Chung-Pai Chang

Strain and stress field in Taiwan oblique convergent system: constraints from GPS observation and tectonic data

Abstract This paper focuses on the stress and strain rate field of the Taiwan area. The strain rate field in the Taiwan region is studied qualitatively and quantitatively, based on the GPS observation in 1990–1995. It reflects the accommodation of the ongoing lithospheric deformation within the seismogenic portion of lithosphere and exhibits zones with contrasting deformation modes and amounts. We then compare the obtained strain rate field with the tectonic information provided by studies of borehole breakouts and earthquake focal mechanisms for the Present, and by fault slip data analyses for the Quaternary period. In the first approximation, the stress and strain rate fields show spatial similarity. The orientation of principal shortening is generally consistent with the compressive stress orientation that reflects the oblique indentation of the Luzon Arc into the Eurasian continental margin. In more detail, significant anomalies in the deformation pattern deserve consideration in that they may reveal ongoing stress accumulation. Despite the short-term variations related to the earthquake cycle, some major features of the strain rate field, including the distribution of extension and compression, highlight the long-term tectonic behavior of the mountain belt at the lithospheric scale. The time and space variations of strain should be a function of local heterogeneity and be transferred between interseismic and coseismic periods.

Origin and evolution of a mélange: the active plate boundary and suture zone of the Longitudinal Valley, Taiwan

Abstract Juxtaposed against the remnant forearc basin sequences along thrust faults, the Lichi Melange of the Coastal Range of Taiwan is composed of exotic ophiolite and sedimentary blocks, metric to kilometric in size, and coherent turbidite beds, all embedded in a sheared scaly argillaceous matrix. The Lichi Melange is controversial in origin, being interpreted either as a subduction complex, or as an olistostrome. By separating four main deformation levels within the Lichi Melange and adjacent sedimentary rocks, we establish detailed geological maps and structural profiles in two key areas of the Lichi Melange. We reconstruct also the evolution in cross-section and calculate the approximate minimum amount of shortening that corresponds to folding and thrusting in these areas. Our field studies suggest that the Lichi Melange most likely arose from the shearing of lower forearc sequences rather than from a subduction complex or an olistostrome. This conclusion is supported by the structural analysis, the clay mineral distribution, and some interfingering sedimentary relationships between the Lichi Melange and the lower Takangkou Formation. We also undertake a comprehensive tectonic analysis of the shear surfaces in the Lichi Melange. The direction of the maximum compressional stress that we obtain is N100° ∼120°E, compatible with that of plate convergence. During the most recent stage of collision, between the Eurasian plate (eastern Central Range of Taiwan) and the Philippine Sea plate (Coastal Range), a major fault zone developed along the innately weak zone of melange, further increasing the shear deformation pattern of the Lichi Melange. This Longitudinal Valley Fault separates the Eurasian plate and the Philippine Sea plate and is one of the most active faults in Taiwan. It can be considered as the present plate boundary in the Taiwan arc–continent collision terrane. According to our reconstruction, this plate boundary of the Longitudinal Valley originated as a submarine arc–prism boundary.

From continental margin extension to collision orogen: structural development and tectonic rotation of the Hengchun peninsula, southern Taiwan

As a result of oblique collision, the Taiwan orogen propagates southward. The Hengchun peninsula in the southern tip of the Taiwan Central Range, preserving the youngest, the least deformed and the most complete accretionary prism sequences, allows therefore better understanding of the tectonic evolution of Taiwan orogen. On the Hengchun peninsula, four main stages of paleostress can be recognized by the analysis of brittle tectonics. After recording the first two stages of paleostress, rocks of the Hengchun peninsula (the Hengchun block) have undergone both tilting and counterclockwise rotation of about 90j. The structural boundaries of this rotated Hengchun block are: the Kenting Melange zone in the southwest, the Fongkang Fault in the north, and a submarine backthrust in the east. The angle of this rotation is principally calculated by the paleomagnetic analysis data and a physical model experiment. Through a systematic back-tilting and back-rotating restoration, the original orientations of the four paleostress stages of Hengchun peninsula are recognized. They are, from the ancient to the recent, a NW-SE extension, a combination of NW-SE transtension and NE-SW transpression, a NE-SW compression, and finally a combination of NE-SW transtension and NW-SE transpression. This result can be explained by a phenomenon of stress axes permutation, instead of a complex polyphase tectonism. This stress axes permutation is caused by the horizontal compression increase accompanying the propagation of the accretionary prism. Combining the tectonic and paleomagnetic data with paleocurrent and stratigraphic data enables us to reconstruct the tectonic evolution of the Hengchun peninsula. This reconstruction corresponds to the deformation history of a continental margin basin, from its opening to its intense deformation in the accretionary prism. D 2002 Elsevier Science B.V. All rights reserved.

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.

Land-surface deformation corresponding to seasonal ground-water fluctuation, determining by SAR interferometry in the SW Taiwan

In this study we implement the InSAR technique for identifying the seasonal surface deformation in the SW Taiwan. The focus of our investigation is on the Pingtung plain, a tectonic valley with a high water-pumping rate. Our preliminary results show that the land subsidence occurs specifically in the dry season and even ceases during the wet season. Whereas local land subsidence is usually caused by dewatering of sediments, we compare therefore our InSAR observations with the groundwater data of this area. This comparison shows that the subsidence rate is associated with the descending trend of groundwater level. Based on our research results, we infer that the dewatering during the dry season removes the buoyant support from the sediments and the effect of sediment compaction simultaneously results in local land subsidence. This successful test exhibits that with suitable images, the InSAR is a useful high-resolution tool for monitoring the earth surface deformation and can provide significant information for the issue of natural hazard mitigation.

Evolution of Subductions Indicated by Mélanges in Taiwan

Two melange zones occur in Taiwan: the Lichi Melange in the Coastal Range of eastern Taiwan and the Kenting Melange in the Hengchun Peninsula of southern Taiwan. Because of the southward propagation of the Taiwan orogen, these two melanges now crop out at the western front of the Coastal Range (Lichi Melange) and the west side of the southern Central Range (Kenting Melange). These two melanges are similar in appearance and controversial in origin, being interpreted either as subduction complexes, or as olistostromes. Remnants of neighbouring strati-graphic units, present as slices or clasts in the melange zones, shed light on the nature and origin of these two melanges and help deciphering the tectonic evolution of Taiwan orogen and its leading subductions.

Coherence Difference Analysis of Sentinel-1 SAR Interferogram to Identify Earthquake-Induced Disasters in Urban Areas

This study proposes a workflow that enables the accurate identification of earthquake-induced damage zones by using coherence image pairs of the Sentinel-1 satellite before and after an earthquake event. The workflow uses interferometric synthetic aperture radar (InSAR) processing to account for coherence variations between coseismic and preseismic image pairs. The coherence difference between two image pairs is useful information to detect specific disasters in a regional-scale area after an earthquake event. To remove background effects such as the atmospheric effect and ordinal surface changes, this study employs the two-step threshold method to develop the coseismic coherence difference (CCD) map for our analyses. Thirty-four Sentinel-1 images between January 2015 and February 2016 were collected to process 30 preseismic image pairs and two coseismic image pairs for assessing multiple types of disasters in Tainan City of southwestern Taiwan, where severe damages were observed after the Meinong earthquake event. The coseismic unwrapping phases were further calculated to estimate the surface displacement in east-west and vertical directions. Results in the CCD map agree well with the observations from post-earthquake field surveys. The workflow can accurately identify earthquake-induced land subsidence and surface displacements, even for areas with insufficient geological data or for areas that had been excluded from the liquefaction potential map. In addition, the CCD details the distribution of building damages and structure failures, which might be useful information for emergency actions applied to regional-scale problems. The conversion of 2D surface displacement reveals the complex behavior of geological activities during the earthquake. In the foothill area of Tainan City, the opposite surface displacements in local areas might be influenced by the axis activities of the Kuanmiao syncline.