Coseismic displacement, bilateral rupture, and structural characteristics at the southern end of the 1999 Chi Chi earthquake rupture, central Taiwan

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi:10.1029/2010jb007760, 2011 Coseismic displacement, bilateral rupture, and structural characteristics at the southern end of the 1999 Chi Chi earthquake rupture, central Taiwan Yuan Hsi Lee 1 and Yi Xiu Shih 1 Received 9 June 2010; revised 14 April 2011; accepted 19 April 2011; published 13 July 2011. [1] The 1999 Chi Chi earthquake (Mw 7.6) was due to slip along the Chelungpu fault: a major N striking thrust fault in the fold thrust belt of western Taiwan. The surface rupture extends over 100 km in a N S trend with NW trending horizontal displacement increasing from 2 to 10 m from south to north. The central segment of the Chelungpu fault is characterized by bedding slip along the Pliocene Chinshui Shale, which has resulted in the development of a monoclinal structure on the hanging wall. At the southern end of the Chi Chi earthquake rupture, however, the Chelungpu fault connects with the NE striking Tachienshan fault and cuts into Miocene strata with complex structures on the hanging wall. In this study, we use digital cadastral data to calculate coseismic horizontal displacement around the Chushan area. Results show the amount and direction of horizontal displacement to be ca. 1.2 to 2.6 m and ca. 220 to 260, respectively, on the hanging wall and ca. 1.4 m and 105, respectively, on the footwall. Coseismic displacement and seismological data pertaining to the hanging wall at the southern end of the Chi Chi earthquake rupture indicate the existence of three distinct blocks. Horizontal displacement indicates the Chushan block s movement trended SW. This result reflects the Chushiang and Luliao faults being reactivated with different faulting mechanisms and slip azimuths. This study considers a complex coseismic displacement field, resulting from a bilateral rupture process whereby a southward rupture with SW trending movement results in right lateral strike slip faulting of the Tachienshan fault and thrusting with a right lateral component on the NS strike of the Chelungpu fault. The northward rupture is associated with NW trending movement that results in activation of the Luliao and Chushiang faults. Citation: Lee, Y. H., and Y. X. Shih (2011), Coseismic displacement, bilateral rupture, and structural characteristics at the southern end of the 1999 Chi Chi earthquake rupture, central Taiwan, J. Geophys. Res., 116,, doi:10.1029/2010jb007760. 1. Introduction [2] The 21 September 1999 Chi Chi earthquake was the largest earthquake (M W = 7.6) to strike Taiwan in the 20th century. The Chi Chi earthquake resulted in a near 100 km surface rupture [Central Geological Survey (CGS), 1999] caused by activation of the low angle Chelungpu fault [CGS, 1999; Kao and Chen, 2000]. GPS data show horizontal displacement across the fault surface trace progressively increasing from 2 to 10 m, south to north. In addition, the displacement azimuth shows clockwise rotation from south to north, which is in a WNW direction for the southern part of the fault and progressively changes to a NW direction in the northern section [Yang et al., 2000; Yu et al., 2001; Dominguez et al., 2003; Lee et al., 2005] (Figure 1). 1 Department of the Earth and Environmental Sciences, National Chung Cheng University, Taiwan. Copyright 2011 by the American Geophysical Union. 0148 0227/11/2010JB007760 Although horizontal coseismic surface displacements are relatively small at the southern end of the Chi Chi earthquake rupture, the structural characteristics are more complex. In the central segment, the Chi Chi earthquake rupture is a bedding slip fault along the Chinshui Shale with a monoclinal structure having developed on the hanging wall [Lee et al., 2003; Yue et al., 2005; Yang et al., 2007]; however, in the southern segment the surface rupture slipped along the Tachienshan Fault, where the Kweichulin Fm. thrusts over the Pleistocene Toukoshan Fm and it developed the NE striking Chushiang fault, NW striking Luliao fault, and EW striking Luko syncline on the hanging wall [Chinese Petroleum Corporation (CPC), 1982, 1986] (Figure 2). [3] Yu et al. [2001] showed the azimuth of regional GPS coseismic displacement to be WNW trending at the southern segment, except for control point M493, where the displacement azimuth is 257 (Figures 1, 2, and 3). Yu et al. [2001] considered the possible causes of an anomalous azimuth at station M493 to be due to local variations in deformed earth material or artificial measurement errors. 1of18

Figure 1. Geological map of central Taiwan (modified from Chinese Petroleum Corporation [CPC, 1974, 1982, 1986]) 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]. 2 of 18

Figure 2. Geological map and coseismic horizontal displacement at the southern end of the Chi Chi earthquake rupture [CPC, 1974, 1982, 1986, Yu et al., 2001, Yang et al., 2000; Lee et al., 2003]. The red arrow and red dashed arrow show the slip vector of the surface rupture and GPS data, respectively. However, they also suggest further field investigations at this site to clarify their measurement, raising the question of whether or not true coseismic displacement occurred at station M493. If it did occur, an active structure should exist between stations M493 and G042, which shows a displacement azimuth of 315. [4] Yang et al. [2000] and LSB [Land Survey Bureau (LSB), 2000] utilized a greater number of GPS stations at the southern end of the Chi Chi earthquake rupture to calculate coseismic displacement (Figures 2 and 3). To the south of the Luliao fault, coseismic displacement occurred in the tens of centimeters with a SW trend. Although the displacement is relatively small, its azimuth is significantly different from the regional NW trend. This indicates the likely existence of a discontinuity structure between the northern and southern boundaries of the Chushan area. To the south of 3of18

Figure 3. Geological map and segmentation at the southern end of the Chi Chi earthquake rupture. According to coseismic displacement, the southern end of the Chi Chi earthquake can be separated into 3 blocks with different displacement azimuths. Chushan, a NW trending seismic cluster occurred during the Chi Chi earthquake sequence. In addition, a large aftershock of Mw6.4 occurred along this seismicity zone [Chi and Dreger, 2004]. It is interesting to consider the tectonic meaning of this seismicity (Figures 2 and 4) and whether or not the Luliao fault was active during the Chi Chi earthquake. [5] From Chaoshui River to Chushan, the coseismic slip vectors across the surface rupture are SW trending dominantly, compared to the regional 300 NW trend in coseismic displacement (Figures 1 and 3) [Lee et al., 2003]. In the Tsaotun area, there is also a 20 difference in the displacement azimuth between the on site measurements of the surface rupture and those from GPS data for the Chi Chi earthquake rupture. Using third order digital cadastral data (infrastructure and land boundaries), Lee et al. [2010] found that the azimuth of displacement progressively rotating from 310 to 280 290 without any significant displacement fault. This result demonstrated that there existed a deformation zone near the surface rupture. However, between GPS stations M493 and G042 there is a difference in the azimuth of horizontal displacement of nearly 50. Such a difference is relatively large and deserves closer examination (Figures 2 and 3). [6] In this study, we combine seismological and coseismic displacement data including: surface rupture slip vectors, digital cadastral surveying, and GPS data with field surveying in order to identify the coseismic displacement field and better understand the deformation process at the southern end of the Chi Chi earthquake rupture. We also discuss the possible mechanisms controlling the southern end of the Chi Chi earthquake rupture, reinterpret the rela- 4of18

Figure 4. Seismicity of the Chi Chi earthquake. The earthquake data (M > 3) was from 21 September 1999 to 21 March 2000. It shows SE and NW trending seismicity at the southern end of the Chi Chi earthquake rupture. tionship between faulting and subsurface structures and present a kinematic model of the rupture process. 2. Geological and Geophysical Background 2.1. Geological Background [7] The causative fault of the 1999 Chi Chi earthquake is the Chelungpu fault: a major N S striking thrust fault in the fold thrust belt of western Taiwan (Figure 1). The fault thrusts Miocene to middle Pleistocene sedimentary rocks on the Quaternary conglomerate of the footwall [Chang, 1971] (Figure 1). The Chi Chi earthquake rupture mostly followed the predefined Chelungpu fault trace (Figure 1) [Lee et al., 2003; Yue et al., 2005]. The total geological slip of the Chelungpu fault is around 14 15 km in its central segment with monoclinal structure developed on the hanging wall [Lee et al., 2003; Yue et al., 2005; Yang et al., 2007]; however, in the southern segment, structures become more complex. To the southwest of the Tachienshan fault is the Meishan fault, which is a right lateral strike slip fault. It was activated in 1906 (M = 7.1) resulting in a 13 km surface rupture [Omori, 1907; Bonilla, 1977] (Figures 2 and 3). 2.2. Slip Vectors of the Surface Rupture [8] Lee et al. [2003] were able to measure surface slip vectors for the Chi Chi earthquake rupture by examining offset in infrastructure across the surface rupture. Slip vectors on the hanging wall of the surface rupture are in a NW to WNW direction north of the Chaoshui River; however, south of this river, the slip vectors trend SW. Figure 3 shows that from the Chaoshui River south to the Chushan area, the surface rupture thrusts SE with a right lateral component (Figures 3, 5a, and 5b). Farther south than Chushan, the surface rupture turns following the NE strike of the Tachienshan fault. In this area, maximum horizontal displacement is 2.4 m in 228, indicating right lateral strike slip faulting (Figures 3 and 5c). 3. Coseismic Displacement Field at the Southern End of the Chi Chi Earthquake Rupture 3.1. Digital Cadastral System [9] Taiwan uses a three tier cadastral control point system whereby first order control points are obtained directly from GPS stations and second order control points, which are more numerous and widespread, are linked to the GPS station system. Second order control points are temporary control points, typically constructed along roads. They are vulnerable to road repair, other anthropogenic activity, and natural disasters. Third order control points use building edges and/or land boundaries as turning points (i.e., positions on a cadastral subdivision). When cartographers utilize this system, they first obtain coordinates from the GPS stations, and then use total station (an electronic theodolite) to connect the GPS with the second and third order control points. In Taiwan, first order control points must be accurate to within ±0.3 1.1 cm and ±0.6 1.1 cm in latitude and longitude; second order control points must be accurate to within ±2 cm; and third order control points (i.e., accuracy of turning points for buildings or land) are location dependent. For example, in cities, the standard error must be within 2 cm with maximum error limited to 6 cm. For farmland, the standard error is to be within 7 cm and the maximum error no more than 20 cm [Land Survey Bureau 5of18

(LSB), 2000]. Lee et al. [2006, 2010] compared digital cadastral data (data source) before and after the Chi Chi earthquake to calculate detailed coseismic displacement. Because the density of first order and availability of secondorder control points can be inadequate to reveal detailed coseismic displacement, third order control points can be very useful in filling the gaps in cadastral data by providing coverage of the hanging wall and footwall in the immediate area of a surface rupture. For example, after the Chi Chi earthquake most of the second order control points were lost in the Chushan area, and therefore, were not available for measuring coseismic displacement. 3.2. Local Coseismic Horizontal Displacement Around the Chushan Area [10] In this study, third order cadastral control points are used to compare changes in the coordinates of cadastral turning points before and after the Chi Chi earthquake to obtain horizontal surface displacement vectors for the Chushan area (Figure 6). The reason being that Yu et al. [2001] showed the azimuth of regional GPS coseismic displacement to be WNW trending at the southern segment, except for control point M493, where the displacement azimuth is 257 (Figures 1, 2, and 3). If it can be shown that true coseismic displacement occurred at this site then an active structure should exist between stations M493 and G042, which shows a displacement azimuth of 315. [11] The use of third order digital cadastral data can give better resolution of horizontal displacement than the more sparsely distributed GPS stations and confirm whether or not true seismic displacement occurred. In order to determine coseismic displacement between GPS control point M493 and the surface rupture zone, we focus our investigation on the southwestern side of control point M493. The study area includes both the hanging wall and footwall. Digital cadastral surveying was undertaken in 1992 and 2001, providing data for before and after the Chi Chi earthquake. As mentioned above, the accuracy of thirdorder control points for cities and rural areas should have a standard error within 2 cm and 7 cm, respectively, with maximum error limited to 6 cm and 20 cm, respectively [LSB, 2000]. Turning point error can be estimated using the Law of Error Propagation [Bevington and Robinson, 2002]. Lee et al. [2010] determined the final standard error to be less than ±11 cm. From 1992 to 1999, preearthquake displacement velocity was estimated to be about 9 mm/yr in 296 for the area around the 1999 Chi Chi earthquake surface rupture [Yu et al., 2001]. Therefore, our correction for preearthquake displacement is 6.3 cm in 296 for the period 1992 to 1999. GPS control point M493 shows postseismic horizontal displacement (15 months after the Chi Chi earthquake) at 8.6 cm in 264 [Yu et al., 2003]. We do not correct for postseismic displacement given that control point M493 is not located in the study area and the extent of 6of18 Figure 5. Surface rupture at the southern end of the Chi Chi earthquake rupture. The precise locations can be found in Figures 3 and 7. The surface deformation shows thrusting with a right lateral component from Chaoshui River to Chushan [Lee et al., 2003]. (a) Vertical displacement near 2.5 m and example of the horizontal displacement at 1.8 m in 255 (point A in Figure 7). (b) Vertical displacement of 1.1 m and horizontal displacement at 2.15 m in 255 according to measure the deformed plastic pipe (point B in Figure 7). (c) The southern end of the Chi Chi earthquake rupture showing near right lateral strike slip faulting (Figure 3). The tea trees were offset by a right lateral strike slip fault (photo was take by Ruey Chyuan Shih). Lee et al. [2003] calculated the offset of the tea trees and obtained a horizontal displacement of 2.4 m in 228.

Figure 6. Third order control points use building edges and/or land boundaries as turning points. The blue and red lines represent the cadastral system before and after the Chi Chi earthquake (the location can be found in Figure 7). We compare the turning points of the cadastral system to calculate horizontal displacements. postseismic horizontal displacement is relatively small compared to coseismic displacement. [12] For the study area, there are a total of 5867 third order cadastral control points covering an area of 2.76 km 2, giving a density of 2100 points/km 2 (see auxiliary material). 1 Figure 7 shows the horizontal displacement vectors for the Chushan area. Given the nearly N S strike of the fault trace, the distribution of horizontal coseismic displacement was plotted in an E W direction to better reveal its distribution. Figure 8a shows that the azimuths of the cadastral results can be separated into three groups. The first group, located on the footwall, shows a homogenous azimuth of about 105. The second group, located on hanging wall, show azimuths ranging from 220 to 260. The third group is also located on hanging wall but exhibits azimuths ranging from 300 to 30. Figure 8b shows horizontal displacement to be 1.4 m on the footwall, and 1.2 m to 2.6 m on the hanging wall. GPS control point M493 is located on the eastern side of the cadastral control points and it shows the displacement to be 2.48 m in 257, which is consistent with the cadastral control points result (Figure 7). In Figure 7 points A and B describe the positions of two different slip vectors along the surface rupture. Figures 5a and 5b give photographs of the two locations, respectively. It is evident from these slip vectors in Figure 7 that the surface rupture is SE trending. This result is consistent with the cadastral data of Figure 3. In addition, the GPS control point data of Figure 3 shows the azimuth of horizontal displacement to be SW trending, indicating that the Chushan block (Block A in Figure 3) moved in a SW direction (Figure 3). 1 Auxiliary materials are available at ftp://ftp.agu.org/apend/jb/ 2010JB007760. 3.3. Landslide Area in Chushan [13] To demonstrate the value of using third order cadastral data, we examine one landslide that occurred within the Chushan area during the Chi Chi earthquake by the Tongpuna River (Figure 9). Coseismic displacements in this landslide area are northwest to northeast trending (Figures 7 and 9). This is inconsistent with the general SW trend described above. Therefore, it is reasonable to attribute the northwest to northeast trending displacements to the landslide. By subtracting coseismic displacement in the surrounding area, we are able to calculate the landslide displacement field. Outside the landslide area, coseismic displacements are about 1.6 m in 250. Subtracting this value, the landslide moved 1.4 m to 3.1 m in a NE direction. It slipped toward the NW trending Tongpuna River, which is consistent with field observations (Figure 9). 3.4. Coseismic Displacement Field at the Southern End of the Chi Chi Earthquake Rupture [14] Figure 3 shows the results of determining horizontal coseismic displacement from GPS stations for the southern end of the Chi Chi earthquake rupture. It gives slip vectors for the surface rupture and the direction of horizontal displacement. Figure 3 also includes the results of using thirdorder cadastral data near GPS station M493. From GPS control point M493 to the surface rupture zone the azimuth of horizontal displacement is SW trending. By contrast, horizontal displacement is NW trending at GPS station G042 on the hanging wall of the Chushiang fault. To the south of the Luliao fault, the GPS data show horizontal displacement in the tens of centimeters with right lateral strike slip faulting in a SW direction of 1 to 2.2 m. Although the GPS gathered displacement is relatively small, the dis- 7of18

Figure 7. Displacement vectors of the Chi Chi earthquake rupture using digital cadastral data. The study area can be found in Figure 3. The yellow arrows show displacement azimuth. The footwall and hanging wall show opposite displacement azimuths. The horizontal displacement is 1.2 to 2.6 m in 220 to 260 on the hanging wall and 1.4 m in 105 on the footwall. The horizontal displacement of GPS control point M493 is 2.48 in 257, which is similar to the cadastral data. Points A and B (Figures 5a and 5b), which also show the slip azimuths to be SW trending, are slip vectors measured for the surface rupture (Figure 3). placement azimuth is the same as the surface rupture, indicating that between the Luliao fault and Tachienshan fault, coseismic displacement is SW trending (Figures 3 and 10). [15] Utilizing this analysis of horizontal coseismic displacement, the hanging wall at this southern end of the Chi Chi earthquake rupture can be divided into three blocks separated as follows: Block A is located between the Chelungpu fault and Chushiang fault; it shows a SW trending azimuth of horizontal coseismic displacement. Block B is located between the Chushiang fault and Luliao fault; it shows a NW trending azimuth of horizontal coseismic displacement. Block C is located between the Tachienshan Meishan fault and the Luliao fault; it also shows a SW trending azimuth of horizontal coseismic displacement. The boundary of the Block B and C is consistent with the NW trending seismicity which extending more than 50 km in length (Figures 3 and 10). In addition, by examining the azimuths of horizontal coseismic displacement and slip vectors for the surface rupture at the southern end of the Chi Chi earthquake rupture, it is evident that the Meishan 8of18

Figure 8. (a) Azimuth of the horizontal displacement of cadastral data. The data plot is EW trending. There are three groups of azimuths, which are related to the footwall, hanging wall and landslide area. (b) Horizontal displacement of the cadastral data. The horizontal displacements are ca. 1.4 m in 105 on the footwall and 1.2 to 2.6 m on the hanging wall. fault forms the southern boundary of the Chi Chi earthquake rupture (Figure 10). It is interesting to note that the different azimuths in horizontal coseismic displacement and slip vectors among these faults suggest that the Chushiang fault and Luliao fault were active during the Chi Chi earthquake. The structural characteristics of these faults are discussed in the following sections. 4. Interpretations 4.1. Structural Characteristics of the Luliao Fault [16] Horizontal coseismic displacement by GPS station shows displacement to be SW trending to the south of the Luliao fault. On site measurements at the surface rupture zone also show the slip azimuth to be SW trending with 2.4 m of horizontal displacement, indicating pure rightlateral, strike slip faulting at the southernmost segment of the Tachienshan fault (Figure 5c) [Lee et al., 2003]. By contrast, north of the Luliao fault, horizontal coseismic displacement is NW trending, indicating left lateral strikeslip faulting with an extension component along the Luliao fault (Figure 10). Figure 11 shows surface deformation along the Luliao fault. NW trending tension cracks, pressure ridges, and normal faults at three sites were observed where the Luliao fault passes through a roadway. Considering all the rupture or fault are NW striking we considered these surface deformation could result from the tectonic force. Especially in site 3 a WNW striking normal fault, more than tens of meters in length, developed on the saddle of the mountain area. The topography map shows that the normal fault passes through the saddle of the mountain and as the steep slopes are nearer the N S trending the normal fault could not have been be induced by landslides. [17] In addition, Figure 11b is an aerial photo, shot 3 days after the earthquake. The photo shows a great deal of landslide activity along the Luliao fault as a result of the Chi Chi earthquake. In combination, this evidence indicates that the Luliao fault was active during the Chi Chi earthquake. Four hrs after the main shock, an Mw 6.4 earthquake occurred with NW left lateral strike slip faulting associated with NW trending seismicity along the southeastward extension of the Luliao Fault [Chi and Dreger, 2004; Kao and Chen, 2000] (Figures 2, 3, and 10). Although the epicenter was 18 ± 6 km, the major asperity with a maximum of 85 cm displacement was located at ca. 5 km in depth [Chi and Dreger, 2004]. This aftershock and seismicity could have been due to rupturing along the Luliao fault. Geologically, north of the Luliao fault are 5000 m deposits of the Chaolan and Toukoshan formations on the hanging wall of the Chelungpu fault. By contrast, south of the Luliao fault is the hanging wall of the Tachienshan fault, which is characterized by the Chaolan Fm. of only a few hundred meters in thickness and no presence of the Toukoshan Fm. The structural styles on the hanging walls of both the Chelungpu 9of18

Figure 9. During the Chi Chi earthquake, one of the landslides occurred near the Tongpuna River. The black arrow shows horizontal displacement calculated by cadastral data. In the landslide area, the horizontal displacement shows to be in a northwest to northeast trend. We subtracted coseismic displacement to obtain the landslide displacement field (red arrows), which shows the landslide slipped in a NE trend toward the Tongpuna River. fault and the Tachienshan fault are also different showing monoclinal structures on the hanging wall of the Chelungpu fault and anticline and syncline structures on the hanging wall of the Tachienshan fault. This evidence indicates the Luliao fault acts as a transfer zone accommodating these two segments (Figures 3 and 10). 4.2. Structural Characteristics of the Chushiang Fault [18] Simoes et al. [2007] surveyed a deformed strath terrace along the Dungpuna river which extended from the footwall and hanging wall of the Chushiang fault. They collected radiocarbon samples and indicate ages of the terrace of 11,231 to 11,850 cal yr BP. Given these ages, the dip angle of the fault, and the vertical throw determined from the offset of the strath terrace across the surface fault traces, they estimated a slip rate of 2.9 ± 1.6 mm/yr. Ota et al. [2004] argued that the Chushiang fault was associated with surface deformation during the Chi Chi earthquake based on the presence of west facing flexural scrap with back tilting on fluvial terrace. At GPS station M493 on the footwall of the Chushiang fault, horizontal coseismic displacement was measured at 2.48 m in 257 while GPS station G042 on the hanging wall of the Chushiang fault registered horizontal displacement of 1.4 m in 315. This difference in horizontal coseismic displacement also indicates that the Chushiang fault was active during the Chi Chi earthquake. [19] In southern part of the Chelungpu fault most structures strike N S or NE, indicating compression that is E W to NW SE. Unlike the surrounding structures, however, the Luko syncline, located between the Luliao and Chushiang faults, strikes near ENE WSW. This would indicate the Luko syncline results from NNW SSE compression stress. GPS station M433 is located at the Luko syncline and it showed horizontal coseismic displacement of 3.48 m in 354 (Figure 3) [Yang et al., 2000]. The direction of horizontal displacement at 354 is nearly perpendicular to the axis of the syncline. Such movement is consistent with near N S local compression stress since GPS M433 is located at a fold termination and because the Luko syncline axis turns anticlockwise to become NE SW trending. Interpreting this result, we think it is possible that the Luko syncline was formed by interaction between the Luliao and Chushiang faults. The Luko syncline is located between these two faults and with right lateral strike slip faulting along the Luliao fault and dominant thrusting at the Chushiang fault a local NNW WSW compression field has resulted leading to the formation of the Luko syncline. 4.3. Subsurface Structure Around the Chaoshui River [20] Yue et al. [2005] constructed 47 structural profiles to constrain the 3D subsurface structure of the Chi Chi earthquake. They found that the Chi Chi earthquake rupture resulted from bedding slip along the Chinshui Shale. Around the epicenter the depth of the detachment of the Chelungpu fault is nearly 5 to 6 km and the epicenter is nearly 8 to 10 km in depth [Kao and Chen, 2000; Chang et al., 2000]; therefore, Yue et al. [2005] suggested there exists duplex structures under the Chinshui Shale or there 10 of 18

Figure 10. The southern end of the Chi Chi earthquake rupture can be subdivided into three blocks. Block A is located between the Chelungpu fault and Chushiang fault; it shows a SW trending azimuth of horizontal coseismic displacement. Block B is located between the Chushiang fault and Luliao fault; it shows a NW trending azimuth of horizontal coseismic displacement. Block C is located between the Tachienshan Meishan fault and the Luliao fault; it also shows a SW trending azimuth of horizontal coseismic displacement. The boundary of the Block B and C is consistent with the NW trending seismicity which extending more than 50 km in length. exists a steep ramp cut into Miocene strata. However, their cross section is located on the northern side of the epicenter and cannot indicate the true subsurface structure of the epicenter. A deep seismic profile (HV2) was conducted along the Chaoshui River [Wang et al., 2002a], which is located near the epicenter of the Chi Chi earthquake. This profile is more informative of the structures beneath the epicenter (Figure 12a) [Wang et al., 2002a]. Previous studies show different interpretations of this deep seismic profile. Wang et al. [2002a] considered the Chelungpu fault to exhibit a dip angle of 27, cutting the bedding plane at its shallower end and Miocene strata at its deeper end. They considered the Chushiang fault (they named as the Tachienshan fault) to be a bedding slip fault that slips along the Chaolan Fm (Pcl of Figure 12b). Hung and Suppe [2002] also considered the Chelungpu fault to be dipping at 27 to the east with slip occurring along the Chinshui Shale and do not show the existence of the Chushiang fault (Figure 12c). However, the geological map shows that in general strata dips at nearly 45 to 50 on the hanging wall. This means that slip along the Chelungpu fault could not have been the result of bedding slip. The geological map also shows the existence of the Chushiang fault; however, this is not present in the interpreted profile of Hung and Suppe [2002] (Figure 12a). In addition, both Wang et al. [2002a] and Hung and Suppe [2002] interpreted profiles showing the Chelungpu fault cutting into Miocene strata; however, there is no Miocene 11 of 18

Figure 11 12 of 18

strata on the hanging wall that is not reasonable (Figures 12b and 12c). Another issue raised by examination of the geological map is the existence of the Kweichulin Fm. on the hanging wall of the Chushiang fault, which means Miocene strata should exist in the interpreted profile. Since the Chushiang fault is NE striking and bedding on the hanging wall is N S striking, it is reasonable to conclude, the Chushiang fault could not be a bedding slip fault (Figures 1 and 12a). In Wang et al. s [2002a] interpretation, the Chushiang fault is a bedding slip fault and this was adopted by Simoes et al. [2007] (Figure 12b). This, however, is not consistent with the geological information. [21] Given the heretofore mentioned inconsistencies in interpretations of geological information by both Wang et al. [2002a] and Hung and Suppe [2002], Figure 12d gives an alternative interpretation of this deep seismic profile. According to the geological map, there exists an imbricate structure on the hanging wall of the Chelungpu fault (Figures 3 and 12a). The shallow drill and seismic profiles show the dip angle to be 40 [Wang et al., 2002b], so we consider the dip angle of the fault plane to be 40 in its shallower section, while at depth, listric fault geometry is apparent. Because the Kweichulin Fm. is exposed on both the hanging walls of the Chushiang and Tachienshan faults, this suggests the Kweichulin Fm. also exists at depth on the hanging wall of the Chelungpu fault. Consistent with geological conditions given in the geological map, the Chushiang fault dips to the east at about 60 and cuts into the Miocene strata. Simoes et al. [2007] also show this high dip angle for the Chushiang fault at shallower depths. Kao and Chen [2000] show the epicenter to be at nearly 10 km in depth and Chang et al. [2000] show it at 8 km. Our interpretation shows the ramp flat boundary to be 9.2 km in depth. Considering the above two descriptions of the epicenter straddle the location of the ramp flat boundary, we think that this ramp flat structure boundary at 9.2 km in depth is the epicenter of the Chi Chi earthquake. Figure 12d shows vertical displacement and displacement vectors across this profile. The Chelungpu fault is N S striking, so we projected the coseismic displacement vectors in an easterly direction. GPS stations: G403, M479, G043, M322, and M402 show GPS vectors plunging at 50 39, 36, 26, and 10 [Yu et al., 2001; Yang et al., 2000]. These results are consistent with listric fault geometry at depth and produce the same fault dip angles as out interpreted fault dip angle (Figure 12d). GPS station M501 shows vertical displacement to be 0.04 m [Yu et al., 2001], which is a result consistent with a flat structure for the Chelungpu fault. After reinterpreting the seismic profile at Chaoshui River, we obtain displacement at nearly 9.2 km and 5.4 km for the Chelungpu and Chushiang faults, respectively. As mentioned earlier on the hanging wall of the Chushiang fault, the Chaolan Fm. and part of the Toukoshan Fm. are 5000 m thick, considering the dip angle of the Chushiang fault and the repeating strata slip displacement along 5.4 km is reasonable. This would give a total displacement of 14.6 km, which is similar to the total displacement recorded in the central segment of the Chelungpu fault at 14 15 km [Yue et al., 2005]. 4.4. Relationship Between the Tachienshan, Chushiang, and Luliao Faults [22] The Kweichulin Fm. is exposed both on the hanging wall of the Chushiang fault and Tachienshan fault and the strike directions of the Chushiang and Tachienshan faults are the same. Both faults show fold structures on the hanging wall. In addition, the Tachienshan fault shows a high dip angle toward the east similar to the Chushiang fault. Given these geological observations, we think it is likely that both faults were at onetime connected but are now cut by the Luliao fault with about 1.3 km in horizontal slip (Figures 3 and 10). The Tachienshan fault shows the Kweichulin Fm. thrusting over the Pleistocene Toukoshan Fm with at least a ca. 9 km offset [Yang et al., 2007]. The Tachienshan fault, however, showed to be strike slip faulting dominantly during the Chi Chi earthquake. In addition, the outcrop of the Tachienshan fault show near 70 dipping to the east. Such a feature is also evidence of strike slip faulting dominantly, which is not consistent with thrust faulting. Therefore, we suspect that the Tachienshan fault was once a thrust fault before becoming a strike slip fault in more recent times. We think it is possible that at some point the Chelungpu and Luliao faults were active and the Tachienshan fault separated into its northern and southern segments. The northern segment is now the Chushiang fault and the southern segment remains the Tachienshan fault. This event possibly changed the faulting characteristics of the Tachienshan fault from thrust to strike slip faulting. 4.5. Effect of Fault Rupture Propagation on Fault Slip Distribution [23] A great deal of research has been conducted on the fault rupture processes, and the spatial and temporal distribution of slip for the Chi Chi earthquake [Kao and Chen, 2000; Ma et al., 2001; Ji et al., 2003; Zeng and Chen, 2001; Huang, 2001]. Depending on the models used, rupture velocity ranges from 2 2.6 km/sec. Using high quality teleseismic records, Kao and Chen [2000] divided the main shock of the 1999 earthquake into five successive subevents, each representing the average properties of a portion of the earthquake rupture at depth (Figure 1). The rupture initiated about 15 km (in horizontal distance) east of the Chaoshui River valley; it was on a fault plane dipping 50 ± 12 to the east with a P axis that shows to be in 250 (subevent 1; Figure 1). The rupture then propagated northward along a dominant 20 to 30 dipping plane down to a depth of 15 km, which was likely the extension of the surface rupture (subevents 2 4; Figure 1). Finally, the center of Figure 11. (a) Geological map around the Luliao fault [CPC, 1982, 1986]. We observed surface deformation of the Luliao fault at stops 1, 2, and 3. To the south of the Luliao fault, anticline and syncline structures developed on the hanging wall of the Tachienshan fault. This is different to the monoclinal structure on the hanging wall of the Chelungpu fault. (b) An aerial photo (shot 3 days after the earthquake) gives surface deformation around the Luliao fault. A landslide occurred along the Luliao fault. Insert boxes are surface deformation or damage along the Luliao fault. Site 1 shows a tension crack and pressure ridges; site 2 also shows developed tension cracks; and site 3 shows NW strike normal faulting. 13 of 18

Figure 12 14 of 18

Figure 13. (a) Time dependent rupture azimuths calculated by Huang [2001]. He made direct measurements of the rupture propagation of the Chi Chi earthquake using SMART 2 array and its nearby accelerometers in eastern Taiwan. The solid red circles are the rupture propagating azimuths from successive snapshots. The vertical scale on the right hand side is the converted distance for locations of energy sources projected on the fault trace. The horizontal scale shows the temporal development of the earthquake. Nucleation is given at time zero. Huang [2001] considered that there was a bilateral fault rupture caused by the Chi Chi earthquake in the southern part of the earthquake fault with major slips at the southern end. (b) Four subevents during the Chi Chi earthquake. Twelve seconds after the initial rupture, clear arrival energy was resolved and this then propagated northward. This can be considered unilateral north south propagation in the northern part of the fault plane. The vertical bars are relative beam peak values which are normalized to the maximum value of the stacked array seismograms, which is taken as 500 [Huang, 2001]. (c) Snapshots of the 1999 Chi Chi earthquake source rupture process about 8 18 s after the Chi Chi earthquake showing a bilateral rupture, which moves southward and northward [Ji et al., 2003]. the rupture shifted eastward and went beneath the high mountain area about 35 km east of the mountain front (subevent 5; Figure 1). In this sequence, slip on the rupture rotated from an almost westerly direction in the south (subevent 1) to a northwest direction in the region east of Wufeng (subevent 4) and to a northerly direction farther east (subevent 5) (Figure 1). Huang [2001] used the SMART 2 array and its nearby accelerometers in eastern Taiwan to make direct measurements of the rupture propagation of the Chi Chi earthquake. He separated the Chi Chi earthquake into four subevents and demonstrated that the source of early arrival coherent energy was close to the epicenter and that the subsequent major energy moved southward (Figures 13a and 13b). He Figure 12. (a) Geological map along the Chaoshui River area [CPC, 1982]. Seismic profile HV2 is beneath the area of the Chaoshui River, which is near the epicenter of the Chi Chi earthquake. (b) Seismic profile interpretation by Wang et al. [2002a]. (c) Seismic profile interpretation by Hung and Suppe [2002]. (d) Reinterpreted seismic profile of the present study, which shows an imbricate structure on the hanging wall and both the Chelungpu fault and Chushiang faults cutting into the Miocene strata. The epicenter is near the boundary of the ramp and flat structure of the slip plane. The projected GPS vectors show coseismic displacement along this section. The dashed line is the distribution of vertical displacement, which progressively decreases from the surface rupture to the east before subsiding on a flat plane. 15 of 18

Figure 14. Schematic model of the bilateral rupture. (a) The rupture reaches the surface after 6 s with 2.1 km/sec velocity [Ji et al., 2003]. (b) From 12 to 24 s, the rupture propagated southwestward with SW trending displacement associated with surface rupturing from Chaoshui River to Tongtou. (c) At the same time (12 to 18 s), the rupture also propagated northward triggering the Chushiang and Luliao faults. thought a bilateral fault rupture produced major slips at the southern end of the Chi Chi earthquake. Figure 13a shows that rupture nucleation lasted 8 s at the epicenter. At 11 to 20 s., it started to propagate south in a direction of 250 ; the rupture then moved in a northwest direction. Waveform and GPS inversions also show similar results in terms of bilateral fault rupturing [Ji et al., 2003; Zeng and Chen, 2001]. Figure 13c gives snapshots of the 1999 Chi Chi earthquake source rupture. It is obvious from Figure 13 that after the initial rupture, the rupture propagated southward and northward for about 9 to 18 s [Ji et al., 2003]. [24] How this bilateral rupture process can be interpreted at the complex coseismic displacement field of the southern end of the Chi Chi earthquake is considered in Figure 14, which gives a schematic kinematic model. Upon nucleation, the earthquake rupture slipped from the epicenter toward the west (Figure 14a). The rupture then moved southwestward relative to the epicenter with bilateral fault rupturing occurring at the southern end of the earthquake fault [Huang, 16 of 18

2001; Ji et al., 2003]. This southwestward propagating rupture triggered the Chelungpu fault south of Chushan. At the Chelungpu fault this resulted in thrusting dominantly with a right lateral component. Consequently, the NE striking Tachienshan fault was then reactivated with right lateral strike slip faulting (Figure 14b). The rupture then propagated in a northwest direction activating the Luliao fault with left lateral strike slip faulting and the Chushiang fault producing thrusting dominantly with a left lateral component (Figure 14c). 5. Conclusions [25] In this study, third order digital cadastral data are used to give detailed horizontal coseismic displacement in the Chushan area at the southern end of the Chi Chi earthquake. [26] For the 2.76 km 2 study area between GPS station M493 and the fault trace of the Chelungpu fault, there are 5867 third order cadastral control points. Horizontal displacement was 1.2 m to 2.6 m in 220 to 250 on the hanging wall and 1.4 m in 105 on the footwall. [27] According to the distribution of horizontal coseismic displacement, the southern end of the Chi Chi earthquake can be divided into three distinct blocks: (1) Block A is between the Chelungpu and Chushiang faults and trends in a SW direction; (2) Block B is between Chushiang and Luliao faults and is NW trending; and (3) Block C is between Luliao and the Tachienshan and Meishan faults and is SW trending. [28] The kinematic relationship between these three blocks suggests all the above faults were active during the Chi Chi earthquake. The Chushiang fault shows thrusting dominantly with a left lateral slip component, while the Luliao fault is a left lateral strike slip fault. [29] Using the geological map, we interpreted seismic profile HV2 beneath the Chaoshui River area. It is evident from the imbricate structure beneath the Chaoshui River that the Chelungpu and Chushiang faults cut into Miocene strata. This finding contradicts the possibility of bedding slip along the Chinshui shale. The ramp flat boundary is interpreted to be at 9.2 km in depth. Kinematically, such a complex coseismic displacement field may have resulted from a bilateral rupture process. Under such a scenario, a southward rupture with SW trending movement results in right lateral strike slip faulting of the Tachienshan fault and thrusting with a right lateral component on the NS strike of the Chelungpu fault. The northward rupture, meanwhile, is associated with NW trending movement that results in activation of the Luliao and Chushiang faults. [30] Acknowledgments. We are grateful to Stephane Dominguez, two anonymous reviewers, and the Associate Editor Rodolfo Console for constructive and helpful reviews of the manuscript. 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