St phane Mazzotti, Xavier Le Pichon, 2 and Pierre Henry

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B6, PAGES 13,159-13,177, JUNE 10, 2000 Full interseismic locking of the Nankai and Japan-west Kurile subduction zones' An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS St phane Mazzotti, Xavier Le Pichon, 2 and Pierre Henry Laboratoire de G ologie, Ecole Normale Sup rieure, CNRS UMR 8538, Paris Shin-Ichi Miyazaki Geographical Survey Institute, Tsukuba, Japan Abstract. We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismicoupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan-west Kurile trench on the other. These "reference free" velocities are close to those predicted by plate motion models with respecto Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km x 30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan-west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku-Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japanwest Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrasto the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long-term deformation of the Japanese islands. 1. Introduction Owing to the large amount of data as well as the long time period of measurements, the instantaneous deformation of the Japanese islands related to subduction processes has been intensively studied in the past several decades using first triangulation and lately spatial geodesy. The dense Global Positioning System (GPS) network deployed in Japan by the Geographical Survey Institute (GSI) since 1994 [Abe and Duff, 1994] has shown that most of the present state of strain 'Now at Pacific Geoscience Centre, Geological Survey of Canada, Sidney, British Columbia, Canada. 2Also at Coll ge de France, Paris, France. Copyright 2000 by the American Geophysical Union. Paper number 2000JB /00/2000JB ,159 of the crust results from interseismicoupling of the island arc with the Pacific and Philippine Sea subducting slabs [Tabei et al., 1996; Karo et al., 1998; Le Pichon et al., 1998; Ozawa et al., 1999]. Using the permanent GPS data obtained for all of Japan by the GSI in 1995 [Miyazaki et al., 1997], Le Pichon et al. [1998] showed that to a first order, the velocity field is parallel to the Pacific subduction vector in northern Japan (about N295 ø [DeMets et al., 1990]) and to the Philippine Sea vector in southern Japan (about N310 ø [Seno et al., 1993]). They used an elastic dislocation model approach to demonstrate thathe high GPS strain rates (of the order 10-7 yr -I) are mostly due to the transient interseismic elastic loading of the island arc by the slabs along the Japan trench and the Nankai trough. They conclude that to a first order, the central Nankai and the Japan trenches were strongly coupled during Because of the relatively small time and space distribution of the GPS data, some important questions were not

2 13,160 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES I,. North American lj 451 / ' /,? /1' -, 11 ', 't. Pla,te?, L.I,._ _ 1 predicted Philippine Sea/Eurasia and Pacific/Eurasia or Pacific/North America vectors [DeMets et al., 1994] confirms that the coupling ratio is close to I along both subduction In a second approach, the high density of GPS stations in 1996 and 1997 (the average distance between stations is of the order of 35 km; see Figure 1) allows us to resolve the ' ' :' / distribution of the coupling ratio within the Philippine Sea 40 /? ' Japan Sea and Pacific seismogenic zones at <100 km scale. This detailed modeling is used to estimate whether the coupling is heterogeneous at this scale. We also test the possibility of extending the postseismic motion related to the 1994 Sanriku- Oki earthquake to the 3 years following the event. The results m t' '. -,'.'.?.,?.,. ' I are coherent over the data time span and indicate that along both northern and southern Japan subduction zones, the seismogenic zone is best described by homogeneous full II ' ' "":,. 5' coupling (ratio of 1) with a few patches of decoupling (ratio of 0) and almost no intermediate value. The free slip areas are ' ' ' J_O ' " ' accounted for in one case by postseismic motion related to the i - F,...-', n.ipp ne '" 'L f'/. 'l ' - r4 Plate 1994 Sanriku-Oki subduction earthquake and in the other case by a downdip extent of the locked zone smaller than the one adopted in the model. This homogeneous elastic loading of Figure 1. Distribution of the GPS stations in 1995 and the seismogenic zone is in contrast with the highly The permanent GPS stations established by the Geographical heterogeneous coseismic slip distribution. This suggests that Survey Institute, Japan, in 1995 (open triangles) and 1996 asperities which may be present in the subduction zones (solid dots) are shown. The 1997 GPS coverage is mostly identical with the 1996 one. The areas of study are outlined by the dashed boxes. cannot be detecte during the interseismic period as zones of heterogeneous elastic loading. Finally, we interpret the residual deformation field obtained after subtracting the transient interseismic effect of elastic loading, within the geodynamicontext of Japan. considered by LePichon et al. [1998]. First, it may be hypothesized that the coupling is, in fact, heterogeneous along 2. GPS Data, Interseismic Phase Modeling the subduction zones, which would make its evaluation and Inversion Method complex. Second, a zone of strong postseismic slip was 2.1. Estimation of the GPS Strain Rates and Errors evidenced after the 1994 Sanriku-Oki earthquake, which The GPS strain rate tensors are derived from the measured affected the GPS data. On the basis the 1995 data set, Heki et al. [1997] showed that the afterslip follows a logarithmic velocities averaged over 1996 and We excluded from decay law for the first year after the earthquake. One may this estimation the 1995 data set because of its small spatial wonder whether this afterslip law can be extrapolated for resolution (about 200 stations in 1995 over the whole Japan longer times and how it would affect the interseismic territory against 600 in 1996 and 900 in 1997 [Miyazaki et al, coupling of the northern Japan trench over such a period. 1997]). The strain rate field was first computed with a Likewise, the relation between postseismic motion and the Delaunay triangulation [Shewchuck, 1996] of the GPS network. Final values of the strain rates were estimated at small seismic coupling in the Japan trench (0.24 after Peterson and Seno [1984] or 0.18 after Pacheco et al. each GPS station by integrating strain over the triangles [1993]), as opposed to the Nankai Trough (1.0 according to having this station as a summit. The strain rate tensors Ando [1975] or 0.5 after Peterson and Seno [1984]), is obtained using this local averaging and projection method another question to be addressed. This discussion especially appear more consistent and less sensitive to data noise than arises since both the southern and northern Japan subduction those computed for individual triangles. zones are similarly locked based on GPS analysis. As mentioned by Le Pichon et al [1998], the GPS-derived In this study, we use a fully three-dimensional elastic strain rates are in good qualitative agreement with those loading model [Flfick et al., 1997] to analyze the GPS obtained by integrating active faults and inland earthquakes in velocity data obtained in 1995, 1996 and 1997 by the northern Japan [Tsukahara and Kobayashi, 1991 ]. The Geographical Survey Institute (Figure 1). We first determine principal axes of the tensor are also consistent with those the effective coupling ratio between the downgoing slab and determined from triangulation data over a century [Shen-Tu the overriding plate in the central Nankai trough and the and Holt, 1996]. The estimated standard error on the GPS Japan-west Kurile trench areas. We estimate the horizontal strains rates is 1.5x10-7 yr - for the Nankai trough area and component of the strain rate tensor at every GPS station and 7x l 0 - yr ' for the Japan-west Kurile trench zone. invert the resulting strain rate field to determine the average regional subduction velocities in southern and northern Japan Forward Problem: Elastic Modeling These velocities are free of any reference frame and can be of Interseismic Loading compared with the relative velocities predicted by plate A widely used model for the interseismic phase of the kinematic models. The comparison with NUVEL-1A zones. subduction process describes the elastic loading along the

3 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,161 seismogenic zone by a combination of steady state slip along criterion (L2 norm) for the misfit function. We systematically the complete subduction interface and an opposite direction invert with both norms for comparison purposes: a large slip (back slip) along the locked portion of the fault [Savage, difference between inversions would sugges that data are 1983]. The virtual interseismic back slip is modeled by an inconsistent or that the elastic model is inappropriate. Using edge dislocation within an elastic half-space. This approach the L2 norm inversion method, we estimate the covariance assumes that the totality of the accumulated interseismic operator of the model and thus the standard error of the deformation is elastic and thus that no long-term deformation effective subduction velocity obtained [Tarantola, 1987]. The accumulates within the upper plate. Although the issue of permanent deformation buildup is still being debated, the standard deviation is of the order of 6 mm yr - for the Nankai trough subduction velocity and of 6-9 mm yr ' for the Japan approximation of a purely elastic behavior of the subduction trench velocity. The velocity vector should be equal to the system during the interseismic phase is a reasonable actual average subduction velocity if the seismogenic zone is approach, and the differences with the deformation estimated fully locked. by more complex models do not exceed a few per cent. Indeed, Cohen [1994] showed that a model that accounts for gravitational forces and allows permanent deformation over earthquake cycles [e.g., Sato and Matsu'ura, 1988] is conceptually similar to the elastic model [Savage, 1983] if the earthquake ruptures the entire elastic lithosphere. Knowing the subduction velocity, we can define the reference frame fixed to the upper plate that should be used for the GPS velocity field. We thus set the subduction velocity and invert the GPS velocity field to resolve the coupling ratio across each of the fault elements that compose the seismogenic zone. In order to keep our inversion from We thus follow the elastic back slip dislocation approach being underdetermined, we group point sources into fault of Savage [1983]. We define the geometry of the subduction elements of about 35 km by 30 km along the Japan and Kurile fault and the locked zone extent along the Japan trench based on the depth contours of the upper seismic plane [Hasegawa et al., 1994]. For the Nankai trough the geometry of the fault surface is based on the depth contour of the upper slab surface, and we test two possible models for the locked zone trenches and about 55 km by 15 km along the Nankai trough. In both cases this grouping leads to a seismogenic surface described by 190 to 200 elements. To avoid anomalous values in the poorly resolved regions, the coupling ratio is preconditioned to be in the [0,1] physically meaningful extent, one following the thermal modeling of the slab interval. With this additional constraint on the solution the [Hyndman et al., 1995], and the other based on coseismic slip modeling (see section 3.1). The precise fault geometry can be fully accounted for using a three-dimensional dislocation model based on the point source solution of Okada [1985]. To simplex optimization method [Nelder and Mead, 1965; Press et al., 1996] was found to give robust results. This method does not require assumption of a starting model or preconditioning other than assuming that the coupling ratio is compute the surface velocity and strain rate fields associated in the [0,1] interval. This method is not commonly used for with the interseismic loading of the upper plate, the slab surface is divided into elements that are approximated by a this type of problem. We thus present in the appendix a description of the inversion method, as well as a series of tests point source at their center of mass. The final deformation that define its robustness and spatial resolution. field is obtained by integrating the solution for the deformation at each source over the whole fault surface. A 3. Regional Strain Rates and Subduction detailed description of the method is given by Flack et al. Velocity [1997]. This method allows one to model relatively complex geometry including zones with moderate bending, such as the 3.1. Nankai Trough transition from the Japan trench to the west Kurile trench. Our first approach consists of estimating the subduction This method also permits us to place a transition zone at velocity based on the GPS strain rates. We define the the upper and lower ends of the seismogenic zone to avoid geometry of the locked zone along the Nankai trough unrealistic edge effects. For the Nankai trough and Japan following the depth contours of the top of the Philippine Sea trench we define both upper and lower transition zones by a plate as given by numerous authors from the seismicity linear decrease of the slip rate magnitude from the full [Hirahara, 1981; Ito, 1990; Kanamori, 1972] or based on subduction velocity (fully locked fault) to zero (unlocked fault modeling [Ando, 1975, 1982; Satake, 1993; Yoshioka, fault). The width of these zones is defined in section ]. We set the downdip limit of the seismogenic zone at 2.3. Inversion Methods 25 km depth with a transition zone down to 33 km depth. The upper limit is set to 8 km depth with a transition zone Using this point source approach, the interseismic phase extending to the trench (see Figure 2). This distribution can be parameterized by the magnitude and direction of slip follows a thermal model of the central Nankai subduction rate at each source defining the seismogenic zone. The slip rate at the point source is defined as the product of the regional subduction velocity with the coupling ratio at this point. We follow two approaches when inverting the GPS zone along two profiles off Shikoku and off Kii peninsula [Hyndman et al., 1995], which shows that the extent of the seismogenic zone is controlled by the 350øC isotherm with a transition extending to the 450øC isotherm and an updip limit data. First, assuming a full coupling across the seismogenic marked by the 150øC isotherm. Hyndman et al. [1995] zone, we invert the GPS strain rates to determine the effective showed further that their estimate of the extent of the locked average regional subduction vector along the southern and zone accounts for both the interseismic and the coseismic northern Japan trenches. The linear relationship between the vertical motion based on leveling data. data (strain rate tensor) and the model (eastward and northward components of the subduction vector) is inverted To estimate the effective regional subduction velocity, we restricted the strain rate data set to the area of extent of using either a least-error criterion (L 1 norm) or a least squares significant interseismic loading by the Philippine Sea plate.

4 13,162 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES,/ ' 2xl 0-07 yr" Predicted Strain Rates ½ ,:, :., 34 SHIKOKU L2 norm Result PHS / EUR...- "< I 20 mm yu Figure 2. GPS strain rates and effective subduction velocity in southwestern Japan. The geometry and extent of the seismogenic zone shown by the isodepth contours (in km) is from Hyndman et al [1995]. The dark shaded area corresponds to the fully locked part, and the light shaded area corresponds to the upper and lower transition zones. The open strain crosses representhe instantaneoustrain rates averaged over 1996 and The solid strain crosses representhe strain rates predicted by the best fit interseismic loading model (L 1 norm). The best fit effective subduction velocities are shown for L 1 and L2 norms, with the associated 95% confidence ellipse (solid and open head arrows, respectively). Philippine Sea/Eurasia motion is shown by the open arrow (after Seno et al [1993]). This area has been shown by Le Pichon et al. [1998] to be limited to Shikoku island and Kii peninsula, south of the Setouchi deformation zone (see Figure 2). A leveling profile across the Kii peninsula confirms that the areas of coseismic subsidence and interseismic uplift do not extend farther than 250 to 300 km away from the trench [Thatcher, 1984]. This data set as well as the best fit strain rates estimated for the L 1 norm inversion are shown on Figure 2. The effective subduction velocities obtained are represented for the L1 norm result and the L2 norm one with its 2 confidence ellipse. They are compared with the Philippine Sea/Eurasia relative motion proposed by Seno et al [1993] (see Table 1). Both vectors agree closely with the predicted velocity, and the velocity obtained with the L 1 norm inversion shows an almost perfect fit with the plate motion vector. Although Seno et al [ 1993] proposed two different rotation poles for the Philippine Sea/Eurasia motion, we have chosen the slowest of the two motions (45 mm yr ' rather than 57 mm yr - ) because recent Table 1. Subduction Velocity Along the Japan-West Kurile Trench and Central Nankai Trough Subduction zone L 1 Velocity a L2 Velocity a (J b V/EUR c V/NAM c V/Az d V/Az d {J!/ 02 / cor V/Az d V/Az d South Tohoku 100 / N / N / 7.17 / / N / N296 Hokkaido 79 / N / N / 9.54 / / N / N298 Both Zones 83/N / N /5.73/ /N293 83/N297 South Japan 45/N312 40/N /5.67/ /N310 a L I and L2 velocity: effective subduction velocity estimated by inversion of the GPS strain rates using a L1 and L2 norm error function. b 95% confidence ellipse of L2 velocity vector (o,o 2, large and small axis of thellipse in mm yr4;cor., correlation factor). c For northern Japan, subduction velocity of the Pacific plate with respecto Eurasia (EUR) and to North America (NAM) as predicted by NUVEL-IA[DeMets et al ]; for southern Japan, subduction velocity of the Philippine Sea plate with respecto Eurasia as predicted by the rotation pole given bsffeno et al. [19 93]. u V, velocity modulus in mm yr' ; Az, azimuth in degrees.

5 .. _ MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,163 Predicted Strain Rates I GP Strain Rates,10-ø7 yr" HOKKAIDO SEA L.. JAPAN ',..,7' -'... ": TOHOKU. ',': ß......? -.:::..:::.;:.::; " ":::' :'" '' ' : " KANTO L1 norm Result (!----- L2 norm Result PAC / NAM PAC / EUR 30 mm yr-',,,, Figure 3. GPS strain rates and effective subduction velocity in northern Japan. Symbols and data set are the same as for Figure 2. The seismogenic zone geometry is constrained by the distribution of seismicity [Hasegawa et al., 1994; Kawakatsu and Seno, 1983; Pacheco et al., 1993; Seno, 1979]. The predicted motions are Pacific/Eurasia (shaded arrow) and Pacific/North America (open arrow) after NUVEL-1A [DeMets et al., 1994]. geodetic surveys on islands within the Philippine Sea plate [Kato et al., 1996] indicate that the measured motion with respect to Eurasia is in better agreement with the "slow solution". These results show that the upper plate and the downgoing slab along the central Nankai trough were essentially locke during the period Japan-West Kurile Trench The geometry of the Japan-west Kurile subduction interface is well known through numerous studies of the interplate seismicity [Hasegawa et al., 1994; Kawakatsu and Seno, 1983; Pacheco et al., 1993; Seno, 1979]. We adopt the lower limit of the fully locked zone at 55 km depth, with a transition zone down to 70 km (see Figure 3), following the precise distribution of interplate earthquakes by Hasegawa et al. [ 1994]. In order to avoid strong interference with the deformation of the eastern Japan Sea convergence zone we restricted the strain rate data set used in the inversion to the zone east of 138øE (Figure 3). We also eliminated sites in northern Tohoku affected by the postseismic effect of the 1994 M 7.7 Sanriku-Oki earthquake [Heki et al., 1997] as well as sites located in the southwestern Hokkaido peninsula. The latter ones may be affected by the 1993 M 7.8 Hokkaido-Nansei- Oki earthquake [Hashimoto et al., 1993; Mendoza and Fukuyama, 1996]. Dividing northern Japan into two zones, we run three independent inversions, one for the northern Kanto and southern Tohoku zone, one for the Hokkaido area,

6 13,164 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES and one for both zones. The results of the strain rate inversions are shown in Figure 3 by the three sets of velocity vectors (see also Table 1). The central set off northern Tohoku corresponds to the inversion using both GPS data sets together and the corresponding predicted strain rates are shown for the LI norm result. For comparison, we show the Pacific/Eurasia (PAC/EUR) and the Pacific/North America (PAC/NAM) relative plate motions predicted by the NUVEL- I A model [DeMets et al., 1994]. These inversions indicate that the average effective subduction velocity along the Japan-west Kurile trench is close to the PAC/EUR and PAC/NAM motions and that these trenches were also essentially locked during the measurement period Comparison With Previous Studies Along both the central Nankai and Japan-west Kurile subduction zones, the effective subduction velocities coincide with the relative plate motions within the 20 confidence ellipse. Thus, in both areas the seismogenic planes are presently essentially locked, as proposed by Le Pichon et al. [1998]. This conclusion had already been reached on the basis of analysis of GPS, long-term geodetic and leveling data for the Nankai subduction [Hyndman et al., 1995; Oza,va et al., 1999; Savage and Thatcher, 1992]. Using the GPS data set, Oza,va et al. [ 1999] estimated that the Nankai trough was strongly coupled during this period, but they found a counterclockwise rotation with respect to the relative subduction vector proposed by Seno et al. [1993]. They suggested that this discrepancy came from the use of velocity vectors referenced to a fixed station in northern Chugoku. Our study confirms this interpretation as the reference frame they adopted has a 10 mm yr ' eastward motion with respect to the Eurasia reference frame obtained in our inversion. In contrast, for the Japan trench subduction zone the study of 1 O0 years of geodesy and leveling data led Hashimoto and Jackson [1993] and Shen-Tu and Holt [1996] to propose that the interseismic phase was best described by a medium coupling coefficient, of the order of 0.4 to 0.6. This discrepancy was discussed by Le Pichon et al. [1998]. They attributed it to the large interplate earthquakes that affected northern Japan during the hundred year measurement period and released a significant part of the stored interseismic elastic deformation. Indeed, the sum of the 0.35 to 0.60 coupling ratio estimated from triangulation and leveling with the small seismic coupling ratio of 0.2 to 0.5 estimated along the Japan trench [Pacheco et al., 1993; Peterson and Seno, 1984] is compatible with the almost full coupling derived from the GPS data. Besides, Scholz and Campos [1995] had proposed that a small seismic coupling value can be explained by a smaller width of the effective locked surface compared to the observed seismogenic zone. To investigate this possibility, we estimate the minimum extent of the Japan-west Kurile locked zones and also of the central Nankai one Width of the Subduction Locked Zones On the basis of the inversion of long-term triangulation data in northern Japan, Shen-Tu and Holt [1996] found that the optimal lower limit of the locked zone is at 55 km depth. This is in good agreement with the 50 to 55 km depth lower limit of the interplate background seismicity [Hasegawa et al., 1994; Kawakatsu and Seno, 1983]. It also agrees with the landward maximum extent of the zone of rupture of large interplate earthquakes there [Nakayama and Takeo, 1997; Pacheco et al., 1993]. However, using depth estimates of interplate large events (M > 5.5) along the subduction zone, Tichelaar and Ruff [ 1993] suggested that the maximum depth of coupling along the Japan and the Kurile trenches is about km, with a deeper extent around the trench junction down to km. We test the possibility of a smaller locked zone by estimating the misfit between the G PS strain rate field and the one predicted by models in which the maximum or the minimum coupling depths of the seismogenic zone are reduced. The predicted error values are then normalized to the ones obtained by our standard model. Figure 4 shows the variation of the error function againsthe coupling depth for the Japan-west Kurile trench (Figure 4a) and the Nankai trough (Figure 4b). In both subduction zones the unlocking of the lower or two upper rows of the seismogenic zone produces a small increase of the error that we do not consider significant. The resolution tests that we presented previously demonstrated that these rows are the least constrained in the model. On the contrary, the unlocking of a larger portion of the upper or lower part of the seismogenic zone leads to a LIJ Coupling Depth (km) (b) W. Kurile i!).?: i}}}:!!i{{ :..: : {{{'"ii{?-::jii!!:i...':!i i i i ] ß downdip extent,.o.-...,-... :: ::' Coupling Depth (km) Figure 4. Minimum and maximum extents of the locked zone estimated by the error function variations. The solid triangles and circles representhe normalized error function versus the coupling depth updip and downdip extent, respectively. The shaded area shows the estimated minimum locked core of the seismogenic zone. (a) Japan-west Kurile trench subduction zone and (b) central Nankai trough subduction zone. (a)

7 MAZZOTTI ET AL ß FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,165 significant error augmentation. These intermediate depth strain rate field is not compatible with a subduction velocity zones are also the best constrained in the inversion model (see smaller than 75 mm yr -, as demonstrated in section 3.2. The appendix, section A3). North American and Okhotsk reference frames thus yield Thus, within the resolution of our model we can estimate inconsistent results when inverting from strain data and from the minimum extents of the fully locked core of the velocity data, respectively. For this reason, we choose the seismogenic zone to correspond to the region between 25 and Eurasia reference frame for the GPS velocities used in the 55 km depth for the Japan trench and between 15 and 24 km coupling distribution determination. depth for the Nankai trough. These depth ranges correspond to an actual surface of the minimum locked zone of about 4. Spatial and Temporal Distribution 65% to 85% of the total seismogenic zone surface (excluding of the Coupling Ratio the upper and lower transition zones that both cover about 15% of the model surface) Central Nankai Trough To confirm this estimation of the minimum surface of the Along the central Nankai trough, we invert the GPS data to locked zone, we invert the GPS strain rates in northern Japan resolve the coupling distribution using subduction velocities and estimate the subduction velocity associated with a smaller ranging between 49 mm yr - toward N310 ø and 44 mm yr - locked zone. We run two tests, the first with a locked section toward N308 ø from west to east, values predicted by the without the upper and lower fault segments rows, the second Philippine Sea/Eurasia rotation pole proposed by Seno et al. with a locked section without the upper and the two lower [1993]. The 1996 and 1997 GPS velocity fields are referred to rows. These two models correspond to fault surfaces of 70% Eurasia, and only GPS sites located southward of the Setouchi and 50%, respectively, of the standard model extent. The deformation zone are used to constrain the inversion with data results indicate that in order to fit the observed strain rates that exclusively represent elastic loading at the Nankai trough. with the smaller locked zone models the subduction velocity As mentioned in section 3.1, the geometry of the has to be 101 and 160 mm yr - respectively. The relative seismogenic zone and its downdip and upper extents are well velocity between the Pacific plate and northern Japan constrained by the analysis of the interseismic and coseismic predicted by NUVEL-1A ranges between 83 mm yr - for geodetic measurements and by the thermal modeling of the PAC/NAM and 90 mm yr ' for PAC/EUR (see Table 1). subducting slab [Hyndman et al., 1995]. The 350øC isotherm Comparison with the small locked zone inversion results coincides with the 25 km isodepth except below the Kii confirm that the maximum possible reduction of the locked peninsula, where the isodepth and isotherm contours diverge zone width in northem Japan is <30% and that the locked slightly. This difference led us to investigate two possible zone extends at least down to 50 km depth. models for the seismogenic zone. The first one is based purely on the isodepth contours of the top of the Philippine Sea plate 3.5. Determination of the Appropriate Reference Frame and the downdip limit of the locked and transition zones are for the Velocity Field set to 25 and 33 km depth, respectively. The second model is Although the GPS strain rate field allows for the estimation characterized by the same geometry of the fault surface, but of reference-free subduction velocities, its resolution is not as the downdip extent of the locked zone is based on the 350øC good as the velocity field because of the noise contamination isotherm as proposed by Hyndman et al. [1995]. The two inherent in the differentiation from velocity to strain rate field. models are similar below western and central Shikoku, but the We thus invert the GPS velocities to resolve in greater detail temperature-based model corresponds to a deeper coupling the coupling distribution along Japanese subduction zones. limit, down to 35 km depth below eastem Kii and to 30 km The use of velocities implies that this data set must be depth below western Kii (see Figure 5). In both models we mapped into a proper reference frame that will lead to a place the upper limit of the locked portion at 8 km depth, with subduction velocity similar to the one estimated from strain a transition zone up to the trench. rate inversion. However, the plate configuration around the The inversion results and the best fit velocity field are Japanese islands is still a matter of debate [DeMets, 1992b; shown on Figure 5 for both models in 1996 (Figures 5a and Seno et al., 1996], and thus the reference frame to be used for 5c) and 1997 (Figures 5b and 5d). The coupling ratio at each the GPS data is not clearly defined. This question has been segment of the seismogenic zone is shown by a circle with discussed by Le Pichon et al. [1998], who showed that the shaded scale fill. The four inversion show that although the Eurasia reference frame is justified in southern Japan by the coupling ratio is free between 0 and 1, the best fit distribution good agreement between the 1995 GPS data and the corresponds to a fully locked seismogenic zone, with some Philippine Sea/Eurasia rotation from Seno et al. [1993] as small areas of decoupling and few intermediate coupling well as the one determined from recent geodetic surveys by values. The misfit is slightly smaller for inversions with the Kato et al. [1996]. This is confirmed by our analysis of the temperature-based model, but the difference may not be 1996 and 1997 measurements. significant (Table 2). The differences in the distribution of Conceming northern Japan, Le Pichon et al. [1998] also coupling between the temperature-based model and the demonstrated that the choice of a reference frame different isodepth one are small but suggest that below the Kii from Eurasia, such as Okhotsk or North America, leads to a peninsula the lower limit of the seismogenic zone is shallower significant misfit between the observed and modeled than indicated by the thermal estimation. Two consistent free velocities. If the northern Japan velocity field is mapped into slip zones can be identified on Figure 5. The first one is the North American or Okhotsk frame, the GPS velocities are limited to the 1996 data inversion and is located between 10 mm yr - smaller than if the reference is Eurasia. This Shikoku and Kii along the two lower segments of the locked velocity field then is best fitted with an effective subduction zone. It is presumably associated with the clockwise deviation velocity of the order of 50 mm yr -, whereas the measured of the GPS vectors southwest of the 1995 Kobe right-lateral

8 13,166 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES Coupling Ratio Full Locking Decoupling I Isodepth Model > ' ' ' - I I depth Model I ' :." %, ".7 ß oo _o /,. ', --,. -. ' o r < -- : n, ' ' '. o o.-. r ', i; <.>., o...,o o,o..,,,' < '% -_, ' 3, 0 u u z. - ) ß --%, ' o '. ' " - - o I.. o '. h I f/.o. o o,' 7 oo ø, ' 1 ly -.-., o o. ½,,'o ' I o g3o t o o. 32 i 0. ',, ' _,. 32' ( 0 'ø ¾0 o,,,3,3 (a) -,,3 (b) I I ' ' to o o 7 o o o o o -0 -,: - o-' I l i ' ' =. ß g o oø /. '? o o. - o _ I g/. o o -l ts // o o. f/,,. o o,,, o o,.. I I' ß-,u. / I > oredicted velocin! I! ß o. o'o ):.,E " ' > predi ed velod 1 I :,,.,, g. II, ø -, (c) (d) Figure 5. Distribution of the coupling ratio in the central Nankai subduction zone. The open vectors representhe measured GPS velocities with respecto Eurasia for the period 1996 (Figures 5a and 5c) and 1997 (Figures 5b and 5d). The solid vectors representhe velocities predicted by the best-fit model. The estimated coupling ratio of each fault segments (circles) is coded in shading: full coupling is white and decoupling is black. (a) and (b) The downdip extent of the locked zone following the 25 km isodepth. (c) and (d) The downdip extent of the locked zone following the 350øC isotherm. Both models show that the best fit coupling distribution corresponds to a fully locked seismogenic zone with small areas of decoupling and few intermediate coupling values. earthquake [Kanamori, 1995]. This earthquake displayed postseismic motion which, near the fault, reached- 20 mm over the year following the earthquake [Nakano and Hirahara, 1997]. Hence this free slip area is an artefact of the inversion process which tries to fit the effect of an intraplate earthquake with a subduction motion model. This conclusion is supported by the disappearance of the free slip zone for the 1997 data inversion and the fact that the 1997 velocity vectors in eastern Shikoku and western Kii do not show the same deviation as the 1996 data. The second free slip zone, located at the lower western comer of the seismogenic zone, west of Shikoku is not constrained properly by data and is probably not significant Japan-West Kurile Trench We investigate the spatiotemporal evolution of the coupling ratio distribution along the Japan trench by inverting separately the 1995, 1996, and 1997 GPS data. To be consistent with the choice of the Eurasia reference frame for the GPS velocity data, we choose the subduction velocity as that predicted by the Pacific/Eurasia rotation pole given by -1 NUVEL-1A [DeMets et al., 1994]. It ranges from 92 mm yr toward N293 ø off Kanto to 89 mm yr - toward N294 ø off Hokkaido. The geometry of the seismogenic zone and the downdip limit of the locked portion are the same as used previously for the regional subduction velocity estimation.

9 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,167 Table 2. Mean Misfit of GPS Data a Subduction zone Nankai trough Temperature model b Depth model b Japan-west Kurile Trench Raw data Corrected data ½ a In mm yr -l. b Temperature and depth controlled lower ends of the seismogenic zone (compare text). ½ GPS data corrected for the 1994 Sanriku-Oki earthquake postseismic motion (compare text). The mean misfit of the inversion for Japan-west Kurile trench is larger than that of the inversion for Nankai trough (Table 2). This is essentially for two reasons. There is a zone of relatively large velocities in the southwestern part of Hokkaido that cannot be explained by elastic loading from the Japan-west Kurile trench, and there is a systematic northward shift of the model velocities with respect to data along the Tohoku coast. A similar increase in misfit would occur in the S = aln (bt+ l ), (1) where t is the time after the earthquake and a and b are the amplitude and time decay parameters at each observation site. The analysis of the measurements at 11 permanent GPS stations led to the estimation of a regional b value (b yr- ), which indicates that about half of the first year postseismic motion was achieved after 3 months. The factor a (amplitude scale) was estimated at each site using the cumulative displacement after 1 year. Following Heki et al. 's [1997] approach, we estimate the postseismic velocity at each GPS station in the affected area for 1995, 1996, and We use the fault plane parameters and the logarithmic afterslip law (1) extrapolated to 3 years to compute the surface velocity field averaged over the period that coincides with the actual recording period of the GPS data. The predicted velocities are shown in Figure 7 for 1995 and We then use this afterslip velocity field to correct the GPS data for the postseismic effect of the 1994 earthquake. The corrected GPS velocities are inverted to find the new coupling ratio distribution. The results of the three inversions (1995, 1996, and 1997 periods) are shown in Figure 8. The decoupling zone identified on the raw data inversions is strongly reduced for the 3 years and actually disappears for the 1995 result. For the 1996 and 1997 inversions the remaining decoupling patch is limited to the lower portion of the seismogenic zone. For the 3 years a decoupling zone remains at the southeastern comer of Hokkaido, north of the 1994 earthquake area. We conclude that the estimation by Heki et al. [1997] of the postseismic slip related to the Sanriku-Oki earthquake does account for the observed temporal behavior of the velocity field in northern Tohoku and southern Hokkaido. The extrapolation of the logarithmic afterslip law (1) seems reasonable for the 3 years following the earthquake. However, the fit between the corrected GPS motion and the predicted velocity is not as good for the sites close to the postseismic fault surface as the one for stations away from this zone (Figure 8). This suggests that the assumption of the same Nankai trough inversion if Chugoku data were included in the logarithmic decay law over the 3 years and constant fault inversion. geometry may be oversimplified. The main feature of the inversion results shown in Figure 6 The coupling distribution evidenced by the afterslipis that as for the Nankai trough area, the best fit distribution of corrected GPS data shows that the decoupling zone identified coupling along the seismogenic zone corresponds to a fully in the inversion of the uncorrecte data probably corresponds locked fault, with some zones of decoupling and almost no to a faster slip within a smaller sector of the subduction zone. intermediate coupling values. A first small decoupling area is Thus the Japan-west Kurile subduction zone appears fully located below the coastline of Kanto and is mostly seen in locked, except for the postseismic slip zone, which seems 1996 and A second, larger, one is located off northern localized within the earthquake rupture area. After correction Tohoku and south of Hokkaido, extending between 39 and for the postseismic motion a small decoupling zone remains 42øN. Its lateral extension is roughly consistent for the 3 years within the zone of bending of the slab, at the junction between (-400 km), but its upper limit varies strongly from 1995 to the Japan and Kurile trenches. Whether it actually exists or is 1997 (Figure 6). This zone coincides with the rupture area of an artefact related to some nonsubduction process is the M, 7.6 Sanriku-Oki thrust earthquake that occurred in unresolved. The only consistent zone of free slip that may be December 1994 [Nakayama and Takeo, 1997; Sato et al., identified is, as for the southern Japan area, located at the 1996]. Heki et al. [1997] have shown that the westward lower end of the modeled seismogenic zone, landward of the component of motion recorded by GPS in 1995 in the area coastline (see Figures 5, 6 and 8). Although we showed west of this earthquake was the result of postseismic motion. previously that the deepest row of point source groups is not They analyzed the time series of 11 GPS sites over the first well constrained, the coherency of these free slip areas over 2 year following the event and estimated the amount of slip as years of measurementsuggests that the lower limit of the well as the fault geometry associated with this postseismic seismogenic zone, in some areas, is less deep than adopted by motion. Following Marone et al. [1991], they proposed that a few kilometers and actually coincides with the coastline. during the first year the afterslip S obeys a logarithmic decay That the coastline follows the lower end of the seismogenic law that can be approximated by: zone is a feature generally observed in subduction zones [Ruff and Tichelaar, 1996; Oleskevich et al., 1999]. In elastic dislocation models the transition between subsidence and uplift induced by tl :.e interseismic loading also roughly coincides with the surface projection of the lower limit of the locked portion of the subduction fault. This suggests that the regions of elastic and plastic deformations coincide and that a small part of the deformation accumulated during the interseismic period is not restored during the subduction earthquakes Comparison Between Interseismic Loading and Coseismic Slip Distribution Because the coupling ratio and the subduction velocity measured by GPS are directly related to the stress buildup

10 ,168 MAZZOTTI ET AL.' FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES (a) 40 0 GPS vel 'ci velocity, predicted - " ::,,O 30 mm yr-' ':,:',:;;t. ; i:. '-:,%,."'"...,-" <= GPS velocity - predicted velocity 30 mm yr-' (c) Full Locking Coupling Ratio :' : ': '.-':l:,:.:. o io :o, 0:::::'5':, :' o :,... o o o: o..' 6(.--:.::6?.". :"::: ß 0 0 i::...'";'?..,'..., ,,'-...-' - ß i L.,. :' i.-[-:'-: < = GPS velocity - predicted velocity 30 mm yr-' Decoupling Figure 6. Distribution of the coupling ratio in the Japan-west Kurile subduction zone. Symbols are the same as for Figure 5. Inversions were performed for (a) 1995, (b) 1996, and (c) 1997 GPS data. The focal mechanism symbols show the location of the 1994, Mw 7.7 Sanriku-Oki interplate earthquake and its major aftershocks (Figure 6a). The best coupling distribution corresponds to a fully locked seismogenic zone with some localized alecoupling areas and few intermediate coupling values.

11 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,169 I i:'""? (:'::,,:,..:...:: : ' '1 "... I.,.,...,, i '". -.,... I-'".'.-'-" 7 I ß...,-' ' ' 'F '" "-'T"I't'""--" '"""--'' ' "." '/,"? II,-' :' Consequently, uniform interseismic loading is indeed associated with heterogeneous coseismic slip in the Nankai and Japan-west Kurile zones, and thus a significant portion of the stored elastic interseismic strain is not released in the following great earthquake. Because this accumulated :.":,.:: '.'.:..':(...:,,,.?- '.,. I..:: :"":.'-::.::..:.'.'.:.i...,.-"-,,-, I I""f":-"./:"::::.'-:::::'"::2:' :' - deformation should eventually be released, the identity I t:.-.:::.'-.'.:-,..">\:".,: J% ' \t t"" "' l.,?:,_," :!:-...,,..- _,.,".',.": I between elastic deformation and seismic slip can only be realized over a cycle of several great earthquakes. If most of the strain release is seismic, as seems to be the case for the Nankai subduction zone, this would require different patterns of slip for successivearthquakes and would suggesthat the asperities are not permanent from one earthquake to the other. I ',:"-.,:i '"' : Residual Deformation Field and Geodynamics of Japan Figure 7. Postseismic velocities associated with the December 1994 Sanriku-Oki thrust earthquake. The shaded rectangleshow the geometry of the postseismic fault surface. Vectors representhe surface postseismic velocity estimated at the GPS sites based on an empirical slip law [HeM et al, 1997] for 1995 (open) and 1996 (solid). elastic deformation As shown by previous authors [Kato et al., 1998; Le Pichon et al., 1998] and our own results, the instantaneous velocity field measured by GPS on the Japanese islands is mostly dominated by the elastic loading of the island arc by the Pacific and Philippine Sea subducting slabs. Our inversion of the GPS data allows us to obtain the elastic loading model that best accounts for the observed transient deformation. This can then be subtracted from the GPS data to obtain residual velocity and strain rate fields in Japan (Figures 9 and 10, respectively, for 1996 period). The along the subduction interface that will lead to earthquakes, resulting deformation field can be regarded as the permanent our study is intrinsically integrated into the spatial and nonelastic deformation that affects the Japan crust if none of temporal distribution of large earthquake slip around Japan. this deformation has been mapped onto the interplate locked We confront here our estimations of the loading distribution zone by the preceding analysis, also bearing in mind that it is along the seismogenic zone and the coseismic slip pattern of subject to the inherent noise and/or network adjustment major and great earthquakes in the same areas. problems of the GPS measurements. The analysis of earthquake records in northern and An example of what we consider as an uncertainty in the southern Japan indicates that the coseismic slip distribution is GPS data is expressed by the coherent residual velocities strongly heterogeneous along both subduction margins. along the eastern coast of Kanto and Tohoku area (near 36øN, Inversion of tsunami and geodetic data for the great øE, Figure 9). In the central part of northern Honshu the Tonankai and 1946 Nankaido earthquakes (Nankai trough) residual vectors are very small (<5 mm yr -1) and scattered, but shows that the coseismic slip resolved for fault segments with in contrast, a consistent motion can be identified along the a typical scale length of 60 to 80 km varies from 0 to 10 m Pacificoast averaging 7-10 mm yr -1 toward the south. This from one segment to its neighbors [Sagiya and Thatcher, southward residual velocity is the cause of the 10 ø 1999; Satake, 1993]. Along the Japan trench the analysis of counterclockwise rotation observed between the predicted seismic data from the 1994 Sanriku-Oki earthquake shows PAC/EUR (or PAC/NAM) motion and the slip direction that the coseismic slip follows the same variations on a estimated by our inversion of the GPS strain rates in the shorter length scale of---i 0 km [Nakayama and Takeo, 1997]. northern Kanto area. In this area the slip direction along the The heterogeneous concentration of high seismic slip is subduction zone is well constrained by earthquake slip vectors generally attributed to asperities where large amount of elastic and corresponds to that predicted by PAC/EUR or PAC/NAM strain has accumulated [Dmowska and Lovison, 1992; plate models [DeMets, 1992b; Seno et al., 1996]. This would Dmowska et al., 1996]. In contrast, our principal result is that tend to exclude the possibility of a strain partitioning effect both the Japan-west Kurile and the central Nankai subduction along the Japan trench as well as a potential error in the systems are characterized by a seismogenic zone that is NUVEL-1A plate motion azimuth. We do not have a homogeneously and fully locked during the short-term geological explanation for this displacement. interseismic period witnessed by the GPS data. In other words, the present homogeneous transient deformation field 5.1. Strain Partitioning Along the West Kurile Subduction Zone of the Japanese island arc implies that the strain loading along the subduction zones was uniform during the 3 years of GPS On the other hand, there are some zones of Japan where we measurements within the <100 km resolution scale of our can interpret the residual vectors in terms of plastic model. We thus show that if asperities influence the elastic loading, their size and spacing are such that the fault plane is kinematically fully locked during the interseismic period. Likewise, our results indicate that large-scale asperities, of 100 km size or more, do not exist in the study areas as far as elastic strain loading is concerned. deformation. As an example, the residual motion in eastern Hokkaido is quite coherent, with a mean velocity with respect to Eurasia of 10 mm yr - toward the west-southwest (Figure 9). This observation is consistent with the strain partitioning model along the Kurile trench proposed by DeMets [1992a] and others. The obliquity of the subduction motion to the

12 13,170 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES (a) (b) ' y... / ' 0 ':...-%.. ß.. ' i.re ' i..8- o io.'?::½w, ' / O: :' ' GPS ' i gp ,0 F.. 99 d t p, ' - ':' ½:: L x,% "- / :Z{' o,!.. *:',, '... - L"'- " ' - "- / Coupling ':... n ""'... :: ' '""'"'""""'""'" / *-. ' ½" :': ½ >;, w r GPS ; '- 30 yr" j o.o Fi[ure 8. Distribution of the couplin8 ratio of the Japan-west Kurile subduction zone a er correction for the ] 994 postseismic motion. Symbols are the same for F isures 5 and 6, with the open vectors representin8 the OPS velocities codected for th estimated postseismic motion (compare text) for (a) 1995, (b) 1996, and (c) For the 3 years the co ection of the OPS data for the predicted postseismic motion leads to a fully locked fault, except for the lower transition zone below eastern Tohoku and a small decouplin8 patch off the southeast comer o Hokkaido. subduction earthquake slip vectors along the Kurile arc led deformation along the eastern margin of the Japan Sea, which DeMets [1992a] to propose that the southern Kurile forearc changes from pure convergence off Honshu to right-lateral deformation can be described by a silver motion of 6-11 transfressive motion at the latitude of Sakhalin island. mmyr -I to the southwest relative to North America. Geological, geophysical, and triangulation data [Hashimoto and Tada, 1988; Kirnura, 1986; Le Pichon et al., 1984] 5.2. Active Deformation Along the Eastern Margin indicate furthermore that this sliver extends southwest off the of the Japan Sea and in Southern Japan Kurile islands into the Hokkaido forearc and is limited by the Using recent GPS measurements in northeast Asia, Heki et Hidaka mountain range at the southeastern corner of the al. [2000] have demonstrated the existence of an Amurian island (see active faults in Figure 9). (AMU) plate that includes northeast China, southeastern In contrast, the strong residual velocities observed in the Russia, and the Japan Sea, with an eastward velocity with central and western part of Hokkaido are more difficult to respect to Eurasia of 9-10 mm yr -l. This motion is confirmed interpret. As we will mention in section 5.2, part of them by GPS data in southern Korea that give a motion of 9 mm yrmight be related to the active transient and/or permanent toward th east with respect to Eurasia (M.B. Heftin et al.,

13 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,171 4O 35 : '? Figure 9. Residual GPS velocity field in Japan. The solid vectors show the residual velocity field with respect to Eurasia after subtraction of the estimated interseismic loading deformation based on the best fit model for the year Thin solid lines show the active faults after Research Group for Active Faults in Japan [ 1991 ]. Plate motion from IGS Global Positioning System, 1998, [Hashimoto and Jackson, 1993]) or seismicity study ( available at mm yr [Shen-Tu and Holt, 1996]). Also, the residual GPS velocity field that we estimate in Japan (Figure 9) shows that the western coast of northern Honshu and the Chugoku district have coherent velocities of the order Thus, in our opinion, most of the Japanese islands, from northern Honshu to Shikoku island and southern Kyushu (Figure 10), should not be regarded as part of the Eurasia, of 5-10 mm yr ' toward the east with respect to Eurasia, North American, or Amurian plates. Rather, these blocks comparable to the AMU/EUR motion. GPS residual strain rates (Figure 10) show that high correspond to an island arc trapped between the converging Pacific, Philippine Sea, and Amurian plates. Furthermore, our compressional strains are concentrated in northern Japan estimations of the subduction velocities show that this island along the east Japan Sea convergence zone, within an area of high seismic activity and active faults [e.g., Nakarnura, 1983; arc is characterized by a motion that happens to be close to the Eurasia one. Tamaki and Honza, 1985]. This compression extends inland in central Japan in a zone of active deformation [Kanaori et 6. Conclusions al., 1992]. In southern Japan, high strain rates are evidenced in the active right-lateral shear zone formed by the Setouchi shear zone [Sugiyarna, 1994] and the Median Tectonic Line fault [Okada, 1980]. These active deformation zones accommodate the relative motion between the Amurian plate and the Japanese island arc (Figure 10). This relative motion The dense permanent GPS network deployed by GSI over the Japanese islands since 1994 allows the estimation, on an instantaneous timescale, of the interseismicoupling between Japan and the subducting Pacific and Philippine Sea slabs. Different studies had previously proposed that the style and is of the order of 10 mm yr - in agreement with the magnitude of the deformation of Japan are directly related to estimations of convergence along the eastern margin of the Japan Sea based on long-term geodesy analysis (12-13 mm yrthe elastic loading by the two subducting plates [Tabei et al., 1996; Kato et al., 1998; Le Pichon et al., 1998; Ozawa et al.,

14 ,172 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE $UBDUCTION ZONES 40 ' plate ø y½' 30 mm yr-' -' Figure 10. Residual GPS strain rates and kinematic setting of Japan. The solid strain crosses show the residual GPS strain rates. This strain rate field was smoothed and interpolated on a regular grid, assuming a Gaussian interpolation function with a 25-km standard deviation. Relative plate motion is represented with respect to Eurasia by the large vectors. The light shading highlights the main tectonic plates. Note the consistency between the active faults and the high residual strain rates. 1999]. In this study, we have used the GPS strain rate fields to determine the velocities of the two subducting plates that best account for the observed deformation of southern and northern Japan. The Pacific and Philippine Sea plate motions thus estimated are in good agreement with those predicted by the relative plate motion models along the Nankai trough (PHS/EUR after Seno et al. [1993]) and the Japan-west Kurile trench (PAC/EUR or PAC/NAM, NUVEL-IA [DeMets et al., 1994]). This demonstrates that the coupling ratio is close to 1 along both subduction zones. Because of the high density of the GPS sites a detailed inversion of the observed velocity field has been able to resolve the spatial and temporal variations of the coupling ratio distribution along both seismogenic zones. The most striking result is that the central Nankai trough and the Japan-west Kurile trench fault zones show the same characteristics during the interseismic period: both seismogenic zones are homogeneously fully locked. Partial coupling does not exist at present, and the only consistent decoupling zones observed are located at the lower end of the seismogenic zone. Furthermore, we have shown that along both the Nankai and Japan-west Kurile trenches the actual locked surface of the subduction interface coincides with the seismogenic zone that extends down to the coastline limit. Thus the small seismic coupling observed along the Japan and Kurile trenches cannot be explained by permanent creep nor by a large reduction of the locked part of the seismogenic zone. Another process that may take up the seismic deficiency is slow seismic motion, associated with slow and "ultraslow" earthquakes or postseismic afterslip. Kawasaki et al. [1995] have shown that following a magnitude 6.9 interplate earthquake along the Japan trench, ultraslow faulting occurred with a time constant of -1 day which corresponds to an equivalent magnitude of 7.3 to 7.7, that is, 4 to 16 times larger than the regular thrust event. On a longer timescale the total postseismic motion associated with the 1994 Sanriku-Oki earthquake discussed in section 4.2 can be estimated following the afterslip decay law proposed by Heki et al. [1997]. Although this logarithmic law is unrealistic for long periods because it does not reach an asymptotic value, it

15 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,173 suggests that afterslip can dissipate up to 3 times as much slip as coseismic slip. This conclusion has also been reached for other subduction zones [Buckham et al., 1978; Zheng et al., 1996]. The "slow motion" associated with slow and ultraslow earthquakes and preseismic and postseismic motion is thus most likely to be the main cause of the large amount of aseismic subduction motion proposed for the northern Japan subduction zone. permanent deformation is evident in Hokkaido, in the direct prolongation of the Kurile silver lateral motion. On a larger scale the residual velocity and strain rate fields indicate that the high deformation zones in Japan (eastern margin of the Japan Sea, central Japan, and southern Japan shear zone) accommodate 5-10 mm yr - of relative motion between the Amurian plate and the Japanese island arc, in agreement with recent estimations of the Amurian plate velocity [Heki et al., 2000]. 0 and 1. The misfit function fassociated with each model is defined as a L I norm: f(m)= ([GPSI/ -mi/ l+lgpsi, (2) where u/,sv//and u/,svj are the east and north components of the ith GPS vector; mvs' and ml/ are the east and north components of the ith velocity vector predicted by the model Because the inversion of the GPS data gives a good m. The search phase consists in a series of steps that will lead estimation of the elastic deformation that affects the Japanese the simplex to contract itself at a local minimum off. islands, we could correct for this first-order transient motion To ensure that the search does not lead to a secondary and determine the residual nonelastic deformation. Such a minimum, we added the following procedure. When a minimum is found, the simplex is "reinflated" and the search starts over. This "reinflation" consists in the reinitialization of Appendix: Simplex Inversion Method A1. Downhill Simplex Method one parameter of each model that defines the simplex. The parameter is set to zero if its former value was larger than 0.5 and to I otherwise. This reinflation allows the simplex to extend to every direction of the model space and thus to potentially converge to a minimum lower than the previous one. We consider that the true minimum is found when the search leads to the same minimum twice. A2. Spatial Resolution of Coupling Ratio Distribution We evaluate the spatial resolution of the inversion from the cross correlation between model parameters (fault element coupling ratio). The cross correlation is computed by multiplying D, the matrix that links the parameter vector (fault elements) to the data vector (GPS velocity) by its The downhill simplex method [Nelder and Mead, 1965; Press et al., 1996] is designed to find the minimum of a function of more than one independent variable. A simplex is a geometrical figure with N+I vertices, where N is the transpose D r. The product is normalized such that the dimension of the model space. The starting simplex is diagonal elements are 1. On average, the cross correlation composed of N+I randomly generated models. There is no between a fault element and its adjacent neighbors (within 55 preconditioning of the starting models and the only constraint is that the parameter values (coupling ratio) must be between km) is -0.7 to 0.8. This implies that adjacent points yield very similar elastic solutions and may not always be properly NORMALIZED WEIGHT 0 0 OA 0.2 0,3 0, Figure 11. Weights of the fault segments in the 1996 model. The normalized weight of each point source group is coded by shading. The and weight intervals are shown by the light and medium shaded zones, respectively. 140

16 13,174 MAZZOTTI ET AL.: FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES (a) (b) Figure 12. Sensitivity to noise of the simplex inversion method. The results of the inversion of a synthetic data set created for a uniform coupling ratio of 0.5 are shown for two levels of white noise' (a) +2 mm yr -, (b) ñ4 mm yr -. Legend is same as for Figure 2 distinguished in the inversion. The cross correlation decreases to 0.3 beyond 100 km. Additionally, we tested the spatial resolution on synthetics. We introduced a small zone of zero coupling within a fully locked fault and inverted the synthetic data set. These tests indicated that the inversion process can retrieve a decoupled fault zone of a size larger than 70 km x 70 km. For smaller zones (30 km x 70 km, for example) a larger patch with an intermediate coupling ratio was found. These tests indicate that the spatial resolution of the inversion is of the order of km and confirms our determination of the spatial resolution from cross correlation. The resolution tests thus show that patches -60 km wide can be resolved. Smaller-scale heterogeneities that may appear in the inversions are not significant. A3. Weight of Fault Elements The weight of each fault element can be estimated by measuring its effect on the predicted deformation of the upper plate. We do this for each element by summing up the modulus of the velocity vectors predicted at every GPS site. These values are then normalized to the highest one, leading to a weight between 0 and 1 for each fault element. This calculation (Figure 11) shows that the weight of the fault elements generally decreases with their distance to the data points. The upper row and the lateral limits of the modeled zone are the least constrained, with a weight <0.2. The second upper and the lowermost rows are characterized by a weight between 0.2 and 0.4, and the intermediate depth zone shows the highest contribution to the results, with a weight >0.5. The lower and upper rows of fault elements have a low weight in the inversion and thus are not as well constrained as the central rows. However, suppression of these rows (equivalent to setting them to zero coupling) would require increasing significantly the coupling ratio of the central rows. This is shown in section 3.5, where we test the minimum size of the locked zone and show that it can at most be reduced to 70% of the zone that we defined as the seismogenic zone. A4. Robustness of Simplex Method In order to test the stability of the inversion method and its sensitivity to data noise we inverted a series of synthetic data sets to which we added different levels of white noise ranging from ñ0 to ñ5 mm yr - on both velocity components. The synthetics were computed from a model with a uniform coupling ratio of 0.5 along the Japan and west Kurile trenches. For no added noise the inversion retrieves the uniform distribution of coupling, even for the elements with the smallest weight. Figure 12 shows the results of the inversion for the ñ2 and ñ4 mm yr ' levels of white noise (see misfit in Table 3). The simplex inversion appears to be stable for noise levels less than ñ3 mm yr -1 but progressively becomes unstable for higher noise levels (Figure 12b). Even in unstable cases, the coupling ratio averaged over larger zones of the subduction plane ( -200 km) is 0.5. Using larger fault elements (and thus less parameters) in the inversion increase stability and reduces sensitivity to noise. The same tests were performed starting from the real GPS data set. Velocities in the real data set are about twice those in the synthetic (computed with 50% coupling), and consequently, higher noise levels are needed to significantly alter the GPS inversion solution. We found that the inversion remains stable and the solutions are similar to those shown in Figure 6 for added white noise levels up to + 10 mm yr -.

17 MAZZOTTI ET AL ß FULL LOCKING OF THE JAPANESE SUBDUCTION ZONES 13,175 oo Coupling ratio 0.8 0,9 1.o (b) 30 mm y½ Figure 13. Comparison between SVD and simplex methods. The 1996 GPS data set is inverted for the coupling distribution over 48 fault elements (legend same as Figure 2). (a) Results of the SVD inversion, the size of the circles indicate the 3 levels of resolution of the parameters (well resolved, 0.9-1; half-resolved, ; unresolved, 0-0.5). (b) Results of the simplex inversion. A.5. Comparison With Singular Value Decomposition Method To further assess the validity of the simplex inversion, we compared results obtained with this method and with the singular value decomposition (SVD) method. These tests were performed with the 1996 northern Japan GPS data. Without preconditioning, the SVD method is highly unstable if all the 192 model parameters are retained. We thus reduced the number of parameters to 48, using fault elements of about 70 km x 70 km. This corresponds to the actual spatial resolution of the inversion. With this geometry the simplex inversion leads to a coupling distribution similar to the one obtained for the 192 elements (compare section 4.2, Figure 13b versus Figure 6b, see Table 3). The zone of decoupling off northern Tohoku is imaged, with intermediate coupling ratio along its boundary Table 3. Inversion Tests Test Misfit, mm yr -I with the fully locked zones. The SVD inversion results are shown on Figure 13a. Following Sagiya and Thatcher [1999], we considered as well resolved the parameters for which the corresponding component of the resolution matrix (VpVp T) is >0.9. The SVD solution has a smaller misfit than the simplex solution (Table 3). However, a coupling ratio >1, which we consider unrealistic, is obtained for well-resolved fault elements. This may reflect a tendency of the SVD method to overestimate the contribution of the best resolved fault elements to the elastic deformation field. Acknowledgments. We are grateful to the Director of the Geographical Survey Institute, Japan, for making available the GPS data used in this study. We thank R. Hyndman and K. Wang for providing us with the 3-D elastic dislocation routines, as well as for discussions on the interseismic phase modeling. We thank A.Lomax for his advice on inversion methods. We are also grateful to R. Dmowska, J. Rice, R. Hyndman, and K. Wang for comments and discussions about the manuscript. Most of the figures in this paper were integrally or partly realized with the Generic Mapping Tools 3.0 software [Wessels and Smith, 1995]. This research was sponsored by Coll ge de France and CNRS UMR Synthetics with 0.5 coupling No added noise +2 mm -i noise +4 mm yy -' noise North Japan, 1996 GPS data 192 faults elements, constrained simplex 48 faults elements, constrained simplex 48 faults elements, S.V.D References Abe, Y., and H. Tsuji, A nationwide GPS array in Japan for geodynamics and surveying, Geod. Info Mag., 8, 29-31, Ando, M., Source mechanisms and tectonic significance of historical earthquakes along the Nankai trough, Japan, Tectonophysics, 27, , Ando, M., A fault model of the 1946 Nankaido earthquake derived from tsunami data, Phys. Earth Planet. Inter., 28, , Bucknam, R.C., G. Plafker, and R.V. Sharp, Fault movement,

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