Shui-Beih Yu

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.

Spatial and Temporal Distribution of Slip for the 1999 Chi-Chi, Taiwan, Earthquake

We investigated the rupture process of the 1999 Chi-Chi, Taiwan, earth- quake, using high-quality near-source strong-motion records, broadband teleseismic displacement waveforms, and well-distributed Global Positioning System (GPS) data. The near-source strong-motion displacement waveforms recorded significant static offsets of up to 8 m. The teleseismic displacement records show a significant pulse with duration of about 18 to 20 sec. Taking into account the surface displacements observed along the Chelungpu fault, we considered two fault geometries: a single planar fault and a two-segment fault with a northeast-striking section near the north- ern end. Using the finite-fault model with variable slip vectors, we derived two models of the temporal and spatial slip distribution of the earthquake. The GPS data provided good surface displacement constraints for the slip-distribution determina- tion. The spatial slip distribution is generally consistent with field observations. The results for the simple fault model show a large asperity located in the region about 25 to 55 km north of the hypocenter with maximum slip of about 15 m. When we use the two-segment model, the asperity further extends to the region where the fault bends toward the northeast with a maximum slip of up to 20 m. A large amount of right-lateral slip beneath station TCU068 is necessary to explain its observed large west movement. It implies a local converging slip at the corner where the fault bends to the northeast. The slip amplitude near the hypocenter is about 3 to 6 m. The seismic moments determined from the various data sets are within the range of 2 to 4 10 27 dyne cm. Most of the slip concentrated at shallow depths (less than 10 km). The total rupture duration is about 28 sec, and the rupture velocity is 75% to 80% of the shear-wave velocity. The slip vector shows a clockwise rotation during the fault rupture. The static stress drop of the large asperity region is comparable with the dynamic stress drop, as observed directly from the slip velocity at the station near the large slip region.

Fault geometry and slip distribution of the 1999 Chi-Chi, Taiwan earthquake imaged from inversion of GPS data

GPS measurements of coseismic displacements from the 1999, Chi-Chi, Taiwan earthquake are modeled using elastic dislocation theory. We find that a single fault plane cannot fit the data, but rather a curved fault surface consisting of multiple segments dipping 20–25° best fits the observations. The model fault exhibits reverse and left-lateral slip on a 75 km long N-S trending segment and reverse and right-lateral slip on a 25 km E-W trending segment at the northern end of the rupture. The 21° dipping E-W segment is inconsistent with previous interpretations of high angle tear faulting.

Coseismic hydrologic response of an alluvial fan to the 1999 Chi-Chi earthquake, Taiwan

Widespread coseismic change in pore-water pressure across a large alluvial fan in central Taiwan in the 1999 Chi-Chi (M w = 7.5) earthquake was captured for the first time by a dense network of hydrologic monitoring wells. The complex, yet systematic, pattern in the water-pressure change appears inconsistent with the existing models; it requires a model that is based on the nonlinear mechanical behavior of sediments under earthquake shaking. This paper presents direct field evidence that earthquake shaking causes rising pore pressure in alluvial fans, which in turn may lead to landslides, even on very gentle slopes.

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.

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 interpretation of the active deformation of southern Taiwan based on numerical simulation and GPS studies

Abstract The fold-and-thrust belt of Taiwan results from oblique convergence between the Eurasia and the Philippine Sea plate, and its front structures are strongly influenced by the presence of large horsts and basins in the foreland. Using a 2-D numerical modelling (finite-element and distinct-element methods), we evaluate the influences on the active deformation of southern Taiwan of: (1) the foreland structural highs; (2) the major fault zones in the belt; and (3) the presence of a subduction zone to the south. To constrain our models, we take into account for the first time the present-day velocity field of southern Taiwan estimated based on new geodetic data obtained through the Global Positioning System (GPS). Particular attention is paid to the role of geological discontinuities, through an evaluation of the presence and role of mechanical decoupling along major faults, which plays an important role in the distribution of the regional and local velocity and stress patterns. This particular analysis of the behaviour and influence of weak shear zones in Taiwan is carried out by using, for the first time, the distinct-element method. Additional 3-D distinct-element modelling allows better consideration of oblique shearing, such as for the Longitudinal Valley Fault of eastern Taiwan. We conclude that the active velocity field and tectonic stress pattern in southwestern Taiwan strongly depend on: (1) the presence and shape of the Peikang High; (2) the presence of the major active regional discontinuities (the Longitudinal Valley Fault and the major thrusts of western Taiwan); and (3) the neighbouring weakness zone of the accretionary prism of the northern Manila subduction zone, and cannot be explained by any of these factors taken solely.

Active deformation of Taiwan from GPS measurements and numerical simulations

Using a two-dimensional distinct element model, we evaluate the relationships between plate kinematics and present-day deformation in Taiwan where active collision occurs. In particular, the distribution of velocity fields calculated in our models is compared with the actual velocity field revealed by the most recent geodetic data (GPS) obtained in Taiwan and the surrounding islands of the Philippine Sea plate and the Eurasian shelf. The main aim of this paper is to produce a mechanically consistent 2-D model that accounts for the observed velocity field taken as whole, within the limits of acceptable rheological parameters and reasonable boundary displacement conditions. We evaluate how the active deformation of Taiwan is influenced by the presence of strong and weak zones such as the structural highs in the foreland and subduction zones with accretionary prisms, respectively, major mechanical discontinuities such as the main fault zones in the mountain belt, and the opening of the Okinawa Trough. Particular attention is paid to the role of preexisting discontinuities since the presence of mechanical decoupling along major faults strongly affects the distribution of the velocity and stress patterns. We show that despite parameter uncertainties, several tectonic factors (the presence of the strong Kuanyin and Peikang highs in contrast to the weak subduction zone to the south, the “weak” active regional shear zones, and the opening of the back arc Okinawa Trough) concur to provide an acceptable mechanical model for this regional deformation. These sources are related not only to the geometry of the plate boundary, the direction of plate convergence, and the shape of the Chinese margin but also to the presence of major zones of relative weakness and mechanical decoupling such as the Longitudinal Valley fault zone and the western thrust belt of Taiwan.

Plate-boundary strain partitioning along the sinistral collision suture of the Philippine and Eurasian plates: Analysis of geodetic data and geological observation in southeastern Taiwan

Crustal deformation and strain partitioning of oblique convergence between the Philippine Sea plate and the Eurasian plate in the southern Longitudinal Valley of eastern Taiwan were characterized, based on geodetic analysis of trilateration network and geological field investigation. The Longitudinal Valley fault, one of the most active faults on Taiwan, branches into two individual faults in the southern Longitudinal Valley. These two active faults bound the Plio-Pleistocene Pinanshan Conglomerate massif between the Coastal Range (the Luzon island arc belonging to the Philippine Sea plate) and the Central Range (the metamorphic basement of the Eurasian plate). A geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983–1990. The strain tensors for polygonal regions (including triangular regions) of the Taitung trilateration network reveal that there are two distinct zones of deformation: a zone of shortening (thrusting) between the Pinanshan massif and the Central Range on the west and a strike-slip movement between the Pinanshan massif and the Coastal Range on the east. The analysis of a discontinuity model consisting of three rigid blocks separated by two discontinuities has been carried out. The results show that the deformation in this region can be characterized by two major faults. A reverse fault is located between the Plio-Pleistocene Pinanshan massif and the metamorphic basement of the Central Range, with a shortening rate of about 12 mm/yr in the direction N280°E. A strike-slip fault is located principally along the river between the Pinanshan massif and island arc system of the Coastal Range with a purely strike-slip component of about 22 mm/yr in the direction N353°E. The analysis of the geodetic data further suggests that substantial deformation (probably strike slip in type) occurs within the Pinanshan massif. Geological evidence of deformation in the Plio-Pliestocene Pinanshan Conglomerate includes regional folding, a conjugate set of strike-slip fractures at the outcrop scale, and morphological lineaments related to fracturing, all indicating that the Pinanshan massif is being deformed within a transpressive stress regime. Regional kinematic data indicate that a significant portion of the 82 mm/yr of motion between the Eurasian plate and the Philippine Sea plate is absorbed in the southern Longitudinal Valley by the decoupling of two distinct major faults. The geometry of the oblique convergence and the rheology of the rock units (the well-consolidated Plio-Pleistocene conglomerate and the sheared melange formation) play the two important roles in the partitioning of crust deformation.