Horng-Yue Chen

Velocity field of GPS stations in the Taiwan area

Abstract The 131 stations of the ‘Taiwan GPS Network’ were surveyed 4–6 times from 1990 to 1995 with dual-frequency geodetic receivers. The standard deviation of an observed baseline length with its linear trend removed is in the range of 6–10 mm for a 3–120 km long baseline. The average rates of length change for all baselines of the network and those from nine continuously monitoring permanent stations are used in a least squares adjustment to estimate the velocities of the GPS stations relative to Paisha, Penghu, situated at the Chinese continental margin. To the south of Fengping, in the northern Coastal Range, the velocity vectors of stations in Lanhsu, Lutao, and the Coastal Range trend in the directions of 306°–322° with rates of 56–82 mm/yr. In contrast, there is a dramatic decrease in the rates to the north of Fengping. This may be caused by the motion along the NE-SW-trending thrusts which obliquely cut the northern Coastal Range. A discontinuity of about 30 mm/yr in the rates along with a remarkable change in the directions of station velocity is observed across the Longitudinal Valley, then the moving directions gradually shift to the west for the stations in the Western Foothills. In the Kaohsiung-Pingtung coastal area, the station velocities are even directed toward the southwest. To the north of the Peikang High, the velocity vectors of the stations change direction from the west gradually to the north and finally to the east and southeast. Significant NW-SE extensional deformation is found in the Ilan Plain and northern Taiwan. In general, the pattern of the velocity field for GPS stations in the Taiwan area is quite consistent with the directions of present-day tectonic stress.

A two-dimensional dislocation model for interseismic deformation of the Taiwan mountain belt

We use a Global Positioning System (GPS)-derived surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by the observed horizontal velocities projected into the direction of plate motion (306‡). The GPS data are divided into northern and southern regions and the velocities in each region are projected into single profiles. The model fault geometry includes a shallowly dipping decollement, based on the balanced geological cross-sections in the Coastal Plain and Western Foothills, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The decollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 and 80 mm/yr, respectively. The optimal geometry of decollement is subhorizontal (2‡V11‡) at a depth of 8V9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu Fault, while in the southern section, it is located beneath the Chukou Fault. The elastic dislocation model successfully matches the horizontal velocity data, and predicts elastic strain accumulation in the Western Foothills that will presumably be released in future earthquakes. However, considered over multiple earthquake cycles, our model cannot explain the topography of the CR and thus fails to predict the active mountain building process in Taiwan. This failure indicates that both horizontal and vertical velocity fields require a more complex rheological model that incorporates inelastic behavior.

AN INSTANTANEOUS AMBIGUITY RESOLUTION PROCEDURE SUITABLE FOR MEDIUM-SCALE GPS REFERENCE STATION NETWORKS

Abstract There is a trend for the establishment of regional-scale GPS permanent receiver networks, for a variety of applications including to support high accuracy, carrier phase-based positioning for surveying and precise navigation. When implemented in real-time, GPS users located within the region enclosed by multiple GPS reference stations can precisely position by using, for example, the ‘correction terms’ generated and transmitted by the reference station network. For such a configuration one of the major challenges is that the integer ambiguities have to be resolved during the real-time processing of the reference network data in order to ensure the generation of the carrier phase corrections, even when the reference receivers are many tens of kilometres apart. Due to the presence of distance dependent errors in the double-differenced data (principally the ionospheric and tropospheric delays) reliable instantaneous (single epoch) ambiguity resolution is difficult in the case of medium-scale reference networks (defined here as where the reference stations are typically in the range 50–100km apart). In practice, the ambiguities among the reference stations can be correctly resolved during an initialization procedure, but the main challenge is to continuously resolve the new ambiguities that result when the tracked satellite experiences cycle slips, or after any long data gap, or when a new satellite rises. In this paper a three-step methodology is proposed which can be implemented in realtime. Firstly, the high correlation of the atmospheric delay between adjacent epochs is used to assist cycle-slip recovery and ambiguity resolution. Then these atmospheric biases are predicted for double-differenced observations on an epoch-by-epoch and satellite-by-satellite basis. Finally these predicted atmospheric biases are applied to an algorithm that can fix the new ambiguities after a long data gap or when a new satellite rises. Data from a set of reference stations spaced 80 km apart were used to test the effectiveness of the algorithm. The results indicate that the proposed methodology can provide reliable integer ambiguities for reference stations spaced many tens of kilometres apart.

Coseismic and postseismic surface displacements of the 10 December 2003 (MW 6.5) Chengkung, eastern Taiwan, earthquake

The MW 6.5 Chengkung earthquake occurred in eastern Taiwan at 04:38 UTC on 10 December 2003. The GPS data from eighteen continuously recording stations (CORS) and 86 campaign-surveyed stations (CSS) collected 18 days to 9 months before and 6 days to 4 months after the main shock are utilized to analyze the coseismic and postseismic deformation associated with the Chengkung earthquake. The earthquake resulted from rupturing of the Chihshang fault, a 25-km-long segment of the Longitudinal Valley Fault (LVF). The coseismic horizontal displacements in the hanging wall showed a fan-shape distribution with vectors towards the west. On the other hand, the movements of the revealed a mirror fan-shape with relatively lesser amounts of displacement. The largest coseismic displacement, which reached 126 mm and 263 mm in the horizontal and vertical components, occurred near the epicenter area in the hanging wall. The largest postseismic displacements in 109 days, which approached 59 mm and 68 mm in the horizontal and vertical components, occurred near the surface trace of the Chihshang fault (TAPO) and near the epicenter area (CHEN), respectively. The stations near the Chihshang fault indicated a more significant postseismic displacement than coseismic one.

Low-cost densification of permanent GPS networks for natural hazard mitigation: First tests on GSI’s GEONET network

Researchers from The University of New South Wales (UNSW), Australia, and from the Geographical Survey Institute (GSI), Japan, have commenced a joint project to develop, deploy and test an innovative hardware/software system design for an automatic, continuously-operated ground deformation monitoring system based on low-cost GPS receiver technology. Conventional continuously-operated GPS (CGPS) networks, such as the one established in Japan by GSI to precisely measure earth surface movement, are very expensive. The high cost being primarily due to the fact that dual-frequency receivers are used. Japan’s nationwide GEONET network is the world’s largest, numbering nearly 1000 receiver stations, with an average station spacing of the order of 30 km. In order to densify such CGPS networks (important when high spatial resolution for the monitoring of the deformation phenomenon is required), and to promote the use of the CGPS technique in lesser developed countries, a significantly cheaper system architecture is needed. The proposed design is an integrated, dual-mode network consisting of low-cost, single-frequency GPS receivers across the area of interest, surrounded by a sparser network of dual-frequency GPS receivers. Initial tests of data collected at selected stations in the GEONET network have already shown that through enhanced data processing algorithms a CGPS network containing both single-frequency and dual-frequency receivers would be able to deliver better than centimetre level accuracies, at considerably lower cost than present systems based exclusively on dual-frequency instrumentation. This paper reports the results of the first field test of this new CGPS system design, in the Tsukuba area of Japan, in August 1999. The test network consisted of: (a) several stations of the GEONET network surrounding (b) an inner network of four single-frequency Canadian Marconi GPS receivers installed by UNSW researchers. The data from both the GEONET and the UNSW receivers were processed using a specially modified version of the Bernese GPS Software Package. The software first processes the GEONET GPS station data in order to generate empirical corrections which are then applied to the double-differenced data of the GPS baselines located within the test area enclosed by the dual-frequency CGPS stations. These corrections have the effect of improving baseline solution accuracy by up to an order of magnitude, even for baselines ranging up to 100 km in length. The baselines connecting the inner network to the surrounding GEONET stations are processed in a number of modes, including 24 hr files (as is the standard practice for geodynamic applications) and hourly data files (as in volcano deformation monitoring applications). The results indicate that single-frequency-with-correction processing can achieve accuracies of better than 5 mm in the horizontal components and 3 cm in height, while the dual-frequency results can achieve accuracies better than 2 mm in the horizontal components and 6 mm in height. In the authors’ opinion, for certain geodynamic applications there are no significant differences between the single-frequency-with-correction results and the dual-frequency results, especially for the horizontal components.

Coseismic displacements and slip distribution from GPS and leveling observations for the 2006 Peinan earthquake (Mw 6.1) in southeastern Taiwan

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (Mw 6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter—around 45 mm movement in the S-SSW direction with +20 to −20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault—approximately 35 mm in a westward movement with −60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter—about 10 mm in a northward displacement with +15 to −10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80° west-dipping fault at a depth of 0.5–20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9–16 km. The total geodetic moment in our optimal model is 2.3 × 1018 Nt-m, which is equivalent to an earthquake of Mw 6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.

Regional-scale multiple reference stations for carrier phase-based GPS positioning: A correction generation algorithm

Continuously operating GPS networks have been used for many years in support of: (a) geodetic goals such as the determination of crustal motion on a variety of spatial scales, and (b) to provide pseudo-range corrections for Wide Area DGPS (WADGPS) implementations. Recently, regional-scale GPS permanent networks have been developed for multi-functional uses, including to support centimetre-accuracy, medium-range, carrier phase-based GPS positioning for surveying or precise navigation applications. In such an implementation the generation of carrier phase correction messages in a manner analogous to WADGPS requires that the integer ambiguities between GPS reference stations be fixed in real-time. Although the ambiguities could be resolved at the beginning of operation the challenge remains: how to recover an integer ambiguity if a cycle slip or data gap occurs, or if a new satellite rises? In this paper, the linear data combination algorithm that has been used is described, and the issue of “ambiguity recovery” techniques for data correction generation purposes is addressed. Three strategies are suggested: (1) using an “ambiguity recovery” technique if the data gap is shorter than a minute or so; (2) re-determining the integer ambiguities using an ionospheric correction derived from the tracking to other satellites when a new satellite rises, or after a long period of data loss, and; (3) re-determining the integer ambiguities with the aid of data corrections generated on previous days. Several 7-day continuous data sets were used for algorithm testing. The corrections can be generated from multiple reference stations in the post-processing mode and then used for comparison purposes with the simulated real-time processing mode using the proposed algorithm. Results confirm that the proposed algorithm can provide reliable carrier phase data corrections for centimetre-accuracy, real-time GPS positioning.

Seasonal, long-term, and short-term deformation in the Central Range of Taiwan induced by landslides

Seasonal movement of GPS stations is often attributed to hydrological loading and other environmental factors. For the first time we observe seasonal motion associated with slow-moving landslides. Eight of 26 continuous GPS (cGPS) sites in the Central Range of Taiwan show long-term landslide-induced motion at rates of ∼3–15 mm/yr, ∼20%–60% of their tectonic interseismic velocities. The directions of movements after heavy rains and in the wet season are consistent with the slope directions derived from a high-resolution elevation model constructed by airborne lidar. Long-term and seasonal interseismic motions are modulated by slow-moving landslides. Seasonal motions of landslides at Lushan show peak to peak amplitudes of ∼3–19 mm. Estimates of interseismic crustal strain can be biased if surface processes are not taken into account. Preliminary analyses indicate that rainfall and topography play strong roles in the occurrence of landslides. Discrimination between surface processes and motion that has a tectonic origin is the key to natural hazard assessments.

Interseismic Deformation and Earthquake Hazard along the Southernmost Longitudinal Valley Fault, Eastern Taiwan

About half of the 8  cm/yr of oblique convergence across the active convergent plate boundaries of Taiwan occurs in eastern Taiwan, across the Longitudinal Valley. Significant shortening and left‐lateral slip occurs across the Longitudinal Valley fault there, both as shallow fault creep and as seismogenic fault slip. The southernmost Longitudinal Valley fault comprises an eastern Peinan strand and a western Luyeh strand. We derive an interseismic block model for these two strands using data from a small‐aperture Global Positioning System (GPS) campaign and leveling. The model provides estimates of fault slip rates and quantifies slip partitioning between the two strands. A 45  mm/yr dip‐slip rate on the northern Peinan strand diminishes southward, whereas the left‐lateral component increases. In contrast, nearly pure dip‐slip motion of about 20  mm/yr on the southern Luyeh strand diminishes northward to about 8  mm/yr and picks up a component of left‐lateral motion of about 15  mm/yr before it dies out altogether at its northern terminus. The Luyeh and the northern Peinan strands record near‐surface creep, but the southern Peinan strand appears locked. The potential earthquake magnitude for the two strands may be as high as M_w 6.5. We anticipate seismic rupture mainly on the locked portion of the Peinan strand.

Continuously Operating GPS Reference Station Networks: New Algorithms and Applications of Carrier Phase-Based, Medium-Range, Static and Kinematic Positioning

Continuously operating GPS networks have been used for many years to address two types of positioning applications. The first, perhaps best known application, is in relation to geodetic objectives such as the determination of crustal motion on a variety of spatial scales — from the measurement of the broad kinematics of tectonic plates to deformation monitoring of local areas undergoing subsidence (e.g. due to fluid extraction and underground mining), surface inflation (e.g. due to magma intrusion under volcano domes), or the complex faulting in seismically active zones. Currently, permanent GPS stations around the world which have been established to address such geodetic applications number well over a thousand. Hundreds of these stations are now formally part of the global network organised under the auspices of the International Association of Geodesy (IAG) as the well-known International GPS Service (IGS).