tectonic Line, which is caused by the westward motion of the dragged forearc sliver due to the oblique subduction of the Philippine Sea plate beneath Shikoku [Kamata, 1992; Tsukuda, 1993; Tabei et al., 7of12

8 increases suddenly around latitude 32 N, where a jump in the displacement rate of about 5 mm/year is observed (Figure 11). This sudden increase is related to the feature by which the southeastward movement that characterizes the crustal deformation in southern Kyushu becomes conspicuous to the south of latitude 32 N (see Figure 8). [18] Figure 12 presents the change in magnitude of the displacement rate in the longitudinal direction of the selected rectangular zone (N150 E) in the southwestern part of Kyushu. The direction N150 E almost coincides with the direction of the principal strain axis of the deformation field (see section 3.1). It is to be noted that the displacement rate in the central part of the zone gradually enlarges toward the southeast, causing an extensional strain rate of about strain/year. The increasing trend in the displacement rate, however, disappears in the southernmost part of the zone near Tanegashima Island. We think it is significant that an appreciable extensional strain exists in the direction of the displacements. This indicates that southern Kyushu is not drifting like a raft, but rather that the crust is being dragged at its base in the direction of the movement of the flow in the mantle. The tectonic meaning of this feature is discussed in the next section. 5. Discussion [19] In the introduction, we enumerated three principal sources that are supposedly producing the crustal deforma- Figure 9. Change in southwestward (N120 W) displacement rate at GPS stations in Beppu-Shimabara graben. The triangle in Figure 9a indicates Aso volcano and arrows in Figure 9b indicate the position of Aso volcano along line A-B. 2002]. Note that the displacement rate suddenly becomes noticeably smaller to the west of the Aso volcano, located in the central part of the graben. The reduction of the displacement rate may be attributed to a balance between the dragging force produced by the oblique subduction of the Philippine Sea plate and the force caused by the flow in the mantle that is dragging the southern half of Kyushu to the southeast. The origin of the flow in the mantle is supposedly in the sea region to the west of Kyushu, which we will discuss in the next section. Before that we introduce some more characteristics of the spatial changes in the velocity field in Kyushu. [17] Figures 10 and 11 illustrate the changes in the northward and eastward displacement rates from the north to south in the western part of Kyushu. Figure 10 shows that a southward displacement starts to appear around the Shimabara Peninsula, situated in the Beppu-Shimabara graben. The displacement gradually becomes larger toward the south, but the trend of the increase stops around latitude 32 N. As a consequence of the increase in the displacement rate toward the south, a north-south extensional strain of about strain/year is observed in the central part of western Kyushu. Contrary to the feature of the southward displacement, the eastward displacement rate is almost constant in the northern and central parts of the zone but Figure 10. Change in northward displacement rate from north to south in rectangular zone in western Kyushu. 8of12

9 graben corresponds to a continuation of the Okinawa Trough [Kimura, 1983], the above mentioned observations indicate that the spreading scheme of the trough should be such that an extensional field is produced not only inside the trough but also outside it as well. We will return to this point again later. Another facet of the Beppu-Shimabara graben that should be noted is a significant movement in the southwesterly direction observed in its eastern part to the east of Aso volcano (Figure 9). [21] A remarkable feature of the crustal movement in western Kyushu is the discontinuity seen in the eastward displacement rate at about latitude 32 N (Figure 11), which is producing a noticeable left-lateral shear strain along the east-west zone. Many large and moderate earthquakes of magnitude 5 or larger, such as the 1997 northwestern Kagoshima earthquakes (M6.6, M6.4), the 1994 earthquake near Okuchi (M5.7), the 1979 earthquake near Hishikari (M5.0), the 1961 earthquake near Yoshimatsu (M5.3), the 1994 southern Miyazaki earthquake (M5.3), the 1911 earthquake near Miyazaki (M5.6), and the 1968 Ebino swarm earthquake (M6.1), are known to have occurred along this zone. Micro-earthquake activity is also high along the westward extension of this zone into the sea region. The mechanism solutions of the two 1997 northwestern Kagoshima earthquakes and the 1994 earthquake near Okuchi, whose principal pressure and tensional axes are in respective north- Figure 11. Same as Figure 10 except showing change in eastward displacement rate. tion in Kyushu Island. They are (1) the interplate coupling beneath Hyuganada between the subducting Philippine Sea plate and the Amurian plate, (2) the spreading of the Okinawa Trough at the back of the Ryukyu Arc, and (3) the westward motion of the forearc sliver on the south side of the Median Tectonic Line in Shikoku. Besides these, there is a local source beneath Sakurajima volcano that produces expanding strain fields around the volcano. In the previous sections, we obtained optimum models for the expansive source beneath Sakurajima volcano and for the interplate coupling in Hyuganada and then looked at the regional characteristics of the deformation fields in Kyushu on the basis of the displacement rates where the effects of those sources are eliminated. Now we would like to discuss their tectonic meanings. [20] HavingfoundanextensionalstrainfieldintheBeppu- Shimabara graben by analyzing the data from triangular measurements, Tada [1984, 1985] presented the idea that Kyushu Island is splitting to the north and south in that zone. Our analysis, which uses recent GPS data, indicates that the southward movement is not only seen in the Beppu- Shimabara graben but also to the south of the zone (Figure 10). Furthermore, the southward movement tends to gradually enlarge toward the south up to around latitude 32 N, resulting in an extensional strain field not only inside the Beppu-Shimabara graben but also to the south of the zone. If we take the hypothesis that the Beppu-Shimabara Figure 12. Change in southeastward (N150 E) displacement rate in rectangular region whose longitudinal side is directed toward N150 E. 9of12

10 east-southwest and northwest-southeast directions, are concordant with the accumulation of the left-lateral shear strain in the zone. Kakuta and Goto [2002] suggest that the high seismicity zone along latitude 32 N might be related to the tearing in the subducted Philippine Sea slab. However, we have a different idea about the origin that will be presented in the following paragraphs. [22] As discussed in section 3.2, the displacement rates in the direction of the relative velocity between the Amurian and the Philippine Sea plates in the northern and central parts of eastern Kyushu are well explained by a model in which interplate coupling does not exist in southern Hyuganada. In order to account for the remarkable southeastward movement in southern Kyushu, Nishimura et al. [1999] assumed an accumulation of the negative back slips on the plate interface in the eastern sea region to the south of the focal regions of the 1996 Hyuganada earthquakes (M6.9, M6.7). However, an unreasonably large back slip rate, one that exceeds the relative velocity between the land and subducting plates, is needed to explain the observed displacement field. This means that we have to look for the cause of the southeastward displacements in southern Kyushu not in Hyuganada but in the sea region to the west of Kyushu. [23] In a study of the velocity structures beneath the Ryukyu arc, Nakamura et al. [2003] found prominent P and S wave low-velocity zones, where the V P /V S values are high, along the Okinawa Trough at about 50 km depth. They suggest that hot mantle material is uprising from depths greater than 50 km along the trough. Therefore we think that a probable candidate for the source of the southeastward displacement in southern Kyushu is the generation of oceanic crust in the Okinawa Trough. Here it is important to note that extensional strain is produced by the southeastward displacement. This feature, in which the crust is stretched in the direction of the motion, is not explained by the idea that the crust of southern Kyushu is pushed away like a raft riding on the spreading oceanic base [Watanabe and Tabei, 2004]. What this does indicate is that southern Kyushu is being dragged at its base by the flow in the mantle. The idea proposed by Seno [1999], in which a viscous force is working at the interface between the crust and the outgoing flow of mantle material, is preferable to the raft hypothesis, though we think it is more likely that the southeastward flow in the mantle originates in the Okinawa Trough rather than in the rising plume that Seno assumes to exist in the sea region far to the west of Kyushu. We think that the retreat of the trench axis suggested by Aoki and Kagiyama [2006] may be caused by the mantle flow. The feature in which the increase in the displacement rate reaches maximum near Tanegashima to the south of Kyushu (Figure 12), indicating the disappearance of the extensional strain field, implies that the flow in the mantle changes its direction downward in this region. We noted that a northsouth extensional strain field is observed over a broad area in western Kyushu, which is not restricted to the zone inside the Beppu-Shimabara graben. This suggests that the crust of central Kyushu is also being dragged at its base by the flow in the mantle produced by an uprising of hot materials beneath the Beppu-Shimabara graben in its western part, which corresponds to a continuation of the Okinawa Trough. [24] The notable decrease in the eastward displacement rate in western Kyushu to the north of around latitude 32 N (Figure 11) indicates that the back arc spreading caused by the uprising of mantle materials in the Okinawa Trough drops its velocity (or more precisely, its eastward component) on the northern side of the zone. Here we need to recall the results of the inversion analysis of the back slip distribution on the plate interface between the Philippine Sea plate and the Amurian plate, in which the interplate coupling is very weak or does not exist south of the focal region of the 1996 Hyuganada earthquake. The line that separates the coupled and uncoupled regions is located near latitude 32 N. This correspondence indicates that the noticeable left-lateral shear zone in the east-west direction at about latitude 32 N is generated by changes in the magnitude of the back arc spreading on the western side of Kyushu and the strength of the interplate coupling on the eastern side. We think it may not be a mere coincidence that the source on the eastern side and the one on the western side, which are together producing deformations in Kyushu Island, change their magnitude and strength at almost the same latitude. Although we cannot presently speak with confidence, we propose the idea that a fast flow in the mantle is bringing about the retreat of the Nansei Island trench and is causing a weakening of the interplate coupling in the southern part of Hyuganada. [25] It is intriguing that the optimum model for the back slip distribution on the plate interface indicates a strong coupling in the deeper part beneath western Shikoku and the Bung Channel because some slow slips have been intermittently observed in the southern Bung Channel [Hirose et al., 1999; Hirose and Obara, 2005], suggesting that the interplate coupling there may be rather weak. No slow slip events have been observed over the analyzed period from April 1998 through April Therefore our inversion results do not necessarily contradict the estimation of a weak coupling in the area made by Miyazaki and Heki [2001], who used data from a 3-year period from 1996 through 1999, during which a notable slow slip event (from March to December 1997) was observed. However, our findings still appear to disagree with the general idea that the interplate coupling in the deeper regions is weak when compared to that in the shallower regions. We think it is most probable that the apparently contradictory result originates in our assumption that all of the northwestward displacements are produced by interplate coupling. It may be that part of the displacement around the Bungo Channel is caused by the westward movement of the forearc sliver being dragged by the obliquely subducting Philippine Sea plate beneath Shikoku. [26] On the other hand, the interplate coupling coefficient for the southern segment in Hyuganada is only 15% of the relative velocity in the optimum model. This especially small back slip rate is considered to be due to the forward slip that continued till 1998 after the two 1996 earthquakes (M6.9, M6.7) in the region [Nishimura et al., 1999]. 6. Summary [27] We investigated the characteristics of the horizontal crustal deformation in Kyushu and have discussed their 10 of 12

11 tectonic significance using GEONET data collected from April 1998 through April We modeled the expansive source beneath Sakurajima volcano and the back slip distribution on the plate interface in Hyuganada. A notable feature of the obtained optimum model for the interplate coupling is that the coupling coefficient in the deeper parts beneath western Shikoku and the Bungo Channel is as large as that in the shallower parts. This result differs from that of Miyazaki and Heki [2001], but we think the two estimations do not necessarily contradict each other, since the analyzed periods are different, and a noticeable slow slip event occurred during the period analyzed by Miyazaki and Heki. Furthermore, we think that the strong coupling in the Bungo Channel may be partly caused by our assumption that the northwestward displacements, a part of which may represent the westward movement of the forearc sliver, are all produced by interplate coupling. After removing the effects of the interplate coupling and the expansive source beneath Sakurajima, we examined the regional characteristics of the displacement field in Kyushu. The following are our findings. [28] 1. Western Kyushu in and to the south of the Beppu- Shimabara graben is in an extensional strain field in the north-south direction. [29] 2. Southern Kyushu, to the south of around latitude 32 N, stretches southeastward in the direction of the displacement. [30] 3. The observation of an extensive strain indicates that Kyushu Island is not drifting like a raft but that the crust is being dragged at its base in the direction of the movement. [31] 4. It is probable that the dragging is caused by a flow in the mantle whose origin is attributable to the back arc spreading at the Okinawa Trough. [32] 5. The notable decrease in the eastward displacement rate to the north of around latitude 32 N indicates that the eastward spreading velocity at the Okinawa Trough decreases substantially in the northern region. [33] 6. A model in which the interplate coupling does not exist to the south of around latitude 32 N explains well the displacements in eastern Kyushu. [34] 7. A noticeable left-lateral shear zone exists in the east-west direction around latitude 32 N. [35] 8. The shear zone appears to be generated by the change in the spreading velocity at the Okinawa Trough as well as the change in strength of the interplate coupling in Hyuganada. [36] 9. A noticeable southwestward displacement is observed to the east of Aso volcano in the Beppu-Shimabara graben. [37] 10. 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