Episodic growth of fault-related fold in northern Japan observed by SAR interferometry

GEOPHYSICAL RESEARCH LETTERS, VOL. 35,, doi:10.1029/2008gl034337, 2008 Episodic growth of fault-related fold in northern Japan observed by SAR interferometry Takuya Nishimura, 1 Mikio Tobita, 1 Hiroshi Yarai, 1 Tomomi Amagai, 1 Midori Fujiwara, 1 Hiroshi Une, 1 and Mamoru Koarai 1 Received 22 April 2008; revised 21 May 2008; accepted 22 May 2008; published 3 July 2008. [1] The interferometric analysis using ALOS SAR data allows us to map the detailed spatial pattern of the deformation associated with the 2007 M6.8 Niigataken Chuetsu-oki earthquake. The interferograms reveal not only a large deformation near the source area of the earthquake but also a local uplift in the region of active folding, 15 km east of the earthquake epicenter. The 1.5-km-wide and 15-km-long band of uplift is located along the anticline axis of an active fold and cannot be explained by the elastic deformation resulting from the mainshock of the earthquake. This uplift suggests the episodic growth of active folds. The analysis of the range changes along two directions and the vertical displacement by leveling reveals that episodic creep on an east-dipping reverse fault just beneath the uplift band has caused the growth of the fault-related fold. The acceleration of this creep has been probably triggered by an increase in static stress due to the earthquake. Citation: Nishimura, T., M. Tobita, H. Yarai, T. Amagai, M. Fujiwara, H. Une, and M. Koarai (2008), Episodic growth of fault-related fold in northern Japan observed by SAR interferometry, Geophys. Res. Lett., 35,, doi:10.1029/ 2008GL034337. 1. Introduction [2] Fold structures resulting from plastic deformations are commonly found in fields. In spite of many studies carried out on the kinematics and mechanisms of fold formation [Dolan and Avouac, 2007, and references therein], the variation in fold growth over different time scales has not been completely understood. One of the reasons is that only few examples reveal the temporal evolution of folds on the geodetic time scale [Fielding et al., 2004]. In this paper, we present a direct evidence of the episodic growth of active folds that have been imaged by the space geodetic technique. [3] The Chuetsu area in Niigata Prefecture, northern Japan is one of the most active tectonic regions, in which a fold-and-thrust belt is well developed (Figure 1). Geological studies [Okamura et al., 1995; Ikeda, 2002; Sato et al., 2004] suggest that thick sediments accumulated in a rift system formed under an extensional stress regime during back-arc spreading are related to the opening of the Sea of Japan during 25 13 Ma BP. Normal faults formed at this stage have been subsequently activated as reverse faults during shortening in the eastern margin of the Sea of Japan 1 Geographical Survey Institute, Tsukuba, Japan. Copyright 2008 by the American Geophysical Union. 0094-8276/08/2008GL034337 in the past 3.5 Ma. Recent GPS observations suggest that the Chuetsu area is part of a 100-km-wide band with a high contraction rate, called the Niigata-Kobe tectonic zone (NKTZ), which is situated in a convergent boundary zone between the Okhotsk and Amurian (or Eurasian) plates [Sagiya et al., 2000; Mazzotti et al., 2001]. The shallowfocus 2007 M6.8 Niigataken Chuetsu-oki earthquake (hereafter Chuetsu-oki earthquake) occurred near the coast of the Chuetsu area on July 16, 2007. The focal mechanism of the earthquake indicates thrust-type faulting with the compressional axis along the northwest-southeast direction, which is concordant with the preseismic regional strain [Sagiya et al., 2000]. The crustal deformation associated with the earthquake is clarified using GPS, InSAR, and leveling data [Nishimura et al., 2008]. The main focus of this study is on the deformation that cannot be explained by the coseismic slip on the source fault of the Chuetsu-oki earthquake. 2. Data and Analysis [4] We have used the L-band SAR data acquired by the Advanced Land Observation Satellite (ALOS), also called Daichi. SAR data obtained on different days in both descending and ascending orbits have been processed in order to generate interferograms (Figure 1). The coherence of the interferograms is excellent, except in the coastal sanddune region where considerable soil liquefaction and lateral soil flow have been observed in the field investigations after the earthquake. The SAR interferograms show the range change between the ground and the satellite, that is, the lineof-sight (LOS) displacement. Because the direction of the LOS is different for the descending and ascending orbits, the patterns of the fringes in the interferograms differ for the two orbits. The unitary vectors along the LOS in the descending and ascending orbits are (0.637, 0.113, 0.762) and ( 0.620, 0.109, 0.777), respectively, in the coordinate set (east, north, up). The interferograms for the period including the earthquake (Figures 1a, 1b, 1c, and 1f) clearly show coseismic deformation. In the interferograms for the ascending orbit (Figures 1a, 1b, and 1c), two peaks of the LOS displacement are found to be located in an area to the north of Kashiwazaki basin and the western part of Nishiyama hill. The deformation zone near Nishiyama hill is a narrow band extending along the NNE-SSW direction and is approximately 1.5-km wide and 15-km long. In contrast, in the interferograms for the descending orbit many fringes are observed to the north of Kashiwazaki basin (Figure 1f). A small offset of fringes can be observed in the deformation band near Nishiyama hill. The signal observed near Nishiyama hill cannot be an artifact of the 1of5

NISHIMURA ET AL.: EPISODIC GROWTH OF FAULT-RELATED FOLD Figure 1. Synthetic aperture radar interferograms for the Chuetsu area. The image is superposed on the top of shaded topography. (a) Image is generated from data acquired from ascending orbit on June 14 and September 14, 2007. It clearly shows the belt-like uplift on the west side of Nishiyama hill, indicated by the two black arrows. The star and red circles represent the epicenters of the mainshock and aftershocks of the Chuetsu-oki earthquake, respectively. The focal mechanism of the mainshock is determined by centroid-moment tensor inversion by NIED. Abbreviations used in the tectonic map (inset) are OK, PA, PH, AM, and NKTZ for the Okhotsk, Pacific, Philippine Sea, and Amurian plates and Niigata-Kobe tectonic zone, respectively. The rectangular area in the tectonic map corresponds to the area shown in the location map. Images generated from data acquired from ascending orbit on (b) June 14 and July 30, 2007, (c) September 11, 2006 and September 14, 2007, (d) July 30 and September 14, 2007, and (e) September 14 and October 31, 2007. Image generated form data acquired from descending orbit on (f) January 16 and July 19, 2007 and (g) September 14 and October 31, 2007. InSAR processing. A possible major source of error in the InSAR analysis is the error in a digital elevation model (DEM) used to remove topographic fringes. However, the DEM error is precluded because the DEMs provided by the Geographical Survey Institute (GSI) and Shuttle Radar Topography Mission (SRTM) have produced almost the same fringe patterns. The other major source of error is heterogeneity in the delay of radio waves in the atmosphere and ionosphere. Although the fringes caused by these effects tend to change rapidly with time, the narrow band of the fringes in Nishiyama hill is clear in all coseismic interferograms (Figures 1a, 1b, and 1c). We therefore conclude that the narrow band of fringes represents actual ground deformation. [5] Most aftershocks of the Chuetsu-oki earthquake occurred off the coast to the west of the epicenter. Very few aftershocks occurred near Nishiyama hill. Clearly, the large LOS displacement near the aftershock area can be attributed to the elastic deformation due to the mainshock; however, the deformation band near Nishiyama hill cannot be attributed to the elastic deformation. In order to isolate the spatial pattern of this deformation band, we first modeled the deformation due to the mainshock of the Chuetsu-oki earthquake using the geodetic data including the InSAR (Figures 1a and 1f), GPS, and leveling data [Nishimura et al., 2008; T. Nishimura, manuscript in preparation, 2008]. The specific geometry of the mainshock of the Chuetsu-oki earthquake is still under debate, but the geometry is not sensitive to the deformation pattern far from the source area of the mainshock. After we subtract the range change predicted by the model of the Chuetsu-oki earthquake from the observed interferograms, a narrow band of a smaller range remains. The overlaying tectonic structures on the residual interferograms (Figure 2) show that the Oginojo anticline is parallel to this band and is at the eastern rim of the band. The Jourakuji fault, an east-dipping active fault, borders the western rim of the northern part of the deformation band. Combining the interferograms for the descending and ascending orbits, we obtain the displacement in quasiupward (elevation angle is 82 from the south) and eastward directions [Fujiwara et al., 2000] (see auxiliary materials1). The displacement vectors along four profiles across the deformation band show that the uplift and westward displacement are bounded by the Oginojo anticline and the southern extension of the Jourakuji fault (Figure 3). The vectors in the surrounding region show compressional deformation. The spatial coincidence of the uplift band and Auxiliary materials are available in the HTML. doi:10.1029/ 2008GL034337. 2 of 5

Figure 2. Residual LOS displacement in and around Nishiyama hill. The coseismic deformation predicted by the fault model of the Chuetsu-oki earthquake is removed from the original InSAR images shown in Figure 1. Active fault traces [Research Group for Active Faults of Japan, 1991], anticline axes [Kobayashi et al., 1995] and leveling benchmarks are also plotted. The four lines indicate the lines of profiles used for plotting two-dimensional displacement. Images from (a) descending orbit data (Figure 1f shows original interferogram) and (b) ascending orbit data (Figure 1a shows original interferogram). Oginojo anticline suggest the growth of active folds. Independent measurements have confirmed the uplift of Nishiyama hill. A first-order leveling route across Nishiyama hill (Figure 2) has been leveled several times in 2006 and 2007. After subtracting the value predicted by the Chuetsu-oki earthquake model from the observed coseismic displacement, the benchmark 3749 in the uplift band of the interferograms is found to have been uplifted by 40 mm relative to the adjacent benchmarks (see Figure S1). [6] We do not have sufficient data to understand the temporal evolution of the uplift because no continuous geodetic observation, for example, using the GPS, has been carried out in the uplift band. Because the fringe associated with the uplift band has been observed in the interferograms for three days after the earthquake (Figure 1f), it can be assumed that the band probably deformed at a time close to the occurrence of the Chuetsu-oki earthquake. However, it is unlikely that the deformation is so rapid that is radiates a seismic wave because few deep aftershocks with a maximum magnitude of 2.9 has occurred in the band, and the source model of the Chuetsu-oki earthquake [Aoi et al., 2007] reasonably explains seismograms in both near and far fields. The interferograms for the postseismic period show no significant deformation in the uplift band (Figures 1d, 1e, and 1g). However, the uplift possibly continued to increase slowly after the earthquake, which is implied by the leveling data (refer auxiliary materials). 3. Modeling of Deformation [7] Next, we determine the causes behind the uplift in the narrow deformation zone. The displacement vectors (Figure 3) can be interpreted in terms of the east-dipping reverse fault under the uplift band. We therefore invert the residual interferograms and residual leveling data to estimate the parameters of a rectangular fault in an elastic half-space [Okada, 1985] using the method described by Matsu ura and Hasegawa [1987]. We selected 1000 data points from the interferograms by visual inspection for the inversion. A Poisson s ratio of 0.35 for an elastic medium reflects a soft sedimental layer. The result indicates the presence of a slip on the east-dipping reverse fault just beneath the uplift band, which can be interpreted as the southern extension of the Jourakuji fault. The rectangular fault is 9.9 ± 0.8 km long and 1.8 ± 0.1 km wide with a strike of 31 ±1 and a dip of 41 ± 3. The fault slips by 100 ± 12 mm with a rake of 93 ± 8. The Figure 3. Two-dimensional displacement vectors along profiles AA 0 to DD 0 shown in Figure 2. The blue and red vectors show the displacements obtained from the residual interferograms and calculated using the creep model. OA and JF indicate the locations of the Oginojo anticline and Jourakuji fault, respectively. The four profiles are shifted to center the location of the Oginojo anticline. 3of5

Figure 4. Stress change determined by mainshock model of the Chuetsu-oki earthquake at depth of 1 km. The area indicated by the dashed line is the main uplift zone clarified by SAR interferometry. The contour interval is 0.02 MPa. (a) Stress change normal to vertical plane with strike of 31. Compressional stress is positive. (b) Coulomb stress change. The focal mechanism for calculating Coulomb stress is the red nodal plane. depth of the fault ranges from 0.1±0.1 to 1.3±0.2 km. The uncertainties in the parameters are introduced by formal errors (1 sigma) of the inversion. Assuming a rigidity of 1.3 GPa, the moment magnitude is found to be 4.2. This fault model provides a logical explanation of the residual interferograms (Figures 2 and S3), displacement vectors (Figure 3), and leveling data (Figure S1). The ratio of the estimated slip to the fault length is very low, which indicates the presence of low stress drops. It implies that slip has occurred aseismically because the creep events are characterized by small slips and low stress drops [Brodsky and Mori, 2007]. [8] Another possible cause of the formation of the deformation zone is elastic inhomogeneity, that is, the presence of zones with reduced elastic moduli [Fialko et al., 2002]. The mainshock of the Chuetsu-oki earthquake increased the compressional stress normal to the deformation band at a depth of 1 km (Figure 4a). If the elastic moduli below the deformation zone had been smaller than those in the surrounding region, the Poissonian extrusion of the zone would have caused the local uplift. In order to estimate the order of magnitude of the difference between the elastic moduli, we assume a simple model of a vertical weak zone sandwiched by the ambient hard rocks [Fialko et al., 2002; equation 5]. Using the width of the uplift zone, increase in the normal stress, and uplift magnitude of 1.5 km, 0.02 MPa, and 50 mm, respectively, the ratio of rigidity below the deformation zone to that of the ambient rock is 0.06. Such a large difference in the elastic moduli is unlikely according to the results of studies on seismic tomography [Kato et al., 2008] and field geology [Kobayashi et al., 1995]. We therefore conclude that the aseismic slip beneath the uplift band satisfactorily explains the observations. 4. Discussion and Conclusions [9] The InSAR data clearly show that the fault-related fold grew at the Chuetsu-oki earthquake. In addition, the leveling data obtained over several decades imply that the narrow band has been undergoing steady uplift before the earthquake (Figure S1). Considering the compressional strain of the contemporary deformation that has been detected by the geodetic observation [Sagiya et al., 2000], the shallow reverse fault described previously may have continued to creep by stable sliding over several decades. The uplift upon the occurrence of the Chuetsu-oki earthquake can be regarded as an episodic slip event on the existing shallow reverse fault. We propose two possible causes of the episodic acceleration. One cause is the static stress triggering by the mainshock of the Chuetsu-oki earthquake. We have calculated the Coulomb stress change [King et al., 1994] on the east-dipping reverse fault in the case of the mainshock model. The stress change at a depth of 1 km shows that the uplift band is in the area of the Coulomb stress increase (Figure 4b). 4of5

Such a stress increase can trigger an episodic creep event on a weak fault in the shallow sedimental layer. The other possible cause involves the fault geometry of the Chuetsu-oki earthquake. Although the major fault ruptured by the Chuetsu-oki earthquake dips to the southeast, a minor fault dipping to the northwest is also ruptured in the northern part of the aftershock area [Kato et al., 2008; Shinohara et al., 2008]. If such a fault extends into the shallow sedimental layer and terminates below the uplift band, the stress on the shallow reverse fault effectively increases. [10] In the Chuetsu area, geomorphologic studies of deformed river terraces and leveling data suggest that a folding process has been ongoing on for 10 10 5 years [Nakamura and Ota, 1968; Mizoue et al., 1980]. Okamura et al. [2007] have suggested that the 2004 mid-niigata Prefecture earthquake has ruptured the subsurface fault responsible for the growth of folds in Chuetsu area. In this study, the InSAR data clarify that the anticline that is not directly related to the earthquake has grown to form uplift. It suggests that the active fold grows at a variable rate as a result of not only the earthquake but local stress field. The uplift observed by InSAR is to the west of the Oginojo anticline. This suggests the possibility of the growth of an uplift to the east of the Oginojo anticline in the future to compensate the average deformation on the geological time scale. A candidate for such an event is a slip and/or plastic flow on and around the down-dip extension of the estimated shallow reverse fault. Detailed investigations of the subsurface structure, including the seismic reflection and refraction, may help us to understand the folding mechanism and earthquake potential in the area. [11] Acknowledgments. We are grateful to Fumi Hayashi for digitizing the geological map. 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Fujiwara, M. Koarai, T. Nishimura, M. Tobita, H. Une, and H. Yarai, Geographical Survey Institute, Tsukuba 305-0811, Japan. (t_nisimura@gsi.go.jp) 5of5