Eos, Vol. 80, No. 50, December 14,1999 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION VOLUME 80 NUMBER 50 DECEMBER 14,1999 PAGES 605 620 The Chi-Chi,Taiwan Earthquake: Large Surface Displacements on an Inland Thrust Fault V 4 Eurasian Plate ; ^ %ukyu Trer^ PAGES 605,611 In the early morning (01:47 local time) of September 21,1999, the largest earthquake of the century in Taiwan (Mw=7.6, ML=7.3) struck the central island near the small town of Chi-Chi.The hypocenter was located by the Central Weather Bureau Seismological Center at 23.87 N, 120.75 E, with a depth of about 7 km. There were extensive surface ruptures for about 85 km along the Chelungpu fault with vertical thrust and left lateral strike-slip offsets. The maximum displacement of about 9.8 meters is among the largest fault movements ever measured for modern earthquakes. There was severe destruction in the towns of Chungliao, Nantou,Taichung, FengYuan,and Tungshi, with over 2300 fatalities and 8700 injuries. The Chi-Chi earthquake was recorded by a dense strong motion network in Taiwan [Liu et al., 1999], providing one of the best strong-motion data sets ever recorded for a destructive earthquake. Tectonic Environment Taiwan is located in a complex, tectonically active region.the Philippine Sea plate is subducting northwestward along the Ryukyu Trench in the north and the Eurasian plate underthrusts the Philippine Sea plate along the Manila Trench in the south (Figure 1). The Philippine Sea plate is moving northwestward at the rate of -7-8 cm/yr [Yu, 1997] relative to the Eurasia plate, creating the Taiwan collision zone. Even though recent seismicity clusters have mostly been in northeast Taiwan, historically, damaging earthquakes have mainly occurred in western Taiwan because events in this region have more shallow focal depths and occur in close proximity to major population centers (Figure 2).The most notable earthquakes this century are the 1906 Chiayi (M7.1), 1935 Hsinchu- Taichung (M7.1), 1941 Chiayi (M7.1),and 1964 Baiho earthquakes (M6.5). Adding the recent Chi-Chi earthquake to the list, this cluster of large M>7.0 earthquakes within one century on the small island of Taiwan is rather striking. Historical records indicate, however, that this seismic activity is not unusual and that the high rate of significant earthquakes reflects the high tectonic stresses in the Taiwan collision zone. Active Faults Western Taiwan is a complex region of faulting, thought to be a stress boundary due to the collision of the Philippine Sea plate to Eurasian plate [Yu et al., 1997].Seismicity is diffuse and, with few exceptions, it is difficult to associate recent moderate earthquakes (M>5.0) to any specific fault. An active fault is defined as one that has moved within the last 100 Ka and possesses potential to move in the future. Figure 2 shows the distribution of the active faults in Taiwan [Tsai et al, 1998].The epicenter of the Chi-Chi event is located between the Chelungpu and Shuantung faults.the hypocenter and focal mechanisms determined from the Harvard moment tensor solution and the first motions from the Central Weather Bureau Seismological Center (CWBSN) are consistent with slips on the Chelungpu fault, which dips shallowly to the east. Interestingly, the Chi-Chi earthquake occurred in the middle of a region where recent background seismicity was relatively low (Figure 2). Surface Rupture The surface rupture from the September 21,1999 Taiwan earthquake extends for about 75 km along the north-south trending Chelungpu fault with vertical displacements 120' 122* Fig. 1. Tectonic setting in Taiwan region. The asterisk indicates the epicenter of the 1999 Chi-Chi, Taiwan earthquake. of 1-8 m.at the fault's northern end, near Fengyuan, it curves toward the northeast and splinters into complex branches.this area of faulting, which trends toward the northeast, extends for an additional 10 km and was not previously identified as an active fault. In this region of complex faulting there are some very large vertical and left-lateral horizontal offsets. At one site, the Tachia River was cut by an 8 m displacement that formed a new waterfall and destroyed a bridge (Figure 3). Seismic reflection data indicate that the Chelungpu fault dips shallowly (25-30 ) toward the east [Wang, 1999; personal communication],so the observed surface displacement may only be about half of the total slip on the fault. Figure 2 also shows the distribution of the observed surface rupture throughout the region. Intensity and Response The Taiwan Rapid Earthquake Information System operated by CWBSN [Wu et al, 1997] reported the earthquake's location, magnitude, and intensity distribution within two minutes. This quick broadcast of information enabled a fast emergency response by the government. The earthquake was also recorded by the dense Taiwan Strong Motion Network, providing one of the best data sets for a damag-
Eos, Vol. 8 0, No. 5 0, D e c e m b e r 14,1999 Fig. 2. The identified active faults and background seismicity distribution. The red asterisk indicates the epicenter of the 1999 Chi-Chi earthquake. The focal mechanisms determined from the Harvard moment tensor solution and the Central Weather Bureau Seismological Center are shown with equal area projection of the lower hemisphere. The black asterisks indicate the historical M>7 events in this century. The bold red line indicates the fault trace ruptured at this earthquake. The broad-up figure to the left shows the measured surface displacement along the fault trace. The casualties from this earthquake are also shown by the number of fatalities of every ten thousand population in the surrounding towns near the fault trace and Taipei. Original color image appears at the back of this volume. ing earthquake (Figure 4 ). T h e instrumental intensity map (Figure 4 ) shows that the max imum horizontal peak ground a c c e l e r a t i o n (PGA) observed for this earthquake is over lg, which is about 1.2 times larger than that observed for the 1995 Kobe earthquake. The eastern region of the fault generally has a PGA value greater than 0.5 g. Hundreds of stations throughout Taiwan had PGA values greater than 25 gal.this extensive distribu tion of significant a c c e l e r a t i o n s produced d a m a g e over much of the island. The most shaking damage was in towns near the e p i c e n t e r along the southern and northern portions of the fault. In general, the hanging wall side of the thrust fault (east of the surface t r a c e ) had more d a m a g e than the foot wall side to the west. The surface faulting in the north had the largest displacements but did not c a u s e the worst shaking damage. The areas around the 8 m vertical displacements showed m u c h less shaking d a m a g e than e x p e c t e d and low er PGA than in the south. Most of the struc tural d a m a g e in the north was limited to areas directly on the surface trace of the fault and the d a m a g e was c a u s e d by ground deformation rather than strong shaking. The lack of extensive shaking damage in the north might b e explained by vertical move ments of the hanging wall that were rather smooth and slow. T h e observed strong motion data recorded along the fault trace as shown in Figure 4 are consistent with this explanation. Stations from the south to the north show a significant c h a n g e of dominant frequency, with stations in the south showing higher frequency c o n t e n t than stations in the north.this topic is being studied further. There were also a few severely affected structures in Taipei City, which is about 150 km away from the epicenter. The collapse of o n e high-rise building in Taipei City is proba bly related to the focusing effects of the re gional basin structure and building con struction problems. Aftershock More than 10,000 aftershocks were record ed in the first three weeks following the mainshock, including over 100 felt events and 5 aftershocks of magnitude greater than 6.0.This aftershock s e q u e n c e is more ener getic than average for an earthquake of this size. Large, felt aftershocks c a u s e d panic, damage, and s o m e additional casualties. The large aftershocks all o c c u r r e d east of the Chelungpu fault and are aligned with the Sanyi-Puli seismic zone to the north (Figure 4 ). T h e s e features imply that the rupture of the Mw7.6 m a i n s h o c k might have activated
Eos, Vol. 8 0, No. 5 0, D e c e m b e r Fig. 3. A photo taken right after the earthquake. The Tachia River was cut by an 8 m that formed a new waterfall and destroyed this bridge. Original color image appears of this volume. 14,1999 displacement at the back other fault systems to the east of the Chelungpu fault.the boundary of the acti vated region appears to b e controlled by the stress system from the collision of the Philip pine S e a and Eurasian plates, which is also s e e n in the patterns of the digital topogra phy map of Taiwan. The abundant data c o l l e c t e d on the ChiChi earthquake will b e valuable for both seismologists and earthquake engineers.this earthquake produced s o m e of the largest surface displacements ever observed from recent events. In addition, it provided o n e of the most c o m p l e t e strong-motion data sets recorded for a damaging earthquake. These new findings will help seismologists, geolo gists, and engineers understand the c o m p l e x pattern of surface faulting and its relation to the earthquake s o u r c e and regional d a m a g e patterns. CWB is s c h e d u l e d to publish all the recorded strong-motion data systematically with digital data files on CD-ROM, so that these data are available to everyone for research. [Lee et al, 1 9 9 9 ]. Acknowledgments The authors thank Chien-Ying Wang, KuoLiang Wen, Horng-Yuen,Yih-Min Wu, and Chien-Hsin Chang, and Masataka Ando for data c o l l e c t i o n, discussion, and significant contributions to this article. Authors Kuo-Fong Ma, Chyi-Tyi Lee, and Yi-Ben Tsai, the Yen Institute of Geophysics, National Central University, Chung-Li, Taiwan, ROC; Tzay-Chyn Shin, the Central Weather Bureau Seismological Center, Taipei, Taiwan, ROC; and Jim Mori, the Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan References %m ttr Fig. 4. The dense Taiwan Strong Motion Stations distribution (triangles) and the instrumental intensity map shown by contours with intervals of 100 gal. The red lines indicate the identified active faults. The bold red line indicates the surface ruptured fault trace during this earthquake. The large felt after shocks are indicated by solid circles. The red asterisk indicates the epicenter of the mainshock. The seismograms to the left are the accelergrams recorded by the stations from north at station TCU068 to the south at station CHY028 along the fault. The numbers on the accelergrams indicate the peak ground acceleration in gal. Original color image appears at the back of this volume. Lee,W. H. K.,T.C.Shin, K.W Kuo,and K. C.Chen, Digital strong-motion data from the 921 Chi-Chi earthquake recorded by the Central Weather Bureau,Technical Report of the Seismology Cen ter, Central Weather Bureau,Taipei,Taiwan, 1999. Liu, K.-S.,T.-C. Shin, and Y-B.Tsai,A free-field strong motion network in Taiwan: TSMIP, TAO, 10,377396,1999. Tsai,Y.-B.,C.Y.Wang, C.T. Lee, M.T. Hsu,and K.S. Liu, Investigation of Taiwan active faults, Report on Ministry of Education, 271,1998. Yu, Shui-Bei, H.-Y Chen, a n d L.-C. Kuo,Velocity field of GPS system stations in the Taiwan area, Tectonophysics, 274,1-3,41-59,1997. Wu,Yih-Min,T.-C.Thin, C.-C. Chen,Y.-B.Tsai, W. H. K. Lee, and T.-L.Teng,Taiwan rapid earthquake infor mation release s y s t e m, S e i s m o l. Res. Lett., 68,6, 931-943,1997.
Eos, Vol. 80, No. 50, December 14, 1999 Fig. 2. The identified active faults and background seismicity distribution. The red asterisk indicates the epicenter of the 1999 Chi-Chi earthquake. The focal mechanisms determined from the Harvard moment tensor solution and the Central Weather Bureau Seismological Center are shown with equal area projection of the lower hemisphere. The black asterisks indicate the historical M>7 events in this century. The bold red line indicates the fault trace ruptured at this earthquake. The broad-up figure to the left shows the measured surface displacement along the fault trace. The casualties from this earthquake are also shown by the number of fatalities of every ten thousand population in the surrounding towns near the fault trace and Taipei. Page 605
Eos, Vol. 80, No. 50, December 14, 1999 Fig. 3. A photo taken right after the earthquake. The Tachia River was cut by an 8 m displacement that formed a new waterfall and destroyed this bridge.
Eos, Vol. 80, No. 50, December 14, 1999 120-121" 122' 50 100 150 200 300 400 600 800 1000 (gal) Fig. 4. The dense Taiwan Strong Motion Stations distribution (triangles) and the instrumental intensity map shown by contours with intervals of 100 gal. The red lines indicate the identified active faults. The bold red line indicates the surface ruptured fault trace during this earthquake. The large felt aftershocks are indicated by solid circles. The red asterisk indicates the epicenter of the mainshock. The seismograms to the left are the accelergrams recorded by the stations from north at station TCU068 to the south at station CHY028 along the fault. The numbers on the accelergrams indicate the peak ground acceleration in gal.