Jyr-Ching Hu

Links between erosion, runoff variability and seismicity in the Taiwan orogen

The erosion of mountain belts controls their topographic and structural evolution and is the main source of sediment delivered to the oceans. Mountain erosion rates have been estimated from current relief and precipitation, but a more complete evaluation of the controls on erosion rates requires detailed measurements across a range of timescales. Here we report erosion rates in the Taiwan mountains estimated from modern river sediment loads, Holocene river incision and thermochronometry on a million-year scale. Estimated erosion rates within the actively deforming mountains are high (3–6 mm yr-1) on all timescales, but the pattern of erosion has changed over time in response to the migration of localized tectonic deformation. Modern, decadal-scale erosion rates correlate with historical seismicity and storm-driven runoff variability. The highest erosion rates are found where rapid deformation, high storm frequency and weak substrates coincide, despite low topographic relief.

Characterization of stress perturbations near major fault zones: insights from 2-D distinct-element numerical modelling and field studies (Jura mountains)

Abstract We consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations. Based on 2-D distinct-element modelling, we reconstruct the local stress field around a vertical discontinuity in various geological contexts. The resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 °. We establish simple relationships controlling stress changes around a pre-existing fault zone as a function of (1) the remote differential stress magnitude, (σ1 − σ3), (2) the friction coefficient on the discontinuity, and (3) the strike of the discontinuity relative to the far-field stress. As a geological example, we present the Morez Fault Zone in the internal Jura. Paleostress reconstruction in forty-two sites indicates that the trends of the Mio-Pliocene compression are N110 ° on average near the fault, whereas they are N130 ° in the surrounding areas. A comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 ° counterclockwise deviation is directly related to the reactivation of this large weak zone. We thus evaluate the role of mechanical decoupling along pre-existing zones of weakness, especially with consideration to the accommodation of the Alpine deformation in the Jura belt.

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.

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.

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.

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.

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.

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.