REPORT #4 on the Chi-Chi (Taiwan) Earthquake

Tectonic Setting and Regional Geology of Taiwan

ByJ.G. Liou and L.Y. Hsiao

Dept. of Geological and Environmental Sciences
Stanford University, Stanford, CA 94305 (Oct. 1st, 1999)



Taiwan is located at a compressive tectonic boundary between the Eurasian Plate and the Philippine Sea Plate; the present convergence rate is about 7 cm/year in a NW-SE direction (Figure 1). To the east of Taiwan, the Philippine Sea plate is subducting beneath the Ryukyu island arc, and to the south of Taiwan, the Philippine Sea plate is overriding the Eurasian plate along the Manila trench (Figure 2). Taiwan, therefore, occupies an unstable region between these two well-defined subduction systems of opposite polarity. One of the profound interactions between these two plates has been the ongoing oblique collision of the Luzon volcanic arc with the Eurasian continent in the vicinity of Taiwan since about 4 Ma. This impacting is currently propagating southward. Reflecting active oblique convergence and crustal indentation, seismicity, folding, and thrusting have been extremely active in this island.

figure 1

Figure 1 (after Ernst et al., 1985). Regional plate tectonic setting of Taiwan at the intersection of Ryukyu and Manila trenches with the continental margin of Asia, largely after Hamilton (1979). The arrow indicates the approximate motion of the Philippine Sea plate relative to Asia (7 cm/yr. [3 in./year] according to Seno, 1977). The Philippine Sea plate is descending obliquely beneath the Ryukyu arc but is being thrust over the South China Sea and Chinese continental margin. Thus, relative to the Asiatic plate, the Manila trench is moving concomitantly westward with time. The Philippine trench appears to be propagating northward (Lewis and Hayes, 1983).

figure 2

Figure 2 (after Ernst et al., 1985). Schematic diagram showing present plate tectonic configuration of Taiwan. Whereas the convergent plate junction located seaward from the Ryukyu arc is a well-defined linear feature, the eastward underflow of the Asiatic plate beneath the Philippine Sea and surmounting Coastal Range (the Luzon calc-alkaline arc) is being choked off because of accumulation of imbricated passive margin wedge (+ forarc basin) and distributory thrust fault system present throughout the main part of Taiwan. The Longitudinal Valley marks the juxtaposition of exotic Coastal Range (including its forarc basin) with the miogeoclinal wedge and its Asiatic basement; a minimum of 150 km (90 mi.) left-lateral slip has taken place along this strike-slip fault system since the Pliocene onset of the collisional event (Tsai, 1978).


Figure 3 shows the distribution of five major lithotectonic terranes of Taiwan. From west to east, they are: (a) the Coastal Plain; (b) the Western Foothills; (c) the Hsuehshan and Backbone Ranges; (d) the Tanano Basement Complex; and (e) the Coastal Range. Units (a)-(d) belong to the eastern margin of the Eurasian lithospheric plate, whereas (e) represents the leading, western edge of the Philippine Sea plate. The Coastal Plain consists of Pleistocene and younger terrace gravel, alluvium, and well-bedded but poorly consolidated clastic sediments.

figure 3

Figure 3. Major tectonic units of Taiwan (after Lu et al., 1998)

Units (b) and (c) represent the active fold-and-thrust belt of western Taiwan. These Cenozoic strata represent continental shelf deposits, and are comprised of sandstone, quartzite, slate, argillite, and tuff of Oligocene to Pliocene age. They are typified by imbricate thrusts and folds with west vengeance, reflecting up to 160-200 km horizontal shortening (Suppe, 1980). The Backbone Range slate is a huge mass of dark gray phyllite intercalated with sandstone and limestone blocks of Eocene to Miocene age, and igneous rocks such as pillow lava, diorite, gabbro, minor andesite, diabase and pyroclastic products. Informally known in aggregate as the slate series, this nonturbiditic sedimentary prism consists of continental shelf to slope deposits.

The Paleozoic/Mesozoic basement is composed of three lithologic units. The first has been interpreted as a Mesozoic melange complex marking an ancient accretionary wedge or suture zone. The second consists exclusively of massive marbles dated radiometrically as Paleozoic. The third unit consists of metamorphosed Cretaceous granitoid rocks.

Eastward across the tectonic boundary along the Longitudinal Valley, which is a zone of modern intraorogenic left-lateral strike-slip vertical faults, lies the Coastal Range. It consists of Miocene Luzon arc calc-alkaline volcanics and associated sediments, forearc-basin flysch deposits, and the trench-basin Lichi melange enclosing abundant olistostromal debris including the fragmented and dispersed East Taiwan Ophiolite.


Imbricated Neogene stratas in the Western Foothills were assembled as a combination of folds and thrust faults during the Pleistocene orogeny. The Tertiary strata consist of mainly unmetamorphosed sandstone and shale, and have been flexed into NE-SW trending asymmetrical folds; at least seven major thrusts have been recognized. For structural and lithological details, see Ho (1986, Figure 7).

The mechanism of thin-skinned tectonics has been used to explain the recognized fold-and-thrusting patterns in western Taiwan (Ho, 1986). According to this model, thrusting was relatively shallow and confined to the cover sequence above the basement, resulting thin-skinned deformation. Related to these decollements, most thrust faults are east-dipping high-angle reverse faults. However, recent studies in many parts of the fold-and-thrust belt of south-central Taiwan demonstrate that some basins (e.g., the Puli Basin; see Figure 3 for the locality) developed within extended terranes; evidently, thrusting took place through reactivation of extensional faults and resulted in the formation of complicated thrust geometries. Hence, reactivated normal faults may play a significant role in the compressive deformation in the fold-and-thrust belt of western Taiwan (Suppe, 1986; Chang et al., 1996). This mechanism has been considered to represent inversion tectonics.

Because of the oblique convergence and the indentation of the Philippine Sea plate, the fold-and-thrust belt of the majority of the Western Foothills developed a combination of contractional, transcurrent, rotational, and extensional deformation features, which resulted in thrusting, strike-slip and normal faulting (Liu et al., 1998). To the north, however, this fold-and-thrust belt is subjected to crustal stretching and rifting due to the flip of subduction of the Philippine Sea Plate (Teng, 1996).


According to Prof. Wang Chian-Yin (in http://www-ses.gep.ncu.edu.tw), the Chi-Chi earthquake (7.3 magnitude on the Richter scale) and many aftershocks with magnitude up to 6.8 on 9-21-99 and continuing may be associated with the Chelungpu and Shuangtung faults; these two faults are 10 km apart and subparallel (see Figure 4 for traces of these and other faults and Figure 5 for a cross section in the vicinity of epicenter areas). The hypocenter at Chi-Chi lies very close to the Shungtung fault and occurred at a depth of about 10 km, near the intersection with the Chelungpu fault. The Chelungpu fault extends for more than 80 km, and changes course from nearly EW trending to nearly NS, whereas the Shuangtung fault trends nearly N-S; these and other faults are E-dipping thrust faults with significant left-lateral strike-slip component. This earthquake caused meters of displacement along certain sections of the Chelungpu fault. See www-ses.gep.edu.tw for short description of these faults related to the 9-21-99 earthquake.

figure 4

Figure 4. Schematic map showing the traces and general geology of the 9-21-99 earthquake region (modified after Ho, 1986)

figure 5

Figure 5. Schematic E-W cross section showing series E-dipping thrusts in the fold-and-thrust belt of the Western Foothill of Taiwan (modified after Lee et al., 1996)

On the other hand, some high-angle reverse faults in central Taiwan have been recognized to be associated with large-magnitude earthquakes [see Report #5 of this Series (Bonilla, 1999)]. This includes the earthquake at Chihhu and Tuntzuchio in 1935 (7.0 Richter magnitude) described by Bonilla (1975). Such high-angle detachment west-dipping reverse faults may have resulted from the reactivation of pre-existing normal faults in the Western Foothills.

Because of the limited time, we did not cover some information from many recent papers from Taiwan. The following references summarize the general tectonics and regional geology of the area. For a review of neotectonics of the Taiwan mountain belt, see Liu et al. (1998), and for the general geology of the epicenter of the 9-21-99 Chi-Chi earthquake, see Ho (1986, p. 112-113). Please also refer to the contents of the following web sites for detailed information about the Chi-chi Earthquake:

http://www.sinica.edu.tw/~jclee/921chichi_main_ch.htm (In Chinese);

http://www-ses.gep.ncu.edu.tw (In Chinese);


Bonilla, M. G., 1975. A Review of recently active faults in Taiwan, U. S. Geological Survey, Open-File Report 75-41, 58 pp and 14 figures.

Bonilla, M. G., 1999. A note on historic and Quaternary faults in western Taiwan, U. S. Geological Survey, Open-File Report 99-447, 5 pp.

Chang, Y. L., Lee, C. I., Lin, C. W., Hsu, C. H., Mao, E. W., 1996. Inversion tectonics in the fold-trust belt of the foothills of the Chiayi-Tainan area, southwestern Taiwan. Petroleum Geology of Taiwan, no. 30, p. 163-176.

Ernst, W.G., C.S. Ho, and J.G. Liou, 1985, Rifting, drifting and crustal accretion in the Taiwan sector of the Asian continental margin. in Tectonostratigraphic Terranes of the Circum-Pacific Region (ed. David Howell), Circum Pacific Energy & Mineral Resource, Earth Sci. Series, no.1, p. 375-390.

Ho, C. S., 19988. An Introduction to the Geology of Taiwan: Explanatory text of the Geologic Map of Taiwan. Central Geological Survey, Taipei, Taiwan, RO China, 192pp.

Lee, J.C., Lu, C.Y., Chu, H.T., Delcaillau, B., Angelier, J., and Deffontaines, B., 1996. Active deformation and paleostress analysis in the Pakua anticline area, western Taiwan: TAO, v. 7, n. 4, 431-446.

Lu, C. Y., Yu, S. B., and C. H. T., 1998. Neotectonics of the Taiwan Mountain Belt. in Flower et al., (eds) Mantle Dynamics and Plate Interactions in East Asia. American Geophysical Union, Geodynamic series, v. 27, p. 301-315.

Teng, L. S., 1996. Extensional collapse of the northern Taiwan mountain belt. Geology, v. 24, p. 949-952.

Suppe, J., 1981. Mechanics of mountain building and metamorphism in Taiwan. Memoir Geol. Soc. China, no. 7, p. 67-8

Suppe, J., 1986. Reactivated normal faults in the western Taiwan fold-and-thrust belt. Memoir Geol. Soc. China, no. 7, p. 187-220

To contact the senior author, email Juhn-Guang Liou (liou@pangea.stanford.edu).

The URL of this page is http://caldera.wr.usgs.gov/chi-chi4.html
Date created: 10/02/1999
Last modified: 10/02/1999

Email technical-support comments to Mike Diggles (mdiggles@mojave.wr.usgs.gov).