Chyi-Tyi Lee

The Chi‐Chi, Taiwan earthquake: Large surface displacements on an inland thrust fault

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.

Site Classification of Taiwan Free-Field Strong-Motion Stations

Based on the available geologic and geomorphologic data, 708 Taiwan Strong-Motion Instrumentation Program (TSMIP) free-field strong-motion station sites are classified using a scheme compatible with the 1997 Uniform Building Code (UBC) provisions. Results show that an extensive usage of geologic maps and geomorphologic data for site classification is satisfactory and is a quick and effective method for categorizing large numbers of strong-motion station sites. The response spectral shapes (RSS) method and the horizontal-to-vertical spectral ratio (HVSR) method both provide good supplements for checking purposes. Field checks are definitely necessary, especially for sites located near geologic boundaries or geomorphologic boundaries. Final comments for each station site have been made after these checks. Some problematic sites do exist and are mentioned in the text. Data from these sites must only be used with care. To increase the accessibity of our results, we have developed a web-based query site at , and an appendix for site classification of the 708 TSMIP stations is given in the CD-ROM attached to this issue. Manuscript received 12 December 2000.

Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan

Subduction zone earthquakes have not been taken into special consid- eration in most previous probabilistic seismic hazard analyses (PSHA) in Taiwan. However, they may be critical to properly analyze the earthquake hazard in metro- politan Taipei, so they need to be studied. Strong-motion data from subduction zone earthquakes, of both interface and intraslab types, obtained by the TSMIP and SMART1 arrays in northeastern Taiwan, are used to establish the attenuation equa- tions for peak ground acceleration (PGA) and response spectral acceleration (SA). The resultant PGA and SA attenuation equations include two site classes and two earth- quake source types. The ground-motion values predicted by these attenuation equa- tions are higher than those obtained from the crustal earthquake attenuation equations previously used in Taiwan but are lower than those predicted by the attenuation equa- tions for worldwide subduction zone earthquakes.

Quaternary transfer faulting in the Taiwan Foothills: evidence from a multisource approach

Abstract The major structures of the Western Foothills of Taiwan mainly consist of NNE-SSW-trending folds and imbricated west-vergent thrust systems. The additional occurrence of N140°E-striking oblique structures was revealed through a multisource approach involving a Digital Elevation Model (DEM), a study of drainage network anomalies, aerial photographs, Side-Looking Airborne Radar (SLAR) images and SPOT-P and Landsat images. These structures are described from north to south based on new field analyses (including stratigraphy and tectonics studies). They are also compared to seismic data and geodetic reconstruction, in order to evaluate their present-day activity. These N140°E major morphostructures are interpreted as left-lateral transfer fault zones, either inherited from the Eurasian passive margin and/or newly formed in the cover in response to the presence of basement highs within the foreland basin (Peikang and Kuanyin highs). The Sanyi and the Chishan transfer fault zones display a high seismic activity; the distribution of earthquakes and the related focal mechanisms confirm the left-lateral movement along N140°E directions. The Chiayi, Chishan, and Fengshan fault zones act presently as transfer fault zones, as indicated by GPS data. The associated N70°E- to N100°E-trending faults result from the reactivation of normal faults of the Eurasian passive margin as right-lateral strike-slip faults in the Foothills during the Plio-Quaternary collision in Taiwan. We conclude that multisource and multiscale geomorphic studies combined with tectonic analysis in the field yield a significant contribution to the understanding of the structural and kinematic development of the Western Foothills at the front of the Taiwan collision belt.

Geometry and Quaternary kinematics of fold‐and‐thrust units of southwestern Taiwan

Structural and paleostress analyses provide new insights into the Quaternary kinematics of the outermost fold-and-thrust units of southwestern Taiwan Foothills. The frontal folds are interpreted as fault-related folds, and their tectonic evolution through space and time is tightly constrained. Fold development is correlated with reef building on top of the anticlines. Moreover, we provide field evidence that NW–SE fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the Quaternary fold-thrust emplacement. Two successive Quaternary stress regimes are evidenced in southwestern Taiwan: A NW–SE compression, followed by a recent nearly E–W compression. The latter shows an along-strike change from pure E–W contraction to the north to perpendicular N–S extension in the south. This southward decrease in N–S confinement probably represents the on-land signature of the incipient Quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present-day by Global Positioning System data.

Repeatable source, site, and path effects on the standard deviation for empirical ground-motion prediction models

In this study, we quantify the reduction in the standard deviation for empirical ground-motion prediction models by removing ergodic assumption. We par- tition the modeling error (residual) into five components, three of which represent the repeatable source-location-specific, site-specific, and path-specific deviations from the population mean. A variance estimation procedure of these error components is developed for use with a set of recordings from earthquakes not heavily clustered in space. With most source locations and propagation paths sampled only once, we opt to exploit the spatial correlation of residuals to estimate the variances associated with the path-specific and the source-location-specific deviations. The estimation proce- dure is applied to ground-motion amplitudes from 64 shallow earthquakes in Taiwan recorded at 285 sites with at least 10 recordings per site. The estimated variance com- ponents are used to quantify the reduction in aleatory variability that can be used in hazard analysis for a single site and for a single path. For peak ground acceleration and spectral accelerations at periods of 0.1, 0.3, 0.5, 1.0, and 3.0 s, we find that the single- site standard deviations are 9%-14% smaller than the total standard deviation, whereas the single-path standard deviations are 39%-47% smaller.

Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along-strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along-strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left-lateral tear fault or transfer fault that offsets two distinct frontal thrust-and-fold sheets.

Coseismic displacements of the footwall of the Chelungpu fault caused by the 1999, Taiwan, Chi-Chi earthquake from InSAR and GPS data

Abstract The differential SAR interferometry technique (interferometric synthetic aperture radar, InSAR) is applied on the Chelungpu fault surface rupture zone of the September 20, 1999, Taiwan, Chi-Chi earthquake using six ERS-2 images covering the period from February 1999 to January 2000. As compared with available geodetic data, InSAR measurements result in more extensive analysis because of high spatial sampling and centimetric accuracy. However, coseismic displacements can be evaluated only on the footwall of the fault. The analysis of interferograms shows the existence of a linear trend in phase difference mainly caused by orbital errors, which we removed from interferograms using GPS data. The corrected interferograms provide a precise map of the InSAR component of the coseismic displacement, showing a continuous decrease over the footwall from a maximum of 36.7 cm at the fault east of Taichung city to a value of about 5 cm at the coastline 30 km further west. The map analysis reveals that the Changhua fault (whose surface trace is located about 20 km west of the Chelungpu one) and the Tuntzuchio fault influence the displacement field. We interpret this in terms of minor reactivation of these faults triggered by the earthquake. A 1.7 cm uncertainty, estimated from the GPS data, is proposed to quantify the precision of the map. Beyond this single value, we highlight the interest of having several coseismic interferograms to evaluate the reliability of the map in a more comprehensive way. Comparisons with displacements inferred from models of slip distribution inverted without InSAR data highlight the advantage of carrying out a joint inversion including our results as new constraints.

Frictional fusion due to coseismic landsliding during the 1999 Chi‐Chi (Taiwan) ML 7.3 Earthquake

Fused materials (pseudotachylytes) generated from landsliding during the 1999 Chi-Chi (Taiwan) ML7.3 earthquake are found on the glide plane in Mio-Pliocene interbedded shale and siltstone. The pseudotachylytes occur as thin layers on the glide plane and as veins injected into cracks in the host rocks, which are a few mm to 1 cm in thickness. Typical melting textures within the pseudotachylyte include vesicles, glassy matrices, flow structures, and rounded and embayed clast shapes. Powder X-ray diffraction analysis has revealed a glass content of up to 50 wt% within the pseudotachylyte. Physical conditions of pseudotachylyte formation are estimated to have been <1.5 MPa corresponding to ca.40 m depth, at a temperature of at least 1100∼1600 °C. The geological and petrological data suggests that these pseudotachylytes formed by frictional melting generated from coseismic landsliding during the Chi-Chi earthquake.

Uplift of Tainan Tableland (SW Taiwan) revealed by SAR Interferometry

Interferometric processing of five SAR-ERS images reveals uplift of the Tainan Tableland (SW of Taiwan) during the period 1996–1998. The maximum measured ground motion for these two years is 2.8 cm along the radar line of sight towards the satellite, indicating for the displacement vector a vertical component of 3.2 cm, and a horizontal component of 1.6 cm towards the WSW considering additional information from GPS data. The reconstructed displacement field is consistent with the geological interpretation of the Tainan Tableland as an actively growing anticline connected to the Taiwan fold-and-thrust belt. This implies that the deformation front is located farther west than usually assumed in the Tainan area. The large Tainan city is thus located in an active deformation zone. Seismic hazard assessment is however difficult because the mechanisms and kinematics are not known in detail.