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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 hasbeen growing since late Pliocene time in a Miocene rift basin along the eastern margin ofthe Japan Sea. We constructed the trajectory of the subsurface faults responsible forthe 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 inclinedantithetic shear with a dip of 85� in the hanging wall above a single reverse fault. The faulttrajectory constructed from the fold geometries nearly coincides with the geometries of thesource fault of the main shock of the 2004 earthquake revealed by aftershocks, whichsupports that the rupture was along a geologic fault that has ruptured repeatedly during thelast a few million years. A three-dimensional fault model based on 12 faulttrajectories constructed along parallel sections revealed that the main shock occurred on aconvex bend in the fault surface and that the southern termination of the aftershockdistribution nearly coincides with a concave bend in the fault. The close relationbetween the source fault and the geologic structure shows that it is possible to constructsource fault geometry assuming inclined shear as deformation mechanism of a hangingwall 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

[2] The concept of fault-related folding suggests that it ispossible to infer the geometry of a subsurface dip-slip faultfrom the geometry of surface folds [e.g., Suppe, 1983;Gibbs, 1983]. This methodology has previously beenapplied to the evaluation of active faults and earthquakepotential in sedimentary basins [Shaw and Suppe, 1994,1996; Shaw et al., 2002; Ishiyama et al., 2004]. Bycomparing faults constructed on the basis of the theory offault-related folding with source faults of earthquakes, wecan examine the reliability of this fault modeling method.Davis and Namson [1994] and Carena and Suppe [2002]showed that the source fault of the 1994 Northridge earth-quake is consistent with a fault inferred from fold geometryin the source area; however, in this case it is difficult toobtain a precise geologic structure related to the source faultbecause the structure is masked by overlying complexdeformation [Carena and Suppe, 2002].[3] The 2004 mid-Niigata Prefecture earthquake occurred

in a fold-thrust belt that has been growing during the

Quaternary, and aftershocks related to this event wereobserved by a dense seismic network deployed after 2 to5 days of the main shock [e.g., Kato et al., 2005a, 2006;Okada et al., 2005]. These data show the precise geometryof the source fault and thus provide an opportunity toexamine whether the method of fault-related folding canbe used to infer successfully the subsurface source fault.Okamura and Yanagisawa [2005] and Sato and Kato [2005]have shown that there is close relation between foldgeometry and the source fault based on construction of abalanced cross section. In this study we constructed three-dimensional subsurface fault models from the detailedgeologic structures in the source area assuming inclinesshear as deformation mechanism of the hanging wall, andexamined the relationship between the fault models and thesource fault of the 2004 earthquake.

2. Seismotectonic Setting

[4] The 2004 mid-Niigata Prefecture earthquake occurredat the southern end of the eastern margin of the Japan Sea,where the convergent plate boundary between the Amurianand Okhotsk plates is thought to be located (Figure 1). Weiand Seno [1998] estimated the convergence rate of theseplates to be about 1.5 cm/yr in this region. The margin wasinitially formed as a rifted passive margin, mainly duringthe early Miocene when the Japan Sea opened, and it has

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

1Active Fault Research Center, National Institute of AdvancedIndustrial Science and Technology, Tsukuba, Japan.

2Institute of Geology and Geoinformation, National Institute ofAdvanced Industrial Science and Technology, Tsukuba, Japan.

Copyright 2007 by the American Geophysical Union.0148-0227/07/2006JB004320$09.00

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accommodated E-W shortening since the late Pliocene[Sato, 1994; Okamura et al., 1995]. Many rift basins formedin the early Miocene have been inverted as a result ofreactivation of normal faults as reverse faults during the last2 to 3 Myr [Okamura et al., 1995]. Okamura [2002] pointedout that contraction is localized mainly in the rifts, whereasthe horsts between the rifts, composed of pre-Neogene rocksare scarcely deformed (Figure 1). The Okushiri ridge is oneof the largest currently contracting zones in the northern partof the margin, and its southern extension is inferred to becontinuous with the Awashima uplift (Figure 1) and theactive fault zone along the Shinano River (Figure 2).

[5] Four major earthquakes larger than M = 7.5 occurredin the twentieth century in the offshore area along theOkushiri ridge and Awashima uplift (Figure 1) [Ohtake,1995]. Along the Shinano River, two destructive earth-quakes occurred in the nineteenth century (Figure 2)[Usami, 2003]. Recent GPS measurements have shown thatcontraction has been focused along the Niigata-Kobe tec-tonic zone [Sagiya et al., 2000] and along the Shinano Riverin the northern part of this tectonic zone.[6] These studies indicate that active contraction has

occurred for the last few million years along the Okushiriridge, Awashima uplift, and Shinano River, and these zonesare still active and have high potential for future earth-

Figure 1. Index and tectonic maps. (right) Plate tectonic framework around the eastern margin of theJapan 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 onwork by Okamura et al. [2005], Tanioka et al. [1995], Satake [1985], and Abe [1975]. Magnitude of theearthquakes is shown by MJMA.

B03S08 OKAMURA ET AL.: FOLDS RELATED TO 2004 MID-NIIGATA EARTHQUAKE

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quakes. Ohtake [1995, 2002] pointed out that there areseveral seismic gaps along this active zone where thepotential for future earthquakes may be even higher. The2004 mid-Niigata Prefecture earthquake occurred in one ofthese gaps.

3. Geology of the Source Area

3.1. Niigata Sedimentary Basin

[7] The Niigata sedimentary basin, one of the largest riftbasins in the eastern margin of Japan Sea, formed mainly inearly Miocene time. The basin is about 50 km wide and100 km long, trends NNE–SSW, and is bounded by theShibata-Koide Tectonic Line (SKTL) on the east and by theItoigawa-Shizuoka Tectonic Line (ISTL) on the west

(Figure 2). The sediments in the basin are up to 6000 mthick and have been folded during the last 2 to 3 Myr [e.g.,Niigata Prefecture Government, 2000]. Suzuki et al. [1974]classified anticlines in the basin into 3 orders and showedthat the first-order anticlines, which have axes longer than30 km, tend to be separated by about 10 km. Okamura[2003] noted that the first-order anticlines in the northwest-ern part of the basin are fault related and lie above thrustfaults that cut through the upper crust. He further suggestedthat other folds of similar scale in the basin are also faultrelated.[8] The 2004 mid-Niigata Prefecture earthquake occurred

in the eastern margin of the basin along one of the first-order anticlinoriums. The anticlinorium deforms Miocene toearly Pleistocene sedimentary sequences and volcanic rocks

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 ofthe 2004 earthquake [Kato et al., 2005a], and crosses show the inferred epicenters of historicalearthquakes [Usami, 2003].


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[Yanagisawa et al., 1985, 1986; Kobayashi et al., 1991].The anticlinorium forms topographic highs that are dividedinto Higashiyama hills to the north and Uonuma hills to thesouth by Uono river valley, although the two hills arestructurally continuous. In this paper, we refer to the entirecomplex as the Uonuma hills.

3.2. Structure of the Uonuma Hills

[9] The Uonuma hills can be subdivided into northern,middle, and southern segments as defined by structuralstyle, as pointed out by Sato and Kato [2005]. North of37�250N, the hills are composed of two anticlines (Figure 3).On the east is the Nigoro anticline, which trends NNE–SSW, parallel to the general trend of the hills, and has agentle symmetrical profile. The western anticline is a west

vergent asymmetric anticline trending N–S. The westernanticline has been called the Higashiyama anticline inprevious papers and geologic maps [e.g., Kobayashi et al.,1991], but we use the name North Higashiyama anticline inthis paper to distinguish it from the Higashiyama anticlinein the middle segment of the Uonuma hills. The westernlimb of the North Higashiyama anticline is steeply inclinedand partly overturned at its base, and these deformation ofPleistocene sediments were attributed to an east dippinghigh-angle reverse fault system, the Yukyuza fault [Tsutsumiet al. 2001; Sato and Kato, 2005]. The North Higashiyamaand Nigoro anticlines merge into the Higashiyama anticlinein the middle segment.[10] In the middle segment of the hills, Yanagisawa et al.

[1986] defined the Higashiyama anticlinorium and the

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


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Horinouchi synclinorium (Figure 4). The anticlinorium iscomposed of two major asymmetric anticlines, theHigashiyama and Tamugiyama anticlines, and other smalleranticlines and synclines. The fold axes of them are discon-tinuous in the anticlinolium, however, the western andeastern limbs of the anticlinorium have no structural offsetalong the anticlinorium (Figure 4). The Horinouchi syncli-norium is a large, gentle basin like structure including small,gentle folds.[11] The southern segment of the Uonuma hills, south of

37�100N, is characterized by a simple east vergent asym-metric anticline with a wide monoclinal western limb[Yanagisawa et al., 1985]. The width of the hills decreasesto the south from 14 km to 10 km (Figure 3).[12] Two active faults and fold scarps were identified

before the 2004 earthquake along the eastern margin of themiddle segment (Figure 3). The Obiro fault, about 6 kmlong, was defined by fold scarps that deforms late Pleisto-cene terrace along the synclinal axis by Watanabe et al.[2001], but no fault or a flexure was observed on a high-resolution seismic profile obtained by Kato et al. [2005b].The Muikamachi fault bounds the eastern margin of themiddle to southern segments of Uonuma Hills [Kim, 2004],and its uplift rate was estimated to be 2 m/ky in maximum.

3.3. Stratigraphy of the Uonuma Hills

[13] Sediments deformed by the anticlines range in agefrom middle Miocene to Pleistocene [Yanagisawa et al.,

1985, 1986; Kobayashi et al., 1991]. Miocene sediments arecomposed mainly of muddy sediments and turbiditesdeposited in deep marine environments. The Pliocene sedi-ments comprise a coarsening upward sequence that thickensto the west, indicating that they were deposited on thewestward deepening slope along the eastern margin of therift basin. The Uonuma Formation consists of latest Plio-cene to middle Pleistocene fluvial to shallow marine sedi-ments. Facies changes in these strata make it difficult tocorrelate them across the Uonuma hills. However, the manyvolcanic tephras contained in the Pliocene to Pleistocenesediments provide useful key horizons for the correlation ofsynchronous stratigraphic surfaces in this area [Kurokawa,1999]. The youngest strata deformed in the Uonuma hillsinclude middle to late Pleistocene terraces [Kim, 2004],preserved in river valleys and fans.

4. The 2004 Mid-Niigata Earthquake

[14] The 2004 mid-Niigata Prefecture earthquake (Mw =6.6) occurred on 23 October 2004 in the southwestern partof the Higashiyama anticline [e.g., Aoki et al., 2005] andcaused severe damage because of strong ground shaking[Honda et al., 2005] and numerous landslides [Yoshimi etal., 2005]. A dense seismic network was deployed in andaround the source area of the earthquake from 24 Octoberand provided precise locations of aftershocks distributedmainly in the middle segment of the Uonuma hills

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 eachother. In contrast, there is no structural offset in the backlimb of the anticlinorium except for a bend ofstrike from NNE in the north of N–S in the south. The forelimb of the anticlinorium is bounded by theSuwatoge flexure, which is continuous through the anticlinorium.


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(Figure 5), several rupture surfaces related with largeaftershocks, and detailed velocity structure (Figure 6) [Katoet al., 2005a, 2006; Okada et al., 2005; Sakai et al., 2005].The main shock had a reverse-fault-type mechanism, andthe aftershock distribution indicated that slip on the mainshock occurred along a west dipping high-angle reversefault (Figures 5 and 6). The aftershocks suggested a flatter,low-angle thrust at shallower level of less than about 5 km(Figure 6) [Kato et al., 2005a, 2006]. Tomographic analysisof aftershock data shows that the P wave velocity of thehanging wall is slower than that of the footwall [Kato et al.,

2005a, 2006; Korenaga et al., 2005; Okada et al., 2005].Analyses of coseismic crustal deformation [Ozawa et al.,2005] and strong ground motion records [Hikima andKoketsu, 2005; Honda et al., 2005] support the high-anglefault model. The aftershocks extended for a distance alongstrike of 35 km, from the northern end of the Higashiyamaanticline to the southern termination of the Tamugiyamaanticline (Figures 3 and 5).[15] The largest aftershock (Mw = 6.3) occurred about

36 min later at the deepest end of a reverse fault parallel toand about 5 km to the east of the source fault of the main

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


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shock (Figure 6). Other aftershock occurred on 27 Octoberalong an ESE dipping thrust fault in the footwall, which isinterpreted as a fault conjugate to those of the main shockand largest aftershock (Figure 6). There were many after-shocks in the hanging wall of the source fault of the mainshock, suggesting complicated deformation there [Aoki etal., 2005].

5. Construction of Fault Models

5.1. Single-Thrust Model

[16] The source area of the 2004 earthquake consists ofmany folds, and the main shock was followed by manyaftershocks, suggesting that several faults may have causedthe growth of the folds. We, however, assumed that a singlereverse fault along the source fault of main shock isresponsible to growth of the folds in the Uonuma hillsbased on following reasons.[17] Surface ruptures were found along the eastern margin

of the Higashiyama Hills [Suzuki et al., 2004; Maruyama etal., 2005]. The displacement at the ground surface was lessthan 30 cm but the trenching survey revealed that thesurface ruptures occurred repeatedly along the active faultthat has slipped [Maruyama et al., 2007]. A high-resolutionseismic profile presented by Kato et al. [2005b] showed thatthe surface rupture continues to a low-angle thrust faultdown to several hundred meters in depth. The fact that thesurface ruptures continues to the thrust fault indicates that

coseismic slip reached the eastern margin of the hills fromone of deep seismogenic faults along the main shock or oneof major aftershocks.[18] Around the location of the surface rupture, two large

aftershocks occurred which accompanied linear alignmentsof smaller aftershocks showing the source faults of the largeaftershocks. The one is the largest aftershock that has asource fault parallel to the main shock and the other is anaftershock that has an east dipping source fault occurred on27th October (Figure 6).[19] Kato et al. [2005a, 2006] have determined the

precise location of aftershocks and detailed velocity struc-ture (Figure 6) and showed that the two large aftershocksoccurred in a high-velocity zone deeper than 8 km. Thevelocity above the upper limit of the aftershocks is higherthan 5 km/s, suggesting that the shallower part has enoughrigidity to generate earthquakes. These observations indi-cate that the slip of the two large aftershocks have notreached the surface rupture.[20] In contrast, the aftershocks along the main shock

continue to the depth 2–3 km and show a bend from high tolow angle around 5 km deep (Figure 6). The aftershocksabove the bend probably occurred in sediments or boundarybetween the sediments and basement that has velocity lessthan 4 km/s. Kato et al. [2005b] and Sato and Kato [2005]showed that the low-angle thrust shown by the shallowseismic profile at the eastern margin of the hill can beconsistently connected with the source fault of the main

Figure 6. Fault models and aftershocks on cross sections. Fault models were constructed along thecross sections from geologic structure along which Kato et al. [2005a] depicted the precise aftershockdistribution and velocity structure. The locations of the cross sections are shown in Figure 5.


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shock. The results of the detailed study of aftershocks andvelocity structure strongly suggest that the surface rupturewere caused by the slip along the source fault of the mainshock.[21] The detailed velocity structure deduced by tomo-

graphic analyses [Kato et al., 2005a, 2006] showed that thesource fault of the main shock nearly coincides withthe boundary between the low-velocity hanging wall andthe high-velocity footwall (Figure 6). This indicates that thereverse fault on the main shock was a major normal faultbounding a rift basin. In contrast, the two aftershocks on 23and 27 October occurred in the high-velocity footwallsuggesting that their source faults were not major normalfaults. Because basin inversion and fault reactivation aredominant tectonic processes in the eastern margin of Japan

Sea [e.g., Sato, 1994; Okamura et al., 1995], it is reasonableto assume that the growth of the folds is mainly attributed toslip of the major reverse fault on the main shock rather thanthe faults related aftershocks in the high-velocity basement.[22] Geologic structure supports our single reverse fault

model. The eastern and western limbs of the Higashiyamaanticlinorium have no structural discontinuity along themargin, whereas the fold axes are discontinuous in theanticlinorium (Figure 4). The back limbs of the Higashiyamaand Tamugiyama anticlines, which are located above thesource fault, have continuous dips and strikes, indicatingthey are underlain by a common reverse fault. In contrast,the forelimbs of the two anticlines are discontinuous. TheSuwatoge flexure, the forelimb of the Tamugiyama anti-cline, continues north to the forelimb of the Komatsukuraanticline, one of the smaller anticlines to the east of theHigashiyama anticline. This strongly suggests that thesmaller anticlines to the east of the Higashiyama anticlineare also related to the source fault. These arrangements offolds and flexures can be simply explained by a singlereverse fault model with lateral ramps. Gentle folds in theHorinouchi synclinorium is also able to be explained by alow-angle thrust.[23] Our single reverse fault model may not be a unique

one, but reasonably explains geologic structure, aftershockdistribution and the surface rupture.

5.2. Inclined Shear

[24] The inclined shear is a simple deformation mecha-nism of a hanging wall above a dip-slip fault, which waspresented as a vertical shear model or a constant heavemodel [Verrall, 1981; Gibbs, 1983] and developed toinclined shear model by White et al. [1986]. Clay and sandbox experiments showed that inclined shear can approxi-mate the deformation of hanging wall [Dula, 1991; Yamadaand McClay, 2003a, 2003b]. In this model, we assume that ahanging wall is composed of many slices bounded by parallelshear plains and is deformed only by slips along the shearplains retaining the thickness of the slices (Figure 7). If wehave the geometries of a key bed before and after deforma-tion and a leading edge of a fault (Figure 7a), passes of thepoint (a, b, . . . i) on a key bed during deformation can bedetermined as one of arrows shown in Figure 7b. Thearrows are parallel to intervals of the fault segmentsbounded by shear planes, then we can construct a faulttrajectory from top to downward by connecting the arrows(a-a0, b-b0, . . . i-i0) as shown in Figure 7b.[25] The parallel shear assumed in this model represents

bulk deformation approximating real deformation [Dula,1991, White, 1992]. The real deformation in a hanging wallis inferred to be complicated, and may have been caused byantithetic and synthetic shears [Dula, 1991]. The foldgeometry constructed by incline shear modeling varies bythe change of the angle of shear plain (Figure 7c), andYamada and McClay [2003b] showed that the angle of shearplain varies by change of three dimensional fault geometry.[26] In this modeling method, geometry of a certain

interval of a fault trajectory is determined by amount of aslip and geometry of a key bed before and after deformationabove the fault interval between the shear planes boundingthe interval (Figure 7b). The amount of slip is a keyparameter to construct the fault trajectory, but it is generally

Figure 7. Cartoon illustrating geometric model fordetermining fault trajectory by inclined shear of hangingwall. (a) Three key geometric elements, (b) passes of points(arrows) on the initial key horizon during the deformation ofhanging wall. Fault trajectory can be constructed byconnecting the arrows from top to down. (c) Fault trajectoryconstructed assuming lower angle (65�) of shear planes.Note that inclination of the fault trajectory becomes steeperas the decrease of the shear angle of antithetic shear plane.


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very difficult to estimate precise slip amount from surfacegeologic structure. To solve this difficulty, we assumed thedepth of the fault to be 13 km from the cutoff depth of theaftershocks. If we can determine the depth of the fault end(D), we can calculate slip amount S by S = A/D, where A isan uplift area that can be measured on sections (Figure 7b).[27] If we can determine the fault depth from the depths

of the aftershocks, we can fix the position of the lower endof the fault. This means that the geometry of deeper part ofthe fault trajectory in a seismogenic zone is mainly deter-mined by the geometry of a backlimb. The Higashiyamaand Tamugiyama anticlines have nearly constant dip in theirback limb from their western margin to the anticlinal axes,indicating that the geometry of the key bed is reliable. Wecan construct the reliable fault trajectory in the seismogeniczone, if we can determine the fault depth and the geometryof the backlimb precisely.

5.3. Construction of Fault Trajectory

[28] We constructed 12 serial cross sections using themethod described above. The section lines were establishedparallel to the cross sections along which Kato et al. [2005a]depicted the precise aftershock distribution and P wavevelocity, and four of them coincide with those cross sections(Figure 5).[29] For the modeling, we needed to determine three

geometric elements: the geometry of folded key beds, theoriginal geometry of the key bed before folding, and thegeometry of the fault interval connecting the tips ofthe folded and original key beds (Figure 8a). We usedcommercial software for balanced cross section modelingto construct fold geometry and fault models. Geologic datawere loaded from a digitized geological map [Takeuchi etal., 2004] based on quadrangle geological maps at a scale of1:50,000 for Nagaoka [Kobayashi et al., 1991], Ojiya

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 mapsplit from the digitized geological map along the section line and (bottom) dips of beds shown on the mapprojected 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 thekey bed. Ko and Ok I are tephra key beds of nearly the same age.


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[Yanagisawa et al., 1986], Tokamachi [Yanagisawa et al.,1985] and part of Suhara [Takahashi et al., 2004]), and partof the Sumondake area [Ijima, 1974; Kageyama andKaneko, 1992].[30] Topographic profiles were constructed from a 50 m

digital elevation model (DEM) presented by GeographicalSurvey Institute of Japan. The dip and strike of beds withinabout 800 m from a section line were projected onto each ofthe topographic profiles (Figure 8b), and references lineswere drawn parallel to the dip of beds projected on thesection (Figure 8c). Because the thicknesses of the sedi-mentary units were not constant through the section, theparallel lines do not indicate synchronous surfaces. Wecorrected for the differences in sediment thickness by usingthe contained tephras. The geometry of the folded key bedwas drawn taking into account the reference lines and thelocation of the key tephra exposure (Figure 8c). We selectedthe youngest tephra exposed on both forelimb and backlimbof the anticline along each of the transects as the key bed.Because there was no tephra on the southern four sections,we used the base of the Uonuma formation as a key bed(Table 1). The Nokogiriyama fault along the axis of theHigashiyama anticlines was ignored in the construction ofthe fold geometry because the stratigraphic offset caused bythe fault is less than a few hundreds of meters, which is oflittle significance on the scale of the Higashiyama anticline.Original geometries of key beds were defined by a linear orsimple concave-upward profile increasing in depth to thewest (Figure 8c). The paleoenvironment indicated by thesedimentary facies of Pliocene and Pleistocene units sup-ports that the western part of the hills was in a deeperenvironment than the eastern part. A dip of inclined shearwas determined to be 85� ESE by comparison of a faulttrajectory and the aftershock alignment along the section onthe main shock (Y = 0 km).

6. Results and Discussion

6.1. Comparison Between Fault Models andAftershocks on Cross Sections

[31] Fault trajectories were compared with the distribu-tion of the aftershocks along the four cross sections pre-

sented by Kato et al. [2005a] (Figure 6). The faulttrajectories are composed of a high-angle segment deeperthan 5 km and a low-angle segment shallower than 5 km,which is consistent with the aftershock distribution alongthe source fault of the main shock. Two of the sections (Y =0 and 10 km) show good agreement between the estimatedfault trajectories and the aftershock distribution, and anothersection (Y = 5 km) is consistent in part with the aftershockdistribution. On the southernmost section (Y = �5 km),there are two cluster of aftershock distribution that does notshow a west dipping plane; thus it is difficult to discuss therelationship between the fault model and the source fault.[32] The result indicates that the geometry of the source

fault of the 2004 earthquake can be constructed from foldgeometry assuming inclined antithetic shear of hanging wallat 85�, although more detailed comparison between the faultmodel and aftershocks are necessary. The high-angle shearof hanging wall presumably indicates that antithetic andsynthetic shears exist in the hanging wall.[33] It is not difficult to apply this method to other active

fold-and-thrust zones and to construct geometry of seismo-genic faults, if detailed geologic structure of ground surfaceand depth of the seismogenic zone in the upper crust can bedetermined. The constructed fault is expected to be a sourcefault of future earthquakes, implying that the model wouldprovide important information to reduce damage by futureearthquakes.

6.2. Lateral Change in Fault Geometry and ItsRelationship to the 2004 Earthquake Rupture

[34] We constructed eight additional fault models by thesame method along sections parallel to the four crosssections discussed above (Figure 9), mainly in the southernpart of the Uonuma hills (Figure 5). The fold geometry,consisting of an asymmetric anticlinorium in the sourcearea, changes to a simple asymmetric anticline in thesouthern part of Uonuma hills, and the fault trajectoriesalso become simpler as the geometry of the folds changes(Figure 9). The depth contours of the subsurface fault planebased on the 12 fault models show that the strike of the faultnorth of the main shock is NNE–SSW; south of the mainshock it bends to strike N–S, and then bends again near thesouthern margin of the aftershock area to strike NNE–SSW(Figure 10). Kato et al. [2006] showed that the NNE–SSWtrend of the aftershock alignment change to roughly N–Sfrom north to south around the main shock, which appearsto be consistent with our fault model based on geologicstructure. If our model is correct, the earthquake rupturestarted at the northern convex bend of the fault surface andstopped at the concave bend at its southern end.[35] We could not construct the fault around the northern

margin of the source area because the northern margin maybe underlain by two oppositely dipping faults. TheHigashiyama anticline in the source area is continuous withthe Nigoro anticline, and the aftershock distribution appearsto follow the trend of the Nigoro anticline (Figures 3 and 5),suggesting that the west dipping source fault extends to thenorthern segment of Uonuma hills. In contrast, theYukyuzan fault along the western margin of the NorthHigashiyama anticline is an east dipping reverse fault underthe northern part of the source area. It is difficult todetermine the relationship between the east dipping and

Table 1. Parameters of Fault Modelinga

Line Key Bed Age, Ma Area, km2 Contraction, km

Y = 10 NA33 4.5 22.2 1.7Y = 5 NA33 4.5 11.6 0.9Y = 2.5 Ko-OkI 2.5 16.2 1.2Y = 0 Ko 2.5 19.6 1.5Y = �2.5 Ko 2.5 14.2 1.1Y = �5 T30 1.3 14.4 1.1Y = �7.5 Kk-Tz 1.8 10.5 0.8Y = �10 Tg 1.6 6.4 0.5Y = �12.5 N/A 2 4.6 0.4Y = �15 N/A 2 7.6 0.6Y = �17.5 N/A 2 5.7 0.4Y = �20 N/A 2 6.0 0.5

aKey beds and age indicate the name of tephra and its age used formodeling of each section. N/A indicates that no tephra exists on the sectionand base of the Uonuma formation was used as a key bed. Area wasmeasured between original and fold geometries of a key bed, and horizontalcontraction was calculated assuming the detachment depth to be 13 km.


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Figure

9.

Faultmodelsalong12cross

sections.Thelocationsofthecross

sectionsareshownin

Figure

5.


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west dipping faults only from the geologic structure at theground surface. The geometric change of the anticlines atthe northern end of the source area suggests that a change infault geometry determined the northern margin of therupture. Sato and Kato [2005] proposed that the east vergentreverse fault in the middle segment abruptly changes into awest vergent fault around the southern margin of thenorthern segment; however, the gradual change in thegeometry of the anticline from the source area to the Nigoroanticline indicates that the east verging fault continues to thenorth for some distance.

6.3. Slip Amount and Rate

[36] On the basis of our fault models, the amounts ofhorizontal contraction along each of the constructed faultswere estimated to be 0.4 to 1.7 km (Table 1). The contractionwas largest at the Higashiyama anticline and decreased to thesouth. The key beds used in the models were tephras ofdifferent ages, ranging from 4.5 to 1.3 Ma, and the largestslip was deduced from the cross section based on the tephraof 4.5 Ma. This suggests that the growth of the fold started inthe early Pliocene; however, the fact that structure of thePliocene sedimentary sequences is almost uniform probably

Figure 10. Depth contours of the fault surface constructed by the fault modeling. The fault plane bendstwice 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) areacoincides with the southern concave bend.


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indicates that the major growth of the folds occurred after thelate Pliocene. If we assume that the growth of the anticlinesbegan about 2 Myr ago, then the average horizontal con-traction rate is calculated to be 0.2 to 0.85 m/kyr (Table 1),but it is difficult to estimate the precise time of onset offolding or the slip rate of the fault from our model.[37] Kim [2004] estimated the uplift rate of the Uonuma

hills to be 0.8–1.3 m/kyr in its middle segment during thelast several tens of thousands of years, based on structuralrelief of folded middle Pleistocene fluvial terraces depositsin the middle segment of the Uonuma Hills. The uplift rateis estimated to be 1.7 times larger than contraction rate, ifthe fault inclination is 60�. Thus the maximum contractionrate 0.85 m/kyr is consistent with the maximum uplift rate1.3 m/kyr. This comparison supports that our fault model isvalid.

7. Summary

[38] The subsurface fault that are responsible for thegrowth of anticlines in the source region of the 2004 mid-Niigata Prefecture earthquake were constructed from foldgeometries, assuming that the folds have been growing by thedeformation of the hanging wall because of inclined shearabove the single reverse fault. The depth of the detachmentwas determined to be 13 km, based on the aftershock cutoffdepth. The fault trajectories agree well with the distribution ofaftershocks along the fault of the main shock. The resultsindicate that the source fault of the earthquake is responsibleto the growth of the geologic structure. The key data for thefault construction are the depth of the detachment and the foldgeometry, especially of the back limb. If these data areavailable, it is possible to infer the geometry of source faultsof future earthquakes in the active fold-and-thrust zone thatis underlain by a simple thrust using inclined shear model-ing. A three-dimensional fault geometry constructed thefrom fault trajectories along 12 section lines indicates thatfrom north to south, the strike of the fault changes fromNNE–SSW to N–S at the site of the main shock and fromN–S to NNE–SSW near the southern termination of theaftershock area. The relationship suggests that the rupturestarted at the convex bend of the fault and stopped at theconcave bend of the fault. The horizontal contraction rate ofthe fault was inferred to be 0.2 to 0.85 m/kyr, which isconsistent with the uplift rates of the hills inferred from theelevation of fluvial terraces by Kim [2004].

[39] Acknowledgments. We are grateful to James Dolan, KarlMueller, and Tom Pratt, reviewers of JGR, for their useful comments andsuggestions, which helped greatly to improve our manuscript. We alsothank Yuichi Sugiyama for his encouragement of our study and to AitaroKoto, who kindly provided precise aftershock data. This work wassupported by the Special Coordination Fund for Promotion of Scienceand Technology offered by the Ministry of Education, Culture, Sports,Science and Technology of Japan (MEXT) under the title of ‘‘UrgentResearch for the 2004 Mid-Niigata Prefecture Earthquake’’. Some figuresare created using Generic Mapping Tools (GMT).

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