Long-Chen Kuo

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.

Preseismic Deformation and Coseismic Displacements Associated with the 1999 Chi-Chi, Taiwan, Earthquake

The destructive 1999 Chi-Chi, Taiwan, earthquake ( M w 7.5) produced an approximately 100-km-long surface rupture, mostly along the previously recognized north-south-trending Chelungpu fault. Preseismic deformation in central Taiwan is realized from annually repeated Global Positioning System (GPS) data acquired during the 1992-1999 period. The total WNW-ESE shortening rate in the vicinity of the epicentral region, that is from the west coast to the western boundary of the Central Range, is up to 25 mm/yr. The crustal deformation before the Chi-Chi earthquake was essentially a uniaxial compressional strain of 0.36 μstrain/yr in the direction of 114°. The GPS measurements taken 0.2-2.7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements. Horizontal movements of 1.1-9.1 m in the NW-NNW directions are observed on the hanging wall (eastern side) of the fault. There is a northward-increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50° toward the east along the fault strike. In contrast, much smaller SE-SEE movements of 0.1-1.5 m are found on the footwall (western side) of the fault. The GPS data show 2.4-10.1 m of total horizontal offsets across the Chelungpu fault. Vertical offsets of 1.2-4.4 m with the eastern side up are also observed along the surface rupture. The uplift on the hanging wall decreases rapidly toward the east. It becomes subsidence at Sun Moon Lake and in the Puli-Wushe area. The stations on the footwall show subsidence of 0.02-0.26 m. The width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north. Manuscript received 13 October 2000.

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.

Spatiotemporal evolution of seismic and aseismic slip on the Longitudinal Valley Fault, Taiwan

The Longitudinal Valley Fault (LVF) in eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of fault slip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five M_w>6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign mode Global Positioning System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolution provided by these data, we complement them with interferometric synthetic aperture radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite images processed using a persistent scatterer technique. The combined data set covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis-based Inversion Method. This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake occurred on a fault patch which had remained partially locked in the interseismic period, (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface, and (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80–90% of slip on the southern section of LVF in the 0–26 km, seismogenic depth range, is actually aseismic. We infer that the clay-rich Lichi Melange is the key factor promoting aseismic creep at shallow depth.

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.

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.

Active back thrust in the eastern Taiwan suture revealed by the 2013 Rueisuei earthquake: Evidence for a doubly vergent orogenic wedge?

Rapid exhumation of 3–10 mm/yr of the Taiwan metamorphic range is often explained as the unroofing of the retrowedge of a doubly vergent mountain belt. Yet, to date, the Central Range fault forming the boundary of the retrowedge has displayed no definitive evidence for recent seismic activity and no unambiguous geomorphic expression over much of the fault. The 2013 M6.4 Rueisuei reverse-faulting earthquake nucleated at the eastern boundary of the retrowedge and appears to illuminate the west dipping Central Range fault. We estimate the fault geometry and coseismic slip distribution using a uniform stress drop slip inversion and surface displacements derived from GPS and strong-motion data. We identify a ~42° dipping blind reverse fault, consistent with the previously proposed buried Central Range fault beneath the highly active Longitudinal Valley fault. This earthquake may be the first indication that rapid exhumation and uplift occur along a distinct fault structure bounding the eastern margin of the Taiwan retrowedge.

IMPACT OF A LARGE EARTHQUAKE ON A GPS NETWORK: THE CASE OF THE 1999 CHI-CHI, TAIWAN EARTHQUAKE

Abstract On 20 September 1999, Taiwan was hit by the largest inland earthquake (Mw=7.6) in the last century, resulting in an 80-kilometre thrust rupture almost directly along the eXisting Chelungpu fault. Here, we estimated the coordinates of the GPS stations by using the annual epoch-observed and permanent continuously-recorded GPS data covering the 1992–1999 period Taking into account the effects of secular motion and post-deformation, we calculated the corrected coseismic displacements. Relatively large coseismic displacements of 8–9 m in the horizontal and vertical components were found in the northern part of the fault. Stations on the hanging wall shifted horizontally 2–9 m in the NW direction and decreased in magnitude from west to east. On the other hand, displacements on the footwall were less than 2 m in the SE direction but increased from west to east. The size of the zone overlapping the hanging wall and the footwall showed a decrease of 796 ppm in 494 km2. This is in direct contrast to areas near the fault which increased in size from 13 to 237 ppm. Significant postseismic deformations were also observed in data from the existing 44 permanent stations and 7 temporary stations. Additionally, large postseismic deformations of 10–20 cm were found at stations SUNM, 1007 and YUSN within 10 months.

Reducing distance dependent bias in low-cost single frequency GPS network to complement dual frequency GPS stations in order to derive detailed surface deformation field

Abstract A total of 17 low-cost single-frequency L1 global positioning system (GPS) receivers with real-time internet transmission have been set up to intensify the pre-existing network of continuously operating reference stations (CORS) in southeastern Taiwan since 2008. The main objective of this study is to investigate the validity and uncertainty of the L1 stations in southeastern Taiwan. It is well known that the main error source of single-frequency GPS relative positioning in low latitude areas comes from an atmospheric delay, even if the relative distance is only a few kilometres. In this study, two methods of correction algorithms, including adopting local ionospheric models and applying correction terms from local CORS, are tested to estimate the long-period accuracy of station positioning. Our results indicate that the standard deviation of calibrated relative positioning is in a linear trend with respect to the baseline length. The derived positioning accuracies from applying correction terms from CORS provide satisfactory results with the linear ratios of standard deviation/baseline of 0·11±0·02, 0·12±0·02, 0·44±0·06 mm km–1 in the north, east and up component, respectively for relative distances under 30 km. The corresponding positioning scatterings amount to 3, 3 and 13 mm, in the north, east and up component, respectively. Although the use of a local ionospheric model algorithm can significantly reduce positioning variation, especially in the north component, the use of the correction terms method yields the best positioning results for three components, horizontal and vertical.

Data Files from “Preseismic Deformation and Coseismic Displacements Associated with the 1999 Chi-Chi, Taiwan, Earthquake”

Tables 1-4 in our article, “Preseismic Deformation and Coseismic Displacements Associated with the 1999 Chi-Chi, Taiwan, Earthquake,” are given as a computer readable file on the …