Ruey Juin Rau

TAIWAN OROGENY : THIN-SKINNED OR LITHOSPHERIC COLLISION ?

Abstract The Taiwan orogeny is young and presently very active. It provides an excellent environment for studying ongoing orogenic processes, especially since the region is monitored intensively with dense seismological and geodetic networks, and new studies aiming at deciphering shallow and deep structures in and around Taiwan have been recently conducted or are being planned. The available data can be used continually to test critically hypotheses of the Taiwan orogeny. Hypotheses dealing with the mechanics of mountain building are basic to the understanding of Taiwan orogeny and are particularly amenable to testing. The widely cited ‘thin-skinned tectonics’ hypothesis was formulated to explain mainly the geologic and relatively shallow (

Tomographic imaging of lithospheric structures under Taiwan

Abstract Tomographic images of the crustal and mantle velocity structures under Taiwan are obtained by simultaneous inversion of local earthquake P-wave arrival times for hypocenters and P-wave velocity structures. In northern Taiwan, a high-velocity zone, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. The imaged zone dips toward the north at an angle of 40° from a depth range of 20–55 to 100–130 km. An upper-mantle low-velocity wedge, ranging from 40 to 80 km in depth, exists above the subducted slab. Above this wedge is the Ilan Plain of northern Taiwan which lies at the west end of the Okinawa Trough, a well recognized back-arc basin; the crustal velocities under the Plain are also relatively low. The well-defined high-velocity zone and the low-velocity wedge provide some constraints on the thermal structures of the subduction system under northern Taiwan. In tomographic images across the central section of Taiwan, thickening of the crust and up-arching of the lower-crustal materials under the Central Range are commonly observed; the crust under the Western Foothills region is clearly thinner and the near-surface low-velocity layers are well developed. The structures under the Central Range show that although the Taiwan orogeny is quite young, a root, deeper in the north and shallowing to the south, has formed. The results of our tomography show that a significant portion of the lithosphere is involved in the Taiwan orogeny.

Seismic image of the Tarim basin and its collision with Tibet

A broadband seismic deployment in 1998–1999 in southwestern Tarim provided data for imaging the crust and upper mantle across the contact between the Tarim block and the Tibetan Plateau. A profile composed of migrated teleseismic receiver functions clearly shows lateral structural changes. The crust under the Tarim basin is relatively simple. The Moho discontinuity is mapped at a depth of 42 km near the northern end of the array and dips gently toward the south to ∼50 km under the Kunlun foreland. The Tarim basin appears to be rigid, with little shortening. Farther to the south, the imaging reveals a complex of reflectors in the lower crust and the upper mantle. There are both north- and south-dipping upper mantle structures under the Kunlun foreland and Kunlun Shan region. We found the observations to be more consistent with a model of lithospheric collision in which the crust and the upper mantle on both sides interpenetrate and deform.

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).

Geodetically observed surface displacements of the 1999 Chi-Chi, Taiwan, Earthquake

The 21 September 1999 Chi-Chi, Taiwan, earthquake of magnitude MW = 7.6 (ML = 7.3) severely deformed the Earth’s crust in the central Taiwan region. The earthquake created an 85-km-long surface rupture along the Chelungpu fault. The epicenter was located at 23.85°N, 120.81°E, near the southern end of the rupture zone. Three-dimensional displacements of 285 geodetic control stations were determined in this study from Global Positioning System (GPS) observations collected before and after the earthquake. The detailed surface displacement field shows that individual stations are vertically uplifted by up to 4 m and displaced horizontally by up to 9 m, with the largest displacement occurring near the northern end of the ruptured thrust fault. The azimuth of the surface displacement field is approximately parallel to the direction of tectonic convergence of the Eurasian and Philippine Sea plates. The maximum three-dimensional displacement of 9.9 m is among the largest fault movements ever measured for modern earthquakes.

Detailed structures of the subducted Philippine Sea plate beneath northeast Taiwan' A new type of double seismic zone

We studied the detailed structure of the subducted Philippine Sea plate beneath northeast Taiwan where oblique subduction, regional collision, and back arc opening are all actively occurring. Simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast, high-quality data recorded by the Taiwan Seismic Network. We further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic P and SH waveforms, a critical step to define the position of plate interface and the state of strain within the subducted slab. The most interesting feature is that relocated hypocenters tend to occur along a two-layered structure. The upper layer is located immediately below the plate interface and extends down to 70–80 km at a dip of 40°–50°. Below approximately 100 km, the dip increases dramatically to 70°–80°. The lower layer commences at 45–50 km and stays approximately parallel to the upper layer with a separation of 15±5 km in between down to 70–80 km. Below that the separation decreases and the two layers seem to gradually merge into one Wadati-Benioff Zone. We propose to term the classic double seismic zones observed beneath Japan and Kuril as “type I” and that we observed as “type II,” respectively. A global survey indicates that type II double seismic zones are also observed in New Zealand near the southernmost North Island, Cascadia, just north of the Mendocino triple junction, and the Cook Inlet area of Alaska. All of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction. We interpret the seismogenesis of type II double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere (originating from oblique subduction) and the downdip extension (from slab pulling force). The upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and/or along the plate interface, whereas the lower layer is associated with events in the uppermost mantle.

The performance comparison between GPs and BeiDou‐2/compass: A perspective from Asia

Abstract The next decade promises drastic improvements to global navigation satellite systems. The USA is modernizing GPS, Russia is refreshing GLONASS, Europe is moving ahead with its own Galileo system, and The Peoples Republic of China is expanding its BeiDou‐1 system from a regional navigation system to a full constellation global navigation satellite system known as BeiDou‐2/Compass, which consists of thirty five satellites including geostationary satellites, MEO satellites and geosynchronous satellites in the coming year. Extra satellites will make possible improved performance for all applications, and especially where satellite signals can be obscured, such as in urban canyons, under tree canopies or in open‐pit mines. The benefits of the expected extra satellites and their signals can be evaluated in terms of availability, accuracy, continuity, and reliability issues. The advent of a hybrid GNSS constellation has drawn a lot of attention to study compatibility and interoperability. A number of performance analyses have been conducted on a global scale with respect to availability, reliability, accuracy and integrity in different simulated scenarios (such as open sky and urban canyons) for each system individually as well as for combined systems with all the possible combinations. Since the BeiDou‐2/Compass has gained more attention from GNSS communities, the main objective of this paper is to study the performance of BeiDou‐2/Compass comparied to GPS in the greater Asia region; and also to explore whether the combination of BeiDou‐2/Compass with GPS would yield performance improvements in this region. The performance analysis can be analyzed by either the signal or the geometrical conditions. However, the scope of this study is limited to investigating the impact of current and future GNSS based on geometrical conditions. Therefore, the satellite visibility and DOP (Dilution of Precision) values of each system or possible combinations between them are used as the major indices for performance evaluation with the emphasis on the addition of Compass. In addition, those indices are further analyzed in terms of their spatial and temporal distributions with the emphasis on the greater Asia region. Moreover, the spatial performance analyses are conducted on both global and regional scales to provide more insightful comparisons to illustrate the importance of future Compass for users in the greater Asia region.

GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi‐Chi earthquake in Taiwan

[1] Using data at 110 continuous GPS stations from 1 January 2003 to 31 December 2005, we characterized the surface deformation in Taiwan after the M w = 7.6 Chi-Chi earthquake of 21 September 1999. In continuous GPS (CGPS) data, the maximum coseismic deformation of the Chengkung earthquake and Ilan double earthquakes reached 165.5 ± 0.5 mm and 35.4 ± 0.5 mm in horizontal displacement and 181.7 ± 1.1 mm and 12.6 ± 1.5 mm in vertical displacement, respectively. With respect to Paisha station, S01R, the stations of the Coastal Range and Lanhsu showed an average displacement of 40.5-93.6 mm/yr with directions of307°-333°. The stations in the Longitudinal Valley and Central Range revealed velocities in the range 19.0―49.3 mm/yr with directions of 285°―318°. In western Taiwan, the velocities in the inner fold-and-thrust belt range from 14.2 to 45.5 mm/yr with directions of 284°―304°. Extensional strain affects the Ilan and Pingtung plains near belt tips, revealing lateral extrusion toward the adjacent subduction zones. Extensional strain also affects the southern Central Range because of the rapid uplift related to the southward propagating collision process. Large and medium size earthquakes affect the strain pattern revealed by CGPS, albeit in different ways: regional extension and displacement were large and rotations were small regarding the M w = 7.6 Chi-Chi earthquake. In contrast, the limited size of the affected area, moderate displacement, and large rotations characterized the 2003 M w = 6.8 Chengkung earthquake. The impact of smaller earthquakes such as the 2005 M w = 5.9 Ilan double earthquakes and the 2005 M w = 5.6 Hualien earthquake was locally significant but regionally minor. The CGPS data provide a snapshot of the deformation that is generally consistent with the long-term history of the collision but should not be directly extrapolated because thrust deformation is migrating along the tectonic boundary. Regarding the Chi-Chi earthquake, the new CGPS data show that the Chi-Chi hanging wall is still recognizable as a kinematic block, whereas in the previous pattern the hanging wall was not discemable.

Crustal deformation and block kinematics in transition from collision to subduction: Global positioning system measurements in northern Taiwan, 1995-2005

[1] We present global positioning system (GPS) measurements for the period 1995 -2005 at 125 campaign-surveyed sites in northern Taiwan. Based on elastic, rotating block modeling analyses derived from the GPS data, we describe the transitional tectonics from arc-continent (Luzon-Chinese) collision to the converging Ryukyu trench subduction and back-arc opening along the Chinese continental margin. Station velocities relative to station S01R, in the Chinese stable continental margin, were estimated from coordinate time series of each station by using the weighted least squares technique. We found two distinct deformation patterns in two geological areas, which are basically separated by the surface projection of the NW-trending boundary of the subducting Philippine Sea plate across northern Taiwan: (1) a waning collision area to the west and (2) a transition zone to the east. In the waning collision area, the horizontal velocity field shows vectors of 0.3-7.3 mm/yr toward the NW in the foothills and the Hsuehshan Range of northwestern Taiwan. The tectonic blocks represent a significant NW-SE internal contraction along with a small block rotation rate (<3.0°/Myr). The transition zone can be further divided into an outer range and inner range with distinguishing rotation rates and deformation behaviors. In the outer range of the transition zone, velocities of 1.0-7.8 mm/yr from south to north rotating from 008° to 143° is found in the northernmost foothills and the Hsuehshan Range. The tectonic blocks within the outer range are characterized by a coherent rotation (low internal strain rate of <0.10 μstrain/yr) with an angular velocity of ∼5.1°/Myr, where the Euler pole is located near its southeastern boundary. In the inner range of transition zone, a larger clockwise rotation from west to east, with horizontal velocities of 9.3-41.2 mm/yr from 053° to 146°, are found in the northernmost Central Range. The tectonic blocks of the inner range reveal a remarkable NW-SE internal extension with an ultrarapid clockwise rotation (∼47.3°/Myr) where the Euler pole is near the southern boundary of the range close to the collision comer with the colliding Luzon arc. The trench roll-back together with back-arc opening are interpreted to be substantially superposed on the arc-continent collision-induced rotation in the transition zone with particular regard to the inner range of the northeast Taiwan mountain belt.

Semiautomatic Object-Oriented Landslide Recognition Scheme From Multisensor Optical Imagery and DEM

Rainfall-induced landslides are a major threat in Taiwan, particularly during the typhoon season. A precise survey of landslides after a super event is a critical task for disaster, watershed, and forestry land management. In this paper, we utilize high spatial resolution multispectral optical imagery and a digital elevation model (DEM) with an object-oriented analysis technique to develop a scheme for the recognition of landslides using multilevel segmentation and a hierarchical semantic network. Four case studies are presented to evaluate the feasibility of the proposed scheme. Three kinds of remote sensing imagery, namely pan-sharpened FORMOSAT-2 satellite images, aerial digital images from Z/I digital mapping camera, and images acquired by a digital single lens reflex camera mounted on a fixed-wing unmanned aerial vehicle are used. An accuracy assessment is accomplished by evaluating three test sites containing hundreds of landslides associated with the Typhoon Morakot. The input data include ortho-rectified image and DEM. Four spectral and one topographic object features are derived for semiautomatic landslide recognition. The threshold values are determined semiautomatically by statistical estimation from a few training samples. The experimental results show that the proposed approach can counteract the commission/omission errors and achieve missing/branching factors at less than 0.12 with a quality percentage of 81.7%. The results demonstrate the feasibility and accuracy of the proposed landslide recognition scheme even when different optical sensors are utilized.