Fault-Related Folds Above the Source Fault of the 2004 Mid-Niigata

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B03S08, doi:10.1029/2006JB004320, 2007

Fault-related folds above the source fault of the 2004 mid-Niigata Prefecture earthquake, in a fold-and-thrust belt caused by basin inversion along the eastern margin of the Japan Sea Yukinobu Okamura,1 Tatsuya Ishiyama,1 and Yukio Yanagisawa2 Received 1 February 2006; revised 10 October 2006; accepted 16 January 2007; published 28 March 2007.

[1] The 2004 mid-Niigata Prefecture earthquake occurred in a fold-and-thrust belt that has been growing since late Pliocene time in a Miocene rift basin along the eastern margin of the Japan Sea. We constructed the trajectory of the subsurface faults responsible for the growth of the folds from the geologic structure and stratigraphy in the source region, assuming that the folds have been growing as fault-related folds because of inclined antithetic shear with a dip of 85° in the hanging wall above a single reverse fault. The fault trajectory constructed from the fold geometries nearly coincides with the geometries of the source fault of the main shock of the 2004 earthquake revealed by aftershocks, which supports that the rupture was along a geologic fault that has ruptured repeatedly during the last a few million years. A three-dimensional fault model based on 12 fault trajectories constructed along parallel sections revealed that the main shock occurred on a convex bend in the fault surface and that the southern termination of the aftershock distribution nearly coincides with a concave bend in the fault. The close relation between the source fault and the geologic structure shows that it is possible to construct source fault geometry assuming inclined shear as deformation mechanism of a hanging wall and to infer the rupture areas from geologic data. Citation: Okamura, Y., T. Ishiyama, and Y. Yanagisawa (2007), Fault-related folds above the source fault of the 2004 mid-Niigata Prefecture earthquake, in a fold-and-thrust belt caused by basin inversion along the eastern margin of the Japan Sea, J. Geophys. Res., 112, B03S08, doi:10.1029/2006JB004320.

1. Introduction Quaternary, and aftershocks related to this event were observed by a dense seismic network deployed after 2 to [2] The concept of fault-related folding suggests that it is 5 days of the main shock [e.g., Kato et al., 2005a, 2006; possible to infer the geometry of a subsurface dip-slip fault Okada et al., 2005]. These data show the precise geometry from the geometry of surface folds [e.g., Suppe, 1983; of the source fault and thus provide an opportunity to Gibbs, 1983]. This methodology has previously been examine whether the method of fault-related folding can applied to the evaluation of active faults and earthquake be used to infer successfully the subsurface source fault. potential in sedimentary basins [Shaw and Suppe, 1994, Okamura and Yanagisawa [2005] and Sato and Kato [2005] 1996; Shaw et al., 2002; Ishiyama et al., 2004]. By have shown that there is close relation between fold comparing faults constructed on the basis of the theory of geometry and the source fault based on construction of a fault-related folding with source faults of earthquakes, we balanced cross section. In this study we constructed three- can examine the reliability of this fault modeling method. dimensional subsurface fault models from the detailed Davis and Namson [1994] and Carena and Suppe [2002] geologic structures in the source area assuming inclines showed that the source fault of the 1994 Northridge earth- shear as deformation mechanism of the hanging wall, and quake is consistent with a fault inferred from fold geometry examined the relationship between the fault models and the in the source area; however, in this case it is difficult to source fault of the 2004 earthquake. obtain a precise geologic structure related to the source fault because the structure is masked by overlying complex deformation [Carena and Suppe, 2002]. 2. Seismotectonic Setting [3] The 2004 mid-Niigata Prefecture earthquake occurred [4] The 2004 mid-Niigata Prefecture earthquake occurred in a fold-thrust belt that has been growing during the at the southern end of the eastern margin of the Japan Sea, where the convergent plate boundary between the Amurian 1Active Fault Research Center, National Institute of Advanced and Okhotsk plates is thought to be located (Figure 1). Wei Industrial Science and Technology, Tsukuba, Japan. and Seno [1998] estimated the convergence rate of these 2Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. plates to be about 1.5 cm/yr in this region. The margin was initially formed as a rifted passive margin, mainly during Copyright 2007 by the American Geophysical Union. the early Miocene when the Japan Sea opened, and it has 0148-0227/07/2006JB004320$09.00

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Figure 1. Index and tectonic maps. (right) Plate tectonic framework around the eastern margin of the Japan Sea. OK, Okhotsk plate; AM, Amurian plate; PA, Pacific plate; PH, Philippine Sea plate. (left) Tectonic map showing the contraction deformation zones (dotted areas) concentrated in several fold belts (modified from Okamura [2002]) and fault models of recent major earthquakes (rectangles), based on work by Okamura et al. [2005], Tanioka et al. [1995], Satake [1985], and Abe [1975]. Magnitude of the earthquakes is shown by MJMA. accommodated E-W shortening since the late Pliocene [5] Four major earthquakes larger than M = 7.5 occurred [Sato, 1994; Okamura et al., 1995]. Many rift basins formed in the twentieth century in the offshore area along the in the early Miocene have been inverted as a result of Okushiri ridge and Awashima uplift (Figure 1) [Ohtake, reactivation of normal faults as reverse faults during the last 1995]. Along the Shinano River, two destructive earth- 2 to 3 Myr [Okamura et al., 1995]. Okamura [2002] pointed quakes occurred in the nineteenth century (Figure 2) out that contraction is localized mainly in the rifts, whereas [Usami, 2003]. Recent GPS measurements have shown that the horsts between the rifts, composed of pre-Neogene rocks contraction has been focused along the Niigata-Kobe tec- are scarcely deformed (Figure 1). The Okushiri ridge is one tonic zone [Sagiya et al., 2000] and along the Shinano River of the largest currently contracting zones in the northern part in the northern part of this tectonic zone. of the margin, and its southern extension is inferred to be [6] These studies indicate that active contraction has continuous with the Awashima uplift (Figure 1) and the occurred for the last few million years along the Okushiri active fault zone along the Shinano River (Figure 2). ridge, Awashima uplift, and Shinano River, and these zones are still active and have high potential for future earth-

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Figure 2. Geologic map of the Niigata sedimentary basin and adjacent areas. ISTL is the Itoigawa- Shizuoka Tectonic Line, and SKTL is the Shibata-Koide Tectonic Line. The star indicates the epicenter of the 2004 earthquake [Kato et al., 2005a], and crosses show the inferred epicenters of historical earthquakes [Usami, 2003]. quakes. Ohtake [1995, 2002] pointed out that there are (Figure 2). The sediments in the basin are up to 6000 m several seismic gaps along this active zone where the thick and have been folded during the last 2 to 3 Myr [e.g., potential for future earthquakes may be even higher. The Niigata Prefecture Government, 2000]. Suzuki et al. [1974] 2004 mid-Niigata Prefecture earthquake occurred in one of classified anticlines in the basin into 3 orders and showed these gaps. that the first-order anticlines, which have axes longer than 30 km, tend to be separated by about 10 km. Okamura 3. Geology of the Source Area [2003] noted that the first-order anticlines in the northwestern part of the basin are fault related and lie above thrust 3.1. Niigata Sedimentary Basin faults that cut through the upper crust. He further suggested [7] The Niigata sedimentary basin, one of the largest rift that other folds of similar scale in the basin are also fault basins in the eastern margin of Japan Sea, formed mainly in related. early Miocene time. The basin is about 50 km wide and [8] The 2004 mid-Niigata Prefecture earthquake occurred 100 km long, trends NNE–SSW, and is bounded by the in the eastern margin of the basin along one of the first- Shibata-Koide Tectonic Line (SKTL) on the east and by the order anticlinoriums. The anticlinorium deforms Miocene to Itoigawa-Shizuoka Tectonic Line (ISTL) on the west early Pleistocene sedimentary sequences and volcanic rocks

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Figure 3. Detailed geologic map of the Uonuma hills (simplified from a digitized geologic map of the Chuetsu area [Takeuchi et al., 2004]).

[Yanagisawa et al., 1985, 1986; Kobayashi et al., 1991]. vergent asymmetric anticline trending N–S. The western The anticlinorium forms topographic highs that are divided anticline has been called the Higashiyama anticline in into Higashiyama hills to the north and Uonuma hills to the previous papers and geologic maps [e.g., Kobayashi et al., south by Uono river valley, although the two hills are 1991], but we use the name North Higashiyama anticline in structurally continuous. In this paper, we refer to the entire this paper to distinguish it from the Higashiyama anticline complex as the Uonuma hills. in the middle segment of the Uonuma hills. The western limb of the North Higashiyama anticline is steeply inclined 3.2. Structure of the Uonuma Hills and partly overturned at its base, and these deformation of [9] The Uonuma hills can be subdivided into northern, Pleistocene sediments were attributed to an east dipping middle, and southern segments as defined by structural high-angle reverse fault system, the Yukyuza fault [Tsutsumi style, as pointed out by Sato and Kato [2005]. North of et al. 2001; Sato and Kato, 2005]. The North Higashiyama 0 37°25 N, the hills are composed of two anticlines (Figure 3). and Nigoro anticlines merge into the Higashiyama anticline On the east is the Nigoro anticline, which trends NNE– in the middle segment. SSW, parallel to the general trend of the hills, and has a [10] In the middle segment of the hills, Yanagisawa et al. gentle symmetrical profile. The western anticline is a west [1986] defined the Higashiyama anticlinorium and the

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Figure 4. Geologic structure in the Ojiya district based on the geologic map by Yanagisawa et al. [1986]. The fold axes in the Higashiyama anticlinorium are discontinuous and often overlapping each other. In contrast, there is no structural offset in the backlimb of the anticlinorium except for a bend of strike from NNE in the north of N–S in the south. The forelimb of the anticlinorium is bounded by the Suwatoge flexure, which is continuous through the anticlinorium.

Horinouchi synclinorium (Figure 4). The anticlinorium is 1985, 1986; Kobayashi et al., 1991]. Miocene sediments are composed of two major asymmetric anticlines, the composed mainly of muddy sediments and turbidites Higashiyama and Tamugiyama anticlines, and other smaller deposited in deep marine environments. The Pliocene sedi- anticlines and synclines. The fold axes of them are discon- ments comprise a coarsening upward sequence that thickens tinuous in the anticlinolium, however, the western and to the west, indicating that they were deposited on the eastern limbs of the anticlinorium have no structural offset westward deepening slope along the eastern margin of the along the anticlinorium (Figure 4). The Horinouchi syncli- rift basin. The Uonuma Formation consists of latest Plio- norium is a large, gentle basin like structure including small, cene to middle Pleistocene fluvial to shallow marine sedi- gentle folds. ments. Facies changes in these strata make it difficult to [11] The southern segment of the Uonuma hills, south of correlate them across the Uonuma hills. However, the many 37°100N, is characterized by a simple east vergent asym- volcanic tephras contained in the Pliocene to Pleistocene metric anticline with a wide monoclinal western limb sediments provide useful key horizons for the correlation of [Yanagisawa et al., 1985]. The width of the hills decreases synchronous stratigraphic surfaces in this area [Kurokawa, to the south from 14 km to 10 km (Figure 3). 1999]. The youngest strata deformed in the Uonuma hills [12] Two active faults and fold scarps were identified include middle to late Pleistocene terraces [Kim, 2004], before the 2004 earthquake along the eastern margin of the preserved in river valleys and fans. middle segment (Figure 3). The Obiro fault, about 6 km long, was defined by fold scarps that deforms late Pleisto- 4. The 2004 Mid-Niigata Earthquake cene terrace along the synclinal axis by Watanabe et al. [2001], but no fault or a flexure was observed on a high- [14] The 2004 mid-Niigata Prefecture earthquake (Mw = resolution seismic profile obtained by Kato et al. [2005b]. 6.6) occurred on 23 October 2004 in the southwestern part The Muikamachi fault bounds the eastern margin of the of the Higashiyama anticline [e.g., Aoki et al., 2005] and middle to southern segments of Uonuma Hills [Kim, 2004], caused severe damage because of strong ground shaking and its uplift rate was estimated to be 2 m/ky in maximum. [Honda et al., 2005] and numerous landslides [Yoshimi et al., 2005]. A dense seismic network was deployed in and 3.3. Stratigraphy of the Uonuma Hills around the source area of the earthquake from 24 October [13] Sediments deformed by the anticlines range in age and provided precise locations of aftershocks distributed from middle Miocene to Pleistocene [Yanagisawa et al., mainly in the middle segment of the Uonuma hills

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Figure 5. Main shocks and aftershocks of the 2004 mid-Niigata Prefecture earthquake [Kato et al., 2005a]. Lines indicate the locations of fault modeling.

(Figure 5), several rupture surfaces related with large 2005a, 2006; Korenaga et al., 2005; Okada et al., 2005]. aftershocks, and detailed velocity structure (Figure 6) [Kato Analyses of coseismic crustal deformation [Ozawa et al., et al., 2005a, 2006; Okada et al., 2005; Sakai et al., 2005]. 2005] and strong ground motion records [Hikima and The main shock had a reverse-fault-type mechanism, and Koketsu, 2005; Honda et al., 2005] support the high-angle the aftershock distribution indicated that slip on the main fault model. The aftershocks extended for a distance along shock occurred along a west dipping high-angle reverse strike of 35 km, from the northern end of the Higashiyama fault (Figures 5 and 6). The aftershocks suggested a flatter, anticline to the southern termination of the Tamugiyama low-angle thrust at shallower level of less than about 5 km anticline (Figures 3 and 5). (Figure 6) [Kato et al., 2005a, 2006]. Tomographic analysis [15] The largest aftershock (Mw = 6.3) occurred about of aftershock data shows that the P wave velocity of the 36 min later at the deepest end of a reverse fault parallel to hanging wall is slower than that of the footwall [Kato et al., and about 5 km to the east of the source fault of the main

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Figure 6. Fault models and aftershocks on cross sections. Fault models were constructed along the cross sections from geologic structure along which Kato et al. [2005a] depicted the precise aftershock distribution and velocity structure. The locations of the cross sections are shown in Figure 5. shock (Figure 6). Other aftershock occurred on 27 October coseismic slip reached the eastern margin of the hills from along an ESE dipping thrust fault in the footwall, which is one of deep seismogenic faults along the main shock or one interpreted as a fault conjugate to those of the main shock of major aftershocks. and largest aftershock (Figure 6). There were many after- [18] Around the location of the surface rupture, two large shocks in the hanging wall of the source fault of the main aftershocks occurred which accompanied linear alignments shock, suggesting complicated deformation there [Aoki et of smaller aftershocks showing the source faults of the large al., 2005]. aftershocks. The one is the largest aftershock that has a source fault parallel to the main shock and the other is an 5. Construction of Fault Models aftershock that has an east dipping source fault occurred on 27th October (Figure 6). 5.1. Single-Thrust Model [19] Kato et al. [2005a, 2006] have determined the [16] The source area of the 2004 earthquake consists of precise location of aftershocks and detailed velocity struc- many folds, and the main shock was followed by many ture (Figure 6) and showed that the two large aftershocks aftershocks, suggesting that several faults may have caused occurred in a high-velocity zone deeper than 8 km. The the growth of the folds. We, however, assumed that a single velocity above the upper limit of the aftershocks is higher reverse fault along the source fault of main shock is than 5 km/s, suggesting that the shallower part has enough responsible to growth of the folds in the Uonuma hills rigidity to generate earthquakes. These observations indi- based on following reasons. cate that the slip of the two large aftershocks have not [17] Surface ruptures were found along the eastern margin reached the surface rupture. of the Higashiyama Hills [Suzuki et al., 2004; Maruyama et [20] In contrast, the aftershocks along the main shock al., 2005]. The displacement at the ground surface was less continue to the depth 2–3 km and show a bend from high to than 30 cm but the trenching survey revealed that the low angle around 5 km deep (Figure 6). The aftershocks surface ruptures occurred repeatedly along the active fault above the bend probably occurred in sediments or boundary that has slipped [Maruyama et al., 2007]. A high-resolution between the sediments and basement that has velocity less seismic profile presented by Kato et al. [2005b] showed that than 4 km/s. Kato et al. [2005b] and Sato and Kato [2005] the surface rupture continues to a low-angle thrust fault showed that the low-angle thrust shown by the shallow down to several hundred meters in depth. The fact that the seismic profile at the eastern margin of the hill can be surface ruptures continues to the thrust fault indicates that consistently connected with the source fault of the main

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Sea [e.g., Sato, 1994; Okamura et al., 1995], it is reasonable to assume that the growth of the folds is mainly attributed to slip of the major reverse fault on the main shock rather than the faults related aftershocks in the high-velocity basement. [22] Geologic structure supports our single reverse fault model. The eastern and western limbs of the Higashiyama anticlinorium have no structural discontinuity along the margin, whereas the fold axes are discontinuous in the anticlinorium (Figure 4). The back limbs of the Higashiyama and Tamugiyama anticlines, which are located above the source fault, have continuous dips and strikes, indicating they are underlain by a common reverse fault. In contrast, the forelimbs of the two anticlines are discontinuous. The Suwatoge flexure, the forelimb of the Tamugiyama anticline, continues north to the forelimb of the Komatsukura anticline, one of the smaller anticlines to the east of the Higashiyama anticline. This strongly suggests that the smaller anticlines to the east of the Higashiyama anticline are also related to the source fault. These arrangements of folds and flexures can be simply explained by a single reverse fault model with lateral ramps. Gentle folds in the Horinouchi synclinorium is also able to be explained by a low-angle thrust. [23] Our single reverse fault model may not be a unique one, but reasonably explains geologic structure, aftershock distribution and the surface rupture. 5.2. Inclined Shear [24] The inclined shear is a simple deformation mechanism of a hanging wall above a dip-slip fault, which was presented as a vertical shear model or a constant heave model [Verrall, 1981; Gibbs, 1983] and developed to inclined shear model by White et al. [1986]. Clay and sand box experiments showed that inclined shear can approxi- mate the deformation of hanging wall [Dula, 1991; Yamada and McClay, 2003a, 2003b]. In this model, we assume that a Figure 7. Cartoon illustrating geometric model for hanging wall is composed of many slices bounded by parallel determining fault trajectory by inclined shear of hanging shear plains and is deformed only by slips along the shear wall. (a) Three key geometric elements, (b) passes of points plains retaining the thickness of the slices (Figure 7). If we (arrows) on the initial key horizon during the deformation of have the geometries of a key bed before and after deforma- hanging wall. Fault trajectory can be constructed by tion and a leading edge of a fault (Figure 7a), passes of the connecting the arrows from top to down. (c) Fault trajectory point (a, b, ... i) on a key bed during deformation can be constructed assuming lower angle (65°) of shear planes. determined as one of arrows shown in Figure 7b. The Note that inclination of the fault trajectory becomes steeper arrows are parallel to intervals of the fault segments as the decrease of the shear angle of antithetic shear plane. bounded by shear planes, then we can construct a fault trajectory from top to downward by connecting the arrows (a-a0,b-b0, ... i-i0) as shown in Figure 7b. shock. The results of the detailed study of aftershocks and [25] The parallel shear assumed in this model represents velocity structure strongly suggest that the surface rupture bulk deformation approximating real deformation [Dula, were caused by the slip along the source fault of the main 1991, White, 1992]. The real deformation in a hanging wall shock. is inferred to be complicated, and may have been caused by [21] The detailed velocity structure deduced by tomo- antithetic and synthetic shears [Dula, 1991]. The fold graphic analyses [Kato et al., 2005a, 2006] showed that the geometry constructed by incline shear modeling varies by source fault of the main shock nearly coincides with the change of the angle of shear plain (Figure 7c), and the boundary between the low-velocity hanging wall and Yamada and McClay [2003b] showed that the angle of shear the high-velocity footwall (Figure 6). This indicates that the plain varies by change of three dimensional fault geometry. reverse fault on the main shock was a major normal fault [26] In this modeling method, geometry of a certain bounding a rift basin. In contrast, the two aftershocks on 23 interval of a fault trajectory is determined by amount of a and 27 October occurred in the high-velocity footwall slip and geometry of a key bed before and after deformation suggesting that their source faults were not major normal above the fault interval between the shear planes bounding faults. Because basin inversion and fault reactivation are the interval (Figure 7b). The amount of slip is a key dominant tectonic processes in the eastern margin of Japan parameter to construct the fault trajectory, but it is generally

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Figure 8. Cartoon showing the fault construction method. (a) Three key geometries for fault modeling. The uplifted area is the product of the amount of slip and the depth of detachment. (b) (top) Geologic map split from the digitized geological map along the section line and (bottom) dips of beds shown on the map projected onto the topographic profile. (c) Parallel lines drawn using the dips of beds as reference lines. The geometry of the folded key bed was built taking into account the parallel lines and the location of the key bed. Ko and Ok I are tephra key beds of nearly the same age. very difficult to estimate precise slip amount from surface 5.3. Construction of Fault Trajectory geologic structure. To solve this difficulty, we assumed the [28] We constructed 12 serial cross sections using the depth of the fault to be 13 km from the cutoff depth of the method described above. The section lines were established aftershocks. If we can determine the depth of the fault end parallel to the cross sections along which Kato et al. [2005a] (D), we can calculate slip amount S by S = A/D, where A is depicted the precise aftershock distribution and P wave an uplift area that can be measured on sections (Figure 7b). velocity, and four of them coincide with those cross sections [27] If we can determine the fault depth from the depths (Figure 5). of the aftershocks, we can fix the position of the lower end [29] For the modeling, we needed to determine three of the fault. This means that the geometry of deeper part of geometric elements: the geometry of folded key beds, the the fault trajectory in a seismogenic zone is mainly deter- original geometry of the key bed before folding, and the mined by the geometry of a backlimb. The Higashiyama geometry of the fault interval connecting the tips of and Tamugiyama anticlines have nearly constant dip in their the folded and original key beds (Figure 8a). We used back limb from their western margin to the anticlinal axes, commercial software for balanced cross section modeling indicating that the geometry of the key bed is reliable. We to construct fold geometry and fault models. Geologic data can construct the reliable fault trajectory in the seismogenic were loaded from a digitized geological map [Takeuchi et zone, if we can determine the fault depth and the geometry al., 2004] based on quadrangle geological maps at a scale of of the backlimb precisely. 1:50,000 for Nagaoka [Kobayashi et al., 1991], Ojiya

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a Table 1. Parameters of Fault Modeling sented by Kato et al. [2005a] (Figure 6). The fault Line Key Bed Age, Ma Area, km2 Contraction, km trajectories are composed of a high-angle segment deeper Y = 10 NA33 4.5 22.2 1.7 than 5 km and a low-angle segment shallower than 5 km, Y = 5 NA33 4.5 11.6 0.9 which is consistent with the aftershock distribution along Y = 2.5 Ko-OkI 2.5 16.2 1.2 the source fault of the main shock. Two of the sections (Y = Y = 0 Ko 2.5 19.6 1.5 0 and 10 km) show good agreement between the estimated Y=2.5 Ko 2.5 14.2 1.1 Y=5 T30 1.3 14.4 1.1 fault trajectories and the aftershock distribution, and another Y=7.5 Kk-Tz 1.8 10.5 0.8 section (Y = 5 km) is consistent in part with the aftershock Y=10 Tg 1.6 6.4 0.5 distribution. On the southernmost section (Y = 5 km), Y=12.5 N/A 2 4.6 0.4 there are two cluster of aftershock distribution that does not Y=15 N/A 2 7.6 0.6 Y=17.5 N/A 2 5.7 0.4 show a west dipping plane; thus it is difficult to discuss the Y=20 N/A 2 6.0 0.5 relationship between the fault model and the source fault. aKey beds and age indicate the name of tephra and its age used for [32] The result indicates that the geometry of the source modeling of each section. N/A indicates that no tephra exists on the section fault of the 2004 earthquake can be constructed from fold and base of the Uonuma formation was used as a key bed. Area was geometry assuming inclined antithetic shear of hanging wall measured between original and fold geometries of a key bed, and horizontal at 85°, although more detailed comparison between the fault contraction was calculated assuming the detachment depth to be 13 km. model and aftershocks are necessary. The high-angle shear of hanging wall presumably indicates that antithetic and synthetic shears exist in the hanging wall. [33] It is not difficult to apply this method to other active [Yanagisawa et al., 1986], Tokamachi [Yanagisawa et al., fold-and-thrust zones and to construct geometry of seismo- 1985] and part of Suhara [Takahashi et al., 2004]), and part genic faults, if detailed geologic structure of ground surface of the Sumondake area [Ijima, 1974; Kageyama and and depth of the seismogenic zone in the upper crust can be Kaneko, 1992]. determined. The constructed fault is expected to be a source [30] Topographic profiles were constructed from a 50 m fault of future earthquakes, implying that the model would digital elevation model (DEM) presented by Geographical provide important information to reduce damage by future Survey Institute of Japan. The dip and strike of beds within earthquakes. about 800 m from a section line were projected onto each of 6.2. Lateral Change in Fault Geometry and Its the topographic profiles (Figure 8b), and references lines Relationship to the 2004 Earthquake Rupture were drawn parallel to the dip of beds projected on the section (Figure 8c). Because the thicknesses of the sedi- [34] We constructed eight additional fault models by the mentary units were not constant through the section, the same method along sections parallel to the four cross parallel lines do not indicate synchronous surfaces. We sections discussed above (Figure 9), mainly in the southern corrected for the differences in sediment thickness by using part of the Uonuma hills (Figure 5). The fold geometry, the contained tephras. The geometry of the folded key bed consisting of an asymmetric anticlinorium in the source was drawn taking into account the reference lines and the area, changes to a simple asymmetric anticline in the location of the key tephra exposure (Figure 8c). We selected southern part of Uonuma hills, and the fault trajectories the youngest tephra exposed on both forelimb and backlimb also become simpler as the geometry of the folds changes of the anticline along each of the transects as the key bed. (Figure 9). The depth contours of the subsurface fault plane Because there was no tephra on the southern four sections, based on the 12 fault models show that the strike of the fault we used the base of the Uonuma formation as a key bed north of the main shock is NNE–SSW; south of the main (Table 1). The Nokogiriyama fault along the axis of the shock it bends to strike N–S, and then bends again near the Higashiyama anticlines was ignored in the construction of southern margin of the aftershock area to strike NNE–SSW the fold geometry because the stratigraphic offset caused by (Figure 10). Kato et al. [2006] showed that the NNE–SSW the fault is less than a few hundreds of meters, which is of trend of the aftershock alignment change to roughly N–S little significance on the scale of the Higashiyama anticline. from north to south around the main shock, which appears Original geometries of key beds were defined by a linear or to be consistent with our fault model based on geologic simple concave-upward profile increasing in depth to the structure. If our model is correct, the earthquake rupture west (Figure 8c). The paleoenvironment indicated by the started at the northern convex bend of the fault surface and sedimentary facies of Pliocene and Pleistocene units sup- stopped at the concave bend at its southern end. ports that the western part of the hills was in a deeper [35] We could not construct the fault around the northern environment than the eastern part. A dip of inclined shear margin of the source area because the northern margin may was determined to be 85° ESE by comparison of a fault be underlain by two oppositely dipping faults. The trajectory and the aftershock alignment along the section on Higashiyama anticline in the source area is continuous with the main shock (Y = 0 km). the Nigoro anticline, and the aftershock distribution appears to follow the trend of the Nigoro anticline (Figures 3 and 5), suggesting that the west dipping source fault extends to the 6. Results and Discussion northern segment of Uonuma hills. In contrast, the 6.1. Comparison Between Fault Models and Yukyuzan fault along the western margin of the North Aftershocks on Cross Sections Higashiyama anticline is an east dipping reverse fault under [31] Fault trajectories were compared with the distribu- the northern part of the source area. It is difficult to tion of the aftershocks along the four cross sections pre- determine the relationship between the east dipping and

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Figure 9. Fault models along 12 cross sections. The locations of the cross sections are shown in Figure 5. B03S08 B03S08 OKAMURA ET AL.: FOLDS RELATED TO 2004 MID-NIIGATA EARTHQUAKE B03S08

Figure 10. Depth contours of the fault surface constructed by the fault modeling. The fault plane bends twice from north to south: from NNE–SSW to N–S and then back to NNE–SSW. The main shock (star) is located on the convex northern bend, and the southern boundary of the aftershock (gray circles) area coincides with the southern concave bend. west dipping faults only from the geologic structure at the 6.3. Slip Amount and Rate ground surface. The geometric change of the anticlines at [36] On the basis of our fault models, the amounts of the northern end of the source area suggests that a change in horizontal contraction along each of the constructed faults fault geometry determined the northern margin of the were estimated to be 0.4 to 1.7 km (Table 1). The contraction rupture. Sato and Kato [2005] proposed that the east vergent was largest at the Higashiyama anticline and decreased to the reverse fault in the middle segment abruptly changes into a south. The key beds used in the models were tephras of west vergent fault around the southern margin of the different ages, ranging from 4.5 to 1.3 Ma, and the largest northern segment; however, the gradual change in the slip was deduced from the cross section based on the tephra geometry of the anticline from the source area to the Nigoro of 4.5 Ma. This suggests that the growth of the fold started in anticline indicates that the east verging fault continues to the the early Pliocene; however, the fact that structure of the north for some distance. Pliocene sedimentary sequences is almost uniform probably

12 of 14 B03S08 OKAMURA ET AL.: FOLDS RELATED TO 2004 MID-NIIGATA EARTHQUAKE B03S08 indicates that the major growth of the folds occurred after the Carena, S., and J. Suppe (2002), Three-dimensional imaging of active structure using earthquake aftershocks: The Northridge thrust, California, late Pliocene. If we assume that the growth of the anticlines J. Struct. Geol., 24, 887–904. began about 2 Myr ago, then the average horizontal con- Davis, T. L., and J. S. Namson (1994), A balanced cross-section of the 1994 traction rate is calculated to be 0.2 to 0.85 m/kyr (Table 1), Northridge earthquake, southern California, Nature, 372, 167–169. Dula, F. W., Jr. (1991), Geometric models of listric normal faults and roll- but it is difficult to estimate the precise time of onset of over folds, AAPG Bull., 75, 1609–1625. folding or the slip rate of the fault from our model. Gibbs, A. D. (1983), Balanced cross-section construction from seismic [37] Kim [2004] estimated the uplift rate of the Uonuma sections in areas of extensional tectonics, J. Struct. Geol., 5, 153–160. hills to be 0.8–1.3 m/kyr in its middle segment during the Hikima, K., and K. Koketsu (2005), Rupture processes of the 2004 Chuetsu (mid-Niigata Prefecture) earthquake, Japan: A series of events in a com- last several tens of thousands of years, based on structural plex fault system, Geophys. Res. Lett., 32, L18303, doi:10.1029/ relief of folded middle Pleistocene fluvial terraces deposits 2005GL023588. in the middle segment of the Uonuma Hills. The uplift rate Honda, R., S. Aoi, N. Morikawa, H. Sekiguchi, T. Kunugi, and H. Fujiwara (2005), Ground motion and rupture process of the 2004 mid Niigata is estimated to be 1.7 times larger than contraction rate, if Prefecture earthquake obtained from strong motion data of K-NET and the fault inclination is 60°. Thus the maximum contraction KiK-net, Earth Planets Space, 57, 527–532. rate 0.85 m/kyr is consistent with the maximum uplift rate Ijima, S. (1974), Nature of the middke Miocene unconformity on the mid- stream region of the R. Aburuma-gawa, Shinano-gawa river group (in 1.3 m/kyr. This comparison supports that our fault model is Japanese with English abstract), Rep. 250-2, pp. 145–154, Geol. Surv. of valid. Jpn., Tsukuba. Ishiyama, T., K. Mueller, M. Togo, A. Okada, and K. Takemura (2004), Geomorphology, kinematic history, and earthquake behavior of the active 7. Summary Kuwana wedge thrust anticline, central Japan, J. Geophys. Res., 109, B12408, doi:10.1029/2003JB002547. [38] The subsurface fault that are responsible for the Kageyama, K., and N. Kaneko (1992), Central part of Niigata Prefecture, growth of anticlines in the source region of the 2004 mid- Geol. Map Oil Gas Fields Jpn. 13, Geol. Surv. of Jpn., Tsukuba. Kato, A., E. Kurashimo, N. Hirata, S. Sakai, T. Iwasaki, and T. Kanazawa Niigata Prefecture earthquake were constructed from fold (2005a), Imaging the source region of the 2004 mid-Niigata Prefecture geometries, assuming that the folds have been growing by the earthquake and the evolution of a seismogenic thrust-related fold, Geo- deformation of the hanging wall because of inclined shear phys. Res. Lett., 32, L07307, doi:10.1029/2005GL022366. above the single reverse fault. The depth of the detachment Kato, N., et al. (2005b), Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the was determined to be 13 km, based on the aftershock cutoff 2004 mid-Niigata Prefecture earthquake (M6.8), central Japan, Earth depth. The fault trajectories agree well with the distribution of Planets Space, 57, 447–452. aftershocks along the fault of the main shock. The results Kato, A., S. Sakai, N. Hirata, E. Kurashimo, T. Iidaka, T. Iwasaki, and T. Kanazawa (2006), Imaging the seismic structure and stress field in the indicate that the source fault of the earthquake is responsible source region of the 2004 mid-Niigata Prefecture earthquake: Structural to the growth of the geologic structure. The key data for the zones of weakness and seismogenic stress concentration by ductile flow, fault construction are the depth of the detachment and the fold J. Geophys. Res., 111, B08308, doi:10.1029/2005JB004016. Kim, H. Y. (2004), Relationship between the upheaval process of the geometry, especially of the back limb. If these data are Uonuma Hills and the cumulative nature of the Muikamachi fault, available, it is possible to infer the geometry of source faults central Japan (in Japanese with English abstract), Active Fault Res., of future earthquakes in the active fold-and-thrust zone that 24, 63–75. is underlain by a simple thrust using inclined shear model- Kobayashi, I., M. Tateishi, M. Yoshioka, and M. Shimazu (1991), Geology of the Nagaoka District (in Japanese with English abstract), quadrangle ing. A three-dimensional fault geometry constructed the series, scale 1:50,000, 132 pp., Geol. Surv. of Jpn., Tsukuba. from fault trajectories along 12 section lines indicates that Korenaga, M., S. Matsumoto, Y. Iio, T. Matsushima, K. Uehira, and from north to south, the strike of the fault changes from T. Shibutani (2005), Three dimensional velocity structure around aftershock area of the 2004 mid Niigata Prefecture earthquake (M6.8) by the NNE–SSW to N–S at the site of the main shock and from double-difference tomography, Earth Planets Space, 57, 429–433. N–S to NNE–SSW near the southern termination of the Kurokawa, K. (1999), Tephrostratigraphy of the Nanatani to Uonuma for- aftershock area. The relationship suggests that the rupture mation of 13 Ma to 1 Ma in the Niigata region, central Japan (in Japanese with English abstract), J. Jpn. Pet. Technol., 64, 80–93. started at the convex bend of the fault and stopped at the Maruyama, T., Y. Fusejima, T. Yoshioka, Y. Awata, and T. Matsu’ura concave bend of the fault. The horizontal contraction rate of (2005), Characteristics of the surface rupture associated with the 2004 the fault was inferred to be 0.2 to 0.85 m/kyr, which is mid Niigata Prefecture earthquake, central Japan and their seismotectonic consistent with the uplift rates of the hills inferred from the implications, Earth Planets Space, 57, 521–526. Maruyama, T., K. Iemura, T. Azuma, T. Yoshioka, M. Sato, and R. Miyawaki elevation of fluvial terraces by Kim [2004]. (2007), Paleoseismological evidence for non-characteristic behavior of surface rupture associated with the 2004 mid-Niigata Prefecture, central [39] Acknowledgments. We are grateful to James Dolan, Karl Japan, Tectonophysics, 429, 45–60. Mueller, and Tom Pratt, reviewers of JGR, for their useful comments and Niigata Prefecture Government (2000), Geological map of the Niigata Pre- suggestions, which helped greatly to improve our manuscript. We also fecture (in Japanese), 200 pp., Niigata, Japan. thank Yuichi Sugiyama for his encouragement of our study and to Aitaro Ohtake, M. (1995), A seismic gap in the eastern margin of the Sea of Japan Koto, who kindly provided precise aftershock data. This work was as inferred from the time-space distribution of past seismicity, Island Arc, supported by the Special Coordination Fund for Promotion of Science 4, 156–165. and Technology offered by the Ministry of Education, Culture, Sports, Ohtake, M. (2002), Earthquake potential along the eastern margin of the Science and Technology of Japan (MEXT) under the title of ‘‘Urgent Japan Sea (in Japanese), in Active Faults and Seismotectonics in the Research for the 2004 Mid-Niigata Prefecture Earthquake’’. 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