Hao-Tsu Chu

A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault

We mapped and analyzed two vertical exposures—exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation—across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 ^(14)C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

Active faulting and earthquake hazard: The case study of the Chihshang Fault, Taiwan

The Longitudinal Valley Fault Zone of eastern Taiwan is the present-day plate boundary between the Philippine Sea Plate and the South China block of Eurasia. Repeated surveys of active deformation were carried out at five sites along its most active segment, the Chihshang Fault. Annual surveys during the period 1990‐1997 reveal a rather constant slip velocity of 2.2 cm/yr in a N408W direction, involving both a thrust component with horizontal shortening of nearly 1.7 cm/yr and a left-lateral component of nearly 1.4 cm/yr. The fault trends N188E and dips 39‐458 to the east. The vertical displacement velocity is about 1.3 cm/yr and the actual oblique oAset of the fault increases at a rate of 2.6 cm/yr. Comparison with GPS data suggests that some additional deformation occurs on the edge of the Valley. Active faulting of the Chihshang Fault and of the entire Longitudinal Valley Fault Zone accounts for 24% and 37% (respectively) of the total shortening across the Taiwan collision in the N548W direction of relative motion between the Philippine Sea Plate and the South China shelf. This distribution of relative displacements illustrates the major role played by this boundary, as a zone of mechanical weakness where tectonic partitioning occurs. Permanent surveying of the displacement on the Chihshang Fault has the potential to detect significant decrease in slip rates, and hence to predict forthcoming locking stages, which would increase earthquake hazard. # 1999 Elsevier Science Ltd. All rights reserved.

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.

KINEMATICS OF CONVERGENCE, DEFORMATION AND STRESS DISTRIBUTION IN THE TAIWAN COLLISION AREA : 2-D FINITE-ELEMENT NUMERICAL MODELLING

Abstract Using a 2-D plane stress finite-element model with elastic and elasto-plastic rheologies, appropriate for deformation within the brittle upper crust, we analyse the relationship between kinematics of convergence, deformation and stress distribution in the present Taiwan collision occurring between the Ryukyu and Luzon subduction zones. The distribution of stress trends calculated in our models is compared with a synthetic map of actual stress trajectories based on the most recent data available in the collision zone. These data combine present-day sources (from borehole breakouts and earthquake focal mechanism) with the reconstruction of Quaternary palaeostress (from fault slip data analyses), resulting in a complete map of compressional stress trajectories which is used to constrain our models. We show that the distribution of stress trajectories in the active Taiwan collision is principally controlled by: (1) the geometric configuration of the boundary between Eurasia and the Philippine Sea plate; (2) the shape and rheological properties of major structural units; (3) the direction of convergence of the Philippine Sea plate relative to Eurasia; and (4) the influence of the opening of the Okinawa Trough. The study of a two-dimensional elastic and elasto-plastic finite-element modelling of the subduction-collision in and around Taiwan allows us to estimate the influences of these different parameters in the stress pattern. Taking into account both the simplifying assumptions of the numerical modelling and the angular uncertainties of field determinations, the fit between the calculated stress pattern of the finite-element model and that determined based on the geometrical synthesis of field analyses is rather good in general, indicating that our model is valid to first approximation. Misfits remain minor and can be explained by data uncertainties and simplifying modelling assumptions (for instance, the shape of the corner of the collision zone is critical but is not accurately known; also limited decoupling in the Longitudinal Valley collision zone was not considered in our models although it certainly plays a role). Some interesting features of our model are: (1) the greater influence of the shape of the collision zone in comparison with that of the direction of convergence; (2) the requirement for a trench retreat related to suction force in the Ryukyu Arc; and (3) the crucial role of the interaction between Okinawa Trough opening and collision at the sharp northwestern corner of the Philippine Sea plate including its influence on the geological evolution of northeastern Taiwan.

POLYPHASE HISTORY AND KINEMATICS OF A COMPLEX MAJOR FAULT ZONE IN THE NORTHERN TAIWAN MOUNTAIN BELT : THE LISHAN FAULT

Abstract The Lishan Fault is a major fault zone in the Taiwan collision belt. It separates two major units, the Hsuehshan Range and the Backbone Range, which differ in lithology, ages of sediments, metamorphic grades and deformation styles. Despite the importance of the Lishan Fault, its evolution and tectonic behaviour remained poorly known and controversial. We therefore carried out a tectonic study which includes both the identification of structures in and along the fault zone and the palaeostress analysis aiming at reconstructing the succession of fault mechanisms. The Lishan Fault zone underwent polyphase evolution with reactivations under different tectonic regimes, consistent with the Cenozoic history of Taiwan. 1. (1) On the Hsuehshan Range, the earliest events reflect the Palaeogene-Miocene extension of the Chinese continental margin. 2. (2) Serial cross-sections and observation of ductile-brittle structures show east-vergent folding, indicating that for the most recent compressional evolution related to late Cenozoic Taiwan collision the Lishan Fault was a steeply dipping east-vergent back-thrust. The compression occurred along NW-SE trends, inducing thrusting, but also along N-S ones, inducing transpression with reverse sinistral slip. The Lishan Fault thus underwent contraction as well as strike slip during the mountain building of the Taiwan orogeny. Reverse and strike-slip fault systems alternated because of permutations σ 2 σ 3 under the same compressional stress regime of NW-SE σ1. Minor compressional events also occurred. 3. (3) A late extension, accommodated by normal faulting, reveals the influence of both the N-S extension in the Okinawa Trough northeast off Taiwan.

Geometry and Quaternary kinematics of fold‐and‐thrust units of southwestern Taiwan

Structural and paleostress analyses provide new insights into the Quaternary kinematics of the outermost fold-and-thrust units of southwestern Taiwan Foothills. The frontal folds are interpreted as fault-related folds, and their tectonic evolution through space and time is tightly constrained. Fold development is correlated with reef building on top of the anticlines. Moreover, we provide field evidence that NW–SE fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the Quaternary fold-thrust emplacement. Two successive Quaternary stress regimes are evidenced in southwestern Taiwan: A NW–SE compression, followed by a recent nearly E–W compression. The latter shows an along-strike change from pure E–W contraction to the north to perpendicular N–S extension in the south. This southward decrease in N–S confinement probably represents the on-land signature of the incipient Quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present-day by Global Positioning System data.

Contractional, transcurrent, rotational and extensional tectonics: examples from Northern Taiwan

Abstract Contraction, transcurrent faulting, block rotation and even extension are four essential tectonic mechanisms involved in the progressive deformation of arcuate collision belts. The neotectonic evolution of the Taiwan mountain belt is mainly controlled by the oblique convergence between the Eurasian plate and the Philippine Sea plate as well as the corner shape of the plate boundary. Based on field observations and tectonic analysis, and taking geophysical data and experimental modelling into account, we interpret the curved belt of northern Taiwan in terms of contractional deformation (with compression, thrust-sheet stacking, folding and transcurrent faulting) combined with increasing block rotation, bookshelf-type strike-slip faulting and extension. As a consequence, the formation of the extensional Taipei Basin, the division of conjugate strike-slip faulted domains and the variable nature and distribution of paleostresses should not be interpreted in terms of distinct Plio-Quaternary episodes but should reflect a single, albeit complicated, regional pattern of deformation. Our study demonstrates that in Taiwan, contractional, extensional and transcurrent tectonics as well as rotations combine together and interact within a single complex framework. The crescent-shaped mountain belt develops in response to oblique indentation by an asymmetric wedge indenter. The distribution, nature and relative importance of these deformation modes are a function of the shape of the indenter and the average direction of convergence.

Quaternary transfer faulting and belt front deformation at Pakuashan (western Taiwan)

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along-strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along-strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left-lateral tear fault or transfer fault that offsets two distinct frontal thrust-and-fold sheets.

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.

Three-dimensional deformation along the rupture trace of the September 21st, 1999, Taiwan earthquake: a case study in the Kuangfu school

Abstract In Central Taiwan, the destructive Chichi earthquake ( M w =7.6) of September 21st, 1999, produced a nearly 100-km-long rupture trace following the Chelungpu Fault. This paper shows how the geometry of the fault slip can locally be determined in three dimensions, based on analyses of earthquake fault in the Kuangfu stadium. The fault at this site is reverse and left-lateral. The three-dimensional geometrical analysis, done with a variety of data through independent determination modes, all applied to the deformation of the surface layer of the stadium tracks, allows complete verification of the results. Not only do these analyses constrain the fault behaviour in terms of trend, dip angle, obliquity and amount of slip, but also they reveal variations in both the horizontal and the vertical fault geometry. The comparison with relative displacements recorded at larger distances from the fault reveals that slip partitioning occurs, with nearly dip-slip thrusting along the rupture trace and distributed left-lateral shear in a deformed zone of the hanging wall, adjacent to the main fault.