Retrodeformation analysis of the Quaternary fault in the southeastern Korean Peninsula

Gondwana Research 17 (2010) 116–124

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Retrodeformation analysis of the Quaternary fault in the southeastern Korean Peninsula Sung-Ja Choi a, Duk-Geun Hong b,⁎, Ueechan Chwae a, Yun Goo Song c, Changryol Kim a, Taekmo Shim d a

Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseonggu, Daejeon 305-350, South Korea Kangwon National University, Department of Physics, 192-1 Hyoja-dong, Chuncheon-si, Gangwon-do 200-701, South Korea Yonsei University, Department of Earth System Sciences, 262 Seongsan-no, Seodaemun-gu, Seoul 120-749, South Korea d Korea Institute of Nuclear Safety, 34 Gwahang-no Yuseong-gu, Daejon 305-338, South Korea b c

a r t i c l e

i n f o

Article history: Received 23 March 2009 Received in revised form 26 July 2009 Accepted 26 July 2009 Available online 11 August 2009 Keywords: Fault restoration Slip rates Quaternary Multiple or three-time faulting event Mw 6–7

a b s t r a c t The Suryum fault (F1) with subsidiary branches (F2, 3) varies in strike from N40°E to N14°E and in dip from b 68 to 8°SE with respect to the surface. New evidence helps redefine the frequency of faulting events and allows calculation of net slip rates. A retrodeformation analysis based on stratigraphic correlation and optical stimulated luminescence (OSL) dates of colluvial deposits reveals four events including three-time faulting during the latest Pleistocene: reverse → landslide → reverse → normal. Net slips are in the range of 100–130 cm for F1 at a dip angle of 68°–45° and 84 cm for F2 at a dip angle of 45°. Based on OSL ages of 23,000± 1200 yr and 12,000 ± 2000 yr, we calculate net slip rates for F1 and F2 ranging from 0.03 to 0.08 mm/yr. The reactivation slip rate on F1 is 0.15–0.4 mm/yr over the last 12,000± 2000–2800± 200 yr. The Suryum reverse faulting might be related to Mw ∼6–7 earthquakes during the late Pleistocene, based on net slip rates comparable to those of other intraplate regime examples. © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The Korean Peninsula is presently located within a stable intraplate regime of the Eurasian continents that has been made by prolonged amalgamation history since the early Mesozoic (Oh, 2006; Chwae and Choi, 1999, Yin and Nie, 1993; Ernst and Liou, 1995; Chang, 1996; Ree et al., 1996; Zhai et al., 2007; Cho et al., 2007; Oh and Kusky, 2007; Kim et al., 2009; Zhang et al., 2009; Sajeev et al., in press). Choi et al. (1999a,b) report five major tectonic events during the Cenozoic in SE Korea, of which the last two Quaternary events included folding and reverse faulting. Since then, over 36 sites, Quaternary reverse faults have been discovered along the Yangsan and Ulsan faults (Chwae et al., 1998; Kyung et al., 1999; Kyung and Chang, 2001; Park et al., 2006). This Quaternary deformation has been interpreted by compression and extensional stresses related to either the subduction of the Philippine Sea and Pacific plates, or to the India– Eurasia collision (Ree et al., 2003; Kim et al., 2006b; Park et al., 2006; Jin and Park, 2007; Park et al., 2007). Of these, the Suryum fault site was first reported by Lee et al. (1999a,b). Since then, it has been studied by several scholars (Ree et al., 2003; Jeong and Cheong, 2005; Jeong et al., 2007; Choi et al., 2009). Lee et al. (1999a,b) and Ree et al. (2003) define the fault as a simple one-time reverse-faulting event, which produced a fault gouge that was forced through a fault plane in the overlying bedrock. ⁎ Corresponding author. E-mail address: [email protected] (D.-G. Hong).

Alternatively, Jeong and Cheong (2005) suggest multiple faulting events by considering a pedogenic origin for the gouge, although no field evidence is presented. We recently found some important field evidence indicating multiple faulting events at the Suryum site; for example: 1) a subsidiary reverse fault associated with the main fault; 2) a dragging colluvial deposit on the hanging-wall side; and 3) an asymmetrical stratigraphic sequence between the hanging-wall and the footwall sides, such as a thick soil deposit (∼60–200 cm) on the footwall side. The upper sedimentary layer of the hanging wall is deeply oxidized, whereas that of the footwalls contains less ferruginous material. We assume that the oxidized layers of the footwall side have been degraded or eroded out because of the collapse of the overhanging-wall tip after reverse faulting. The reconstruction of faults is fundamental to the understanding of the behavior and extent of paleoearthquakes, the knowledge of which is crucial in estimating the impact that future earthquakes would have on this area. The Suryum fault outcrop is a kind of trench log from which we can identify the critical structure and stratigraphy indicating a paleoearthquake. In this study, we attempt to restore the stratigraphic units to know the predeformation position. The sequence of deformation was geometrically reconstructed by tracing the fault movement sense in reverse order from the recent to the past and by correlating the stratigraphic units with optically stimulated luminescence (OSL) age data. We provide the number of fault movements and the timing of each fault event, and calculate the net

1342-937X/$ – see front matter © 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2009.07.008

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slip and its rate for the fault for better understanding the active tectonic processes within the Korean peninsula.

2. Paleoseismological investigation 2.1. General view of Suryum fault The Suryum fault is situated 10.8 km and 25 km away from the Ulsan and Yangsan faults, respectively (Fig. 1a). Its exposure is 10 m long and 3 m high. The fault cuts a sediment bed of marine terrace divided into 45–50 m (T3a) above mean sea level (Choi et al., 2008) (Fig. 1b). We have discovered three fault lines (F1, F2 and F3) at the site (Fig. 2), whereas the Suryum fault was previously thought to be a single reverse fault. Here, F1 is convex upward and varies in strike from N40°E to N14°E and in dip from b68°SE to 8°SE toward the surface. It is splayed into the subsidiary faults, F2 and F3, which pass through unconsolidated sedimentary strata. F2 is oriented subparallel (N67°E to N40°E) to the major fault (F1) with a dip of 65–45°SE but does not penetrate the overlying soil layers. Fig. 2 also shows that the fault line F3 results from a bifurcation of F2. Antiform strata on the hanging wall (Fig. 2), clast rotation on both sides, and an asymmetry of strata are indications of reverse movement on F1. The tip of F1, which collapsed in the past through gravitational force, is now displayed as a low-angle slope ranging from 8° to 6°. The maximum vertical throw of F1 within the bedrock is approximately 90–95 cm, and that of the subsidiary F2 is ∼ 60 cm. The throw of F1 gradually decreases upward to 4–6 cm. The gouge along the fault plane of F1 has a variable width ranging from ∼15 cm in the lower bed to 1 cm in the uppermost layer. This is generally

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thought to be a result of slip in the deep bedrock being repeatedly concentrated in the same narrow fault zone during the Quaternary (Fig. 3b, c). Through interpretation of an aerial photograph, Lee et al. (1999b) claim that the Suryum fault extends 40 m toward the NE and also that its lineament reaches 400–500 m. Lee et al. (2000) later trace the fault along a length of up to ∼ 200 m by quick trenching. In a study of ground-penetrating radar (GPR) and electrical resistivity, Kim et al. (2006a) report the fault to be up to ∼ 120 m long. In summary, the length of the Suryum fault varies from approximately 40 to 200 m. 2.2. Determination of OSL age Two components, equivalent dose and dose rate, are required for OSL dating, with ages assessed from the well-known equation Age ðkaÞ =

Equivalent dose ðGyÞ ; Dose rate ðGy = kaÞ

where luminescence measurements are used to determine the equivalent dose, and the dose rate is obtained by measuring the radioactive content of samples. 2.2.1. Sample preparation Seven soil samples were collected from the site; namely, SRY 1, SRY 1-1, SRY 2, SRY 3, SRY 4, SRY 5, and SRY 6 (Fig. 3a). The samples were obtained by hammering steel cylinders into the face of the beds. After being removed from the section, the steel cylinders were immediately covered tightly at both ends with thick black tape. In the laboratory dark room, material from both ends of the steel cylinders, which might

Fig. 1. (a) Landsat image of the southeastern Korean Peninsula, courtesy of the Korean Ministry of Environment. Displayed are the Ulsan and Yangsan faults, source 10.8 km and 25 km away from the Suryum fault site. (b) Map with marine terraces in the vicinity of the Suryum fault.

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Fig. 2. Above: Outcrop photo of the Suryum fault. Numbers in white quadrangles indicate units from 1 to 8. Red lines represent the major fault (F1) and two subsidiary faults (F2 and F3). White open circles are sample sites for OSL age dating, and solid white circle is for 14C date. Black asterisk is for OSL ages reported by Jeong et al. (2007). Blue rectangle indicates close-up photograph for Units 1, 2 and 3 in the vicinity of the reverse fault line F1.

have been exposed to daylight during wrapping, was extracted for beta and gamma dosimetry measurement. The sample material remaining in the cylinders was processed for luminescence analysis. Quartz samples were used for determination of the equivalent dose. All sample preparations were undertaken under subdued red light to prevent the dating signal being affected otherwise. Density separation at 2.60 and 2.70 g cm− 3 using a sodium polytungstate solution was employed to obtain quartz from selected sample grains (125–150 µm). The quartz was then treated with concentrated HF acid for 1 h followed by washing with HCl and distilled water. The resultant quartz was sieved to select grains ∼ 125 µm in size. Some aliquots of the separated quartz were spread as a monolayer over 10-mm diameter stainless steel disks and tested for feldspar content, the most probable contaminant, using infrared stimulated luminescence. 2.2.2. Experimental equipment All measurements were made using an automated Riso TL/OSL system (Model TL/OSL-DA-15) installed at the central laboratory of Kangwon National University. The system comprises a precision rotatable wheel, a 90Sr/90Y beta irradiator delivering a dose rate of 0.12 Gy/s at the sample position, a heater, a Nichia blue light-emitting diode (LED) array for optical stimulation, and a photomultiplier (EMI type 9635QA) for luminescence measurement. The Nichia blue LEDs (type NSPB-500S) produced about 50 mW/cm2 at the sample position, with a peak emission at 470 nm (full width at half maximum 40 nm). The short wavelength end (b400 nm) of the blue LED emission spectrum, which can cause cross talk between luminescence and light from the LEDs, was removed by placing a long-pass GG-420 filter (Schott) in front of the blue LED array. Luminescence was detected in the spectral region of 340–380 nm by the photomultiplier preceded by a Hoya U-340 filter (transmission band: 280–370 nm).

2.2.3. Equivalent dose determination The single-aliquot regenerative-dose (SAR) protocol introduced by Murray and Wintle (2000) was used for the determination of the equivalent dose. The procedure employed in this study involves using the same aliquot; the natural signal is measured, successively larger but known laboratory regenerative doses are administered, and the resulting luminescence is measured. In this procedure, the sensitivity changes of the ensuing luminescence caused by successive measurements are corrected by monitoring the luminescence response of the sample to a subsequent small radiation dose (the so-called ‘test dose’). This radiation dose is kept constant throughout the experiment, after the measurement of the luminescence due to the natural dose and also after each regeneration step. Samples received a total of 40 s blue-light exposure, and the first 0.16 s of the luminescence signal measured at 125 °C was integrated in each case to develop growth curves. Fig. 4 represents an example of the growth curve for sample SRY 3 constructed using this approach. In order to investigate the thermal charge transfer induced by preheating, four different preheat temperatures (range 220–280 °C with 20 °C intervals) for different sets of aliquots were employed. Fig. 5 shows that the equivalent doses of all samples used in this study are independent of the preheat temperature. This implies that for given samples, the thermal transfer of charge caused by preheating does not significantly affect the determination of the equivalent dose. The results are summarized in Table 1, and the uncertainty quoted is equivalent to the standard error for the number of samples used. 2.2.4. Dose rate measurement The total dose rates for age calculation comprise the beta dose rate, the gamma dose rate, and a cosmic ray component, as shown in Table 1. The beta and gamma dose rates were estimated from the concentrations of the major radioactive isotopes: here the uranium

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Fig. 3. (a) Black circles are OSL and 14C sampling locations. (b) Gravel-cobbles of Unit 7, overlying the Tertiary volcanogenic sedimentary beds, display offset and drag structures caused by reverse fault motion. (c) Dragged pebbles under the fault zone show Z- or concave-upward layering (white dotted line). (d) Under the reverse fault plane, the poorly compacted landslide zone (red dotted line) contains a fabric of sheared pebbles (yellow dotted line) along the landslide line. (e) Gravel-cobbles of Unit 7 display a small amount of displacement and drag structure caused by the reverse motion of F2 in stage 1.

and thorium series and potassium comprised within the surrounding materials. We used a high-purity germanium detector (Well-type HPGe detector, Canberra Ltd.) installed at the central laboratory of

Fig. 4. An example of the determination of the equivalent dose based on the single-aliquot regenerative-dose (SAR) protocol for sample SRY 3. Four different regeneration doses (L x, x=1, 2, 3, 4) are given, with regenerated OSL data corrected for sensitivity changes through division with the subsequent OSL test dose response (Tx, x=1, 2, 3, 4); ratios (L x,/Tx, x=1, 2, 3) are shown as open circles. Also shown are the sensitivity-corrected natural OSL response (LN/TN) and repeat point (L x,/Tx, x= 4), indicated by open square and open triangle. The curve is fitted by a single saturation exponential function to the regenerated data points.

Fig. 5. Plot of equivalent dose values as a function of preheat temperature for sample SRY 3. The line indicates an average of the equivalent dose values.

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Table 1 Luminescence dating results. Sample Equivalent dose (Gy) Gamma dose rate (mGy/yr) Beta dose rate (mGy/yr) Cosmic dose rate (mGy/yr) Total dose rate corrected (mGy/yr) Age (year, 1σ SE) SRY SRY SRY SRY SRY SRY SRY SRY

1 2 3 4 5 6 7 8

5.8 ± 0.4 26.1 ± 3.7 52.0 ± 2.4 215 ± 16 229 ± 15 225 ± 14 5.03 ± 0.30 5.15 ± 0.31

0.90 ± 0.01 0.91 ± 0.01 0.90 ± 0.01 0.90 ± 0.01 0.95 ± 0.01 0.81 ± 0.01 0.80 ± 0.01 0.79 ± 0.01

1.03 ± 0.02 1.13 ± 0.03 2.13 ± 0.03 1.20 ± 0.03 1.25 ± 0.03 1.15 ± 0.06 0.97 ± 0.03 0.96 ± 0.03

0.15 ± 0.01 0.15 ± 0.01 0.15 ± 0.01 0.10 ± 0.01 0.10 ± 0.01 0.10 ± 0.01 0.15 ± 0.01 0.15 ± 0.01

2.08 ± 0.03 2.19 ± 0.03 2.18 ± 0.03 2.20 ± 0.04 2.30 ± 0.03 2.06 ± 0.06 1.92 ± 0.03 1.90 ± 0.03

2800 ± 200 12,000 ± 2000 23,000 ± 1200 98,000 ± 7000 100,000 ± 6000 110,000 ± 7000 2600 ± 200 2700 ± 200

Notes: Total dose rate consists of beta and gamma dose rates and cosmic ray component. Total dose rate is corrected for water content using the formula of Zimmerman (1971) and for beta dose attenuation through the grains etched in conc. HF (48%). Cosmic ray contribution was considered according to Prescott and Hutton (1994).

Kangwon National University. A correction for the saturated water content was applied to both the measured beta and gamma dose rates, using the formulae of Zimmerman (1971). For calculations of the beta dose rates, a factor of 0.9 was considered to allow for beta dose attenuation within the grains of quartz. The cosmic ray contribution was calculated using the equation given by Prescott and Hutton (1994). 2.3. Stratigraphy A marine terrace platform 43 m above mean sea level at the Suryum site underlies stratified beach sediments ∼ 2 m thick and soil ∼ 1 m thick, which are classified into seven units (Fig. 2). The beach sediments consisting of highly weathered pebbles and coarse sands are divided into three units (labeled 7, 6, and 5 in ascending order); the soil strata are classified into four units (4, 3, 2, and 1 in ascending order). Unit 8 is the basement forming a platform to the marine terrace, composed of consolidated volcanogenic and fluvial sedimentary rock of late Miocene age. The volcanogenic clasts that originated from various volcanic sources are well rounded to angular in shape. The top of Unit 8 shows an irregular, unconformable boundary, which involves wedge-shaped fractures filled with beach sediments (Figs. 2 and 3e). Unit 7 unconformably overlying Unit 8 is composed of unconsolidated beach sediments and alluvium derived from the collapse of an old sea cliff. Coarse gravel to cobbles with a fine gravel matrix constitutes the sediments. Andesitic or rhyolitic gravels and cobbles are mostly well rounded, poorly sorted, and ellipsoidal to spheroidal in shape. The thickness of the unit is 21–64 cm at the hanging wall and 12–41 cm at the footwall, but thins away from the fault line until it dies out at the northwestern part of the footwall side (Fig. 2). Unit 6 is an unconsolidated marine sediment bed, which is mainly composed of granules less than 5 mm in diameter (mostly 1–3 mm), very well rounded, well sorted, and of elongate-oblate shape. It rarely includes coarse-medium pebbles (10–15 mm). This unit comprises a thickness of 10–20 cm and 40–60 cm at the hanging wall and at the footwall, respectively. It yields OSL ages ranging from 111,000 ± 7000 yr to 98,000 ± 7000 yr, which is consistent with the formation age (MIS 5e) of the terrace as defined by Choi et al. (2008). Ree et al. (2003) report OSL ages ranging from 32,000 ± 4000 yr to 58,000 ± 12,000 yr for the Unit 6 deposits; much younger than those obtained in this study (Table 1). A main factor for the discrepancy in these OSL ages comes from the equivalent dose values rather than the dose rates. In order to determine the equivalent dose, they used the SAR method employing the 110 °C TL peak to monitor changes in sensitivity during repeated measurements (Murray and Roberts, 1998), whereas this study employed the improved SAR method using the OSL test dose signal for correction of the sensitivity changes (Murray and Wintle, 2000). Very recently, using the same SAR method as the one in this study, Choi et al. (2009) also discuss OSL ages for samples taken from Unit 6. Their ages are in reasonably good agreement with our OSL ages, given the

error limits. However, there are limitations regarding the application of the OSL method in highly weathered beach deposits, as discussed by Jeong et al. (2007). In comparison with Unit 6, Unit 5 is a clast-supported sediment bed composed of medium to coarse well-rounded pebbles, moderately sorted and blade shaped, and cemented by strongly oxidized yellowishbrown clay. The grain size of the clasts ranges from 5 to 30 mm, mostly 10–20 mm. The unit is 64–140 cm thick but is obliquely truncated at the footwall side. At the hanging wall, a number of vertical fractures and cracks crossing the horizontal unit are filled with reddish-brown mud. Unit 4 is a soil member ∼90 cm thick, and classified into five components: a grayish-brown (7.5 YR 4/3) A-bed; a light-grayish-brown (7.5 YR 6/6) EB-bed; a brown (7.5 YR 5/5) B-bed; a reddish-brown (5 YR 5/4) BC-bed; and a light-reddish-brown (2.5 YR 6/4) C-bed. This unit with an OSL age of 23,000±1200 yr is only observed at the hanging-wall side. It dies out at the crest, which is part of an anticlinal structure on the hanging wall (Fig. 2). Unit 3 is a colluvial deposit b25 cm thick, identified as a brownishyellow (10 YR 6/6) Bt-bed. It consists of sand and silt with floating beach pebbles 5–20 mm in size. The Bt-bed obliquely overlies the horizontal Unit 5 near the fault zone on both sides. The somewhat inclined bed is slightly displaced with regard to the hanging wall and also truncates Unit 5 at the footwall. The unit is dated at 12,000 ± 2000 yr by OSL. Unit 2 is also a colluvial deposit composed of two components: a brownish-yellow (10 YR 6/6) Bt-bed and a yellowish-brown (10 YR 5/ 4) A-bed. The Bt-bed is 50–60 cm thick and is mainly composed of sand and silt with less than 5% medium beach pebbles, locally scattered in certain parts of the unit. The A-bed shows a very irregular thickness ranging from 0 to 20 cm (Fig. 2). Bt and A beds of Unit 2 yield an OSL age of 2800 ± 200–2600 ± 200 yr (Table 1) and a 14C age of 400 ± 50 yr (Table 2), respectively. The boundary between Units 2 and 3 is unclear. With regard to the results of OSL dating for both units, one can anticipate vertical mixing of soil materials because of the sloping vegetated surfaces. However, judging from the error limits of the evaluated OSL ages, this process is deemed rather insignificant.

Table 2 Result of radiocarbon analysis. Lab. no.

Sample Material Method δ13C (‰) Conventional Calibrated 14 C yr BP age (BC/AD) no. 1σ SD 2σ SD (68.2%) (95.4%)

KR08-319 SRY 9

Organic AMS sediment

− 30.42

400 ± 50

1430 1420 (53.7%) (95.4%) 1520 AD 1640 AD 1590 (14.5%) 1620 AD

Lab. no.: Chungcheong Research Institute of Cultural Heritage, Gongju, S. Korea. AMS measurement was carried out at Seoul National University. Conventional 14C age was corrected by δ13C = − 25% and calculated using the Libby half-life of 8033 years.

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Table 3 Calculation of the net slip and net slip rate from the Late Quaternary faulting event at the Suryum site, SE Korean peninsula. Stage

Age (yr)

Event

1st

23,000 ± 1200

Pre-faulting stage Reverse faults

2nd 12,000 ± 2000 3rd 4th

2600 ± 200 400 ± 50

Landslide Colluvium (Unit 3) Reactivated reverse faults Degradation/colluvium (Unit 2) Normal faulting along the reverse fault Degradation/colluvium (Unit 1)

A-bed of Unit 2, from which we collected the sample for 14C age dating, contains accumulated organic matter and is typically dark because of decomposition and humification. We carefully removed plant roots from the sample for age dating. Thus, we are confident that the 400 yr 14C date is the best available indication of the age of formation of the soil horizon. Unit 1 is the uppermost colluvial deposit with a thickness of 0–20 cm. At the hanging wall, it contains only a light-yellowish-brown (10 YR 6/4) Bt/C-bed, whereas at the footwall it consists of a reddishyellow (7.5 YR 6/6) Bt-bed and a light-yellowish-brown (10 YR 6/4) Bt/ C-bed. 2.4. Retrodeformation analysis Based on the stratigraphic correlation and OSL ages, we have reconstructed a sequence of multiple faulting events for the Suryum fault. The faulting events that occurred during the late Pleistocene to Holocene are identified in four stages: reverse, landslide, reactivated reverse, and normal faulting (Table 3). (A) The initial stage shows that the primary geomorphic surface covered by Unit 4 of 90–100 cm thickness inclines northwestward at an angle of 6° (Fig. 6a). We think that there should have been a preexisting tectonic movement that resulted in the tilted surface; this is because the marine terrace platform is here supposed to incline at an angle of 2–3° toward the sea to the southeast. (B) Stage 1 is a reverse-faulting event, which is defined by the stratigraphic offset of Units 5 to 8. We assume that the two secondary faults (F2 and F3) splay out from the major reverse fault (F1) as a result of reverse faulting (Figs. 2 and 6b). These accompanying faults converge into F1, which refracts to a lower angle (b10°) toward the surface. The low dip angle of F1 near the surface is likely to have created the initial fault scarp in the form of a pressure ridge of D type rather than C type (Carver and McCalpin, 1996). Reverse faulting of F1 also generated a convexupward bedding structure on the hanging wall because of the compressional force. Vertical displacement of F1 and F2 is ∼90– 95 cm along Unit 8 and 60 cm along Unit 7, respectively (Fig. 3e). F3 has no visible displacement. (C) Stage 2 is an event of landsliding with a dip of ∼60° showing a counter direction to the reverse fault of stage 1 (Fig. 6c). There are three lines of evidence for the landslide: 1) the sheared pebble layer N5 cm thick oblique to the reverse fault line; 2) truncation of the horizontal Unit 5 at the foot wall of the reverse fault; and 3) the gravelly colluvium of Unit 3 residing on the hanging wall of the reverse fault. A part of the convex layers on the hanging wall of the reverse fault collapsed and slid northwestward along the landslide plane into a

Vertical slip(cm)

Dip (°)

Net slip (cm)

Net slip rate (mm/yr)

F1:∼90–95 F2:∼60 –

F1: 68–45 F2: 45 60

F1: 100–130 F2: 84 –

F1: 0.04–0.08 F2: 0.03–0.07 –

F1:∼6?

F:10

F1: ∼43 ?

F1: 0.15–0.4

F1:∼6

F1:10

F1: ∼43

F1: N 0.15

small valley (Fig. 6c). This process is a result of gravitational force affecting unconsolidated sediments, and thus induced the largest amount of mass movement among the four events. The landslide caused the inclined surface under the reverse fault plane. When such a surface is formed by landsliding, it produces sheared pebbles along the fault plane (Fig. 3d). The free face of the landslide is degraded to build up the colluvium of Unit 3, and its slope declines with a dip of ∼27° (Fig. 6d). The timing of this event is loosely constrained by the age obtained from the colluvial deposit (Unit 3) to about ∼12,000 ± 2000 yr. (D) Stage 3 is an event of reactivated reverse faulting on F1 and is defined by the colluvial deposit of Unit 2. It is suggested that the faulting led to slope degradation (dip ∼16°) on the upper half of the retreating landslide scarp and colluvial aggradation on the lower half (Fig. 6f) (Carver and McCalpin, 1996). Vertical displacement amounts to ∼6 cm (Fig. 6e). The thick colluvial deposit on the lower half slumps slightly toward the small valley because of gravitation and poor compaction. This event might have happened after the 2800 ± 200 yr date obtained from Unit 2. (E) Stage 4 is the last normal faulting event along the F1 plane with a date of 480 ± 200 yr (Fig. 6g). Evidence for the reactivation is seen in the dragged and thin Unit 3 of the hanging wall (Fig. 2) and the Zshaped pebble rotations of the footwall (Fig. 3c). In this stage, constant degradation on the scarp of the landslide forms a lowangle slope of 8–16° and deposition of the colluvium of Unit 1 (Fig. 6h). The normal faulting caused by rebound of the reverse fault is diminutive and weakly expressed. It has not been generated by a distinct tectonic force.

3. Discussion and conclusions All four events of the Suryum fault happened after the deposition of Unit 4, which has an OSL age of 23,000 ± 1200 yr. The landsliding event triggered by the first reverse faulting engendered the Unit 3 colluvial deposit, dated at 12,000 ± 2000 yr. After the second reverse faulting, the fault scarp collapsed again, and the landslide scarp then retreated, becoming a gentler slope. Debris and wash element colluvium of Unit 2 was produced during 2800 ± 200 yr to 480 ± 200 yr. Most previous researchers (Lee et al., 1999a,b, 2000; Ree et al., 2003) regard the Suryum fault as merely a single event because of the simple stratigraphic expression where it is exposed. However, the retrodeformation analysis of the Suryum fault based on stratigraphic correlation and colluvial deposits reveals four events including threetime faulting during the latest Pleistocene: reverse → landslide → reverse → normal. With the exception of the landslide, all of them occurred within the same narrow fault zone. The result of this restoration is corroborated by Jeong and Cheong's (2005) investigation postulating three-time faulting events during the late Quaternary. In fact, even though the Suryum fault is less than 1.6 km long, it

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Fig. 6. Reconstruction of faulting and deposition on the Suryum fault. (a) Initial topographic slope inclines northwestward at an angle of 6°. (b) Major fault (F1) refracts to a lower angle (b10°), and produces convex-upward bedding structure at the hanging wall, the low dip angle of F1 near the surface being more likely to create an initial fault scarp in the form of a D-type pressure ridge rather than a C-type (Carver and McCalpin, 1996). (c) The initiation of the landslide. Dip angle is N 60°. Convex-upward layers on the hanging wall of the reverse fault slide northwestward along the landslide, then most of the deposits on its hanging-wall side slump into the northwestern small valley producing sheared pebbles along the landslide scarp zone, which is poorly compacted. Two units, 4 and 5, are truncated by this event. (d) The free face of the landslide degrades to build up colluvium, named here Unit 3 (slope dips with a dip ∼ 27°). (e) Reactivation of the reverse fault that occurred along the same zone of F1 is concomitant with subtle vertical displacement. (f) Reactivated reverse faulting leads to slope degradation with a dip of ∼16° on the upper half of the retreating normal fault scarp and colluvial aggradation (Unit 2) on the lower half. The thick colluvial deposit on the lower slightly slumps toward the small valley because of gravitation and poor compaction (black dotted line). Unit 3 is buried by Unit 2. (g) Normal faulting is reactivated along F1, dragging older colluvial deposits. The vertical displacement is ∼6 cm at Unit 3. (h) Constant degradation on the landslide scarp creates a gentle slope of 8–16° and deposits the colluvium of Unit 1.

might still be considered a capable fault because multiple faulting events occurred within the past 50,000 yr. Knowledge of the fault slip rate is an important factor required to make more accurate predictions of future seismic activity and related hazards arising from a deformation zone. The maximum vertical displacement of F1 in the first event ranges from 90 to 95 cm, and that of F2 is ∼60 cm (Table 3). Using trigonometry, net slips are calculated to 100–130 cm for F1 at a dip angle of 68°–45° and 85 cm for F2 at a dip angle of 45°. Assuming these net slips occurred between 23,000 ± 1200 and 12,000 ± 2000 yr BP, the net slip rates range from 0.03 to 0.08 mm/yr (Table 3). Furthermore, if reactivation of F1 in stage 3 occurred from 12,000 ± 2000 to 2800 ± 200 yr BP, its net slip rate equals 0.15–0.4 mm/yr.

By comparison, Okada et al. (2001) estimate the average vertical slip rate for the Ulsan fault during the Quaternary at 0.1 to 0.08 mm/ yr. Cheong et al. (2003) report a minimum slip rate of 0.52 mm/yr for the Wangsan fault using the OSL age of fluvial terrace sediments. Ree and Kwon (2005) later reestimate slip rate for the Wangsan fault to 2.8 mm/yr, about five times higher than the previous values of Cheong et al. (2003). In addition, the vertical slip rate of the Eubchun fault, 1 km north-northeastward of the Suryum site, is calculated at 0.01–0.05 mm/yr during the Quaternary (Kyung and Cho, 2005). The net slip rate of the Suryum fault is smaller than that of the Wangsan fault and is compatible with that of the Eubchun fault, whereas the vertical displacement of the Eubchun fault is four to six

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times larger than that of the Suryum fault. We also notice that previous researchers have only used the age of terrace sediments for the calculation of slip rate, but did not consider the age of the colluvium generated by the faulting events. Therefore, we claim that the short-term slip rate of the Eubchun fault should be greater than the 0.01 to 0.05 mm/yr given by Kyung and Cho (2005). Most studies in high-seismicity areas report a vertical slip rate higher than 0.4 mm/yr; for example, the vertical slip rate at the Muikamachi–Bonchi–Seien fault zone, Niigata in central Japan (Maruyama et al., 2007), during Holocene time, lies at 0.4 mm/yr, corresponding to a net slip rate of 0.8–1.2 mm/yr. In addition, the vertical slip rate of the Tym–Poronaysk fault, Sakhalin (Russia) is 0.9– 1.4 mm/yr, consistent with a net slip rate of 1.0–2.8 mm/yr (Tsutsumi et al., 2005). Other vertical slip rates are 0.64–0.9 mm/yr for the Zhangye thrust zone, Qilian Mountains, China (Hetzel et al., 2004), and 0.24–0.5 mm/yr for the central Italian Avezzano faults (Galadini and Galli, 1999). These studies also show that movement along faults as described above, except for the Zhangye thrust zone, is often linked to earthquakes of magnitude Mw 6–7. On the other hand, only a few reports exist of slow slip rates generated by Mw 6–7 intraplate earthquakes. According to Perea et al. (2003), the vertical slip rate of the Elcamp normal fault in Spain is 0.05–0.08 mm/yr during the last 1–1.5 Ma. The Azambuja reverse fault in Portugal has experienced a slip of 0.05–0.06 mm/yr over the last 125,000 yr (Cabral et al., 2004). In conclusion, we suggest that the Suryum reverse-faulting event might have been associated with an Mw 6–7 earthquake during the late Pleistocene. Acknowledgements We thank G.Y. Jeong and M. Chitambo for careful reviews and associate editor S. Kwon for comments and editorial suggestions. We are also grateful to M. Landes for comments and corrections. This research was supported by the Nuclear R&D Program funded by the Ministry of Education, Science and Technology and by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge and Economy, Republic of Korea. References Cabral, J., Ribeiro, P., Figueiredo, P., Pimentel, N., Martins, A., 2004. The Azanbuja Fault: an active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal). Journal of Seismology 8, 347–362. Carver, G.A., McCalpin, J.P., 1996. Paleoseismology of compressional tectonic environments. In: McCalpin, J.P. (Ed.), Paleoseismology. Academic Press, San Diego, pp. 183–270. Chang, E.Z., 1996. Collisional orogene between north and south China and its eastern extension in the Korean Peninsula. Journal of Southeast Asian Earth Sciences 13, 267–277. Cheong, C.S., Hong, D.G., Lee, K.S., Kim, J.W., Choi, J.H., Murray, A.S., Chwae, U., Im, C.B., Chang, C.J., Chang, H.W., 2003. Determination of slip rate by optical dating of fluvial deposits from the Wangsan Fault, SE Korea. Quaternary Science Reviews 22, 1207–1211. Cho, M., Kim, Y., Ahn, J., 2007. Metamorphic evolution of the Imjingang belt, Korea: implications for Permo-Triassic collisional orogeny. International Geology Review 49, 30–51. Choi, P.Y., Kwon, S.K., Hwang, J.H., Lee, S.R., 1999a. Paleostress analysis of southeast Korea: tectonic sequence and timing of block rotation of the Pohang–Ulshan area. Gondwana Research 2, 532–537. Choi, P.Y., Kwon, S.K., Hwang, J.H., Lee, S.R., 1999b. Deformation in and around Korea in relation to the opening of the East Sea. Gondwana Research 2, 537–540. Choi, S.J., Merritts, D.J., Ota, Y., 2008. Elevations and ages of marine terraces and late Quaternary rock uplift in southeastern Korea. Journal of Geophysical Research 113, B10403. doi:10.1029/2007JB005260. Choi, J.H., Kim, J.W., Murray, A.S., Hong, D.G., Chang, H.W., Cheong, C.S., 2009. OSL dating of marine terrace sediments on the southeastern coast of Korea with implications for Quaternary tectonics. Quaternary International 199, 3–14. Chwae, U., Choi, S.J., 1999. On the possible extension of the Sulu Belt toward the east through the Korean Peninsula. Gondwana Research 2, 540–542. Chwae, U., Lee, D.Y., Lee, B.J., Ryoo, C.R., Choi, P.Y., Choi, S.J., Cho, D.L., Kim, J.Y., Lee, C.B., Kee, W.S., Yang, D.Y., Kim, I.J., Kim, Y., Yoo, J.H., Chae, B.G., Kim, W.Y., Kang, P.J., Yu, I.H., Lee, H.K., 1998. An investigation and evaluation of capable fault — southeastern part of the Korean Peninsula. KIGAM Research Report KR-98(c)-22. Korea Institute of Geology, Mining and Materials, Daejeon. 301 pp.

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