Yuan-Hsi Lee

Surface Rupture of 1999 Chi-Chi Earthquake Yields Insights on Active Tectonics of Central Taiwan

The 1999 Chi-Chi earthquake was caused by rupture of the Chelungpu fault, one of the most prominent active thrust faults of Taiwan. This largest of Taiwans historical fault ruptures broke the surface for over 90 km at the western base of the rugged mountain range. A short right-lateral tear extended southwestward from the southern end of the Chelungpu fault, and a complex assemblage of shallow folds and faults ran northeastward from the northern end. Vertical offsets averaged about 2 m along the southern half of the Chelungpu fault and about 4 m along the northern half, and offsets of 5 to 7 m were typical along the northern part of the major thrust. The sinuous nature of the surface trace is consistent with seismographic data that indicate a dip of about 30°. The 1999 rupture draws attention to the fact that this active fault system is highly segmented and that this segmentation influences the characteristics of seismic ruptures. Active faults to the south, north, and west of the Chelungpu fault have distinctly different characteristics. Faults to the south and north broke the surface during earthquakes in 1906 and 1935. The active Changhua fault to the west, a blind thrust similar in length to the Chelungpu, has not ruptured in the historical period and should be considered a prime candidate for generating a future earthquake. Manuscript received 4 October 2000.

The Arc–Continent Collision in Taiwan

We present a new compilation of magnetic, geologic, GPS and seismic data and propose that the geometry and kinematics of the Taiwan arc–continent collision are dominated by the partial subduction of a continental margin promontory and associated fracture zone. A prominent magnetic high in the pre-collision zone southwest of Taiwan serves as proxy for the edge of the continental crust of normal thickness (i.e., ~30 km). The high ends abruptly in central Taiwan, suggesting truncation by a NW-striking fault zone interpreted as a rift-related transfer zone. The NW-striking fault zone correlates with a steeply dipping, crustal-scale cluster of earthquakes recognized as the Sanyi-Puli seismic zone, indicating reactivation of the transfer zone. The truncated anomaly and transfer zone define a triangular-shaped continental margin promontory partially subducted beneath Taiwan. Island-wide GPS data show: (1) divergent flow around the promontory; (2) significant shortening between the promontory and the arc in the central part of Taiwan; and (3) significant lateral extrusion of the orogen south of the promontory–arc collision. Collision of the relatively rigid promontory resulted in a tripartite division of the fold and thrust belt as it developed from about 2 Ma to present and the development of a recess or syntaxis in the central part of the Central Range. The syntaxis correlates with the highest topography, thickest crust and highest values of P-wave attenuation, suggesting that it may be an area of high rates of rock uplift and exhumation, consistent with other orogenic syntaxes recognized around the world. This interpretation is also consistent with geomorphic parameters from the central part of the Central Range that indicate high rates of uplift and with the absence of seismicity in this area. Although detailed thermochronologic and kinematic data are limited in central Taiwan, the available data suggest a recent increase in rates of exhumation consistent with the interpretation that the promontory collided relatively recently (~2 Ma).

Influence of Surface-Normal Ground Acceleration on the Initiation of the Jih-Feng-Erh-Shan Landslide during the 1999 Chi-Chi, Taiwan, Earthquake

The 1999 Chi-Chi, Taiwan, earthquake triggered numerous landslides throughout a large area in the Central Range, to the east, southeast, and south of the fault rupture. Among them are two large rock avalanches, at Tsaoling and at Jih- Feng-Erh-Shan. At Jih-Feng-Erh-Shan, the entire thickness (30-50 m) of the Mio- cene Changhukeng Shale over an area of 1 km 2 slid down its bedding plane for a distance of about 1 km. Initial movement of the landslide was nearly purely trans- lational. We investigate the effect of surface-normal acceleration on the initiation of the Jih-Feng-Erh-Shan landslide using a block slide model. We show that this ac- celeration, currently not considered by dynamic slope-stability analysis methods, significantly influences the initiation of the landslide.

Slip vectors of the surface rupture of the 1999 Chi-Chi earthquake, western Taiwan

About 100 slip vectors were surveyed along the 1999 Chi-Chi earthquake rupture (100 km long), western Taiwan, to understand the coseismic reverse-faulting processes. The surveyed slip azimuths vary locally (over 80°) where the rupture is irregular or associated with flexure. Even where the rupture is straight and has consistent slip azimuths, the fault-scarp heights, horizontal slips and fault dip-angles oscillate in a distance of hundreds of meters along the rupture. Despite these local variations, the net slip (up to 11.5 m), vertical and horizontal slips are significant greater in the north, on which maximum shortening (6 m; by pure thrust) and strike slip (8 m; by left-lateral fault) occur in association with the bend of the rupture. The slip azimuths also progressively rotate from north/northwest in the north to west/southwest in the south. These patterns generally agree with those revealed by GPS surveys, except that the slips by GPS in the south move to the west/northwest and have values unexpectedly smaller than our measurements. Our obtained SW-directed slips resulted in dominantly right-lateral faulting (up to 2.4 m) where the rupture ends to the southeast. This, however, does not correspond to focal mechanisms of any main shocks or aftershocks.

Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology

The Longmen Shan, located at the boundary between the Tibetan Plateau and the Sichuan Basin, has received considerable attention following the 2008 Wenchuan earthquake. However, the tectonic history of the southwestern segment of the range has remained poorly constrained. We present zircon fission-track, zircon (U-Th)/He, and apatite (U-Th)/He data from the Baoxing region in the southwestern Longmen Shan that provide the first constraints on the cooling and exhumation history of the region. All of the measured ages are Cenozoic, and the data suggest that exhumation of the Baoxing region was ongoing by ca. 15 Ma. Zircon (U-Th)/He ages from several samples appear to be affected by radiation damage, suggesting that damage may be a concern even in samples with Cenozoic cooling ages. Samples were collected from two bodies of Precambrian crystalline rocks separated by the Wulong fault, and for all three thermochronometers, ages west of the Wulong fault are systematically younger than ages to the east, indicating that the fault has accommodated differential exhumation since 8–10 Ma. The regions east and west of the Wulong fault have experienced 7–13 km and at least 7–10 km of exhumation, respectively. The magnitude of exhumation in the southwestern Longmen Shan is similar to that reported in the central Longmen Shan, indicating consistency along strike. The thermochronology data also suggest that the Erwangmiao fault in the southwestern Longmen Shan is analogous to the Beichuan fault in the central Longmen Shan, and therefore may represent a source of seismic hazard.

1999 Chi-Chi Earthquake: A Case Study on the Role of Thrust-Ramp Structures for Generating Earthquakes

The 21 September 1999 Chi-Chi earthquake ( M w 7.6) occurred on east-dipping shallow thrust faults that produced a high-relief surface rupture. Extraordinary surface breaks appeared that could be clearly traced for about 100 km across many counties. These thrust faults, the Chelungpu and Shihkang, are part of an active fold-and-thrust belt related to ongoing recent arc-continent collision. Measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a westward direction (N70-90°W) on the Chelungpu fault to a northwestward direction (N30-40°W) on the Shihkang fault. The stress field underwent a clockwise rotation of about 40° during the Chi-Chi earthquake. Near-rupture vertical displacements in the hanging wall of the Shihkang fault have more cumulative displacement than on the Chelungpu fault, which is consistent with Global Positioning System (GPS) measurements. Maximum vertical offset on the rupture was found to be about 10 m by the surficial rupture and GPS measurements. In addition, analysis of crustal deformation by GPS measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure. Our synthesis of geological and geodetic analyses shows the importance of ramp structures associated with thrust faults for generating large earthquakes and provides a general framework for understanding earthquake in fold-and-thrust belts. Large surficial coseismic uplift and strong asperities appear to be a function of fault ramp geometry. Our analysis also indicates that, in general, ramp structures in fold-and-thrust belts may have a high potential in generating large earthquakes. Manuscript received 14 November 2000.

Simultaneous mountain building in the Taiwan orogenic belt

The obliquity of the arc-continent collision in Taiwan is thought to have produced a southward-propagating orogenic system starting as early as 8 Ma. We combine 62 new zircon fission-track ages with timing of rapid subsidence in the foreland basin to identify the timing of the onset of exhumation and orogenesis. The oldest completely reset zircon fission-track ages and the onset of rapid tectonic subsidence in both the northern and southern parts of the foreland basin are ca. 5–6 Ma. We propose that north-south rifting in the South China Sea created a north-trending continental margin before the collision rather than the commonly assumed northeast-trending margin. Consequently, both this north-trending continental margin and the Luzon arc of the Philippine Sea plate were subparallel, resulting in an initial collision that was simultaneous along strike.

Structures Associated with the Northern End of the 1999 Chi-Chi Earthquake Rupture, Central Taiwan: Implications for Seismic-Hazard Assessment

Abstract The surface rupture of the 1999 Chi-Chi earthquake ( M w 7.6) trendsmore than 100 km in a north–south direction. Surface deformation at the northernend stops abruptly at an area between the Tachia River and the Taan River where abroad pop-up structure with east to northeast strike can be found that has a trenddifferent from the north–south-striking main thrust. We combine the absolute ele-vation data before and after the Chi-Chi earthquake to obtain the regional verticaldisplacement and the magnitude of the pop-up structure. The greatest uplift couldreach as high as 15–16 m. Using deformation magnitude and the area-balancingmethod, we measure the depth of the detachment to show the subsurface geometryof the Chelungpu fault at its northern end. This shows that the geometry of theChelungpu fault controls termination of the surface rupture and the depth of thedetachment controls the amount of deformation.Introduction Perhaps the most unexpected geological feature asso-ciated with the Chi-Chi earthquake (

Pleistocene onset of rapid, punctuated exhumation in the eastern Central Range of the Taiwan orogenic belt

The Taiwan orogenic belt is often treated as a steady, southward-propagating orogenic system with an essentially constant erosion rate of 4–6 mm/yr over the past 5 m.y. We present 4 new age-elevation transects from the Central Range based on 19 new and 86 previously published fission track and (U-Th)/He dates of completely reset detrital zircon and apatite grains. The age-elevation curves and thermal models imply slow cooling prior to ca. 2–1.5 Ma (at exhumation rates of ∼0.1 mm/yr), an increase in exhumation rates from ca. 2–1.5 Ma to ca. 0.5 Ma (2–4 mm/yr), and possibly a further acceleration in exhumation from ca. 0.5 Ma to present (4–8 mm/yr). Three transects from three different latitudes in the eastern Central Range yield similar results, each showing punctuated exhumation with progressively faster rates.

Revealing Surface Deformation of the 1999 Chi-Chi Earthquake Using High-Density Cadastral Control Points in the Taichung Area, Central Taiwan

The 1999 Chi-Chi earthquake, M w 7.6, ruptured a major thrust fault along the western foothills of the Central Mountain Range of Taiwan. Here, we use cadastral control points to detect horizontal displacement in Taichung, central Taiwan. The cadastral control points were used to identify the coordinates of buildings and acreage. The coordinates of each point are measured by total station and connected to Global Positioning System (gps) control points. The density of these control points is nearly 36 points/km2 and the accuracy of each point is within less than 2–3 cm. The government of Taiwan measured cadastral control points before and after the Chi-Chi earthquake in the Taichung area (northern end of the Chelungpu fault); 1269 control points were measured over a 35-km2 area with most control points being located on the hanging wall of the Chelungpu fault. On the footwall, the displacement is about 1.1–1.2 m toward 123° to 128°, which is consistent with gps data. At the hanging wall, the displacement direction concentrates at 326° to 330°. In the eastern section, the displacement is from 7.5 to 8 m, which is consistent with gps and strong- motion data, but in the western part displacement changes from 8 m to 4–6 m and then increases again to 8 m along the 326° trending. We conclude that the changes in displacements were controlled by the geometry of the fault plane. Online material: Horizontal displacement of cadastral control points.