Late Quaternary left-lateral slip rate of the Haiyuan fault, northeastern margin of the Tibetan Plateau

TECTONICS, VOL. 28,, doi:10.1029/2008tc002302, 2009 Late Quaternary left-lateral slip rate of the Haiyuan fault, northeastern margin of the Tibetan Plateau Chuanyou Li, 1 Pei-zhen Zhang, 1 Jinhui Yin, 1 and Wei Min 1 Received 10 April 2008; revised 14 January 2009; accepted 9 July 2009; published 3 October 2009. [1] Slip rates of major active strike-slip faults within and around the Tibetan Plateau provide constraints for understanding the dynamics of continental deformation in general because large slip rates can be taken to imply localized deformation between rigid blocks and low slip rates on faults are more consistent with distributed deformation. Several major strike-slip faults have been studied during the last 3 decades. As one of important active strike-slip faults in and around the Tibetan Plateau, the slip rate of the Haiyuan fault has been discrepantly estimated to be high by proponents of escape tectonics or low by others and GPS observations. To better constrain its slip rate and then to better understand the mechanics of intracontinental deformation, we try to more carefully consider offset geomorphic features and age constraints along the Haiyuan fault. In this paper, we select three sites where both upper and lower bounds of slip rate can be obtained. We find that slip rates are 4.2 ± 0.8 mm/yr at the Shaomayin, 4.5 ± 0.7 mm/yr at Gaowanzi, and 5.0 ± 2.5 mm/yr at Huangliangtan site. Combinations of these rates with those published previously yield an average slip rate of 4.5 ± 1.0 mm/yr on the Haiyuan fault. This rate agrees with the present-day slip rate of 4.3 ± 1.5 mm/yr measured by GPS, 4.2 8 mm/yr by interferometric synthetic aperture radar, and 5 mm/yr through paleoseismological studies. The temporal consistency suggests a steady state process of strain accumulation and release along the Haiyuan fault. The low slip rate suggests that the Haiyuan fault, similar to the Altyn Tagh fault, does not transfer a significant portion of the convergence between India and Asia out of India s path into Eurasia but merely redistributes crustal thickening. Citation: Li, C., P.-z. Zhang, J. Yin, and W. Min (2009), Late Quaternary left-lateral slip rate of the Haiyuan fault, northeastern margin of the Tibetan Plateau, Tectonics, 28,, doi:10.1029/2008tc002302. 1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China. Copyright 2009 by the American Geophysical Union. 0278-7407/09/2008TC002302 1. Introduction [2] In their pioneering work on late Cenozoic tectonic deformation of the Tibetan Plateau, Molnar and Tapponnier [1975] postulated that more than 2000 km convergence between the two colliding plates has been accommodated by a combination of crustal thickening and lateral extrusion of crustal blocks bounded by large-scale strike-slip faults. Slip rates of major active strike-slip faults within and around the plateau provide important constraints on the dynamics of continental deformation, because large slip rates can imply localized deformation between rigid blocks [e.g., Tapponnier et al., 1982, 2001; Avouac and Tapponnier, 1993], whereas low slip rates on faults are more consistent with distributed deformation [England and McKenzie, 1982; England and Houseman, 1986; England and Molnar, 1997]. [3] Several major strike-slip faults have been studied during the last three decades (Figure 1). Based on the assumption that ages of offset features are 10 ka [Peltzer et al., 1989], and more recently from cosmogenic dating of offset Quaternary landforms, slip rates averaged over several thousand years on the Altyn Tagh fault have been reported to be in the range of 20 30 mm/yr [Peltzer and Saucier, 1996; Van der Woerd et al., 2002; Mériaux et al., 2004, 2005; Molnar et al., 1987a, 1987b]. Through 10 Be and 26 Al cosmogenic nuclide dating of offset moraines, Lasserre et al. [2002] reported 19 ± 5 mm/yr late Pleistocene slip rate on the Leng Long Ling fault (Figure 1) in the middle of Qilianshan Mountains east of the Altyn Tagh fault. Estimated slip rates on the Maomaoshan and Laohu Shan faults (Figure 1), a major strike-slip fault system immediately west of the Haiyuan fault, have been estimated to be either 11.6 ± 1.1 mm/yr [Gaudemer et al., 1995; Lasserre et al., 1999, 2001] or 4 5 mm/yr [Liu and Zhou, 1986; Yuan et al.,1998]. [4] Slip rates estimated by recent GPS surveys are lower than at least some of those reported for most of the strikeslip faults in northeastern Tibetan Plateau. For example, the Altyn Tagh fault has a slip rate of 10 ± 2 mm/yr [Bendick et al., 2000; Shen et al., 2001; Wallace et al., 2004; Zhang et al., 2004, 2007]. [5] As one of important active strike-slip faults in and around the Tibetan Plateau, slip rate of the Haiyuan fault has been estimated to be 8 ± 2 mm/yr 20 years ago by Zhang et al. [1988a] by measuring Holocene offsets of six stream valleys and dating their 14 C ages. Subsequently, from the offsets and 14 C ages of the alluvial terraces and risers, Lasserre et al. [1999] derived a most likely minimum postglacial slip rate 11.6 ± 1.1 mm/yr on the Maomaoshan 1of26

Figure 1. Map showing major active faults in northeastern part of the Tibetan Plateau. Dotted box in inset shows the region in Figure 2. 2of26

Figure 2. Geometric pattern of the Haiyuan fault overlain on Landsat imagery. White lines are strands of the active Haiyuan fault that are marked from F1 to F9. Three sites are also marked on the map. 3of26

Figure 3. Aerial photo showing geomorphology and drainage system of the Shaomayin site. Shaomayin Creek is a major stream in the region. Minor streams labeled by A, B, and C are tributaries of the Shaomayin Creek. T1, T2, and T3 mark terraces of Shaomayin Creek. View is to the southwest. 4of26

Figure 4. (a) Photos showing terraces of the Shaomayin Creek and the minor streams B and the offsets of the minor streams; view is to the southwest. (b) The terrace, outlined by box in Figure 4a, inside the minor stream B; view is to the west. fault immediately west of the Haiyuan fault, while Liu and Zhou [1986] reported 5 mm/yr slip rate on the same fault, from measurement of the offsets of many later late Pleistocene and Holocene creeks and 14 C ages of corresponding alluvial terraces. [6] There were two problems in our previous study [Zhang et al., 1988a]. First, the amount of displacement was estimated using tape measure, which was somewhat subjective and difficult to reproduce. Second, the radiocarbon sample, collected 20 years ago, from the most reliable site near Shaomayin was taken from the face of a terrace riser. Therefore, it might have been subjected to subsequent contamination by younger material sliding down the face of the riser. Such contamination would, in turn, result in a young age for the onset of the displacement. In this paper, we select three sites where both upper and lower bounds of slip rate can be obtained, so that slip rate along the Haiyuan fault can be well constrained. 2. Tectonic Setting and Geometric Pattern of the Haiyuan Fault [7] The Haiyuan fault is a major active tectonic feature in the northeastern margin of Tibetan Plateau that connects the seismically active Qilian Shan in the west to the tectonically active Liupan Shan, which abuts against the relatively stable Ordos block (Figure 1) [Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1977; Deng et al., 1984, 1986; Burchfiel et al., 1991; Zhang et al., 1991a; Institute of Geology, State Seismological Bureau, 1991], in the east. Deformation along the Haiyuan fault included northeastward compression in early Quaternary time that resulted in pre-silurian metasedimentary rocks to be thrust onto Quaternary conglomerate and formation of a series of northeast to east trending reverse faults and folds. Thrusting was followed by left-lateral strike-slip motion that has offset a series of geological units, ridges, and streams. The measured total Quaternary left-lateral displacement is between 12 to 14.5 km, shown by matching pre-silurian marbles, amphibolite, and Tertiary basal conglomerate on opposite of the fault [Burchfiel et al., 1991]. The left-lateral displacement along the fault has been accommodated by crustal shortening of similar amount at the eastern end of the Haiyuan fault zone in the Liupan Shan area [Zhang et al., 1991a]. [8] The Haiyuan fault zone consists of nine subparallel fault strands separated by pull-apart basins of different sizes [Deng et al., 1986] labeled in Figure 2 from F 1 in the west to F 9 in the east. The average strike of the Haiyuan fault is about N75 80 W. Individual fault strands mostly trend N60 65 W and intersect the average strike of fault zone at about 15,exceptforF 1. The Haiyuan fault can be divided into three segments according to fault geometry, geomorphology, and paleoseismology [Zhang et al., 2003]. [9] The western segment of the Haiyuan fault is composed of fault strands F 1,F 2,F 3,F 4, and F 5 (Figure 2). Deformation along each of the strands is dominated by leftlateral strike slip, and as much as 4 m average left slip during the 1920 Haiyuan earthquake has been observed [Deng et al., 1984, 1986; Zhang et al., 1987]. No matter what direction the fault strand dips, reverse slip characterizes the vertical component (Figure 2). The length of each of these strands is too short to be a potential seismic source to generate large earthquakes of magnitude 6 and over [Wells and Coppersmith, 1994]. These strands are separated from each other by four pull-apart basins (Figure 2). These pull-apart basins are usually less than 2 km wide (perpendicular to the fault segments) and are not large enough to 5of26

Figure 5. (top) A SW looking photo showing of a cross section of terrace T2 of the Shaomayin Creek. (bottom) An interpretation of the stratigraphy in the cross section with the location of the sample shown: 1, grayish yellow to grayish black silt layers, with lots of roots of dry land plants; 2, yellowish white gravel layers, the pebbles are mostly about 0.2 0.5 cm in diameter; 3, grayish yellow sandy silt, containing a few scattered gravels; 4, grayish white gravel layers, the pebbles are mostly about 1 3 cm in diameter; 5, grayish yellow sandy silt, silty clay; 6, light grayish green gravel layers, the pebbles are mostly about 1 cm in diameter; 7, massive homogeneous reworked loess; 8, light grayish green gravel layers, interbedded with thin sandy silt lenses. 6of26

Figure 6 7of26

Table 1. Radiocarbon Analytical Data a Sample Site d 13 C(%) 14 C Ages (years B.P. ± 1s) Calendar Years (Cal years B.P. ± 1s) Terrace Material SM-2 Shaomayin 25 11560 ± 180 13480 ± 310 Upper part of T2 charcoal SM-4 Shaomayin 25 15170 ± 190 18140 ± 350 Lower part of T2 charcoal Shaomy-2 Shaomayin 25 8930 ± 90 10150 ± 160 T1 charcoal GW-3 Gaowanzi 25 3210 ± 65 3430 ± 60 Upper part of T2 peat GW-4 Gaowanzi 25 11470 ± 130 13440 ± 300 Upper part of T3 charcoal HLT-3 Huangliangtan 25 2990 ± 70 3200 ± 125 Young terrace peat HLT-4 Huangliangtan 25 7150 ± 150 7960 ± 180 Upper part of alluvial fan charcoal a The 13 C values are the assumed values ( 25%) according to Stuiver and Polach [1977]. The quoted age is in radiocarbon years using the Libby half-life of 5568 years and following the conventions of Stuiver and Polach [1977]. Analytical uncertainties are reported at 1s. The calendar dates were calculated by using CALIB 5.0.1 [Stuiver and Reimer, 1993; M. Stuiver et al., CALIB radiocarbon calibration 5.0, program and documentation, 2005, available at http://calib.qub.ac.uk/calib/]. form a structural discontinuity that can significantly impede a propagating earthquake rupture [Zhang et al., 1991b, 1999]. The Jingtai pull-apart basin is 5 km wide at the western end (Figure 2). The pull-apart basin at the eastern end of this segment, in the Salt Lake basin (between F 5 and F 6 ), ruptured during the 1920 earthquake. The Salt Lake basin is 8 km wide, the largest basin within the entire Haiyuan fault zone. [10] The middle segment of the Haiyuan fault zone (F 6 ) follows the northern range front of the Xihua Shan and Nanhua Shan east of the Salt Lake basin (Figure 2). The fault dips southwest where pre-silurian rocks of Xihua Shan and Nanhua Shan have been thrust onto Quaternary sediment in the Haiyuan basin [Burchfiel et al., 1991; Institute of Geology, State Seismological Bureau, 1991]. The surface ruptures of the 1920 earthquake are commonly defined by small scarps and grabens. A maximum left-lateral displacement of 10 m occurred in the middle part of this segment in 1920 [Deng et al., 1986; Zhang et al., 1987; Institute of Geology, State Seismological Bureau, 1991]. The western end of the middle segment connects to the western segment through the Salt Lake pull-apart basin. The southeastern limit to the middle segment occurs at Luanduizi, about 15 km east of Haiyuan (Figure 2). Here, another fault strand steps to the left to form a compressional step over; some low fault scarps (less than 1 m high) and minor slip are observed across this step over (Figure 2). [11] The eastern segment consists of fault strands F 7,F 8, and F 9 (Figure 2). These three fault strands form a zone trending N35 W, which differs by about 20 from the overall trend of the Haiyuan fault (Figure 2). This segment can be followed more than 40 km to the southeast. Although this segment shows a significant component of left-lateral strike slip, its oblique orientation with respect to the main fault zone requires a large compressional component to absorb at least part of crustal shortening due to left slip along the Haiyuan fault farther west [Burchfiel et al., 1991; Zhang et al., 1991a]. Compressional ridges and reverse fault scarps formed during the 1920 earthquake rupture attest to the crustal shortening [Deng et al., 1986; Burchfiel et al., 1991]. [12] East of the eastern segment, there is a region characterized by anticlinal folding that is attributed to most of late Cenozoic left-lateral displacement along the Haiyuan fault [Zhang et al., 1991a]. Although most of displacement of the 1920 Haiyuan earthquake has been accommodated by the eastern segment, surface ruptures of the middle segment did not end at the step over area, but propagated eastward for another 10 km [Deng et al., 1986; Institute of Geology, State Seismological Bureau, 1991]. This suggests that the area east of the eastern segment may still accommodate left slip along the Haiyuan fault. 3. Method and Logic of Estimating Fault Slip Rates [13] Terrace risers are erosional escarpments that separate two stream terraces of different age and elevation. Offset terrace risers have commonly been used to determine longterm slip rates along a strike-slip faults through dating the age of abundant of terrace tread either above or below the riser [Mériaux et al., 2004, 2005; Van der Woerd et al., 1998, 2002; Lasserre et al., 1999]. However, controversies surround the discussion of which terrace ages most accurately dates initiation of riser displacement [Cowgill, 2007]. Large uncertainties thus can arise from interpretations of the timing at which the riser began to accumulate displacement. The best example highlighting this problem is the Cherchen site on the western section of the Altyn Tagh fault, where Figure 6. (top) Photo and (bottom) cross section of terrace deposits of minor stream B. Sample locations are also shown. The length of hammer for scale is 70 cm. Legend: 1, grayish yellow to light green gravel layers; the larger cobbles are about 10 20 cm in diameter, filled with sandy silt, and covered by 5 10 cm thick loess layer; 2, grayish yellow loess-like sandy silt, silty clay, with gravel lenses; the pebbles are mostly about 1 2 cm in diameter; 3, light gray, grayish green gravel layers; 4, grayish yellow reworked loess interlaced with thin sand and silt lenses. 8of26

Figure 7. Topographic map of the minor stream B, (a) made using a total station, and (b) the possible restoration before the offset. The yellow, blue, and orange lines in the lower panel show likely places of the eastern bank, center of channel, and the western bank before the offset. Mériaux et al. [2004] reported a rate of 26.9 ± 6.9 mm/yr by using age of lower terrace as proxy for the initiation of offset. By contrast, Cowgill [2007] and Zhang et al. [2007] independently estimated slip rates of 9.4 ± 2.3 and 10.0 ± 2.4 mm/yr by assuming that the age of upper terrace more accurately dates the beginning of the riser offset. [14] It has been suggested that erosion of the riser occurs continuously during fluvial occupation of the adjacent terrace tread [Van der Woerd et al., 2002], and thus, displacement does not accumulate until abandonment of the adjacent lower terrace. This interpretation assumes that all riser displacement is removed by lateral erosion and scarp refreshment as long as the lower tread is active. Thus, riser displacement is presumed to accrue only after the lower terrace surface has been abandoned [Cowgill, 2007]. If this assumption is in error, then the calculated rate will be too fast as in the case considered by Mériaux et al. [2004]. This condition applies if erosion is the only agent in the river and where slip on the fault displaces terraces into the path of the river erosion [Kirby et al., 2007]. [15] Zhang et al. [2007] proposed another scenario, which is similar to the upper terrace reconstruction model by Cowgill [2007], to estimate the onset of accumulation of terrace risers. Suppose a stream has just incised into its floodplain, and the remaining part of that floodplain has been abandoned to form the terrace. Subsequent left-lateral slip on the fault displaces the downstream terrace on the left side so that it lies in the path of the river, which in turn subjects the downstream riser to erosion. The downstream terrace on the right side and its riser, however, may become protected by topography on the upstream side of the fault. Because lateral erosion is unlikely to completely refresh 9of26

Figure 7. (continued) Figure 8. Simplified geological map of the Gaowanzi site, with the Huangjiawa Shan to the north. Location of the Gaowanzi site is also shown in Figure 2. 10 of 26

Figure 9. Aerial photo shows stream offsets along the Haiyuan fault at the Gaowanzi. Black arrows indicate location of the fault. terraces on both sides of the downstream especially the shielded side of the stream, the age of upper terrace abandon appears to be closer to the initiation of displacement of the terrace riser [Cowgill, 2007; Kirby et al., 2007; Zhang et al., 2007]. [16] In an ideal situation, the ages of both upper and lower terraces would be sufficiently similar that they would place nearly equal upper and lower bounds on the slip rate, as Kirby et al. [2007] found for part of the Kunlun fault. In any case, the association of initiation of riser offset with the age of upper and lower terrace abandonment yields lower and upper bound on the slip rate along the fault, respectively. This way will give more reliable estimate of slip rate on a strike-slip fault. Sieh and Jahns [1984] used this method to obtain the slip rate on the San Andreas fault, and this rate has been replicated by various methods. We identified sites along the Haiyuan fault where the stream riser offset can be measured, and ages of both upper and Figure 10. Photo shows left-lateral offset and terraces along the stream G at Gaowanzi site. View is to the northwest. 11 of 26

Figure 11. (a) Photo showing left-lateral offset of the stream G; view is to the southwest. (b) and (c) Cross sections of terraces T3 and T2, respectively, with sample locations shown. In Figure 11b, 1, coarse sand and gravel layers interbedded with fine-grained sand, silt to clay layers, the pebbles are sharp angular and covered by calcic film on the surface; 2, reworked loess with a few fine-grained gravel layers; 3, grayish green coarse sand and a little rounded fine-grained gravel layers; 4, reworked loess with some calcispheres; 5, mixed sand and gravel, filled with silty clay and sandy silt; 6, yellow silty clay with green sand lump layers. In Figure 11c, 1, reworked loess with a few scattered gravels; 2, grayish green fine-grained gravel layers filled with sandy silt; 3, dark brown to black coarse sand and fine-grained gravel layers, with brown sandy silt lenses; 4, grayish green coarse-grained, poorly rounded gravel interbedded with thin grayish yellow sedulous clay layers. lower terraces can be determined, so that we can place both upper and lower bounds on the rate, and where possible constrain the slip rate within a narrow range. 4. Holocene Slip Rate Determinations 4.1. Shaomayin Site 4.1.1. Observations and Results [17] Shaomayin is located on the northern flank of the Xihua Shan, where the Haiyuan fault bounds its northern pediment (Figure 2). The fault strikes N70 W and steeply dips southwest. The fault zone is about 100 m wide and consists of cataclastite, breccia, gouge, and mylonitic rock. Pre-Cenozoic bedrock crops out southwest of the fault to form the Xihua Shan, on which the gray-green Precambrian schist is extensively exposed. These Precambrian rocks were thrust northward onto Tertiary red beds north of the fault [Burchfiel et al., 1991]. Zhang et al. [1988a] obtained strike slip rate at this site to be 8 ± 2 mm/yr. As mentioned above, the 14 C sample was collected from the face of a terrace riser, and possible contamination may have resulted in a large error in the age of the offset. 12 of 26

Figure 12. Topographic map of the stream G at the Gaowanzi site surveyed with a total station and estimates of displacements. (a) A topographic map of the stream G. (b) A restoration of terrace T3 on the eastern bank of the channel. (c) A restoration of terrace riser T3/T2 on western side of the stream channel. 13 of 26

Figure 12. [18] The Shaomayin Creek is the primary drainage system at Shaomayin site (Figure 3). The upstream channel extends several kilometers southward into the Xihua Shan with a sizable drainage basin. It flows westward along the fault for 800 m and then turns to southward flowing away from the fault (Figure 3). The downstream channel can also be followed for several kilometers before it merges in the Yuan River, a major river in the Haiyuan region. Three terraces developed within the channel of the Shaomayin Creek (Figures 3 and 4). The highest terrace T 3 is a strath terrace about 25 m above the creek bed. The basement of terrace T 3 is Precambrian rock and is overlain by alluvial deposits and reworked loess up to 15 m thick. Terrace T 2 is well developed along the Shaomayin Creek, especially in the westward flowing section along the fault (Figure 3). The surface of terrace T 2 is flat and covered by loess. Although it is extensively farmed, the surface of terrace T 2 still maintains its original shape. Terrace T 1 can be found on both sides of the creek, although it is only 4 m above the creek bed (Figures 3 and 4). [19] Three minor streams (labeled A, B, and C in Figure 3) have cut through terrace T 2 of the Shaomayin Creek in its westward flowing section (Figure 3). They have been clearly displaced by left-lateral fault movement where they cross the Haiyuan fault. The ages of minor streams are predicted to be younger than the abandonment age of terrace (continued) T 2 because they have deeply incised into it (Figure 4). If the age of terrace T 2 can be determined, it would give a maximum age for the displacement of the minor streams. These minor streams have a roughly V-shaped morphology but with a narrow terrace (2 3 m wide) in one of them (minor stream B). The age of the minor stream terrace postdates the initiation of displacement and therefore provides a minimum age of the minor stream offset. If the ages of terrace T 2 of Shaomayin Creek and terrace of the minor stream can be determined, we would obtain maximum and minimum ages for offset of the minor stream, and therefore place lower and upper bounds of the slip rate of the Haiyuan fault. Because minor streams A and C do not have terraces within their channels, we chose stream B for detailed study. [20] Terrace T 2 of the Shaomayin Creek is about 10 m above the channel bed, and consists of interbedded gravel and reworked loess layers (Figure 5). The bottom of the section (unit 8 in Figure 6) consists of 4 m of thick gravel. The pebbles in the gravel are well rounded and sorted. Several sandy and silty interlayers are present within the gravel. Immediately above the gravel is 3 m of reworked loess (unit 7). The reworked loess is coarse grained with some sand and silt lenses indicating a fluvial origin of the deposit. Three thin gravel layers that are interbedded with silt layers and reworked loess characterize the upper part of terrace T 2 (units 1 6) (Figure 5). 14 of 26

Figure 12. (continued) [21] We collected organic material from a silt layer 1.1 m below the surface of terrace T2. To avoid recent radiocarbon contamination, we dug about a half meter into the surface of terrace riser to collect the sample. The calendar age of the sample is determined to be 13,480 ± 310 years B.P. (Table 1). All the ages quoted hereafter are calendar ages (see Table 1). This suggests that terrace T2 was probably abandoned after this age, and the onset of minor stream incision must be later than this age. [22] Minor stream B has a well developed strath terrace along its downstream channel (Figure 6). The terrace itself is 2 to 3 m above the channel bed. The lower part of terrace is composed of coarse grained reworked loess (unit 4 of Figure 6), with some sand and silt lenses. This reworked loess is interpreted to be at the same stratigraphic level as the reworked loess (unit 7 of terrace T2 of Figure 5) of terrace T2 of the Shaomayin Creek. Thus, we think that the lower part of the minor stream terrace is a strath that incised into terrace T2 of Shaomayin Creek. There is also an erosional surface on top of the reworked loess (between units 4 and 3 in Figure 6), which further suggests the terrace is a strath terrace. The unsorted and coarse grained gravel (unit 3) and reworked loess with gravel blocks within it are newly formed terrace deposits above the strath. We collected two 14 C samples (Figure 6). One consists of pieces of charcoal located in upper part of the reworked loess of the strath. Its age is dated to be 18,140 ± 350 years B.P. which is consistent with the inference that it is part of terrace T2 of the Shaomayin Creek. The other sample was obtained within upper reworked loess layer (unit 2) 5 cm above gravel layer (unit 3) and gave an age of 10,150 ± 160 years B.P. of early terrace deposition associated with the minor stream B development. This age is younger than the age of upper part of terrace T2 of the Shaomayin Creek and is further support its strath origin of the minor stream B. [23] Studies of the terrace deposition allow us to understand the evolution of minor stream B. A broad and flat floodplain had been incised and abandoned to leave terrace T2 near the time after 13,480 ± 310 years B.P. along the Shaomayin Creek. Minor streams initiated only after the 15 of 26

Figure 13. (a) Simplified geologic map of the Huangliangtan site region. (b) A photo showing how the northwest trending Haiyuan fault bounds the southeastern side of the Huangliangtan basin. View is to the east-northeast. 16 of 26

Figure 14 17 of 26

abandonment of terrace T2 because they incised into the terrace tread. Lateral erosion formed a strath along the channel of the minor stream B. Subsequent aggradation caused deposition on top of the strath. This aggradation process took place near about 10,150 ± 160 years B.P. After the aggradation, incision occurred again to form the terrace in the channel of the minor stream B. The offset of stream B could have started to accumulate only after the abandonment of terrace T2 and surely was underway before formation of the terrace along stream B. [24] To determine the displacement, we surveyed a topographic map of the minor stream B using a total station (Figure 7). The amount of the offset can be measured by restoring the likely position of the stream before the offset (Figure 7). We use three markers to measure the displacement. First, both upstream and downstream channels are similar in shape and width as shown by Figure 7. The displacement can be measured by matching the centers of upstream and downstream channels which gives about 57 m displacement. Second, the western wall of minor stream B can also be used to estimate the displacement although there is significant erosion nearby the fault trace. By restoring the upper and downstream channel walls across the fault to its possible preoffset position, we obtain about 58 m left-lateral displacement. Third, the eastern wall of stream can also be used as a displaced marker. The eastern downstream channel wall may have subjected to more erosion than its western counterpart due to its movement into the stream course. But the straightness of it suggests that the lateral erosion may not be significant. Matching of the eastern wall yields about 56 m displacement (Figure 7). Combining the three offset markers and considering the lateral erosion of the stream, we estimate the average displacement to be 57 m and add 1 m (from the averaged amount of offsets of the three markers) as uncertainty. 4.1.2. Inferences of Fault Slip Rates [25] The measured offset contains coseismic displacement of the 1920 Haiyuan earthquake which elapses only 88 year to present. Paleoseismological studies on the Haiyuan fault reveal that the average recurrence intervals between major earthquakes are in the range of 1000 to 2000 years [Zhang et al., 1988b, 2003; Ran et al., 1997]. Liu et al. [2007] report about 1000 year recurrence interval along the section of the Haiyuan fault, west of our studied area. Thus, for 1000 2000 year recurrence and 88 year elapse time, the total slip will be strongly biased by inclusion of the most recent event. To remove earthquake cycle effects on the slip rate estimates, we subtract the 1920 coseismic displacement from the measured offset at the site [Zhang et al., 1988a]. The coseismic displacement near this site is about 8 m [Zhang et al., 1987], and thus, the pre-1920 displacement should be about 49 m and add 2 m as uncertainty (1 m from the averaged amount of offsets of the three markers and 1 m from the coseismic displacement of 1920 earthquake). The upper bound of slip rate at the Shaomayin site can be determined by dividing the maximum amount of displacement by its minimum age of initiation. Since we have determined the pre-1920 displacement to be 49 ± 2 m, the upper bound of pre-1920 displacement would be 51 m. The age of terrace of the minor stream B gives the minimum age for the displacement of the stream, which is 10,150 ± 160 years B.P. Thus, we obtain the upper bound of slip rate to be 5.0 mm/yr. The minimum amount and the maximum onset age of the displacement yield lower bound of the slip rate. The minimum pre-1920 displacement is measured to be about 47 m. As we have discussed above, displacement on the minor stream B started to accumulate only after the abandonment of terrace T2 of the Shaomayin Creek. This places the maximum age for the displacement of the minor stream B. For the 13,480 ± 310 years B.P. age of terrace T2, the lower bound of slip rate would be about 3.4 mm/yr. Thus, the Holocene slip rate at the Shaomayin site can be constrained to be larger than 3.4 and <5.0 mm/yr, or 4.2 ± 0.8 mm/yr if one prefers. [26] Considering that the rationale for removing slip related to the 1920 Haiyan earthquake coseismic offset is still debating when calculating the long-term slip rates, we also calculate slip rate by including the slip related to the 1920 earthquake. For measured offset at Shaomayin, 57 ± 1 m, the resultant Holocene slip rate would be larger than 4.15 and <5.7 mm/yr. 4.2. Gaowanzi Site 4.2.1. Observations and Results [27] The Gaowanzi site is located in the south slope of the Huangjiawa Shan, near the east end of the western segment of the Haiyuan fault (Figure 2). The fault strikes N70 W and displays a simple linear strand at this site. To the east, the fault splays into several branches adjacent to the Salt Lake basin (Figure 8), and the active fault bounds the southwestern edge of the basin [Zhang et al., 1987; Burchfiel et al., 1991]. To the west, the fault trace veers to N50 W orientation and steps to another strand (Figure 8). [28] Along most of this segment, Precambrian grayish green schist and slate of the Huangjiawa Shan were thrust southward onto the Quaternary loess, sands and conglomerates near Gaowanzi, or onto the Jurassic red sandstones west of this site (Figure 8). The southeastern wall of the fault also consists of Precambrian and Jurassic rock, but extensively covered by alluvial and aeolian sediment of various thicknesses that range from several meters to less Figure 14. (a) Photo showing stream offset of the stream H at the Huangliangtan site. White rhombuses show sample locations. View is to the northwest-north. (b) and (c) Cross sections of terraces T3 and T2, respectively, with sample locations shown. Figure 14b, 1, gray to grayish green angular gravel layers; 2, grayish yellow reworked loess, sand silt; 3, grayish green sharp angular gravel layers, interbedded with thin yellow reworked loess, fine sand, and fine-grained gravel layers. In Figure 14c, 1, dark red, light gray gravel layers; 2, yellow sand silt, reworked loess, interbedded with thin finegrained gravel layers; 3, grayish green sharp angular gravel layers. 18 of 26

Figure 15. Geomorphic map of the stream H at the Huangliangtan site. (a) A topographic map of the stream H, surveyed using a total station. (b) The restoration of stream H before the offset. 19 of 26

than one meter. The fault zone itself is marked by dark gray fault gouge and fault breccia. Although the bedrock geology shows southeastward thrusting, the fault scarps along this segment of the Haiyuan fault face northeastward indicating a minor vertical component with northeastern wall dropped down. This is confirmed by trench exposures 300 m to the southeast along the fault [Ran et al., 1997]. [29] A series of streams incised into broad alluvial fans and bedrock ridges have been offset left-laterally across the Haiyuan fault in Gaowanzi (Figure 9). Amounts of strikeslip offsets determined by measuring terrace risers and stream channels range from 20 to 120 m. We selected the stream G to study the slip rate at this site because of its welldeveloped stream terraces and clearly preserved offset features (Figure 10). Surface ruptures associated with the 1920 Haiyuan earthquake are still obvious along this segment of fault, and coseismic displacements near the Gaowanzi site were measured to be 6 to 8 m [Deng et al., 1984, 1986; Zhang et al., 1987; Institute of Geology, State Seismological Bureau, 1990]. [30] The stream G flows from approximate northwest to southeast (Figure 9). Offsets of terrace risers are present on both of its sides. As shown in Figure 11, stream G has developed three levels of terraces along its channel. Terrace T3 is the highest along the foot of the south slope of Huangjiawa Shan. The surface of the alluvial fan terrace is generally 5 10 m above the present channel bed. The terrace is Capped by aeolian loess, secondary loess, sand and gravel. The lower portion of the terrace deposition is primarily yellow, calcareous clay with cloddy structures and grayish green sandy lumps (unit 6 in Figure 11b), and the terrace deposition gradually turned upward to be coarse sands and sandy gravels (units 2 5 in Figure 11b). The uppermost level is loess or secondary loess, containing some gravel (unit 1 in Figure 11b). Except for the aeolian loess, the materials originate mainly from erosion of the bedrock of the Huangjiawa Shan and transport of loess by water. A radiocarbon sample collected about 50 cm below the surface yields a calendar age of 13,440 ± 300 years B.P. (Figure 11). [31] Close to the channel of stream G are the terraces T2 and T1 (Figure 10). Terrace T2 is a strath terrace whose tread is about 3 m above the present channel bed. The lower part of the terrace deposits (which seem to overlie a strath) mainly consists of poorly rounded gravel interbedded with thin grayish yellow sedulous clay (unit 4 in Figure 11c), which is found below parts of terrace T3. The upper part of the terrace deposits consists of sand and gravel interbedded with brownish black sedulous clay lenses (unit 2 and 3 in Figure 11c), capped by reworked loess (unit 1 in Figure 11c). The upper part is clearly aggradational sediment above a strath. We obtained a radiocarbon age near the surface of terrace tread of T2 of 3,430 ± 60 years B.P. This age should be close to abandonment age of T2 because it is only about 40 cm below its surface (Figure 11). [32] Ran et al. [1997] excavated three-dimensional trenches on terrace T2 of neighboring stream about 300 m east of the stream G. That stream also has three terraces. They obtained a radiocarbon age of the highest terrace T3 of 13,220 ± 1060 years B.P. [Ran et al., 1997], which is indistinguishable from the age of terrace T3 of the stream G. Their excavation also reveals that T2 is a strath terrace. Approximately 4 m of aggradational sediment lies above the strath of T2. Ran et al. [1997] also collected samples from the bottom of the aggradational sediments and obtained ages of 7070 ± 100 B.P. at the base and 3430 ± 60 years B.P. for the top. [33] We suggest that the stream G and the neighboring stream studied by Ran et al. [1997] shared similar evolutions, because the ages of terrace T3 and of the top of terrace T2 are almost identical. Therefore, we adopt the onset age (7070 ± 100 B.P.) of sedimentary aggradation from the neighboring stream as the inception of deposition on top of the strath terrace T2. [34] Terrace T1 is only about 1 m above the current channel bed (Figure 11). Dark green angular conglomerate composed of different sized cobbles are overlain by gray, darkish sand, and soil underlie terrace T1. We did not obtain samples to date terrace T1, but its age has to be younger than 3430 ± 60 years B.P., the age of sedimentary deposit near top of terrace T2. [35] The stream channel and all terraces on both sides of stream G at the Gaowanzi site are clearly offset left-laterally by the Haiyuan fault as well as by the 1920 Haiyuan earthquake rupture. Figure 12 shows a topographic map of the stream surveyed with a total station (Figure 12a). Amounts of displacements can be determined by restoring features to their respective positions before the offset. On the eastern side of the stream channel, terrace riser T3/T2 is well preserved both upstream and downstream of the fault trace. The terrace risers T3/T2 either upstream or downstream depict a simple and straight geometry that allows for their restoration. We obtained a left-lateral displacement of terrace riser T3/T2 of about 71 m (Figure 12b). Terrace T1 is present along the upstream channel on eastern side of the stream, but is absent along the downstream channel on the same side. Thus, the riser T2/T1 cannot be used for displacement determination. Persistent left-lateral displacement on the stream G moves terraces on western side of the channel into the river course and subjects them to erosion. Only about 37 m left-lateral displacement on the terrace riser T3/T2 on the western side of the stream can be measured, which is 34 m less than the offset of riser T3/T2 on the opposite side of the stream channel (Figure 12c). This difference concurs with the scenario presented by Zhang et al. [2007] in which offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace. 4.2.2. Inferences of Fault Slip Rates [36] A critical step in slip rate determination is to assign the onset time for offset of the risers. For displacement of terrace riser T3/T2 on eastern side of the stream, we argue the age of upper terrace (T3) to be the appropriate date for slip rate calculation. The first reason follows the argument given by Zhang et al. [2007] that the downstream terrace and its riser became protected by topography on the upstream side of the fault, and the age of upper terrace 20 of 26

appears to closer to the initiation of displacement of the terrace riser. Terrace riser T3/T2 on eastern side of the stream G meets this situation. The second reason is that terrace riser T3/T2 offset on eastern side of the stream G is suitable to apply one of Cowgill s [2007] criteria. In an effort to resolve the question of which terrace should be used to define the age of an adjacent terrace riser, Cowgill [2007] suggested six criteria. Cowgill noted that the product of the slip rate and the age of the lower terrace must be less than sum of the offset of the riser above the terrace and the width of the active stream and floodplain. The age of terrace T3, 13,440 ± 300 years B.P., should be regarded as a maximum age of the displacement on T3/T2. In the region near this site, the 1920 Haiyuan earthquake rupture displaced a series of streams and ridges with an average offset of 7 m [Deng et al., 1986; Zhang et al., 1987; Ran et al., 1997]. For a long-term average slip rate, contribution of the Haiyuan earthquake rupture should be removed from the total offset because it occurred only 87 years ago. We subtract 7 m slip from the total offset as upper and lower bounds to calculate slip rate. The resultant lower and upper bounds of slip rate are 4.4 and 5.2 mm/yr, respectively. [37] Lateral erosion reduced the apparent slip of terrace riser T3/T2 on the western side of the stream as it was gradually displaced into the river course. As argued also by Zhang et al. [2007], the onset of the sedimentary aggradation on lower terrace tread could also approximate the age of a riser above it. Since we have obtained a displacement of about 37 m and the age of onset sedimentary aggradations of terrace T2 to be 7070 ± 100 years B.P. [Ran et al., 1997], this allows slip rate calculation using inception time of deposition on lower terrace. Similar to the calculation made on the eastern side of the stream, we also subtract 7 m coseismic offset due to the 1920 Haiyuan earthquake. The lower and upper bound slip rates are 3.9 and 4.6 mm/yr. [38] In summary, we use the lowest bound from western side (3.9 and 4.4 mm/yr) and the uppermost bound (4.6 and 5.2 mm/yr) from the eastern side of the stream to set up a range for the slip rate for the Gaowanzi site to be from 3.9 to 5.2 mm/yr, or 4.5 ± 0.7 mm/yr. [39] Similarly, we also offer an estimated slip rate by including the slip related to the 1920 earthquake as we do at the Shaomayin site. Displacement of terrace riser T3/T2 of about 71 m and the age of terrace T3, 13,440 ± 300 years B.P. yields a slip rates of 5.3 mm/yr. Using displacement of about 37 m and the age of 7070 ± 100 years B.P. of onset sedimentary aggradations of terrace T2, the slip rate can be calculated to be 5.2 mm/yr. 4.3. Huangliangtan Site 4.3.1. Observations and Results [40] The Huangliangtan site is located in the western section of the Haiyuan fault (Figure 2). The Huangliangtan basin, 5 km long and 1.5 km wide, is a pull-apart basin bounded by two subparallel strands of the Haiyuan fault zone (Figure 13). The surface of basin slopes southward, and the fault scarp faces northward and accumulates late Quaternary sediments along its base. North of the basin, the exposed bedrock is chiefly Silurian green slate and phyllite. South of the basin, the basement consists mainly of Carboniferous yellow sandstone, Permian black sandstone, Neogene red beds, and early Pleistocene conglomerate. The basin itself exposes late Pleistocene and Holocene alluvial sand and gravel that form several alluvial fans fed by the streams that flow out of the Hasishan range north of the basin (Figure 13). [41] The northern edge of the basin is bounded by a 9 km long inactive fault segment and was truncated by the active normal fault trace along the southeastern edge of the basin (Figure 13). The active fault strand along eastern edge of the basin strikes east-northeast, obliquely to the regional strike of the Haiyuan fault zone. The main active Haiyuan fault lies along the southern edge of the basin with a strike of N70 W (Figure 13). Features of left-lateral displacement include a series of small ridges, and offsets of streams. The main fault also shows a component of normal faulting as a north facing scarp is present with the southern wall dropped down. The Huangliangtan site is located near the eastern end of this part of the strike-slip strand. The 1920 Haiyuan earthquake ruptured the Huangliangtan site with 4 to 6 m of left-lateral slip and 0.5 to 1.5 m vertical components [Deng et al., 1986]. [42] As shown by Figure 14, a number of stream channels incised into the alluvial fans, which fill the Huangliangtan basin, have been offset left-laterally by the Haiyuan fault. We selected one of the offset streams, the stream H, at the Huangliangtan site to conduct a detailed study. The alluvial fans into which the stream H incised consists of gravels with thin sheets and lenses of sand, silt, and reworked loess (Figure 14b). The pebbles and cobbles within the gravel layers imbricate to the north indicating southwestward flow. The abandonment age of the alluvium at the site gives the maximum age of the stream H. A 14 C age of a sample collected 30 cm below the surface of the alluvial fan (upper part of unit 2 in Figure 14b) gives an age of 7960 ± 180 years B.P. (Figure 14b). [43] The stream H has a simple channel geometry. Both upstream and downstream channels are straight and narrow (10 m) except the area near the fault on the downstream side where a depositional terrace developed (Figure 15). This young terrace stands only 1 to 1.5 m above the current channel bed, and consists of loosely cemented sandy gravel (Figure 14c). The sediment is similar to that exposed on the alluvial fan, suggesting that they are reworked alluvial fan deposits. Formation of the young terrace inside the channel is clearly due to lateral erosion and sideward cutting into the channel wall to widen the downstream channel. Because the stream H does not have a large catchment to maintain enough power to erode material along the entire downstream channel, the young terrace formed only near the fault (Figure 15). The age of this young terrace thus postdates the offset on the stream channel. The radiocarbon age of a sample obtained from the young terrace deposits (floor of unit 2 in Figure 14c) is 3200 ± 125 years B.P. [44] The simple geometry and straight walls of the stream H allow restoration of the channel to its original position prior to the displacement (Figure 15). By ignoring the subsequent lateral erosion near the fault on eastern wall of 21 of 26

Table 2. Displacements, Ages of Offset, and Slip Rate Estimations at Shaomayin, Gaowanzi and Huangliangtan Sites a Site Min D (mm) Max D (mm) Min T (years) Max T (years) Min S (mm/yr) Max S (mm/yr) Error (mm/yr) Shaomayin 46000 51000 10150 13480 3.4 5.0 0.8 Eastern Gaowanzi 60000 68000 13140 13740 4.4 5.2 0.4 Western Gaowanzi 28000 32000 6970 7170 3.9 4.6 0.3 Huangliangtan 20000 24000 3200 7960 2.5 7.5 2.5 a Long-term slip rates are also calculated at each site by removing coseismic displacement associated with the 1920 earthquake. Min D and Max D are minimum and maximum offsets at each site. Min T and Max T are minimum and maximum ages of displacement inception. Min S and Max S are minimum and maximum slip rates at each site. down stream channel, we can measure amount of displacement to be 27 ± 2 m on both channel walls (Figure 15). 4.3.2. Inferences of Fault Slip Rates [45] To calculate long-term slip rate, we need to remove the effect of 1920 Haiyuan earthquake coseismic offset. The coseismic displacements near Huangliangtan site were measured to be about 5 m by Deng et al. [1986]. Thus the displacement prior to the 1920 Haiyuan earthquake was 22 ± 3 m. [46] The initiation of left-lateral strike slip on the stream H occurred after abandonment of the alluvial fan deposit and before deposition of the young terrace. Since there is no additional evidence to further constrain the inception of displacement on the stream H, we can only use the age of alluvial fan to estimate lower bound of slip rate and the age of young terrace to obtain its upper bound. By assuming 5 m coseismic offset, we obtain the upper and lower bounds to be 7.5 and 2.5 mm/yr (or 5.0 ± 2.5 mm/yr). [47] Including the slip related to the 1920 earthquake, the upper and lower bounds of the slip rate at this site are 3.1 and about 9.0 mm/yr, respectively. 5. Discussion 5.1. Average Slip Rate Estimation [48] We calculate slip rates at each site both with and without coseismic displacement associated with the 1920 Haiyuan earthquake. Because of long recurrence interval, short elapsed time, and the over 10 ka offset marker that this study selects, we think that removing such an effect of the 1920 earthquake may be more appropriate to reflect the average long-term slip rate, and then we will adopt this set of slip rates in the following discussion. However, the coseismic effect due to the 1920 earthquake would only change the slip rate at each site by less than 20%, which is within the uncertainty of the slip rate estimate. [49] The combination of all of the lower and upper bounds, that we obtained at the three sites, allows estimation of an average rate along the Haiyuan fault. As shown in Table 2, the lower and upper bounds of slip rates are 3.4 and 5.0 mm/yr at Shaomayin site, 4.4 and 5.2 mm/yr at Gaowanzi site on its eastern channel bank, 3.9 and 4.6 mm/yr at Gaowanzi site on its western bank, and 2.5 and 7.5 mm/yr at Huangliangtan site. The amount of average slip rate should lie between these lower and upper bounds. [50] The Shaomayin site has well-defined displacement. As mentioned above, pre-1920 left-lateral displacement of the minor stream B is measured to be 49 ± 2 m by matching all of the three offset marks of the center, the eastern and the western wall of the stream channel (Figure 7). The initiation of offset on the minor stream B must be younger than the age of terrace T2 of the Shaomayin Creek because the minor stream B incises into T2. The inception of offset on the minor stream B should also be older than the age of terrace within itself. Thus, the rate of slip on the minor stream B can be well constrained within the range from 3.4 to 5.0 mm/yr (or 4.2 ± 0.8 mm/yr). This revised rate is better constrained than the 8 ± 2 mm/yr rate we obtained 20 years ago [Zhang et al., 1988a]. We believe rate at the Shaomayin site may represent average slip rate of the Haiyuan fault. In the following we evaluate if rates from other site are consistent with this preferred average rate. [51] At East Shaomayin 2 site, Zhang et al. [1988a] reported large slip rates of 13.3 ± 4.8 and 16.4 ± 5.9 mm/yr. But they doubted the reliability for two reasons. First, because the stream has been flowing roughly parallel to the fault since before the organic material was deposited, the measured offset might not be representative of the displacement since that age. Second, because the present stream flows at an angle of only 20 to 25 to the fault trace, meanderings of the stream could make the measured offset more uncertain than we had estimated in the field. Thus we will not include this site in our slip rate estimation. [52] Three lower bounds of slip rates reported by Zhang et al. [1988a], 3.5 ± 0.9 mm/yr at Fangjiahe site, 4.1 ± 0.5 mm/yr at East Shaomayin 1 site, and 3.4 ± 0.7 mm/yr at Dagoumen site agree with our new results. [53] The lower bound at Yehupo site, however, deviates from our new results. Zhang et al. [1988a] used a tape measurer to estimate 90 m left-lateral offset of a stream at Yehupo by matching upstream to downstream channels, and dated high terrace within the channel to be 12,280 ± 1000 B.P. as the initial of the displacement accumulation. From this, they estimated slip rate at this site to be at least 6.7 ± 1.0 mm/yr. We doubt the uncertainty associated with the amount of displacement at this site because at Shaomayin site also tape measure result yields 9 m slip difference from the more accurate topographic restoration. In addition, the relationship was not well understood between slip accumulation and terrace development then. We include this data 22 of 26

Figure 16. Average slip rate estimation on the Haiyuan fault. Red squares and black diamonds are upper bounds and lower bounds of slip rates given by this study. Green diamonds are lower bounds obtained by Zhang et al. [1988a]. Grey area brackets the lowest and uppermost bounds. Yellow area brackets the range that envelops most (9 of 12) of the bounds of slip rates. Blue line, separating the lower bounds from the upper bounds, represents an average slip rate of 4.5 mm/yr. point of slip rate in Figure 16 because it is part of the data set, but we doubt it. [54] We plot all of the bounds of slip rate on Figure 16. All of the upper and lower bounds of slip rates are located within the gray area bracketed by 2.5 and 7.5 mm/yr. Average between the lowest and the uppermost bounds yields 5.0 ± 2.5 mm/yr slip rate. Uncertainty associated with this average rate however is too large and can be further reduced. It is obvious from Figure 16 that 9 of the 12 bounds located within the yellow area bracketed by 3.4 and 5.2 mm/yr. Although an average slip rate can be simply obtained to be 4.3 ± 0.9 mm/yr by taking the average, but much tighter constraints can also be placed for the slip rate estimation. The closest lower and upper bounds are 4.4 and 4.6 mm/yr, respectively (Table 2 and Figure 16). A slip rate of 4.5 mm/yr not only separates the closest lower and upper bounds, but also almost all lower and upper bounds with the only exception of the Yehupo site reported by Zhang et al. [1988a]. We therefore can take 4.5 mm/yr as the average slip rate and the range bracketed by 3.4 and 5.2 mm/yr can be approximated as 4.5 ± 1.1 mm/yr. This is also consistent with our preferred rate at the Shaomayin site. [55] The low slip rate suggests the Haiyuan fault, similar to the Altyn Tagh fault [Zhang et al., 2007], does not transfer a significant portion of the convergence between India and Asia out of India s path into Eurasia, but merely redistributes crustal thickening. If strike-slip extrusion occurs, its amount must be in the range of no more than 15 km for the Haiyuan fault region [Burchfiel et al., 1991; Zhang et al., 1991a]; it must occur with a low speed of a few millimeters per year and be largely limited to the confines of the Tibetan Plateau. 5.2. Comparison With GPS, Interferometric Aperture Radar, and Paleoseismological Data [56] Geological slip rate, based on measurements and dating the offset features, is long-term slip rate because it averages over several thousands or even longer year time span [Schwartz and Coppersmith, 1984]. Long-term slip rate represents the releases of the strain accumulated over many earthquake cycles. Short-term slip rate obtained from the GPS measurements and interferometric aperture radar (InSAR) observations spans time period of decades, and it 23 of 26

Figure 17. (top) Left-lateral slip rate across the Haiyuan fault and adjacent region constrained by GPS measurements. Thick black lines represent approximate the locations of the West Qinling, Haiyuan, and Zhongwei faults. All of the GPS velocities (N75 E components) are relative to a fixed Alashan block. (bottom) Velocity profile; the location is shown by black box in Figure 17 (top). 24 of 26