JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B03419, doi: /2009jb006397, 2010

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jb006397, 2010 Revealing coseismic displacements and the deformation zones of the 1999 Chi Chi earthquake in the Tsaotung area, central Taiwan, using digital cadastral data Yuan Hsi Lee, 1 Kun Che Wu, 1 Ruey Juan Rau, 2 He Chin Chen, 2 Wei Lo, 3 and Kai Chien Cheng 1 Received 19 February 2009; revised 29 September 2009; accepted 16 October 2009; published 31 March [1] The 1999 Chi Chi, Taiwan, earthquake (M w 7.6) was the largest earthquake to strike Taiwan in the twentieth century. This earthquake is associated with a 100 km long surface rupture. In order to reveal the details of displacement near the surface rupture, we use a digital cadastral system to calculate coseismic displacement around the Tsaotung area, central Taiwan. The digital cadastral system was originally conceived to survey land and building boundaries. In the Tsaotung area, Taiwan authorities have taken digital cadastral measurements before and after the Chi Chi earthquake. The cadastral system affords high density control points that reach 1421 points/km 2, a system denser than that of the GPS. Accuracy is to within ±11 cm, a level that is higher than spot imaging and one that allows us to study surface deformation in detail. Coseismic displacement is m at distance from the surface rupture and decreases to 3 4 m near the surface rupture. The azimuth of horizontal displacements is and rotates to near the surface rupture. This produced a compression, left lateral deformation zone with 10 3 compression strain near the surface rupture. Coseismic displacement of the footwall is m in 110, which is similar to that from using GPS data. In the Tsaotung thrust slice, we observed that the azimuth of horizontal displacement rotates from a NW trend to a south trend as a result of slip partitioning and gravity slide effect. Citation: Lee, Y. H., K. C. Wu, R. J. Rau, H. C. Chen, W. Lo, and K. C. Cheng (2010), Revealing coseismic displacements and the deformation zones of the 1999 Chi Chi earthquake in the Tsaotung area, central Taiwan, using digital cadastral data, J. Geophys. Res., 115,, doi: /2009jb Introduction [2] The 1999 Chi Chi, Taiwan, earthquake (M w 7.6) was the largest earthquake to strike Taiwan in the twentieth century. GPS data show that horizontal displacement progressively increases from south to north with 2 m displacement in the south, 5 m in the central part of the fault, and near 10 m at the northern part. The azimuth of horizontal displacement shows progressive clockwise rotation from south to north [Yang et al., 2000; Yu et al., 2001]. Besides control point M493, which shows the azimuth of horizontal displacement in a SW direction, most of the control points show the azimuth of horizontal displacement being in a WNW direction for the southern part of the fault with the azimuth progressively changing to a NW direction in the northern section [Yang et al., 2000; Yu et al., 2001] 1 Department of Earth and Environmental Sciences, National Chung Cheng University, Minhsiung, Taiwan. 2 Department of Earth Sciences, National Cheng Kung University, Tainan City, Taiwan. 3 Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan. Copyright 2010 by the American Geophysical Union /10/2009JB (Figure 1). The surface rupture shows similar characteristics of progressively increased displacement and changes in the azimuth of displacements from south to north [Lee et al., 2003]; however, azimuth rotation angles show larger variation than those shown in the GPS data. The azimuth of the surface rupture is SW trending in the southern part of the fault, near west trending in the central part, and NW trending in the northern part. This means that results from GPS data and observations of the surface rupture are similar north of Wufeng, but become progressively disparate to south of Wufeng (Figure 1) [Lee et al., 2003]. Not only is the azimuth of the slip direction different, the slip amounts of the surface rupture are usually smaller than that given by GPS data [Lee et al., 2003; Angelier et al., 2003]. This raises the question as to why the displacement azimuths are different between the surface rupture and GPS data. Angelier et al. [2003] studied the slip vector of the Chi Chi earthquake in the Wufeng area. They also observed that slip vectors between the GPS data and surface rupture are different and considered the possibility of slip partitioning effect, whereby nearly dip slip thrusting occurred along the rupture trace with left lateral shear developing in the deformation zone at the hanging wall adjacent to the main fault. This finding, however, is hard to be verified by field surveying. 1of13

2 Figure 1. Geological map of central Taiwan (modified from Chinese Petroleum Corporation [1974, 1982]) including the location of the surface rupture during the 1999 Chi Chi earthquake [CGS, 1999], horizontal slip of surface rupture (red arrows) and GPS displacement on the hanging wall side of the rupture (black arrows) [Yang et al., 2000; Yu et al., 2001; Lee et al., 2003]. Inset shows map location and tectonic setting of Taiwan. Bold arrow with rate shows the current movement of the Philippine Sea plate relative to the Eurasian plate [Yu et al., 1997]. 2of13

3 [3] During the Chi Chi earthquake 2500 people died, more than ten thousand were injured and at least 100 thousand buildings were destroyed. Most of the human casualties and destroyed buildings occurred in sites close to the surface rupture. Consequently, it is important to know what style of deformation occurred around the surface rupture. Although GPS data give a reasonable indication of regional displacement, most GPS control points are at distances far away from the surface rupture and these data points are not dense enough to give detailed deformation information about the surface rupture (Figure 1). InSAR or SPOT imaging can be used to obtain regional displacements, however, the displacements are too large to calculate displacement using the InSAR method and resolution is too low to calculate local displacement for SPOT imaging [Dominguez et al., 2003; Chang et al., 2004; Hsieh and Shih, 2006]. Field surveying can directly observe surface deformation but is limited to specific locations [Lee et al., 2003; Angelier et al., 2003]. We have found that the digital cadastral system with its high density of control points affords detailed horizontal displacement (see the auxiliary material). 1 Using the digital cadastral system, it is possible to understand the disparity between surface rupture observations and GPS results to the south of Wufeng, both in terms of the difference in the azimuth of the slip direction, and what the deformation style was in those areas where most destruction occurred along the surface rupture. Cadastral data are the fundamental information used to delineate property boundaries [Land Survey Bureau (LSB), 2000]. After the occurrence of large earthquakes, such as the 1999 Chi Chi earthquake or the recent 2008 Sichuan earthquake, government authorities will typically conduct cadastral surveys. We can use this before and after cadastral data to reveal details of the surface deformation pattern. 2. Geological Setting [4] The 1999 Chi Chi earthquake was due to slip along the Chelungpu fault, a major north striking thrust fault in the fold thrust belt of western Taiwan (Figure 1). Seismological data shows the main shock continued for about 30 s and slip azimuths rotate clockwise from south to north and displacement increases from south to north [Kao and Chen, 2000; Huang, 2001]. The resultant rupture exhibited complicated surface faulting along approximately 100 km of the Chelungpu fault (Figure 1) [Lee et al., 2003; Central Geological Survey (CGS), 1999]. The fault thrusts Miocene to mid Pleistocene sedimentary rocks on the hanging wall over Quaternary conglomerate on the footwall [Chang, 1971] and roughly coincides with the mountain front that defines the eastern margin of the Taichung basin (Figure 1). The strata on the hanging wall side of the Chelungpu fault form an east dipping monoclinal structure; they generally dip at but >40 near the fault [Chinese Petroleum Corporation, 1974, 1982]. The total slip of the Chelungpu fault is around km [Yue et al., 2005; Yang et al., 2007]. The Chi Chi earthquake did not follow the predefined Chelungpu fault trace completely. To the north of 1 Auxiliary materials are available at ftp://ftp.agu.org/apend/jb/ 2009jb the Wufeng, surface rupturing is the result of slip along the Chinshui shale with both the hanging wall and footwall being of the Chinshui shale. The predefined Chelungpu fault trace in this northern part merges with the lower Sanyi fault and is the result of the Kueichulin formation (late Miocene) thrusting on the Toukengshan formation with larger total slip (Figure 1). Total slip in the northern part of the Chi Chi earthquake rupture is only 300 m [Yue et al., 2005]. Consequently, the width of the fault zones in the northern part of the Chelungpu fault trace is only in the tens of meters wide while in the central and southern parts of the fault trace it is several hundred meters wide. [5] The study area is located on the Wu River flood plane and several different elevated river terraces were caused by the Chelungpu fault. Around the Tsaotung area the main surface ruptures separated into two faults that consist of a small thrust slice, named the Tsaotung thrust slice (Figure 2). In this thrust slice the scarp highs are m and 1.6 m, respectively (Figure 2b) [Lee et al., 2003; CGS, 1999]. Instead of the main surface rupture, an eastern verging subordinate fold scarp developed 2 km to the eastern side of the main fault [Lee et al., 2003; CGS, 1999; Chen et al., 2007]. This fault is roughly parallel to the main fault and the scarp is less than 1 m high. Leveling surveys have been conducted across the main surface rupture and the subordinate rupture before and after the Chi Chi earthquake; these surveys indicate the existence of a pop up structure [CGS, 1999]. Figure 2b shows vertical displacement across the main and subordinate rupture surfaces showing a pop up structure of m high at the main rupture to less than 2.5 m high east of the subordinate rupture. This eastern verging fold scarp was caused by a change in the dipping angle of the Chelungpu fault (Figure 2c). [6] Horizontal displacement of the main surface rupture is 3 m at while the slip vector of the subordinate rupture is m at 90 [Lee et al., 2003]. There are two GPS control points near the Tsaotung area. M482 is located on the southern side of the pop up structure with 5.05 m of horizontal displacement in 313 and AF23 is located on the eastern side of the eastern verging fold scarp with 5.29 m of horizontal displacement in 304 (Figures 1 and 2) [Yang et al., 2000; Yu et al., 2001]. 3. Digital Cadastral System [7] The digital cadastral system is currently utilized by the Taiwan government to manage cadastral information including positions, boundaries and ownership of land parcels and buildings. In order to efficiently manage the cadastral records and improve accuracy, a new digital cadastral system, based on digital instruments including GPS and total stations, was initialized by the government to replace the former cadastral system, which has been established more than 50 years ago using traditional cartography [LSB, 2000]. The first class control points are regularly maintained by episodic GPS survey and the control is consequently expanded to the second class control points with high spatial resolution. The measuring period of cadastral data were conducted from 1983 to 2001 and 1995 to 2001 in the northern and southern parts, respectively, of the Tsaotung area (Figure 2). Lee et al. [2006] used the second class 3of13

4 Figure 2 4of13

5 control points to calculate horizontal displacement in the Taichung area. They obtained consistent results with GPS data and concluded that variation in horizontal displacement is controlled by the fault geometry. Typically, authorities measure land or building boundaries using the total station data system based on the second class control points. Turning points are control points for the cadastral system. The coordinates of each turning point are measured and the combinations of the turning points compose the boundaries of buildings or land. After the Chi Chi earthquake most of the second class control points were lost in Tsaotung areas, so were not available for measuring the coseismic displacement. On the contrast the turning points remained, so we can compare the coordinates of turning points before and after the Chi Chi earthquake to calculate coseismic displacements. The digital cadastral system only records twodimensional plane coordinates, as a result, only horizontal coseismic displacements of the Chi Chi earthquake can be inferred from the system. Figure 3 shows examples of the cadastral data before and after the Chi Chi earthquake. The locations of Figure 3 are shown in Figure 2. The red lines represent post Chi Chi earthquake locations, and the green lines are the prior positions. We compared the coordinates of turning points before and after the Chi Chi earthquake and calculated the displacement vectors. Figure 3a is the footwall area; it shows displacement progressively increasing from 0.9 m to 1.14 m from west to east. Figure 3b includes the hanging wall and areas at distances away from the surface rupture area; it shows displacement is near 4.85 m in 310. Both examples show coseismic displacements as being consistent in this small area. In such a manner, we can use the digital cadastral system to reveal details of surface deformation near the surface rupture and discuss the deformation mechanism. [8] Taiwan s authorities have rigorous codes governing the accuracy of cadastral surveys. The accuracy of the firstclass control points and the GPS control points must be within ± cm and ± cm, respectively in the latitudinal and longitudinal components. The accuracy of the second class control points must be within ±2 cm. Accuracy of turning points for buildings or land parcels depends on the locations. For example, in urban areas, the standard error is within 2 cm with a maximum error of 6 cm. In the rural areas, the standard error is within 7 cm with a maximum error less than 20 cm [LSB, 2000]. The error of turning points can be estimated p using the Law of Error Propagation, that is M x = M x ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffi P i M i 2, where M i is the each error source, assuming noncorrelation, and M x is the overall error contributed by all errors. In the Tsaotung area, most properties are either in the city or the farmland, so in our study we assumed all properties were farmland and calculated the measurement error of the turning points. The error relating to turning points, assuming all error pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sources to be uncorrelated, is estimated to be 7.4 cm ( 1:1 2 þ 1:1 2 þ 2 2 þ 7 2 ). Since the turning points were measured before and after the Chi Chi earthquake, the error of horizontal displacements estimated using pcoordinate ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi change of the turning points can reach 10.5 cm ( 7:4 2 þ 7:4 2 ). In addition, the coordinates before the Chi Chi earthquake refer to the TW67 datum, whereas the ones after the earthquake refer to the TW97 datum. In order to calculate the coordinate change, an error of coordinate transformation from the TW67 into the TW97 systems in the amount of less than 1 cm was introduced. Consequently, the overall standard error of turning points, including errors from GPS control points, cadastral surveys, and the coordinate transformation, is concluded to be less than ±11 cm. In the Tsaotung area, horizontal displacement is nearly 5 m at the hanging wall and 1.2 m at the footwall. These amounts are much larger than the overall measurement error of turning points. Instead of measuring error, we corrected the preseismic and postseismic displacements to obtain the coseismic displacement. The preseismic displacement velocity of the Tsaotung area is 1.2 cm/yr in 294 from 1992 to 1999 [Yu et al., 2001], indicating that we should correct the 13.2 cm and 4.8 cm of the preseismic displacement in the northern and southern parts of the Tsaotung area. The postseismic displacement of AF24 located in northern Tsaotung is 8.1 cm in 310 and is rather small on the footwall for the displacement of 15 months after the Chi Chi earthquake [Yu et al., 2003]. Finally, we corrected 8.1 cm of the postseismic displacement on the hanging wall and we do not correct postseismic displacement on the footwall considering the postseismic displacement is rather small. 4. Coseismic Horizontal Displacement Around the Tsaotung Area 4.1. Regional Horizontal Displacement [9] This study area is about km 2 and we obtain 11,371 control points to calculate displacement (as shown in appendix). The density of control points is 1421 points/km 2, enough to calculate detailed displacement (Figure 2). Figure 4 shows horizontal displacement in the study area. Horizontal displacement is rather uniform at the footwall and at distance from the surface rupture area. The yellow lines show the azimuths of displacements are consistently NW trending, except near the surface rupture where they show distinct anticlockwise rotation. Around the Tsaotung thrust slice, larger variations in displacement are observed. Here displacement decreases and the azimuth of displacements rotate to become south trending. Figure 5a shows contouring of the Figure 2. Location of the digital cadastral data and displacement vector of the surface rupture. (a) Shadow relief map of 40 m DEM that shows the Tsaotung area being located on the flood plane of the Wu River. The small black dots are the control points of the cadastral system. There are two GPS control points, AF23 and M482, close to the Tasotung area and both azimuth show in 312. The azimuths of the surface rupture are The measuring period of cadastral data were conducted from 1983 to 2001 and 1995 to 2001 in northern and southern part of Tsaotung area. (b) Vertical displacement of the Chi Chi earthquake according to leveling survey [CGS, 1999]. The vertical displacement is near 3.3 m in the pop up structure and 2.4 m at the eastern side of the pop up structure. (c) Structural profile of the Chelungpu fault and the Chelungpu fault slip along the Chinshu Shale. The fault plane changes the dipping angle that produces the folded scarp on the eastern side the main scarp. 5of13

6 Figure 3. The digital cadastral system around the Tsaotung area. The green and red lines represent the cadastral system before and after the Chi Chi earthquake. We compare the turning points of the cadastral system to calculate the horizontal displacements. Location of Figures 3a and 3b are shown in Figure 2. (a) The study area of the footwall shows displacement at nearly 1.1 m in 110. (b) This area is located at the hanging wall, which shows displacement near 4.85 m in 310. The displacement amounts are consistent for this small area, indicating the system is stable. horizontal displacement. At the hanging wall horizontal displacement is about m at distance from the surface rupture area but decreases to 3 4 m near the surface rupture. In the Tsaotung thrust slice, horizontal displacement decreases to m. At the footwall, horizontal displacement is m in Figure 5c shows contours of azimuths of displacement. The azimuths of horizontal displacement are and rotate to near the surface rupture. Displacement vectors, located at the footwall and at distance from the surface rupture, are similar to that of the GPS data, which is used to identify the accuracy of these findings. The GPS control point M467 near the study area shows horizontal displacement to be 1.04 m in 111, which is similar to the cadastral data at the footwall area (Figure 2) 6of13

7 Figure 4. Displacement vectors of the Chi Chi earthquake using digital cadastral data. The footwall and hanging wall show opposite displacement azimuths. The gray lines show the displacement direction. [Yang et al., 2000; Yu et al., 2001]. The GPS control point AF23, located east of the subordinate rupture, shows horizontal displacement to be 5.3 m in 308 (Figure 2). Cadastral data located on the eastern margin of the subordinate rupture show horizontal displacement to be near 4.4 m in 315, a result that is smaller than that for GPS control point AF23. The horizontal slip vector of this eastern verging fold scarp is near 1 m [Lee et al., 2003], which indicates that the horizontal displacement is consumed by this scarp resulting in a smaller displacement in the pop up structure compared 7of13

8 Figure 5 8of13

9 to the eastern side of the subordinate rupture. GPS control point M482 is located on the southern side of the study area; it shows that the horizontal displacement is near 5.1 m in 314 (Figure 2). Cadastral data, near this control point, show horizontal displacement is 4.5 m in 312. Once again, in this case, horizontal displacement is somewhat smaller than that for GPS data. GPS control point M482 is on the southern side of the pop up structure and there is no eastern subordinate to consume displacement; this results in slightly larger displacement. In all, the GPS data show similar horizontal displacement to that from the cadastral data, giving veracity to the accuracy of using the cadastral system Compression With the Left Lateral Deformation Zone Near the Surface Rupture [10] The strike of the surface rupture is near N S trending. We, therefore, plot all of the data and construct an E W trending displacement profile to show variation in displacement (Figure 5b). Horizontal displacement is 4.2 to 4.8 m on the eastern side of the study area. East of the surface rupture, within m, horizontal displacement starts to decrease from 4 mto 3 m around the surface rupture. This indicates that a deformation zone exists near the surface rupture. The azimuths of the displacement also show a similar phenomenon. The azimuths of displacement are approximately the same at 310 to 315 at distance from the surface rupture and near 110 at the footwall. Near the surface rupture azimuths of displacement show anticlockwise rotation from 310 to 280 (Figure 5). The variation of the horizontal displacement indicates that there exists a deformation zone near the surface rupture. In order to clarify the characteristics of the deformation zone, we decomposed the horizontal displacement into thrust and strikeslip components, respectively. Because the fault trace is roughly N S trending, we decomposed horizontal displacements in east and north directions to indicate thrust and strike slip components (Figures 5e and 5f). We found that the strike slip component decreases more significantly than the thrust component indicating that the deformation zone is dominated by left lateral shearing. Figure 6a shows how the azimuths of displacement rotate anticlockwise near the surface rupture. The displacement vectors are consistent at the footwall and the displacement azimuth rotates from 305 on the eastern side to 280 near the surface rupture. Displacement also decreases from 4.2 m to 3.4 m from east to west. This identifies a compression, left lateral deformation zone near the surface rupture. In this area, the deformation zone is nearly 200 m wide. The scarp high along the surface rupture of the Tsaotung area usually is less than 3.4 m, which is similar to the vertical displacement in the pop up structure (Figures 2b and 6b). Figure 6b shows the scarp is nearly 2 m high, which is even less than the 3.4 m vertical displacement at the pop up structure. Figure 6c shows a schematic 3 D model of the deformation zone. This deformation zone is limited in the near surface rupture area and further explanation of the deformation zone will be given in the section 4.3. Figure 7 shows another example of the existence of the deformation zone. To the northern part of the Tsaotung thrust slice, a slip vector of the surface rupture was obtained by measuring offset nonparallel lineaments (Figures 7a and 7b). The horizontal slip vector of the main surface rupture is 3.21 m in 271. The cadastral data show the displacement azimuth to be 4.2 m in 303 in the eastern side of the surface rupture. This result also indicates a compression, left lateral deformation zone existing near the surface rupture. In this area some minor faults developed to the east of the main fault. Compression and left lateral components could have been consumed by these minor faults. [11] We also calculate the coseismic horizontal strain of the Chelungpu fault. At distance from the surface rupture area, coseismic strain is 10 4 dilation strain and the maximum dilation direction is NW trending (Figure 8a). Around the Tsaotung area, however, coseismic horizontal strain shows compression strain ( 10 3 ) and the maximum strain direction is approximately NW trending near the surface rupture (Figure 8b). Compression strain of 10 3 could explain why so many buildings being damaged in the area of the surface rupture (Figure 8). In the northernmost area, there is higher anticlockwise rotation that results in the dilation strain being near N S trending. With all the evidences pointing to a several hundred meter wide compression and left lateral deformation zone developed near the surface rupture area, what the possible mechanism could have resulted in such a deformation zone? We consider slip partitioning occurring, with near dip slip thrusting along the rupture trace and distributed left lateral shear in the deformation zone of the hanging wall, adjacent to the main fault as suggested by Angelier et al. [2003]. [12] The deformation zone is limited to several hundred meters in width. We consider that could be the result of there being a m wide fault zone along the central to southern segments of the Chelungpu fault, which experienced 15 km of total slip. Confined pressure is rather low near the surface due to the strength of faulted rock being weak and easily deformed. As the fault slipped to the surface, most displacements were consumed in the main fault plane with some displacements being consumed in the fault zone. This is a possible explanation of the mechanism of the Figure 5. (a and c) Displacement contour and profile. At distance from the surface rupture area, horizontal displacements are from 4 to 4.9 m. Near the surface rupture, horizontal displacement decreases to m. Displacement is near m at the footwall. (b and d) Contour and profile of the azimuths of displacements. In the eastern side of the pop up structure, the displacement azimuths are near , and they decrease to near the surface rupture. The azimuth of horizontal displacement is near 110 at the footwall. The deformation zone seems wider in Figures 5c and 5d this is the result of plotting the data in an easterly direction; however, the surface ruptures are not exactly N S trending and consequently the deformation zone seems wider. The real width of the deformation zone is shown in Figures 5a and 5b. We decomposed the horizontal displacement of components in the east and north directions to indicate the thrust and strike slip fault components. (e) East direction displacement (negative sign). (f) North direction displacement (positive sign). Near the surface rupture, the north direction component decreases significant indicating a left lateral shear zone. 9of13

10 Figure 6 10 of 13

11 Figure 7. (a) Measuring the slip vector of the surface rupture, two nonparallel lineaments were offset by the surface rupture. (b) The slip vector of the surface rupture is 3.21 m in 271. The locations of Figures 7a and 7b are shown in Figure 7e. (c and d) Surface rupture cutting the road with about a 1.5 m right lateral separation, and the slip direction is south trending; this result is similar to the cadastral data. Both locations are shown in Figure 7e (e) Horizontal displacements around the Tsaotung thrust slice. The displacement azimuths progressively rotate from a NW trend to a south trend from north to south. The yellow lines are elevation contours. The white letters are slip amounts at the front and rear faults of the Tsaotung thrust slice. In the front fault, the slip amounts progressively decrease from north to south. By contrast the slip amount of the rear fault progressively increases from north to south. deformation zone (Figure 6c). We observed that the hanging wall has developed minor faults though some are not near the surface rupture and suspect that the deformation mechanisms were ductile deformation or minor slip faults. Comparing displacement amounts of the surface rupture with GPS data, displacement of the surface rupture is generally rather small for GPS data on the hanging wall in the central to southern segments of the Chelungpu fault, indicating that further deformation zones might exist in other areas (Figure 1). [13] The other possible mechanism to explain decreasing horizontal displacement near the surface rupture is that the geometry of the fault plane changes from low to high angle dip. If this were to be the case, it should have been associated with high vertical displacement near the surface rupture in some sections, such as the Taichung area [Lee et al., 2006]. The leveling survey shows that the vertical displacement is similar being m at the pop up structure, and the field survey shows the scarp high of the surface Figure 6. (a) Horizontal displacement near the surface rupture. The azimuth of displacement rotates progressively from 310 to near the surface rupture. Horizontal displacement is consistent at the footwall, 1.1 m in 110. (b) The surface rupture shows 2 m vertical displacement. In the Tsaotung area, the scarp high of the surface rupture is near 3 m or less than 3 m. (c) We consider the deformation zone to be several hundred meters wide which would be relative to the existing m thick fault zone. The strength of the fault zone is rather weak near the surface that results in the development of small branch faults or a ductile deformation zone near the surface rupture resulting in slip partitioning effect. 11 of 13

12 Figure 8. (a) Coseismic surface strain of the Chi Chi earthquake. The regional coseismic strains are 10 4 dilation strain and the maximum dilation direction is NW trending. (b) Coseismic strain near the Tsaotung area. The coseismic strain is 10 3 compression strain close to the surface rupture, and the maximum principal strain direction is approximately west to northwest trending. The northernmost part shows near N S trending dilation strain resulting from there being anticlockwise rotation of the displacement field in this area. rupture is also 3 m and less (Figure 6b) [CGS, 1999; Lee et al., 2003]. In addition, if the decreasing horizontal displacement results from changes in the fault dip angle, it should be associated with decreases in the east component (thrust component) rather than north component (strike slip component); however, our studies show it decreases the north component dominantly (Figure 5f). Combining all these information, we conclude that this deformation zone does not result from change in the fault geometry but rather from the slip partitioning effect Deformation Style in the Tsaotung Thrust Slice [14] Elevation of the Tsaotung thrust slice is 9 12 m higher than the footwall. The surface rupture of the Chi Chi earthquake is along the preexisting fault trace [Ota et al., 2004] (Figure 7c). In the northern part of the thrust slice, horizontal displacement is nearly 3.5 m with a northwest trend and progressively decreases to 1.7 m with a south trend in its southern part (Figure 7d). Field surveying also confirms this displacement distribution. At site A, the strike of the road is 288 and the surface rupture cuts the road and produces a 1.5 m right lateral separation (Figure 7c). Around this site, the azimuths of displacement trend southward with 1.5 m of displacement, matching the field survey (Figure 7e). At site B, the road strikes E W and the surface rupture cuts this road with 1.5 m right lateral separation, similar to the cadastral data (Figure 7d). In concordance with the field survey, we confirm that the displacement azimuth progressively rotates from a NW to south trend from north to south for the Tsaotung thrust slice. [15] To explain the large variation in slip vectors, we examine whether such variations could result from slip partitioning, left lateral shearing and gravity slide effect. We calculate the amount of slip at the rear fault of the thrust slice by comparing displacement vectors between the hanging wall and footwall. Slip amounts progressively decrease from 3 m to 1.7 m along the front thrust from north to south. By contrast, slip amounts at the rear fault progressively increase from 0.5 m to 1.5 m from north to south (Figure 7e). This difference from slip partitioning along the front and rear faults results in the variation in horizontal displacements at the Tsaotung thrust slice. [16] We then examine why the azimuth of displacement begins to trend southward. While we still hold that a compression zone with left lateral shear developed near the surface rupture this should have produced a near east 12 of 13

13 trending displacement azimuth. However, the displacement azimuth rotated to become southward trending. This means there may have been another mechanism at play that rotated the displacement field. Elevation of the thrust slice is 9 12 m higher than at the footwall and this difference in elevation could cause gravity slide. In addition, the northern part of the thrust slice has higher elevation than the southern part, a situation that would likely result in a southern trending gravity slide and produce a displacement change from eastward trend to a southward trend. 5. Conclusions and Discussions [17] The accuracy of the digital cadastral data is within ±11 cm, sufficient for calculating the coseismic displacement of large earthquakes. For the case of the Chi Chi earthquake, the cadastral data shows horizontal displacement to be m in at distance from the surface rupture, decreasing to m in near the surface rupture. The horizontal displacement is m in at the footwall. These displacement vectors are similar to other measurements of slip vectors at the surface rupture and GPS data. Near the surface rupture a several hundred meter wide compression, left lateral deformation zone exists. The width of the deformation zone relates to the width of the fault zone. And the deformation zones exist not only in the Tsaotung area, but also in the central to southern segments of the Chelungpu fault. Variations in displacement vectors at the Tsaotung thrust slice are associated with slip partitioning and gravity slide effect. [18] Typically, the offset of linear features or slickenside of fault planes is used to determine displacement vectors at a surface rupture [Lee et al., 2003]. However, such measurements are limited by location availability. For example, Lee et al. [2003] only measured approximately 100 sets of slip vectors of the surface rupture for the Chi Chi earthquake in the entire 100 km length of the fault. By contrast, if we measure displacement vectors by comparing cadastral data before and after an earthquake, we can obtain enough detail on displacement vectors to reveal the deformation mechanism. [19] Acknowledgments. We are grateful to Stephane Dominguez and one anonymous reviewer and the Editor Patrick Taylor for constructive and helpful reviews of the manuscript. This project was supported by the National Science Council, Taiwan, under grant NSC M The GMT software of Wessel and Smith [1995] was used in constructing most of the figures. References Angelier, J., J. C. Lee, H. T. Chu, and J. C. Hu (2003), Reconstruction of fault slip of the September 21st, 1999, Taiwan earthquake in the asphalted surface of a car park, and co seismic slip partitioning, J. Struct. Geol., 25, , doi: /s (02)00038-x. Central Geological Survey (CGS) (1999), Report of the Geological Survey of the 1999 Chi Chi earthquake (in Chinese), Cent. Geol. Surv., Minist. of Econ. Affairs, Taipei. Chang, S. L. 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Lee, and K. C. Wu, Department of Earth and Environmental Sciences, National Chung Cheng University, 168 University Road, Minhsiung, Chiayi County 62102, Taiwan. (seilee@eq. ccu.edu.tw) W. Lo, Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Sec. 3, Chung hsiao E. Rd., Taipei, 10608, Taiwan. 13 of 13