articolo ran.pdf Key words Daqingshan piedmont fault (China) – Hohhot Segment – paleoearthquakes – seismic history – completeness of paleoseismic record 1. Introduction The Daqingshan Piedmont Fault (DPF) is one of the major active normal faults in Hetao Late Quaternary history of paleoseismic activity along the Hohhot Segment of the Daqingshan piedmont fault in Hetao depression zone, North China Yongkang Ran, Peizhen Zhang and Lichun Chen Institute of Geology, China Seismological Bureau, Beijing, China Abstract The Daqingshan Piedmont Fault (DPF) is one of the major active normal faults in the Hetao depression zone in the northern part of Ordos Block, North China. It extends in NEE direction along the Daqingshan piedmont zone in the eastern part of the depression, dipping to the south, for a length of 223 km. The fault formed in the Eocene and under- went strong movement during the Cenozoic time. Its vertical displacement amplitude has exceeded 2400 m since the Quaternary. The fault can be divided into 5 active segments. Paleoseismological studies were concentrated on its western part from Baotou to Tumdzuoqi whereas the Hohhot Segment to the east was scarcely studied. To fill this gap of knowlegde, the authors carried out in-depth study on the Daqingshan piedmont fault during recent years. Exca- vation of trenches at Kuisu, Ulanblang, and Bakouzi sites on the Hohhot Segment of the Daqingshan piedmont fault and study of geomorphic surfaces allow us to identify and date paleoearthquakes and to evaluate the completeness of paleoseismic activity history. This was done both for the individual sites and for the entire segment since the Late Quaternary using the «method for displacement confining» along the fault and «method for correlation between mul- tiple trenches». In this paper we present the geological loggings of two trenches at Kuisu site, provide the evidence for 6 events since 19 ka BP and the cumulative displacement amount produced by them is around 7 m. But the cumu- lative displacement amount obtained from difference in heights of geomorphic surfaces is 5. .5.5 m. Results of tests using the method of displacement confining show that the event sequence revealed at this site can be considered com- plete. The data supplemented with information obtained in the Ulanblang and Bakouzi trenches show that 7 paleo- seismic events occurred on the Hohhot Fault Segment since 19 ka BP, i.e. they occurred at 18.75 ± 0.75 ka, 16.97 ± ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 6.83 ± 0.26 ka, and 4.50 ± 0.23 ka BP, respectively, and the average recurrence interval is 2.375 ± 0.432 ka. These results basically reflects the history of paleoseismic acti- vity on the fault segment in this period of time. depression in the northern part of Ordos Block, North China (fig. 1). A M 7.5 earthquake occur- red on the fault in 849 A.D. An important indu- strial-agricultural economic zone and two ma- jor cities of Inner Mongolia Autonomous Region, Baotou and Hohhot, are distributed bas- ically along the fault zone. Because the fault is a typical active normal fault and its movement can seriously threaten people’s lives and pro- perties in the region, many scientists have stu- died its paleoseismic activity, fault movement behaviors, and recurrence intervals of strong 1053 ANNALS OF GEOPHYSICS, VOL. 46, N. 5, October 2003 Mailing address: Dr. Yongkang Ran, Institute of Geology, China Seismological Bureau, Beijing 100029, China; e-mail: ykran@263.net earthquakes along the fault (Research Group on Active Fault System around Ordos Block, 1988; Li et al., 1994; Wu et al., 1995; Nie et al., 1996; Jiang et al., 2001). A large number of data have been accumulated and provided a good basis for new studies. However, there exist some weakly studied problems for the fault, for example, the previous paleoseismological studies were largely concen- trated on its western part from Baotou to Tum- dzuoqi and less on the other segments of the fault. Are the other fault segments potentially seismoge- nic too? Can the revealed paleoearthquakes be con- sidered representative of a complete history of pa- leoseismic activity on the fault in the Late Quater- nary? If it is incomplete, how can the seismic hazard be evaluated using the available paleosei- smological data? These questions led the authors to conduct a detailed study on the Daqingshan pied- mont fault during recent years, in particular, on the Hohhot Segment. This paper deals with the paleo- seismic events revealed along the segment since the Late Quaternary and discusses the complete- ness of its paleoseismic activity history. 2. Brief geology and geomorphology DPF is one of three major active normal faults in the Hetao depression zone in the nor- thern part of Ordos Block, North China. It ex- 1054 Yongkan Ran, Peizhen Zhang and Lichun Chen Fig. 1. Map showing active faults in North China and the position of study area: 1 - active faults in Late Pleistocene and Holocene; 2 - active faults in Early and Middle Pleistocene; 3 - hidden active faults in Qua- ternary; 4 - normal fault; 5 - reverse fault; 6 - strike-slip fault; 7 - Quaternary sedimentary basin; 8 - Quaternary uplifted area. tends in a ENE direction, dip to the south, along the southern piedmont of Daqingshan Range for a length of 223 km. The Daqingshan Range in the north of the DPF has been uplifted to an elevation of 1000 m above the Hohhot Basin, a vertical displacement of more than 2400 m since the Quaternary was estimated (Research Group on Active Fault System around Ordos Block, 1988). The range, consisting mainly of Achean metamorphic rocks and Jurassic debris rocks is formed by a tilted block. The southern slope of the range is far steeper than the nor- thern slope. The Tertiary planation surface on the range top dips to the north. As the Daqingshan Range intermittently uplifted due to the fault movement, gorges were formed by creeks flowing southward, along which at least 3-4 terraces are well developed. The most stri- king geological-geomorphic features along the Daqingshan piedmont fault are three surfaces, which are widely distributed on the footwall of the fault. The first-step surface, formed ca. 8000 10.000 years ago (Li et al., 1994), con- sists mostly of the raised alluvial fan deposits controlled by fault scarp. Its height is several to over ten meters above the present surface of the basin. The second- and third-step surfaces are mostly covered with Late Pleistocene alluvial- diluvial or alluvial-lacustrine sediments (the youngest lacustrine sediments were dated to be 23.0 26.0 ka BP; Li et al., 1994) and are loca- ted at 120 170 m above the present basin. The piedmont tableland along the Hohhot Segment is not well preserved, but the spur surface 60-90 m high was found. The elevation of surface in the west of Baotou gradually decreases down to 15 m. The fault scarps usually appear to cut the surfaces. The DPF consists of step-like normal faults along its strike and the spatial distribution of their movement appears to have been uneven since the Late Pleistocene (Research Group on Active Fault System around Ordos Block, 1988). Li et al. (1994) claim that the DPF can Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China Fig. 2. Principal characteristics of the Daqingshan piedmont fault: 1 - active faults in Late Pleistocene and Holocene; 2 - pre-Pleistocene active fault; 3 - buried fault; 4 - depth contour of the Quaternary deposits; 5 - segment boundary, dotted lines were decided by predecessors, but given up now. 1055 be divided into 5 segments according to its geo- metry, kinematics, and distribution of paleo- seismic events, i.e. Yellow River-Xuehaigou, Xue- haigou-Tumdyouqi, Tumdyouqi-Meidaiqiao, Meidaiqiao-Tumdzuoqi and Tumdzuoqi-Kuisu segments. Jiang et al. (2001) divided the DPF into three segments, i.e. West Baotou, Baotou- Tumdzuoqi, and Tumdzuoqi-Hohhot segments. The main evidence for this interpretation is that the 849, M 7.5 earthquake rupture extended from Xuehaigou to Tumdzuoqi, and thus the three segments in the Li et al. (1994) model can be considered as a single segment. We prefer the division of the fault into 5 segments, but by introducing some changes in the the bound- ary locations and lengths of the segments. According to the distribution of cumulative dis- placement amounts reflected by the isopach of Quaternary deposits along each segment (fig. 2), we consider that the DPF should be better divided into 5 segments, the Yellow River-Xue- haigou (Baotou) Segment 37 km long, Xue- haigou-Tumdyouqi (West Tumdyouqi) Segment 35 km long, Tumdyouqi-Tumdzuoqi (West Tumdzuoqi) Segment 56 km long, Tumdzuoqi- Usutu (Bikeqi) Segment 49 km long, and Usutu- Kuisu (Hohhot) Segment 46 km long. According to Li et al. (1994), the displace- ment rate since the Late Pleistocene is 4.75. .6.46 mm/a along the West Tumdyouqi, West Tumdzuoqi and Bikeqi segments, 2.4. .3.5 mm/a along the Baotou Segment, and 2.4-3.5 mm/a along the Hohhot Segment. 3. Trench logs at several sites and paleoseismological analysis Three main trenches and their supplemen- tary trenches were excavated at the sites of Kuisu, Ulanblang and Bakouzi (fig. 3) to dis- close evidence for paleoseismic events in adja- Yongkan Ran, Peizhen Zhang and Lichun Chen Fig. 3. Geological sketch showing the Hohhot Segment of Daqingshan piedmont fault: 1 - Upper Holocene; 2 - Lower Holocene; 3 - Upper Pleistocene to Holocene; 4 - Upper Pleistocene; 5 - Cretaceous; 6 - Jurassic; 7 - Paleozoic; 8 - Archean; 9 - Granites; 10 - Late Pleistocene active normal fault; 11 - section with clear fault scarp; 12 - Quaternary Fault; 13 - geologic boundary; 14 - recent alluvial fan; 15 - location of trench; 16 - boundary between fault segments. 1056 cent trenches at the same site and along seg- ments to identify the whole rupture extent. 3.1. Kuisu site 3.1.1. Analysis of deposits and evidence of paleo- seismic events exposed in the main trench (Tc3) The excavated trenches are located in the west of Kuisu village, where the Kuisu gully runs from north to south and resulted in the for- mation of a huge alluvial-diluvial fan accumu- lated on the mountain spur (fig. 3). The alluvial- diluvial fan was offset by the DPF and its sur- face was abandoned on the upthrown side of the fault (fig. 4). The surface was partially des- troyed by the gully of regressive erosion. It can be seen on the section cut by the gully that the tableland on the upthrown side is composed of boulders or large-sized cobbles covered by eolian or secondary loess. The downthrown side is more complicated, as some deposits are from gully terrace sediments and others are loess or secondary loess with several meters in thick- ness. We attempted to reveal as many paleoseis- mic events as possible on the one hand, and to collect samples for 14C dating on the other. A large trench (20 m long, 3 m wide, and 5 m deep in maximum) was excavated in front of the ta- bleland, where the fault scarp was less disturb- ed by recent human activity. A supplementary trench (Tc4) was excavated very close and along the gully (20 m long and 4 m deep) to collect samples for 14C dating where the most recent sediments were accumulated. The sediments exposed in trench Tc3 were divided into 9 units (fig. 5). Unit 1, exposed on the upthrown side of the fault, consists of gra- vel layers from the stream, they show clear bed- ding and sorting. The grain size reaches maxi- mum 50 cm and visible thickness is 4.2 4.6 m. Unit 2 is found only on the downthrown side of the fault on the western wall of the trench. It is composed of yellow silt and sandy soil, litholo- gically similar to eolian loess, weakly pedoge- nic, corresponding to sediments on the surface. Units 3, 4, 5, 6 and 7 are only exposed on the downthrown side of the fault and lithological- ly are not significantly different, composed of loess and secondary loess. All these units are commonly characterized by a higher content of gravel close to the fault, variegated, and loess or brown loess clumps found near the fault and at the bases of the units, but they gra- dually change into pure loess or secondary loess at a distance from the fault. Obviously, they are the mixed products of accumulation in front of the scarp after faulting and during eolian sedimentation. Unit 8 covers the top of the fault and consists mainly of blackish sandy soil near the fault, but pale due to the concen- tration of calcium, gradually changes into loess or secondary loess at a distance from the fault. Unit 9 is composed of yellow-black sandy soil with gravel. Five offset scarp-front deposit units possi- bly representing 6 paleoearthquakes can be dis- tinguished in accordance with the above-de- scribed lithological characteristics and the dis- tribution of strata in combination with the dis- turbance and dislocation of the strata by the fault (fig. 5). Unit 2 represents an ancient land surface, a sample (KT1) dated by thermolumi- nescene (TL) method to be 18.80 ± 1.45 ka BP Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China Fig. 4. Topographic map of the area west of Kuisu village: spacing between topographic contours is 1 m; square shows bore pit for sampling, at 56 cm below the surface. 14C sample (Kc5) from thin-layer carbonaceous sandy clay was dated to be 19.990..±..395 years BP; rectangle shows location of excavated tren- ches. 1057 1058 Yongkan Ran, Peizhen Zhang and Lichun Chen Fig. 5. Geological logging of the Kuisu large trench (Tc3): 1 - gravel; 2 - yellow sandy soil; 3 - loess or soil lumps; 4 - surface soil; 5 - stratigraphic unit and its number; 6 - fault and its number; 7 - TL sample site and its number. Stratigraphic sequence: unit 1 - gravel layer, layered and sorted; unit 2 - yellow silt and sandy soil, weakly pedogenic; unit 3 - mixed sediments of yellow sandy soil with gravel and loess or brown loess clumps, gradually changing into loess or secondary loess with distance from fault; unit 4 - yellow sandy soil, content of gravel increases near fault and at base of the unit, variegated, containing brown loess clumps and gradually chan- ging into loess or secondary loess; units 5, 6, and 7 - mixed sediments of yellow sandy soil with gravel and loess or brown loess clumps, content of gravel increases near fault and at their base, variegated, containing loess or brown loess clumps, which gradually change into pure loess or secondary loess with distance from fault, simi- lar to units 3 and 4. Unit 8 - mainly pale sand, rich in calcium; unit 9 - yellow silt with small-sized gravel. (table I). It reflects the oldest paleoseismic event occurring some time later than the age value. Then unit 3 was deposited. Unit 3 was offset by fault F2, providing clear evidence for a second event. The subsequent unit 4 covers fault F2. TL dating of samples from the top part of unit 3 and the base of unit 4 yielded age va- lues of 17.92 ± 1.38 ka BP (KT2) and 16.01 ± ± 1.20 ka BP (KT3), respectively. Thus, the se- cond paleoseismic event occurred between these two age values. The third paleoseismic event caused fault Fl to offset unit 4 and resul- ted in formation of a filled wedge near the fault, as found on both eastern and western walls of the trench. Then, unit 5 was accumulated and covers the filled wedge. TL samples from the top part of unit 4 and the base of unit 5 were dated to be 15.31 ± 1.06 ka BP (KT4) and 13.77 ± 1.06 ka BP (KT5), respectively. The fourth event led unit 5 to be in direct contact with the fault on the western wall of the trench and a filled wedge to be developed. It expresses an offset of unit 5 by the fault on the eastern wall of the trench and unit 6 covers a branching fault. TL sample from the top of unit 5 was dated to be 12.51 ± 0.96 ka BP (KT6). Afterwards, unit 6 was accumulated. TL sample from the base of unit 6 was dated to be 11.13 ± 0.85 ka BP (KT7). Thus, the fourth event occurred between 12.51 ± 0.96 and 11.13 ± 0.85 ka BP. The fifth event is expressed as a possible dislocation of unit 6 and led to accumulation of unit 7 on both walls of the trench. This event is less con- trained than the others. It is known from age values of TL samples (KT8 and KT9) from the top part of unit 6 and the middle part of unit 7 that the event occurred between 9.72 ± 0.75 ka BP and 7.09 ± 0.55 ka BP. The sixth event has 1059 Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China Table I. Age values confining paleoseismic events. Trench# Sample# Material Dating Measured age Calib. age method (years BP) (years BP) TC1 Qc7 Humus 14C 9820 ± 160 10 990 ± 150 Qc8 Humus 14C 9270 ± 130 10 230 ± 150 Qc6 Humus 14C 10 140 ± 190 Qc5 Humus 14C 5780 ± 85 6570 ± 110 TC2 TW9 Silty soil TL 19 750 ± 1520 TW7 Silty soil TL 15 320 ± 1180 TW8 Silty soil TL 13 890 ± 1070 TW1 Silty soil TL 10 710 ± 820 TW2 Silty soil TL 8110 ± 620 TW3 Silty soil TL 7720 ± 590 TW4 Silty soil TL 6490 ± 490 CW1 Humus 14C 4030 ± 10 TC3 KT1 Silty soil TL 18 800 ± 1450 KT2 Silty soil TL 17 920 ± 1380 KT3 Silty soil TL 16 010 ± 1200 KT4 Silty soil TL 15 310 ± 1060 KT5 Silty soil TL 13 770 ± 1060 KT6 Silty soil TL 12 510 ± 960 KT7 Silty soil TL 11 130 ± 850 KT8 Silty soil TL 9720 ± 750 KT9 Silty soil TL 7090 ± 550 TC4 WC2 Humus 14C 13 570 ± 140 WC3 Humus 14C 9210 ± 180 bore pit Kc5 Humus 14C 19 990 ± 395 Notes: TL samples and 14C samples were dated in the Neogeochronological Laboratory of Institute of Geology, China Seismological Bureau. dislocated unit 7 and then the unit 8 was accu- mulated. At present, only the time of the sixth event is known to be later than 7.09 ± 0.55 ka BP. Moreover, the traces of three later events can be found in trench Tc4 too. We attempted to estimate coseismic displa- cement amount by analysis of thickness and lith- ological composition of scarp-front deposits. In a simple model of normal faulting, the thick- ness of fault-scarp colluvial wedges near the normal fault is commonly assumed to be half of the initial scarp height (McCalpin, 1996). But if the scarp-front deposits were not completely the colluvium from initial scarp, then their double thickness should be regarded as a maximum displacement amount. We measured the maxi- mum thickness of scarp-front deposits formed by every event (measurement was done perpen- dicularly to bed plane and the thickness of de- posits on the upper breaking point was measu- red for the last event). The obtained thickness of them is 0.50 ± 0.03 m, 0.85 ± 0.05 m, 1.1 ± ± 0.1 m, 0.65 ± 0.03 m, 0.8 ± 0.3 m, and 0.6 ± ± 0.02 m, respectively. We know from the above that all the scarp-front deposits formed by events disclosed in the Tc3 contain some eolian loess, except for the last event, so they are the mixed deposits of colluvial and eolian mate- rials. Therefore, we tend to regard the thickness of scarp-front deposits formed by events I V as the minimum coseismic displacement a- mount, i.e. 0.50 ± 0.03 m, 0.85 ± 0.05 m, 1.1 ± ± 0.1 m, 0.65 ± 0.03 m, and 0.8 ± 0.3 m, respec- tively. Whereas, double thickness of scarp-front deposits are regarded as the maximum cosei- smic displacement amount; these are ca. 1 m, 1.7 m, 2.2 m, 1.3 m, and 1.6 m, respectively, and their mean values represent the inferred displa- cement amounts produced by the events I V at this site, i.e. 0.7 ± 0.25 m, 1.28 ± 0.43 m, 1.65 ± ± 0.55 m, 0.95 ± 0.35 m, and 1.2 ± 0.4 m, respectively. Because the deposit by event VI does not contain eolian loess, double thickness of the deposit covering the fault is used to re- present the inferred coseismic displacement amount, i.e. ca. 1.2 m. Thus, the minimum cu- mulative displacement amount by the paleosei- smic events revealed at this site is ca. 5.1 m and the maximum is ca. 9 m, mean value is ca. 7.03 m. 3.1.2. Paleoseismological analysis of supplementary trench (Tc4) A supplementary trench was excavated on the eastern wall of the gully east of the main trench (fig. 6). It mainly revealed part of a small graben formed by fault movement on several faults. As the trench was excavated along the 1060 Yongkan Ran, Peizhen Zhang and Lichun Chen Fig. 6. Geological log of Kuisu supplementary trench (Tc4): 1 - sandy gravel; 2 - brown-black clayey sand; 3 - yellow clayey sand; 4 - fault; 5 - fissure and fillings; 6 - stratigraphic unit and its number; 7 - site of 14C sam- ple; 8 - analyzed part of the profile in detail. gully wall, the relation between the exposed strata and displacement is more complicated, a correlation between fault scarp and collapse wedge is not clearly found, but some charac- teristics of deposits in the small graben in the front of scarp can be distinguished. If the gravel layer-brown-black sandy soil-yellow sandy soil sequence is regarded as a sedimentary unit, and other sequences are stratified in terms of their lithological characteristics. A total of 10 strati- graphic units are identified. Units 1 3 are the gravel layer-brown-black sandy soil-yellow sandy soil sequence. Unit 5 is composed of about 85 cm thick brown sandy soil. Unit 7 is relatively pure eolian loess. Units 9 and 10 are recently ditch-filling yellow sandy soil and sediments on the ditch surface. The units 2 5 are characte- rized by deposition in a small graben in the front of the scarp and well correlated with fault movement traces. In the trench log the top part of fault cutting unit 2 is covered with unit 3 and the fault cutting unit 3 is covered with units 4 7, which were also offset. It indicates that at least 3 events can be distinguished in the log. 14C sample (WC2) from the top part of brown- black soil of unit 2 was dated to be 13.570 .±.140 years BP. 14C sample (WC3) from the top part of brown-black soil of unit 3 was dated to be 9210 ± 180 years BP. It follows that 3 events occurred since 13.57 ka BP, two of them bet- ween earlier than 9.21 ka BP and 13.57 ka BP, the third after 9.21 ka BP. These appear to cor- relate well with the fourth, fifth, and sixth events revealed in trench Tc3. 3.2. Ulanblang site (Tc2) A large trench (Tc2) was excavated (20 m long, 3.5 m wide, and 4-4.5 m deep) at about 400 m east of Ulanblang village. At this site the fault scarp is well developed and two steps of scarp can be distinguished because of gully erosion. Moreover, there is a clear change in the slope grade on the recent wide gully bot- tom. Here actually there are three fault scarps F with different heights. They are about 7.2 m, 3.6 m and 1.2 m high, respectively. The trench Tc2 was excavated across the highest step of scarp. The strata exposed in the Ulanblang large trench (Tc2) can be divided into 16 units (fig. 7). Units 1 3 are exposed only on the footwall of the fault. Unit 1 consists of Cretaceous con- glomerate and mudstone and units 2 and 3 con- sist of Late Pleistocene fluvial-lacustrine sedi- ments. Units 4 11 are characterized by scarp- derived deposits caused by fault movement. Unit 4 was deposited near fault F1 and consists of brown-black sandy clay with gravel in a fault contact with unit 5, which is gray-yellow sandy clay with gravel and then gray-yellow sandy soil with massive structure was accumu- lated and formed unit 6. TL sample (TW9) from the upper part of unit 6 was dated to be 19.75 ± 1.52 ka BP (table I). This stratigraphic contact relation indicates that the first event revealed in the trench occurred before the de- position of unit 6, earlier than 19.75 ± 1.52 ka BP. Unit 7 is a wedge-shaped calcium-rich pale deposit near fault Fl. Unit 8 is gray-yellow sandy soil with gravel in its lower part and upward becomes pedogenic brown sandy soil. It indicates that the unit was exposed at the sur- face for long time and then has offset and buried. Thus, it is conceivably the result of a paleoseismic event. TL sample (TW7) from the top part of the unit 8 was dated to be 15.32 ± ± 1.18 ka BP. Later, unit 9 was accumulated, TL sample (TW8) from its base was dated to be 13.89 ± 1.07 ka BP. It indicates that the second event disclosed in the trench occurred between 15.32 ± 1.18 ka BP and 13.89 ± 1.07 ka BP. Unit 9 has covered the paleosol buried by event 2 and is a chaotic soil and gravel deposit, with more gravel in lower part, and calcium-rich pale deposit in middle part. Unit 9 represents a scarp-derived deposit mainly formed of collu- vial material and was offset by event 3. Soon after the event, the wedge-shaped unit 10 was formed (near faults F1 and F5). TL samples (TW8 and TW1 ) from the base of unit 9 and the top part of unit 10 were dated to be 13.89 ± ± 1.07 ka BP and 10.71 ± 0.82 ka BP, respecti- vely, indicating event 3 occurring between the two age values. Event 4 caused fault F5 to off- set unit 10, as found on both eastern and west- ern walls of the trench, which was covered with unit 11. Age value of TL samples (TW2 and TW3) from the base of unit 11 and the top 1061 Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China part of unit 10 are 8.11 ± 0.62 ka BP and 10.71 ± ± 0.82 ka BP, respectively. Thus, the event occurred between 10.71 ± 0.82 ka BP and 8.11 ± ± 0.62 ka BP. Unit 11 was offset by event 5, as found on the west wall of the trench. Unit 12 was mainly accumulated near fault F5. Age values of TL samples (TW3 and TW4) from the top part of unit 11 and the base of unit 12 indi- cate the event occurring between 7.72 ± 0.59 ka BP and 6.49 ± 0.49 ka BP. Unit 12 was off- 1062 Yongkan Ran, Peizhen Zhang and Lichun Chen Fig. 7. Geological log of large trench (Tc2) in Ulanblang village: 1 - interlayers of conglomerate and mudsto- ne; 2 - sand-gravel with clear layering; 3 - sandy soil with dispersed gravel; 4 - buried soil; 5 - loess or soil lumps; 6 - surface soil; 7 - stratigraphic unit and its number; 8 - fault and its number; 9 - location of TL sample and its number; 10 - location of 14C sample .and its number. Stratigraphic sequence: unit 1 - weathered and semi-wea- thered Jurassic conglomerate with grayish mudstone containing carbonized matters; unit 2 - lacustrine gray-yel- low and orange-yellow silt clayey sand with interlayers of coarse sand-small-sized gravel and clayey sand; unit 3 - red clayey sand with gravel lenses; unit 4 - brown-black sandy clay with gravel; unit 5 - gray-yellow sandy clay with dispersed gravel; unit 6 - gray-yellow sandy soil with massive structure; unit 7 - gray-yellowish calcium-rich sandy soil; unit 8 - gray-yellowish sandy soil with gravel in lower part and upward becomes dark- brown sandy soil (pedogenic); unit 9 - yellow sandy soil with gravel; unit 10 - yellow sandy soil; unit 11 - sandy soil with gravel; unit 12 - brown-yellow sandy soil with dispersed gravel, brown-black and black southward; unit 13 - thin black soil; unit 14 - deposits in ditch (may be man-dug ditch); unit 15 - brown-yellow and brown- black sandy soil with dispersed gravel; unit 16 - layering and well sorted small-sized gravel bed in lower part and brown-black soil rich in humic matter in upper part. set by event 6 and faults F3. .F5 and was cove- red with sediments of unit 13 and later. 14C sam- ple (CW1) from unit 13 was dated to be 4030 ± ± 10 years BP. Part of unit 13 filled the cracks in unit 12, reflecting its accumulation following event 6. It indicates that the event occurred slightly earlier than 4030.±.10 years BP. 3.3. Bakouzi site (Tc1) The Bakouzi trench (Tc1) was excavated at about 1 km east of Bakouzi. Correlative geomor- phic surfaces at this site are clearly different in elevation, though the fault scarp is poorly preser- ved due to recent human activity. At a mountain spur, terraces on a series of gullies flowing toward the basin were offset. Elevation of the highest terrace is more than 10 m above gully beds, the third-order terrace is 5-6 m, and that of the se- cond-order terrace is 3-4 m. The sediments on the second-order terrace were offset by fault at many sites. We analyzed the exposure of the gully east of Bakouzi (fig. 8). The strata exposed on the gully wall can be di- vided into 7 stratigraphic units. Unit 1 is red clay, earlier sediment. Units 2 4 are gully deposits . 14C sample (QC8) from the base of unit 4 was dated to be 10.230 .± .150 years BP (table I) after dendrochronological correction. Units 5. .7 repre- sent scarp-derived deposits mainly formed of col- luvial material. Unit 5 is a wedge-shaped deposit formed in a fissure in front of the scarp and is composed of mixed gravel and sandy soil. 14C sample (QC7) from its top brown-black soil was dated to be 10.990 .±.150 years BP after dendro- Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China Fig. 8. Geological log of the trench (Tc1) excavated on gully wall east of Bakouzi: 1 - gravel; 2 - soil lumps; 3 - stratigraphic unit and its number; 4 - location of 14C sample and its number. Stratigraphic sequence: unit 1 - red clay locally containing gravel; unit 2 - gravel bed intercalating a brown-yellow sandy clay layer; unit 3 - interlayers of small-sized gravel and sandy clay with brown-black soil clumps; unit 4 - gravel and sandy soil; unit 5 - wedge-shaped deposit of mixed gravel and sandy soil; unit 6 - colluvial wedge, more gravel in lower part and brown-black soil with gravel in upper part; unit 7 - brown-black sandy soil intercalating a gravel layer, more cobbles in base part and black soil containing more humic matter between gravels. 1063 chronological correction. Unit 6 is a scarp-derived colluvial wedge, with more gravel in its lower part and brown-black soil with gravel in the upper part. 14C sample (QC6) from its top part was dated to be 10.140.± .190 years BP (without dendrochro- nological correction). Unit 7 is brown-black san- dy soil intercalating a gravel layer and covers the fault. Cobbles are more concentrated at the base of the unit and, meanwhile, black humic matter was more found between the cobbles. 14C sample (QC5) from black soil at the base of the unit was dated to be 6570.±.110 years BP after dendrochro- nological correction. The above-described lithologic characteristics of the stratigraphic units and their structural rela- tions with faults indicate that 3 paleoseismic e- vents can be distinguished in the trench. The ear- liest event offset the deposits up to unit 3, then unit 4 was accumulated in the gully. The second event offset unit 4, resulting in the formation of mixed deposit of colluvial gravel and sandy soil in the lower part (or cleft in filled wedge), i.e. unit 5, and deposit in upper part of the colluvial wedge, i.e. unit 6. The third event offset unit 6 and led unit 4 on footwall of the fault to be in a fault con- tact with unit 6 on the hanging wall. Age value of 14C sample from the base of unit 4 is slightly higher than that from the upper part of unit 5, the colluvial material of which might come from the older strata. Therefore, it is inferred from dating results of the described statigraphic sequence that the first event occurred earlier than 10.990 ± ± 150 years BP, the second event was later than 10.230.±.150 years BP, and the third event was ear- lier than 6570.±.110 years BP. 4. The completeness of paleoseismic activity history The completeness of paleoseismic activity his- tory along a fault is crucial for understanding the sequence of a fault and for a correct estima- tion of future seismic hazard. At present, no tested method for the evaluation of the complete- ness of paleoseismic activity history has been pre- sented. But according to the characteristics of sur- face ruptures produced by normal faults and their evolution, we propose to use methods for «co- nfining displacement amount» and for «correc- tion between events in many trenches» to deter- mine the completeness of paleoseismic activity history along the fault segment. 4.1. Method for confining displacement amount Many studies (Wallace, 1977; Bucknam and Anderson, 1979; Hanks, et al., 1984; McCalpin, 1987; Deng and Liao, 1996) highlighted that in addition to the direct determination of amount of displacement of the same marker horizon on both sides of a fault, the difference in elevation of the correlative geomorphic surface on both sides of a fault is a good indicator reflecting cumulative displacement. Thus, both observations can be used as a measure for cumulative displacement amount along the fault in a given time period. The method for confining displacement is based on the measurement of the cumulative displacement produced by the paleoseismic events recognized in the trench and that cumulated the same site for the same time interval. If the results obtain- ed using both the methods fit, the paleoseis- mic history for this time interval can be consider- ed complete. The difficulty in using this technique is mainly related to the approach to correctly esti- mate the different types of displacement. Detail- ed quantitative studies of elevation difference of geomorphic surfaces produced by normal fault (Wallace, 1977; Deng et al., 1984; Ran et al., 1992, 1996) indicate that in most cases, the elevation dif- ference of geomorphic surfaces obtained in field surveys is equal to (when the geomorphic surface is in a stable environment, the upper and lower primary surfaces are intact) or less than the real cumulative displacement amount along a fault since the formation of geomorphic surfaces (at least only 2/3 of the real displacement; in fact the geomorphic surface on the upthrown side may have been eroded in the long-term whereas on the downthrown side partial filling may have occur- red. Moreover, it should also be considered that except for the coseismic displacement directly ob- tained from offset sediments, the displacement amount obtained by other methods has uncertain- ties, especially when the coseismic displacement is obtained by measuring the double thickness of 1064 Yongkan Ran, Peizhen Zhang and Lichun Chen the colluvial wedge or different height of gully ter- races. Considering these uncertainties, whatever method is used to estimate the cumulative displa- cement of a geomorphic surface, this is expected to be a minimum. Figure 9 presents a graph of time versus slip accumulation. An area with fluctuating value en- closing the range of the measured height of the geomorphic surface is shown. If this fluctuating di- splacement of geomorphic surfaces truly reflects the real displacement amount along the fault, com- paring it to the coseismic displacement can provi- de insights on the completeness of the paleosei- smic record. When the cumulative displacement is approximately equal to or larger than the dis- placement in the fluctuating area, then the paleoseismic activity history can be considered complete (fig. 9 case A and B). On the contrary, if the coseismic cumulative displacement is lower than that in the fluctuating zone, then the paleoseismic record appears to be incomplete (fig. 9 case C). 4.2. Method for correlation between events in many trenches The method for correlation between events in many trenches is another useful method for de- creasing the potential incompleteness of reco- gnized paleoseismic events on the same segment caused by uneven distribution and different pre- servation degree of surface rupture traces at a single site produced by paleoearthquakes. The essence of the method is principally consistent with that of the «progressive confining method» for correlation between paleoearthquake occur- rence times (Mao and Zhang, 1995). The basis for the method is that a single trench site is not enough to define a complete paleoseismic record and, as far as it is possible, it is recommended to investigate more than one site. Even if the ruptu- res produced by some events did not extend to one site, or were removed by erosion, evidence of the missing event can be derived from diffe- rent sites. It is also possible to use many trenches for gathering more information supplementing each other, so that any event which occurred along the segment was not missing. For this method, it is assumed that several trenches were excavated along a fault segment and the paleoseismic events revealed in the tren- ches are different in number, but some events found in different trenches overlap each other in their age values. Thus, by using a progressive confining method we can constrain in age some events having only their upper- or lower-limit age value determined or having their upper- and lower-age limit within their corresponding time intervals and then can better determine the age of these paleoseismic events. 4.3. Completeness of paleoseismic activity history Identification of the completeness of paleo- seismic activity history using the two above- described methods follows the principle that the first is to use the method for confining displa- cement to determine whether the events revea- led in each trenched site are complete and then to use the method for correlation between events in many trenches for supplementing and correc- 1065 Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China Fig. 9. Conceptual scheme for determination of the completeness of paleoseismic activity history from displacement amount. Stepped line with arrow indi- cates an assumed revealed paleoseismic activity hi- story. Line A, extending into the completeness area, shows a complete activity history; line B, into the un- certainty area, remains to be further determined by other methods; line C, into the incompleteness area, indicates a revealed incomplete paleoseismic activity history. ting the paleoseismic activity history along the fault segment. 4.3.1. Test of the completeness of paleoseis- mic activity history revealed in Kuisu trenches Critical in the method for confining displa- cement amount is that the time of the oldest e- vent should be equal to the time when the geo- morphic surface was formed and offset. Study on the regional geomorphic surfaces disclosed that a lacustrine tableland formed 26-23 ka BP is widely developed in Hetao Region (Li et al., 1994; Wu et al., 1995). Then another two or three diluvial tablelands developed. In the bound- ary zone between Shanxi, Hebei and Inner Mon- golia, the Late Quaternary diluvial tablelands were formed in the ranges 3 0 ka, 10 7.5 ka, 13.5-10.3 ka, and 18.7-15.6 ka BP (Xu et al., 1996). Stratigraphic correlation of the deposits on the tablelands along the Hohhot Segment suggests that the geomorphic surfaces at the Kuisu trench sites were formed later than the lacustrine tableland surface, approximately in 18.7 15.6 ka BP. The age of 14C sample from a thin-layer carbonaceous sandy clay at 56 cm depth below the geomorphic surface at the Kuisu site is 19.990 ± 395 years BP, represen- ting basically the formation time of the geo- morphic surface. It corresponds roughly to the time when the oldest event were revealed in the trenches. Therefore, the identification of the completeness of paleoseismic events at the two sites by using the method for confining displa- cement amount is feasible. Six paleoseismic events were revealed in the trenches (Tc3, Tc4) at Kuisu site on the Hoh- hot Segment. The measured geomorphic surfa- ce shows displacement of 5 5.5 m, while the cumulative displacement obtained from collu- vial wedges is 5.1 m in minimum and 9 m in maximum, and is taken to be 7.03 m produced by paleoearthquakes. The cumulative displace- ment produced by the paleoearthquakes is lar- ger than that measured from geomorphic surfa- ce. The paleoseismic activity history revealed at this site by using method for confining displa- cement amount is complete (fig. 10a-c). 1066 Yongkan Ran, Peizhen Zhang and Lichun Chen c a b Fig. 10a-c. Scheme showing the application of the dis- placement confining method for identifing the com- pleteness of paleoseismic activity history at Kuisu site. Stepped line with arrow indicates the paleoseismic acti- vity history: a) shows confining result by using the mini- mum displacement; b) shows confining result by using the maximum displacement; c) shows confining result by using the mean displacement. Figure 10a-c shows that even the minimum cumulative displacement does not fall inthe incompleteness area, and both the mean and maximum cumulative displacement fall in the com- pleteness area. On this basis the paleoseismic events revealed at Kuisu site are considered to be complete. event does not have its confined age value, but it can be deduced by correlation from the up- per-limit age value of the third event and chro- nological information obtained in Kuisu trench Tc3, i.e. the age values of the two earliest events in the two trenches are approximately equal. The difference is that the latest event in Ulanblang trench occurred in little earlier than 4000 years BP. This event was not found in Kuisu trench Tc3 and other trenches. The e- vents revealed in Kuuisu trench Tc4 and in Bakouzi trench Tc1 are fewer in number, but these events and their age values supplemented those in Kuisu trench Tc3 and Ulanblang trench Tc2. In general, the paleoseismic events reveal- Late Quaternary paleoseismicity along the Hohhot Segment of the Daqingshan piedmont fault, North China 4.3.2. Correlation between paleoseismic events along the fault segment It is clear that the above-described three sites on the fault segment are different in tecto- nic positions and geomorphic surfaces where the trenches were excavated, thus, the number of paleoseismic events revealed in the trenches and age values of dated samples fluctuate (fig. 11). It can be found in fig. 11 that 6 paleoseismic events revealed in Kuisu trench Tc3 (Kiusu 2 in fig. 11) yielded well confined their age values. 6 paleoseismic events were revealed in Ulan- blang trench Tc2, but the second from the last Fig. 11. Correlation between paleoseismic events at different sites on the Hohhot Segment of the Daqingshan piedmont fault. 1067 ed in 4 trenches and their age values are confi- ned within 7 time windows. It indicates that in all 7 paleoseismic events occurred on the Hohhot Segment since about 19 ka BP, i.e. they occurred in 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 7.41 ± 0.315 ka, and 4.50 ± 0.23 ka BP, respectively. This would suggest an average recurrence interval of 2375 ± 432 years. It is clear that by using correlation between events found in many trenches, the events and their age values revealed in 4 trenches can be corrected and supplemented to each other, so we can eventually determine 7 paleoseismic ev- ents which represent the seismic activity history along the Hohhot Fault Segment since about 19 ka BP. Moreover, the latest event may be mis- sing at Kuisu site, as the site is located near the boundary of the studied fault segment. Paleo- seismic traces in the Bakouzi trench were remo- ved by erosion from gully, so the loss of seismic events is expected. 5. Conclusions Excavation of trenches at Kuisu, Ulanblang, and Bakouzi sites on the Hohhot Segment of the Daqingshan piedmont fault and detailed study of geomorphic surfaces permitted us to identify the completeness of paleoseismic activity history at single sites and on the fault segment using methods for confining displacement amount and method for correlation between events in many trenches and provide an improved dataset. Two trenches at Kuisu site have mutually co- nfirmed the revealed 6 seismic events since 19 ka BP. The cumulative displacement amount produced by these events is around 7 m and that obtained from difference in height of geomor- phic surfaces is 5. .5.5 m. The result of the test using method for confining displacement a- mount shows that the paleoseismic events re- vealed at this site is complete. The Ulanblang and Bakouzi trenches revea- led 6 and 3 events, respectively. The second e- vent from last in Ulanblang trench has no age value limit, so its age can be determined from the upper-limit age value of the third event and chronological information obtained in Kuisu trench. The latest event in Ulanblang trench oc- curred a little earlier than 4000 years BP, but this event was not found in Kuisu and other trenches. Events revealed in Bakouzi trench are fewer in number, but the events and their age values in this trench well supplement those in Kuisu and Ulanblang trenches. The integration of data from different trenches at 3 sites shows that a total of 7 paleoseismic events occurred on the Hohhot Segment since 19 ka BP, i.e. they occurred in 18.75 ± 0.75 ka, 16.97 ± 0.96 ka, 14.65 ± 0.67 ka, 11.82 ± 0.69 ka, 9.45 ± 0.26 ka, 7.41 ± 0.315 ka, and 4.50 ± 0.23 ka BP, re- spectively, and the average recurrence interval is 2375 ± 432 years. 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