Tectonophysics 470 (2009) 319–328

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Tectonophysics

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Subsurface geometry and structural evolution of the eastern margin fault zone of theYokote basin based on seismic reflection data, northeast Japan

Kyoko Kagohara a,⁎, Tatsuya Ishiyama b, Toshifumi Imaizumi b, Takahiro Miyauchi c, Hiroshi Sato d,Nobuhisa Matsuta e, Atsushi Miwa f, Takeshi Ikawa g

a Japan Atomic Energy Agency, Toki 509-5102, Japanb Graduate School of Science, Tohoku University, Sendai 980-8578, Japanc Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japand Earthquake Research Institute, the University of Tokyo, Tokyo 113-0032, Japane Department of Geosciences, National Taiwan University, Taipei, Taiwanf OYO Corporation, Saitama 336-0015, Japang Geosys Inc., Tokyo 112-0012, Japan

⁎ Corresponding author.E-mail addresses: kagohara.kyoko@jaea.go.jp (K. Kag

t-ishiyama@m.tains.tohoku.ac.jp (T. Ishiyama), imat@m.tatmiya@faculty.chiba-u.jp (T. Miyauchi), satow@eri.u-tokyonobumatta@ntu.edu.tw (N. Matsuta), miwa-atsushi@oyonikawa@geosys.co.jp (T. Ikawa).

0040-1951/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.tecto.2009.02.007

a b s t r a c t

a r t i c l e i n f o

Article history:

High-resolution seismic refl Received 8 March 2007Received in revised form 31 October 2008Accepted 5 February 2009Available online 15 February 2009

Keywords:Thrust faultSubsurface structureSeismic reflection profilingCross-section balancingA.D. 1896 Rikuu earthquake

ection profiles across coseismic fault scarps reactivated during the A.D. 1896Rikuu earthquake along the eastern margin fault zone of the Yokote basin (EFZYB) in northeast Japan,correlated with borehole stratigraphy and geologic mapping, provide insights into its detailed kinematichistory and structural evolution. In spite of along-strike variations of thrust geometries both at groundsurface and at shallow depth, the EFZYB has commonly formed as forward breaking imbricate thrust systems.Near surface complexity of thrust geometries appears strongly affected by mechanical decoupling betweenlayers within middle to late Miocene mudstone. Cross-section balancing across the Mahiru Mountains showsstrong correlation of the millennial uplift rates with mountain topography but its weak correlation with thelate Pleistocene uplift rates. Considering the thrust trajectories estimated by the balanced cross sections, themillennial dip-slip rate are consistent with the late Pleistocene dip-slip rate, suggesting that the EFZYB hasaccumulated strains at a constant rate since the onset of its fault activity.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Coseismic behavior of active fold and thrust belts is crucial forprediction of the magnitudes and locations of future earthquakes andthus for appropriate evaluation of their future seismic hazards. Recentkinematic analyses of active fold and thrust belts show deformation ofgeomorphic surfaces can be successfully used to define styles ofcoseismic growth of active folds (Molnar et al., 1994; Lavé and Avouac,2000; Dolan et al., 2003; Ishiyama et al., 2004, 2007; Hubert-Ferrariet al., 2007; Leon et al., 2007). In addition, along-strike variations inthrust fault geometry within an en échelon active thrust system aretraditionally viewed as a key factor to limit the extent of coseismicsurface rupture by individual earthquakes. Structural analyses onnumerous seismic sections with boreholes (cf. Shaw et al., 2002) andnumerical simulations (Magistrale andDay,1999) also suggest that the

ohara),ins.tohoku.ac.jp (T. Imaizumi),.ac.jp (H. Sato),et.oyo.co.jp (A. Miwa),

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structure of fault segment boundaries in en échelon, active fold andthrust belts may control on slip transfer from one segment to the next.Nevertheless, the structural link of coseismic fault scarps withunderlying thrust geometries is still little understood except forintensive studies on the 1999 Chi-Chi earthquake (Lee et al., 2002; Leeet al., 2003; Chen et al., 2007) and thus remains a focus for futurework.

In this paper, we present new structural models of coseismic faultscarps along the easternmargin fault zone of the Yokote basin (EFZYB)associated with a large thrust earthquake that occurred in A.D. 1896(Yamazaki, 1896; Imamura, 1913; Otsuka, 1938; Matsuda et al., 1980)in northeast Japan, to define their subsurface geometry and structuralevolution. In this study, we focus on underlying thrust trajectories andtheir structural relationship to coseismic fault scarp evolution and thekinematic history of active thrusting along the EFZYB. High-resolutionseismic reflection data obtained across the EFZYB (Kagohara et al.,2006a,b) are combined with geologic mapping and boreholestratigraphy, to define deformation mechanisms that have acted tobuild it since Pliocene to early Pleistocene time. In our view, thisintraplate active thrust fault system is interesting in that the A.D. 1896coseismic fault scarps are tightly coincident with an east-dippingemergent thrust ramp formed within the Miocene marine mudstone.In addition, we also point out the discrepancy between long-term and

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short-term uplift rates along the EFZYB, which are attributed to thealong-strike variation in dips of the shallowest thrust ramps.

2. Regional setting of the EFZYB

2.1. Tectonic and geologic settings

Northeastern Honshu Island is an actively deforming island arc onthe hanging-wall block above the Pacific Plate which is subducting at arate of about 8 cm/yr (Fig. 1a; DeMets et al., 1990). The innernortheast Japan arc is characterized by arrays of north-trending thrustsheets, which have deformed Neogene basin deposits (Sato, 1994) toaccommodate the strong east–west intraplate strains. The innernortheast Japan arc is overlain by an extensive cover of Miocenesedimentary and volcanic rocks deposited in rift basins formed duringmiddle Miocene back-arc spreading and subsequent regional sub-sidence associated with lithospheric cooling (Sato and Amano, 1991).

The EFZYB extends for about 60 km (Research Group for ActiveFaults of Japan, 1991) and separates the Yokote basin from itspiggyback Yuda basin. The Ou Backbone Range forms the strippedcore of the largest structure in northeast Japan, and exposes lower tomiddle Miocene strata that were translated along an east-dippingthrust above a 400-m thick sequence of Pleistocene strata deposited inthe Yokote basin. On the east side, the Ou Backbone Range has alsobeen thrust over a greater than 200-m thick sequence of Pleistocenestrata deposited in the Kitakami lowland, along a west-dipping thrust(Fig. 1c; Sato et al., 2002).

The deep seismic reflection profile across the Ou Backbone Rangetied with the P-wave velocity structure and surface geology suggestedthat active thrusts along the western margin of the Yuda basin(Kawafune fault) and along thewesternmargin of the Kitakami lowland

Fig. 1. (a) Tectonic setting of Japan. Solid lines with barbs indicate plate boundaries. AbbreviaSea plate. (b) Topographic map of the northern Honshu Island. Shaded relief map was conInstitute. Active fault traces are based on the Research Group for Active Faults of Japan (199margin fault zone of the Yokote basin; WFZKL, Western margin fault zone of the Kitakami loacross the Ou Backbone Range after Sato et al. (2002).

(Uwandaira fault) are west-dipping, reactivated Miocene normal faultssince Pliocene (Sato et al., 2002). Contrastingly, the EFZYB (Senya fault)has no evidence of reactivation of a preexisting Miocene normalfault and has been interpreted as formed as a newly nucleated thrustfault formed at∼2.4Ma associatedwith east–west compressional stressregime since Pliocene (Sato et al., 1997; Sato et al., 2002).

2.2. Eastern margin fault zone of the Yokote basin

The EFZYB is one of the largest active thrust fault system innortheast Japan (Fig. 2). It is located along the western flank of theMahiru Mountains which comprise the western flank of the OuBackbone Range. The EFZYB generated the Rikuu earthquake in A.D.1896 (magnitude 7.2; Utsu,1979). The northernportion of the EFZYB iscomprised of a pair of east-dipping thrust faults that lie at the base ofthe western flank of the Ou Backbone Range and a frontal thrust faultthat lies at the base of the foothillswest of themountains. The southernportion is comprised solely of an east-dipping range front boundarythrust fault (Fig. 2; Ikeda et al., 2002; Nakata and Imaizumi, 2002). TheKawaguchi fault lies along the western flank of the Mahiru Mountains(Fujiwara, 1954) and separates the Miocene strata from the Pliocene-Quaternary sediments deposited in the Yokote basin and Senya Hills(Fig. 3). The frontal thrust consists of three fault segments (Shiraiwa,Ota and Senya faults) located along thewesternmargin of the foothills(Nakata, 1976; Matsuda et al., 1980). West facing fault and fold scarpsalongwhich the lateMiocene strata of theMiroku Formation are thrustover the late Pleistocene and Holocene fluvial terrace and alluvial fandeposits, have been interpreted as formed byeast-dipping thrust faults(Matsuda et al., 1980; Ikeda, 1983; Ikeda et al., 2002).

Trench excavations revealed that the vertical component of averagedHolocene slip rates and the recurrence interval of past earthquakes on

tions are EU, Eurasian plate; NA, North American plate; PA, Pacific plate; PHS, Philippinestructed using a 250-m digital elevation model published by the Geographical Survey1). Line X–Y is the location of the cross-section in (c). Abbreviations are EFZYB, Easternwland. (c) Cross-section shows geologic interpretation of deep seismic reflection profile

Fig. 2. Topographic map showing locations of active fault scarps in the EFZYB (Ikeda et al., 2002; Nakata and Imaizumi, 2002). Solid and dashed lines indicate precisely located andinferred active fault scarps, respectively. Barbs indicate hanging-wall side of active thrust faults. Open circles indicate locations of the coseismic surface ruptures during the A.D. 1896Rikuu earthquake (Matsuda et al., 1980). The shaded relief map was constructed using a 50-m digital elevation model published by the Geographical Survey Institute.

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the EFZYB were estimated to be 0.5 to 0.8 mm/yr and 3500 years,respectively (ResearchGroup for the Senya Fault,1986; Akita PrefecturalGovernment,1999). The average dip-slip rate of the Senya fault was alsoestimated to beabout 1.2 to 1.6mm/yr based on the analysesof boreholetransects (Imaizumi et al., 1997). In addition, high-resolution seismicreflection profiling revealed that the shallow structure of the Senya faultis an emergent, easterly 35° dipping thrust fault, which flattens at about800-m depthwithin the lateMiocenemudstone and steepens at greaterdepth (Sato and Hirata, 1998; Sato et al., 2006; Fig. 4).

2.3. The A.D. 1896 Rikuu earthquake

The Rikuu earthquake occurred in A.D. 1896 and its epicenter liesbeneath the Mahiru Mountains (Yamazaki, 1896; Imamura, 1913;Otsuka, 1938). Coseismic surface ruptures during the A.D. 1896earthquake are evident by the 35-km long, west facing fault and

fold scarps along the base of thewestern flank, and the 5-km long, eastfacing fault scarps along the eastern flank (i.e., western margin of theYuda piggyback basin) of the Mahiru Mountains (Fig. 2; Yamazaki,1896; Matsuda et al., 1980). These main coseismic surface ruptureshave highly sinuous en échelon patterns with vertical displacementsas high as 1–3m. Detailedmapping indicates that locations of the A. D.1896 coseismic surface rupture were almost coincident with the threefrontal thrust faults within the EFZYB (Matsuda et al., 1980; Fig. 2).

2.4. Stratigraphy of the Mahiru Mountains and Yokote basin

The Mahiru Mountains are composed mainly of Miocene volcanicand sedimentary rocks (Figs. 3 and 5). The middle Miocene Yuda andMahirugawa Formations are composed of lavas and pyroclastic rocksand are the oldest deposits in the rift basin. These units areconformably overlain by the middle Miocene Yoshizawagawa

Fig. 3. Geologic map of the study area modified from Ozawa and Suda (1980), Ozawa et al. (1988) and Usuda et al. (1976, 1977, 1980). Red lines indicate locations of the coseismicsurface ruptures during the A.D. 1896 Rikuu earthquake (Matsuda et al., 1980).

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Fig. 4. Depth converted seismic profile C–C′ across the fault scarp of the Senya fault and Senya Hills after Sato and Hirata (1998) and Sato et al. (2006). Location of the profile C–C′ isshown in Fig. 3. There is no vertical exaggeration. See Fig. 3 for abbreviations.

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Formation composed of deep marine siliceous mudstone. This unit isconformably overlain by themiddle to lateMioceneMiroku Formationcomposed of mudstone in the lower, and tuffaceous rocks in the uppersections. Pliocene to early Pleistocene fluvial and lacustrine conglom-erate, sandstone and siltstone (Senya Formation) unconformablyoverlie the Miocene units in the Yokote basin. Fission-track dating ofinterbedded volcanic ash layers indicates that the Senya Formationwas deposited from 2.7 Ma (Fig. 5; Kagohara et al., 2006c).

3. Interpretation of high-resolution seismic reflection data

3.1. Data acquisition and processing

Two high-resolution seismic reflection profiles across the fault andfold along the Shiraiwa and Ota segments were acquired to furtherimage the detailed subsurface geometry of the structures (Figs. 6and 7; Kagohara et al., 2006a,b; see data acquisition parameters inTable 1). A 10-m source and geophone spacing was used and yields a5-m common midpoint (CMP) spacing on the final sections. Seismic

Fig. 5. Schematic stratigraphic column of the Neogene strata in the Yokote bas

reflection data were processed by use of the standard CMP stackingmethod (Yilmaz, 1987; Sheriff and Geldart, 1995).

3.2. Seismic data interpretations

Reflectors imaged in these high-resolution seismic profiles (Figs. 6and 7; Kagohara et al., 2006a,b) correlate with strata defined inboreholes drilled in the Yokote basin (see locations of profiles andboreholes in Fig. 3) and stratigraphic contacts on the geologic maps(Fig. 3).

Depth section A–A′ (Fig. 6; Kagohara et al., 2006a) contains flatand continuous reflectors west of the coseismic fault scarp associatedwith the A.D. 1896 earthquake at CMP No. 200, to a depth of 0.4 km.These reflectors are correlated with the Senya andMiroku Formations,based on stratigraphy in boreholes WS-14 and WS-20 drilled in theYokote basin. The eastern limits of these reflectors are adjacent toeast-dipping reflectors that are correlated with the Miroku Formationexposed in the Konuma Hills, indicating an east-dipping thrust fault.Upward projection of the thrust trajectory revealed by footwall cutoffs

in (modified from Usuda et al., 1976, 1977, 1980; Kagohara et al., 2006c).

Fig. 6. (a) Location map of the A–A′ seismic survey line and CMP numbers. (b) Depth converted seismic section A–A′ (Kagohara et al., 2006a) and (c) interpreted section. See Fig. 3for locations of the seismic section and borehole. There is no vertical exaggeration.

Fig. 7. (a) Location map of the B–B′ seismic survey line and CMP numbers. (b) Depth converted seismic section B–B′ (Kagohara et al., 2006b) and (c) interpreted section. See Fig. 3 forlocations of the seismic section and borehole. See Fig. 6 for geological legend. There is no vertical exaggeration.

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Table 1Data acquisition parameters used for seismic reflection profilings shown in Figs. 6 and 7.

Seismic line Kotaki 2005 (A–A′) Kawaguchi 2003 (B-B′)

Length of seismic line 3.8 km 7.1 kmSource parametersSource (type) Minivib (IVI, T-15000)Sweep frequency 10–100 Hz 10–100 Hz and 10–60 HzSweep length 20 s 15 s and 24 sNo. of sweeps 10 or 7 7No. of shot points 368 558Shot interval 10 m 10 m and 20 m

Receiver parametersNatural frequency 10 Hz 10 HzReceiver interval 10 m 10 mNo. of channels 180 ch 180 ch

Recording parametersInstruments JGI, GDAPS-4Sampling rate 2 ms 2 msStandard CMP fold numbers 98 80

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corresponds to the base of the A.D. 1896 coseismic fault scarp along theShiraiwa fault. In contrast to the tabular beds with uniform thickness inthe Miroku Formation that are characterized by strong reflectors withlower frequencies, overlying strata aremarkedbyweaker reflectorswithhigh frequencies that thicken eastward. The Miroku and lower SenyaFormations are exposed on the hanging-wall of the Shiraiwa fault,indicative of their pre-growth origin. These observations suggest thatthe middle and upper Senya Formations were deposited concurrentwith fault activityand foldgrowthabove theShiraiwa fault. Fission-trackdating of the volcanic ash layer contained within the lowermost sectionof the middle Senya Formation (Fig. 5; Kagohara et al., 2006c) indicatesthat active thrusting of the Shiraiwa fault began at ca. 1.6 Ma.

Discontinuous reflectors with lower frequencies and high intervalvelocities (∼3500 m/s) are correlated with Mahirugawa Formationbased on geologicalmapping and borehole data. Furthermore, reflectorsbetween CMP No.130 and No.180 up to a depth of 0.6 km are uniformlywestward dipping and bounded by east-dipping axial surfaces to form awest-dipping panel. We interpret this subsurface fold as formed abovean east-dipping blind thrust on the footwall of the Shiraiwa fault.

Correlation of reflectors on the section B–B′ (Fig. 7; Kagohara et al.,2006b), with stratigraphy intersected by borehole WS-16, indicate

Fig. 8. Balanced cross-sections based on projected seismic sections A–A′, B–B′ and C–C′ based

that the gently east-dipping and continuous reflectors west of CMPNo. 450 between 0.4 and 0.7 km depth correlate with hard shale of theYoshizawagawa Formation. Underlying discontinuous reflectors withlow frequencies and high interval velocities (∼3500 m/s) alsocorrelate with the Mahirugawa Formation, based on boreholestratigraphy east of the seismic section. The Mahirugawa Formationexposed west of CMP No. 750 is also marked by discontinuousreflectors similar to those in the Yokote basin. Flat and continuousreflectors with low interval velocities (1500 to 2500 m/s) and highfrequency correspond to the late Pliocene to Quaternary SenyaFormation. The upper and middle Senya Formation appears to thickeneastward, whereas the lower Senya and Miroku Formations maintaintheir uniform thickness, indicating that the Senya Formation recordsQuaternary structural growth and fault activity of the Ota fault.

West-dipping reflectors of the Miroku and Yoshizawagawa Forma-tions are juxtaposed with subhorizontal reflectors of the SenyaFormation from the ground surface at CMP No. 600 to a depth ofabout 0.7 kmat CMPNo. 850.We interpreted these footwall cutoffs as aneasterly dipping thrust fault. Upward projection of the thrust ramp isconsistent with the A.D. 1896 coseismic fault scarp along the Ota fault.

West-dipping reflectors between CMP No. 450 and No. 600 at depthof 0.1–0.7 km that correlate with the Miroku to lower Senya Formationappear to be underlain by gently east-dipping reflectors correlatedwiththe Yoshizawagawa Formation. Thus we interpreted the Miroku andlower Senya Formations to have been thrust over the YoshizawagawaFormation along a shallow west-dipping thrust fault. In addition, weinterpreted cutoffs between CMP No. 450 and 500 at shallow depth as ashallowly east-dipping thrust fault that cuts across the west-dippingforelimb of the thrust fold at the footwall of the Ota fault. Upperprojection of these fault cutoffs is coincident with a west-facing foldscarp that deforms late Quaternary fluvial terrace deposits.

4. Discussion

4.1. Structural growth and kinematic history of the EFZYB

Balanced cross-sections of profiles A–A′, B–B′, and C–C′ arecorrelated with borehole stratigraphy and geologic mapping, based onfault-bend fold theory (Suppe, 1983), define the kinematic evolution

on fault-bend fold theory (Suppe, 1983). Section C–C' modified Sato and Hirata (1998).

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and structural growth of the EFZYB (Fig. 8).We firstmade line drawingsof reflectors on each depth sections, and then we subdivided them intoseveral segments and projected reflectors on these segments onto across section which is perpendicular to the trend of the EFZYB. Weconstructed balanced cross-sections based on these projected geologiccross sections.

Active thrusting of the Shiraiwa fault initiated as a blind, west-dipping thrust that flattens within the Miroku Formation (Fig. 8a)after the deposition of the lower Senya Formation. Subsequent coevalfaulting of hinterland- and foreland-ward thrusts since ca. 1 Ma (afterthe deposition of the middle Senya Formation) has formed overallfault-related fold structures. Thus, this segment can be interpreted as aforward breaking imbricate thrust system, though geomorphicexpression of the range front boundary thrust is not clear.

Similarly, active thrusting of the Ota fault has produced a forwardbreaking imbricate thrust system, comprised of a steep thrust rampand pairs of frontal, shallowly-dipping thrusts (Fig. 8b). Upwardprojection of the Ota fault coincides with the location of the coseismicfault scarp associated with the A.D. 1896 earthquake.

In contrast to the Ota segment, upward projection of the Senyafault is coincident with the location of a west-facing fold scarp thatdeforms late Quaternary to Holocene alluvial fan deposits. Asindicated by Sato and Hirata (1998), simple flat-ramp fault-bendfolding occurred during the earliest stage of active thrusting of the

Fig. 9. (a) Topographic contour map of the study area. (b) Structural contour map of the top oon geologic mapping and seismic sections A to D are shown in Fig. 9 (b). The shallower part

Kawaguchi fault, followed by a basin-ward migration of activethrusting along a bedding-parallel decollement and a thrust rampon the footwall of the Senya fault (Sato and Hirata, 1998; Fig. 8c).

In summary, structural analyses of cross-sections across theShiraiwa, Ota, and Senya segments indicate that they have formed asforward breaking, active imbricate thrust systems, although they areseparated from each other by along-strike structural discontinuities.Three seismic sections indicate that imbrication of thrust ramps hasoccurredwithinmiddle to lateMiocenemudstoneand siltstone (MirokuandYoshizawagawa Formations), alongwhichdecollementor shallowlydipping thrust ramps formed subparallel to bedding. This indicates thatmechanical decouplingbetween layers has stronglyaffected the styles ofbranching of thrust faults. In contrast to the complex geometry ofshallow structures, basal thrust ramps commonly dip eastward at about30°. This is consistent with emergence of coseismic fault scarps alongthree fault segments, in spite of along-strike variations of thrustgeometries at shallow depth.

4.2. Horizontal shortening, tectonic uplift, and mountain topographyabove the EFZYB

We constructed fault-related fold models on four geologic cross-sections across the EFZYB (Fig. 9) based on results of cross sectionbalancing (Fig. 8). Dip of the deeper section of the thrust fault is

f the Miroku Formation. (c) Balanced cross-sections across the Mahiru Mountains baseds of cross-section apply to high-resolution reflection profiles (Kagohara et al., 2006a,b).

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constrained by deep seismic reflection profiles across the Ou BackboneRange (Sato et al., 2002). The total horizontal shortening wasestimated to range from about 2.1 to 3.2 km. The estimated structuralcontour map of the top of the Miroku Formation suggests that theamount of uplift associatedwith active thrusting of the EFZYB since ca.2–3 Ma have two elongated peaks (N1750 m). Patterns of uplift seemto correspond to overall topography of the Mahiru Mountains and OuBackbone Range.

Using structural relief on the pre-growth strata obtained by cross-section balancing and the age of the lowermost growth strata, weestimated millennial uplift rate across the EFZYB. The millennial upliftrates along the four balanced cross sections are calculated to be about0.8±0.2 mm/yr since the onset of the sedimentation of the growthlower Senya Formation, 2.7±0.4 Ma (Fig. 10). These values are almostcomparable to the incision rate based on altitudes of fluvial terraces(Tajikara and Ikeda, 2005; Fig. 9) and denudation rate based onsediment accumulations in drainage basins (Fujiwara et al., 1999;Fig. 9), suggesting that the Mahiru Mountains has achieved topo-graphic steady state.

Topography of the Mahiru Mountains is consistent with the spatialpattern of structural relief on the top of the pre-growth Mirokuformation on the hanging-wall of the EFZYB (Fig. 9), in spite ofcomplex traces of active faults at the ground surface. This suggeststhat long-term accumulation of slip along the entire EFZYB has formedthe mountain topography.

In contrast to this consistency, 0.5±0.1 and 0.5±0.2 mm/yr ofuplift rates across the Shiraiwa and Senya faults during estimated byoffset of fluvial terraces formed during the late Pleistocene (Imaizumi,1992) are apparently smaller than themillennial uplift rates (Fig. 10a).

Fig. 10. Along-strike distribution of the millennial and the late Pleistocene slip ratealong the EFZYB. (a) uplift rate. (b) dip-slip rate. The late Pleistocene uplift rates afterImaizumi (1992).

This apparent discrepancy is possibly explained by along-strikevariation in dips of the shallowest thrust ramps. Based on interpretedseismic depth sections (Figs. 4 and 6), dips of the shallowest thrust rampof the Shiraiwa and Senya faults are 25° and 35°, respectively. Using thelate Pleistocene uplift rates of the Shiraiwa and Senya faults for theshallowest thrust ramp, late Pleistocene dip-slip rates are calculated tobe 1.2±0.2 mm/yr for the Shiraiwa and 0.9±0.3 mm/yr for the Senyafault, respectively. Similarly,millennial dip-slip rate for thedeeper thrustramp dipping about 45° (Fig. 9) is calculated to be 1.1±0.3 mm/yr.

We argue that the millennial dip-slip rates are almost consistentwith the late Pleistocene slip rates (Fig. 10b), in spite of smalldiscrepancies probably due to spatial and/or temporal fluctuations ofslip rates and thrust trajectories, This consistency suggests that strainaccumulation along this active structure has been constant since theonset of fault activity.

5. Conclusions

In conclusion, combining high-resolution seismic reflection pro-files across fault scarps, which ruptured during the A.D. 1896 Rikuuearthquake along the EFZYB in northeast Japan with boreholestratigraphy and geologic mapping, provides insights into the detailedkinematic history and structural evolution. Along-strike variation ofthrust geometries both at the ground surface and at shallow depthssuggests that the EFZYB has in general formed as a forward breakingimbricate thrust system. Near surface complexity of thrust geometriesappear strongly affected by mechanical decoupling between layerswithin the middle to late Miocene mudstone. Cross-section balancingacross the Mahiru Mountains shows strong correlation of millennialuplift rates with mountain topography but its weak correlation withlate Pleistocene uplift rates. Considering the thrust trajectoriesestimated by the balanced cross sections, the millennial dip-slip rateare consistent with the late Pleistocene dip-slip rate, suggesting thatthe EFZYB has accumulated strains at a constant rate since the onset ofits fault activity (ca. 3 Ma).

Acknowledgements

We are grateful to many participants in the seismic reflectionprofiling for their help in the field. Our thanks also go to associatededitors Tanio Ito, Hans Thybo and Takaya Iwasaki, and reviewersKelvin R. Berryman and the anonymous reviewer for their construc-tive reviews, which greatly improved the manuscript. This work wassupported by Japan Society for the Promotion of Science researchfellowship for young scientists (05 J06455) and the Japan AtomicEnergy Agency.

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