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The southernmost Ryukyu subduction zone may have a geodetically inferred Mw 7.5 to 8.7 megathrust earthquake in a shallow locked region, the Ryukyu fault. Paleoseismological evidence of historical earthquakes available from the last 417 years indicates that only a 1920 Mw 7.7 earthquake occurred within this magnitude range, near the downdip end of the Ryukyu fault. As slow slip events downdip the locked seismogenic zone may trigger a large subduction earthquake, we investigate how the first observed slow slip events in 2005, 2009, and 2015 initiated downdip in the Ryukyu fault interface affect the occurrence of a megathrust. We establish possible megathrust earthquake cycles from Mw 7.5 to 8.7 on the Ryukyu fault using constraints from the magnitude-frequency relation based on local historical earthquakes. This analysis shows a b value of 1.2 for magnitudes greater than Mw 7.0, which is higher than the empirical 1.0 value. This indicates that the recurrence of an event up to Mw 8.7 is longer than previously thought if the megathrust events follow the observed magnitude-frequency relation. Then, we quantify the influence of slow slip events on the triggering of a potential megathrust earthquake by calculating the static stress increase. We find that stress perturbations caused by the three slow slip events are generally consistent with the values that have triggered the large interplate earthquakes in several subduction zones. However, a large earthquake has not yet been triggered on the Ryukyu fault after a sequence of slow slip events. If the 1920 Mw 7.7 earthquake is the last rupture of the Ryukyu fault, the earthquake cycle on the Ryukyu fault is very likely in an early stage. However, this is not true if the slow slip events occur toward the end of the earthquake cycle and there has been no megathrust earthquake at the fault interface in the last 417 years, as the 2011 Mw 9.0 Tohoku earthquake. Thus, higher potential for a megathrust earthquake may occur in the southernmost Ryukyu subduction zone.

A natural hillslope developing into a landslide shows ground cracks and topographic deformation. Geomorphological and subsurface investigations using appropriate methodology are essential to understand the failure mechanisms and stability of a hillslope. A dip slope in sedimentary rock in northern Taiwan has been observed to have a potential landslide hazard for the development of ground cracks and persistent deformation of local buildings and facilities on the slope. To monitor the movement of the dip slope and then understand its movement pattern, 144 ground monitoring points was set in 2001, and its coordinates were measured using conventional traverse surveying twice a year until 2017. In addition, 6-year surficial surveying results as time series of displacements and velocity field are revealed by each GPS station on the slope surface. The long-term surveying results point out different displacement patterns of the dip slope depending on rainfall duration and amount. The surficial surveying results are presented as a time series of displacements with constraints of geometry and distribution of ground cracks and underground observations. The long-term surveying results reveal multiple potential sliding blocks within the Huafan University campus. A model of landslide movement with a listric sliding surface is proposed. Finally, the continuous GPS stations show the average velocity of 0.396~0.528 x 10-7 mm/sec, being classified as “Extremely slow” in the global “velocity scale of landslides” proposed by Cruden and Varnes in 1996. The long-term surface monitoring of a potential landslide slope in this study provides a reliable and economical way to understand the mechanism of movement behavior of the slope and evaluate slope stability.

Slow-moving landslides can transform into catastrophic landslides at certain conditions. In recent decades, catastrophic landslides around the world have caused widespread damage to buildings and threatened human lives. Thus, the detection of surface displacement variation and the definition of precursory motion for extensive slow-moving landslides become a primary goal of current researches. This study dedicates to detect and monitor surface displacement of multiple slow-moving landslide areas in Taiwan by the multitemporal interferometric SAR (MTInSAR) technique. A new package was established to generate interferograms of multiple slow-moving landslide areas at the same time. Thus, the surface displacement information in large spatial coverage and a long period of time can be obtained efficiently. The landslide characteristics in space and time domain of those slow-moving landslides were investigated and analyzed. Especially, the significant acceleration movements of certain slow-moving landslides were observed and agreed with the in situ monitoring data. The results of this study provide important information of landslide hazard assessment and show the opportunity of detecting the landslide precursory motions by the MTInSAR technique in the future.

We use 12 years of InSAR time-series and 22 years of piezometric records to track the spatiotemporal evolution of land subsidence andgroundwater depletion in Hanoi. Our result shows diminishing subsidence in central Hanoi, while the southern and western outskirts (HaDong and Hoai Duc district) become the new subsidence center duringthe past few years. The maximum subsidence rate reaches up to 50 mm/yr, and the maximum cumulative subsidence can be as high as 500 mm.The maximum groundwater storage decrease is ~25 m over 22 years.

Located in northern Vietnam, the Fansipan mountain range is the highest topography in the Indochina area. Until recently, there has been limited research regarding the tectonic deformation of the Fansipan mountain range and whether there are notable surface ruptures or active faults in the area. This study provides the first insight into the deformation patterns and potential active movements of the Fansipan mountain range, and may improve our understanding of the overall tectonic activities in northern Vietnam. Our observations from ASTER 30-m DEM show a symmetrical mountain form in the north and an asymmetrical form in the south. The analysis of river profiles indicates similar patterns on both sides in the north and different patterns in the south with short and steep rivers on the SW side and long and gentle rivers on the NE side. The results of the normalized steepness index (ksn) also show higher values on the SW side of the south part than the values from the other sides. These results suggest a spatial variation in rock uplift patterns of the Fansipan mountain range. Notably, the mountain front of the SW side of the southern Fansipan mountain range is associated with the Phong Tho-Nam Pia normal fault, suggesting that the normal faulting activity likely played an important role in tectonic uplift of this high mountain range. Our investigation of the stress field using fault kinematics indicates that the Fansipan mountain range and its surrounding areas are undergoing an inhomogeneous mixture of strike-slip and normal faulting. It is proposed that the strike-slip and normal motions alternated because of a permutation of σ1/σ2 under the same extensional stress regime of approximately NE–SW σ3. This kinematic pattern may effectively control the recent tectonic movements in northern Vietnam.

The recurrence intervals of slow slip events may increase gradually after a large earthquake during the afterslip. Stress perturbations during coseismic and postseismic periods may result in such an increase of intervals. However, the increasing recurrence intervals of slow slip events are rarely observed during an afterslip. The evolution process along with the afterslip remains unclear. We report an observation of slow slip events following the 2002 Mw 7.1 Hualien offshore earthquake afterslip in the southernmost Ryukyu subduction zone. Slow slip events in 2005, 2009, and 2015 are adjacent to the Mw 7.1 earthquake hypocenter. An increasing slow-slip interval of 3.1, 4.2, and 6.2 years has been observed after the earthquake. We calculated coseismic and postseismic slips from the Mw 7.1 earthquake and then estimated the Coulomb stress changes in the slow slip region. The Mw 7.1 earthquake has contributed positive Coulomb stresses to both the 2005 slow-slip region and 2009/2015 repeating slow-slip region. The coseismic and postseismic Coulomb stress change on the 2005 slow-slip region is approximately 0.05 MPa and 0.035 MPa, respectively. However, both Coulomb stress changes on the 2009/2015 repeating slow-slip region are not over 0.03 MPa. The ongoing afterslip following the Mw 7.1 earthquake last for at least five years, evolving with a decaying stress rate with time. The long-term stress perturbations may be able to trigger the 2005 slow slip event during the afterslip. The 2009 slow slip event seems to be influenced by the afterslip as well. Postseismic stress evolution and frictional and stressed conditions of the slow-slip region can be a reason to affect the evolution process of slow slip events intervals.

Balancing the demand of groundwater resources and the mitigation of land subsidence is particularly important, yet challenging, in populated alluvial fan areas. In this study, we combine multiple monitoring data derived from Multi-Temporal InSAR (MTI), GNSS (Global Navigation Satellite System), precise leveling, groundwater level, and compaction monitoring wells, in order to analyze the relationship between surface displacement and groundwater level change within the alluvial fan of the Choshui River in Taiwan. Our combined time-series analyses suggest, in a yearly time scale, that groundwater level increases with the vertical surface displacement when the effect of pore water pressure dominates. Conversely, this relationship is negative when the effect of water-mass loading predominates over pore water pressure. However, the correlation between the vertical surface displacement and the groundwater level change is consistently positive over the time scale of two decades. It is interpreted that the alluvial fan sequence in the subsurface is not fully elastic, and compaction is greater than rebound in this process. These findings were not well reported and discussed by previous studies because of insufficient monitoring data and analyses. Understanding the combined effect of groundwater level change and vertical surface displacement is very helpful for management of land subsidence and usage of groundwater resources. The spatial and temporal integration of multi-sensors can be applied to overcome the limitations associated with the single technique and provides further insights into land surface changes, particularly in highly populated alluvial fan areas.

The topography effect has been thriving investigated based on numerical modeling. It impacts the seismic ground shaking, usually amplifying the amplitude of shaking at top hills or ridges and de-amplifying at valleys. However, the correlation between the earthquake-induced landslide and the topographic amplification is relatively unexplored. To investigate the amplification of seismic response on the surface topography and the role in the Chi-Chi earthquake-induced landslide in the JiuJiu peaks area, we perform a 3D ground motion simulation in the JiuJiu peaks area of Taiwan based on the spectral element method. The Lidar-derived 20m resolution Digital Elevation Model (DEM) data was applied to build a mesh model with realistic terrain relief. To this end, in a steep topography area like the JiuJiu peaks, the designed thin buffer layers are applied to dampen the mesh distortion. The three doubling mesh layers near the surface accommodate a more excellent mesh model. Our results show the higher amplification of PGA on the tops and ridges of JiuJiu peaks than surrounding mountains, while the de-amplification mostly occurs near the valley and hillside. The relief topography could have a ±50% variation in PGA amplification for compression wave, and have much more variety in PGA amplification for shear wave, which could be in the range between -50% and +100%. We also demonstrate that the high percentages of the landslide distribution right after the large earthquake are located in the topographic amplified zone. The source frequency content interacts with the topographic feature, in general, small-scale topography amplifies the higher-frequency seismic waves. It is worthy of further investigating the interaction between the realistic topography and the velocity structure on how to impact the seismic response in the different frequency bands. We suggest that the topographic seismic amplification should be taking into account in seismic hazard assessment and landslide evaluation.

Landslides are commonly triggered by heavy rainfall events, but how the loose landslide debris is redistributed through time and how fast the landslide scars are healed by vegetation are not well and precisely documented. Due to recent advances in airborne Lidar-derived digital elevation models, we are able to obtain precise DEMs at different time periods and analyze the redistribution of landslide debris that was once difficult to measure because of relatively minor elevation changes. Three periods of Lidar-derived DEMs were used to analyze a drainage basin that was affected by a heavy rainfall event and generated several landslide deposits and scars within the drainage basin in Taiwan. We selected a single drainage basin to better constrain the source of landslide debris for subsequent observations of landslide debris removal. How the landslide debris is transported and redistributed remains an important topic for understanding debris removal and evaluating post-landslide hazards in downstream areas. The multi-period high-resolution Lidar DEMs give the necessary accuracy to calculate small but significant volume changes that were not easily detectable from previous measuring techniques. Our results show that the landslide debris redistributed most effectively during later large rainfall events, and the landslide materials are minimally redistributed during small rainfall events. Areas without existing landslides were also insignificantly affected in terms of volume change even during large rainfall events. The standard deviation of elevations in the drainage basin is used to show how the topography was changed due to heavy rainfall events within the drainage basin. The concept of surface roughness may be useful to characterize the dissipation of landslide debris because the roughness values become lower during the debris redistribution process. The redistribution of landslide debris over the observed years suggests that the dissipation of landslide debris is mainly affected by by episodic heavy rainfall events and the landslide scars recover relatively quickly for smaller affected landslide regions.

Several remote sensing techniques, namely traditional aerial photographs, an unmanned aircraft system (UAS), and airborne lidar, were used in this study to decipher the morphological features of obscure landslides in volcanic regions and how the observed features may be used for understanding landslide occurrence and potential hazard. A morphological reconstruction method was proposed to assess landslide morphology based on the dome-shaped topography of the volcanic edifice and the nature of its morphological evolution. Two large-scale landslides in the Tatun volcano group in northern Taiwan were targeted to more accurately characterize the landslide morphology through airborne lidar and UAS-derived digital terrain models and images. With the proposed reconstruction method, the depleted volume of the two landslides was estimated to be at least 820 ± 20 × 106 m3. Normal faulting in the region likely played a role in triggering the two landslides, because there are extensive geological and historical records of an active normal fault in this region. The subsequent geomorphological evolution of the two landslides is thus inferred to account for the observed morphological and tectonic features that are indicative of resulting in large and life-threatening landslides, as characterized using the recent remote sensing techniques.

Slope failure is a widely observed phenomenon in the mountainous areas in Taiwan due to rainy climatic and fragile geological conditions. Landslides easily occur after intense rainfall, especially from typhoons, and, accordingly, cause a great loss of human life and property. At the northern end of the Western Foothill belt in northern Taiwan, Huafan University is founded on a dip-slope about 20° toward the southwest composed of early Miocene alternations of sandstone and shale. Data from continuous monitoring using inclinometers and groundwater gauges reveal that 6–10 mm/month of slope creeping occurs, and a potential sliding surface is then detected about 10–40 m beneath the slope surface. To understand the potential runout process of the dip-slope failure at the campus, particle flow code 3D models based on a discrete element method are applied in this study. Results of the simulation reveal a critical value of the friction coefficient to be 0.13 and that more than 90% of the campus buildings will slide down in 100 s when the friction coefficient is reduced to half the critical value. The weakening of the shear zone due to the rise of groundwater during rainstorms is assumed to be the main factor. Some suggestions for preventing landslide disasters are to construct catchpits to drain runoff and lower the groundwater table and to install a sufficient number of ground anchors and retaining walls to stabilize the slope.

Dip-slope mapping is a fundamental task for landslide investigation and mitigation. However, most dip-slope mapping methods involve visual interpretations and manual processes that are inevitably subjective and time consuming. The advent of high-resolution digital elevation models (DEM) and increases in computing power have provided opportunities to improve the dip-slope mapping process. This study proposes a polygon auto-tracing method for generating dip-slope maps based on airborne Light Detection and Ranging (LiDAR) data and a customized spatial analysis toolset developed in Python. This method requires the input of strata boundaries produced for sedimentary terrain based on 2 m resolution LiDAR DEMs. The method begins by deriving the raster layer of the dip direction of the bedding, and it then executes a series of raster calculations among the three raster layers slope, aspect, and dip direction to extract the dip-slope raster cells. Using the clustering pattern of the dip-slope raster cells, we implement the Point-Density analysis tool to determine the dip-slope areas. The ArcGIS ModelBuilder platform is used to lay out an automated workflow for the proposed polygon auto-tracing method using the customized toolset. For demonstration purposes, we successfully mapped 298 dip slopes in the study area, which frequently experiences dip-slope landslides and is located in the sedimentary terrain of northern Taiwan. The dip-slope mapping results were compared and validated against two government-funded visually interpreted dip-slope maps. Our dip-slope mapping results were also used in a daylight analysis along major freeways to identify potential locations of daylighted dip slopes.

Mica porphyroblasts, a common metamorphic microstructure, are analyzed in the slate belt of northern Taiwan where large fish-like growths are found within a meta-pyroclastics. With constraints on the time-temperature history from deposition through peak metamorphic state to exhumation, in-situ 40Ar/39Ar laser microprobe dating was carried out on muscovite and corrensite fibers of mm-scale mica porphyroblasts grown on a pressure-solution seam. Because the peak metamorphic temperature and the porphyroblast formation condition (∼250 °C) remained far below the closure temperature of the K-Ar radioisotope system in muscovite, and the absence of muscovite in the mafic protolith, the dating results likely document the growth of the mica porphyroblast fabrics. The syn-kinematic nature of the analyzed porphyroblasts is confirmed by the ∼6 to ∼2.5 Ma growth ages, suggesting that the host rock was continuously deformed during the earlier two-thirds of the Taiwan Orogeny. The pattern of fiber growth, in contrast to outward-decreasing ages normally observed in peripheral recrystallization, appears random and resembles void fills in boudin openings. We postulate that syntaxial crack-seal following tensile micro-boudinage, along with slips on sub-grain boundaries, as a viable mechanism for the development of mica porphyroblasts and fish especially in lower-grade metamorphic rocks.

International audience

A natural hillslope developing into a landslide shows ground cracks and topographic deformation. Geomorphological and subsurface investigations using appropriate methodology are essential to understand the failure mechanisms and stability of a hillslope. Huafan University campus located on a dip slope in northern Taiwan is facing a potential landslide hazard. Slope movement was detected through the development of ground cracks and persistent deformation of campus buildings and facilities. To monitor the sliding behavior of the dip slope, a nail network consisting of 144 ground monitoring points was set in 2001, and its coordinates were measured using conventional traverse surveying twice a year until 2017. The 17-year surficial surveying results were presented as a time series of displacements with constraints of geometry and distribution of ground cracks and underground observations. The long-term surveying results reveal multiple potential sliding blocks within the Huafan University campus. A model of landslide movement with a listric sliding surface is proposed. Additionally, from the velocity field derived from the monitoring points, the horizontal strain rates of the slope are estimated. The pattern of strain rates indicates that a plausible fault passing through the campus may have affected the movement of the dip slope. The long-term surface monitoring of a potential landslide slope in this study provides a reliable and economical way to understand the mechanism of movement behavior of the slope and evaluate slope stability.

International audience; Deep tectonic processes are key integral components in the evolution of mountain belts, while observations of their temporal development are generally obscured by thermal resetting, retrograde alteration and structural overprinting. Here we recorded an integrated rock time-temperature history for the first time in the pro-wedge part of the active Taiwan arc-continent collision starting from sedimentation through cleavage-forming state to its final exhumation. The integrated thermal and age results from the Raman Spectroscopy of Carbonaceous Material (RSCM) method, zircon U-Pb laser ablation dating, and in-situ 40Ar/39Ar laser microprobe dating suggest that the basal accretion process was crucial to the development of the Taiwanese orogenic wedge. The basal accretion process commenced early in the mountain building history (∼6 Ma) and gradually migrated to greater depths, as constrained by persistent plate convergence and cleavage formation under nearly isothermal state at similar depths until ∼ 2.5 Ma recorded in the early-accreted units. Such development essentially contributed to mountain root growth by the increased depth of the wedge detachment and the downward wedge thickening during the incipient to full collision stages in the Taiwan mountain belt.

Earthquakes can generate seismic disturbances that propagate vast distances and trigger landslides that can achieve high-speeds. It remains difficult to identify the co-seismic ground motion of these landslides and their triggering earthquakes. In this paper, we report on the analysis of co-seismic ground motions generated by the initiating fracture and deposition impact of the Tsaoling landslide. The landslide, with a source volume of 125 × 10 6 m 3 , was triggered by the 1999 Chi-Chi earthquake in Taiwan. The ground motion was recorded by a strong ground motion station, CHY080, near the scar area. The polarization of the seismic waves indicates that the peak acceleration was parallel to the dip direction. Modified ensemble empirical mode decomposition (EEMD), with additional clustering analysis, was applied to decompose the seismic signals. Two instances were found in the seismic records of a series of peculiar wave packets, with the first being associated with the landslide initiation and the second the landslide impact on the deposit valley. To confirm the first landslide breakage, the decomposed signals were compared with the predictions of the analytic elastic wave model and Newmark analysis. The landslide impact was verified with a computational fluid dynamic simulation. Comparison between the EEMD decomposed signals, elastic wave theory, Newmark rigid body analysis, and numerical simulation demonstrates the claimed landslide motion.

Catastrophic landslides and related phenomena are commonly facilitated by the subtropical climate with frequent typhoons and recurrent earthquakes in Taiwan's mountainous areas. One area susceptible to potentially catastrophic landslides is located at Lushan in central Taiwan, which is famous for its hot springs and tourism. The northern slope above the hot spring district slips gradually and frequently due to heavy rainfall. For safety reasons, the slip-affected area has been under constant borehole monitoring by government agencies, and a controversial public debate has arisen over permanent evacuation of the hot spring district. In this study, we attempt to simulate possible scenarios of catastrophic slope failure using slip geometries derived from the monitoring data, and assess potential landslide impact areas by a discrete element method using the PFC3D code. In the worst case scenario, the Lushan hot spring district is predicted to be destroyed by debris in 20 s. Besides, the planned regional emergency refuge is rapidly jeopardized by flooding resulted from landslide-dammed lakes. This study addresses catastrophic slope failure under heavy rainfall conditions given a range of friction coefficients and varied continuity of the failure surfaces. It is noted that the PFC3D code has limitations in modeling all complex mechanisms of landslide, particularly in modeling loss of material shear strength due to increases in pore pressure. Nevertheless, the numerical simulation results by the 3D discrete element method provide scenario-based runout paths, particle velocities and landslide-affected areas, which are useful information for decision support and future landslide hazard assessment.

The Shanchiao Fault is an active normal fault with documented paleoearthquakes in the Taipei metropolis, Taiwan. While posing direct seismic threat on the multimillion population, its crustal-scale fault plane configuration has not been constrained. This study presents the first attempt to resolve the fault plane dip changes of the Shanchiao Fault within the upper crust by forward modeling late Quaternary deformation. Tectonic subsidence over the last ~23 ka is estimated from vertical displacements of a rapidly formed alluvial fan horizon deformed into a dramatic rollover monocline. A 2-D profile across the Shanchiao Fault is chosen for elastic half-space dislocation modeling, and the results suggest that the fault is listric in the shallow crust with an abrupt change from subvertical ramp (85°–75°) to near-horizontal flat (10°–15°) at 3–4 km depth, consistent with an origin from the inversion of an orogen-related thrust detachment. Given the presence of rift-related fabrics in the underthrust Chinese Continental Margin basement beneath the Taiwanese orogenic wedge, listric ramp-flat-ramp models with a second deeper bend to 60° dip are also tested. Reasonable fits with the geological observations are produced when the lower ramp is located at greater than 8 km depth, which correlates with the hypocentral location of a moderate earthquake in 2004. Joint reactivation of preexisting thrust and rift faults by the Shanchiao Fault is therefore plausible with implications for seismic hazard in the Taipei area.

The accuracy, reliability and best practises of Ti-in-quartz thermobarometry (TitaniQ) in greenschist facies rocks have not been established. To address these issues, we measured Ti concentrations in rutile-bearing samples of moderately deformed, partially recrystallized quartzite and vein quartz from the Hsüehshan range, Taiwan. The spread of Ti concentrations of recrystallized grains in quartzite correlates with recrystallized grain size. Recrystallized quartz (grain size ~100–200 μm) that formed during early deformation within the biotite stability field shows a marked increase in intermediate Ti-concentration grains (~1–10 ppm) relative to detrital porphyroclasts (Ti ~0.1–200 ppm). Fine recrystallized quartz (~5% of the samples by area, grain size ~10–20 μm) has a further restricted Ti concentration peaking at 0.8–2 ppm. This trend suggests equilibration of Ti in recrystallized quartz with a matrix phase during deformation and cooling. Unlike previously documented examples, Ti concentration in the quartzite is inversely correlated with blue cathodoluminescence. Deformation was associated with a minimum grain boundary diffusivity of Ti on the order of 10−22m2 s−1. Vein emplacement and quartzite recrystallization are independently shown to have occurred at 250–350 °C and 300–410 °C, respectively, with lithostatic pressure of 3–4 kbar (assuming a geothermal gradient of 25° km−1), and with hydrostatic fluid pressure. Estimates of the accuracy of TitaniQ at these conditions depend on whether lithostatic or fluid pressure is used in the TitaniQ calibration. Using lithostatic pressure and these temperatures, the Thomas et al. (2010) calibration yields Ti concentrations within error of concentrations measured by SIMS. If fluid pressure is instead used, predicted temperatures are ~30–40 °C too low. TitaniQ has potential to yield accurate PT information for vein emplacement and dynamic recrystallization of quartz at temperatures as low as ~250 °C, however clarification of the relevant pressure term and further tests in rutile-present rocks are warranted.

article i nfo The Tatun Volcano Group (TVG) represents the main volcanic center in northern Taiwan and based on various observations it is considered a potentially active volcano. TVG has been monitored since 2003 by a seismic network that consists of eight stations equipped with three-component, short-period and broadband seismometers. In this study, we use waveform data of high frequency earthquakes in order to investigate the stress field orientation beneath the TVG area. The focal mechanisms of 35 selected events have been derived using P-wave polarities and amplitude ratios, assuming a double-couple source. These fault plane solutions and various subsamples derived from them, were subsequently inverted for the best fitting stress tensor using a linear inversion method. The results show stress homogeneity beneath TVG for depths larger than 3 km while the stress tensor is characterized by a subvertical σ1 and a subhorizontal NW-SE trending σ3 axis consistent with the regional stress field in northern Taiwan. On the other hand, Chihsinshan which is an area of vigorous hydrothermal activity, exhibits a localized stress field with horizontal NW-SE trending σ3 axis and NE-SW trending horizontal σ1. Such an axes orientation is likely to be causing opening of microcracks and thus favour the ascent and circulation of fluids in the upper crust. Shear wave splitting measurements seem to confirm these results, showing fast polarization directions along NNE-NE, subparallel to the main fracture system in TVG. Shear wave anisotropy averages about 2.9% and is probably caused by an anisotropic volume of fluid-saturated cracks within the upper 2.5 km of the crust.

article i nfo In the village of Tsaoling (in Yunlin County, Taiwan), a major landslide was triggered by the Chi-Chi earthquake in 1999 with more than 125×10 6 m 3 of rock displaced. The kinematic behaviour of this landslide is simulated using a 2D discrete element model (PFC2D code). Our numerical model is composed of discs bonded together. The initial boundary conditions are applied along the ball-wall contacts by using derived velocities integrated from the strong motion data with a duration of 160 s including the peak acceleration near Tsaoling. The constraints are mainly issued from the final geometry of the landslide including its capacity to cross the river valley and reach a significant elevation on the opposite mountain flank. They also result from a variety of geological and hydrological observations, including the local levels of material disruption and the location of survivors. Our modelling thus indicates that a low-friction coefficient (about 0.15) and a medium strength are required to account for the actual landslide characteristics. A self-lubrication mechanism probably accounts for the low residual friction. Our model also suggests that the maximum velocity of sliding reached 50 m/s, a result that cannot be checked in the absence of actual measurements. In addition to friction, the strength of sliding block is of special importance because it controlled the possibility for the upper layer fragments to roll and get buried, and hence the probability of survival.

The Chi-Chi earthquake occurred in the western foothills of central Taiwan, which triggered coseismic and catastrophic Chiufengershan dip-slope landslides, Tsaoling rockslides and surface stripping of the Jiujiufeng area. After a few days of the Chi-Chi event, the Hongtsaiping area in central Taiwan was reported to have several meters of slide in a small scale area. The reported landslides are dissimilar to the catastrophic events. To better understand the characteristics of such non-catastrophic and unobvious landslide, we try to characterize in detail the sliding directions, their magnitudes and region of the landslide. We analyzed three orthorectified aerial photographs of the Hongtsaiping area, which were taken in 1998/06/13, 1999/11/18 and 2002/10/26, using the Particle Image Velocimetry (PIV) technique. Through the method we correlated the image pixels of the two aerial photographs in order to derive the displacement vector field. The results of the PIV analysis have helped us not only find the areas where sliding had happened, but also evaluate the azimuth and magnitudes of the sliding event. The PIV analysis in the Hongtsaiping area covers a dimension of about 3228 × 2448 pixels in an area of about 1.2 km2. Our results show that the maximum horizontal displacement of the landslide is about 24 m towards NW in the study area. This unobvious but wide-coverage landslide may need further investigation in mechanisms combining with available or new geological data.

Data sets of collapsed earthquake locations, earthquake focal mechanisms, GPS velocities and geologic data are integrated to constrain the geometry and kinematics of a crustal block within the accreted continental margin rocks of Taiwan's northeastern Central Range. This block is laterally extruding and exhuming towards the north-northeast. The block is bound on the west-southwest by the previously recognized Sanyi–Puli seismic zone and on the east by a vertical seismic structure that projects to the eastern mountain front of the Central Range. Focal mechanisms from the Broadband Array of Taiwan Seismicity (BATS) catalog consistently show west-side-up reverse displacements for this fault zone. A second vertical structure is recognized beneath the Slate Belt–Metamorphic Belt boundary as a post-Chi-Chi relaxation oblique normal fault. BATS focal mechanisms show east-side-up, normal displacements with a minor left-lateral component. The vertical and lateral extrusion of this crustal block may be driven by the current collision between the Philippine Sea Plate and the Puli basement high indenter on the Eurasian Plate and/or trench rollback along the Ryukyu subduction zone. In addition, the vertical extent of the two shear zones suggests that a basal decollement below the eastern Central Range is deeper than previously proposed and may extend below the brittle–ductile transition.

Several levelling routes in the metropolitan Taipei Basin have been repeatedly conducted during the past decades, mainly in order to monitor the anthropogenic ground subsidence due to massive pumping of groundwater. We analysed the levelling data released from government and investigated the rate of ground level change from 1975 to 2003, which postdate the massive groundwater exploitation in Taipei area. Based on the contour maps created from the levelling data of 406 benchmarks, the overall subsidence rate in the Taipei Basin gradually decreased since 1975, and around 1989 the basin switched to slight uplift throughout a large part of the basin. Three mechanisms are proposed to be responsible for the observed land elevation changes, including shallow soil compaction, deformation within aquifer, and tectonic subsidence. The trend of the ground level change in 1975–2003 essentially demonstrated the effects of natural recharge to previously depleted aquifers, and is explained by the hydro-mechanical coupling of aquifer materials, i.e., elastic rebound, to the rising piezometric level. The rate of shallow soil compaction is estimated about 1–8 mm/yr throughout the basin according primarily to the shallow clay thickness. Asymmetric tectonic subsidence related to the Shanchiao Fault was estimated to be 1.75 mm/yr and 0.9 mm/yr in the western part and the central part of the basin, respectively. By subtracting the components of the soil compaction and tectonic subsidence from the surface land elevation change, the rebound of aquifer strata was estimated to be about 6.7 cm and 16 cm in western margin and Central Taipei, respectively. The amount of rebound is approximately 10% in magnitude comparing to the amount of previous anthropogenic subsidence due to massive groundwater pumping, totally about 2 m. © 2007 Elsevier B.V. All rights reserved.

We summarize the structural characteristics of the surface ruptures of the 1999 Mw 7.6 Chi-Chi earthquake in western Taiwan and discuss the geological interaction of the Chelungpu fault with adjacent faults. Based on geological investigations, seismological analyses, and GPS measurements of surface co-seismic displacements, we describe the regional 3-D fault plane and slip distribution of the Chi-Chi earthquake and compare these to the geological features of the Chelungpu fault. We find that one key feature of the Chelungpu fault is the stratigraphy-controlled slip surface: at the level of the uppermost few kilometers, the Chelungpu fault slip plane generally follows the bedding plane of the Pliocene Chinshui shale. The second key feature of the Chelungpu fault is the difference in structurally geometric configuration between its northern and southern segments. The northern Fengyuan segment shows a bedding-parallel thrust fault within east-dipping strata in both footwall and hanging wall. In contrast, the southern Tsaotun segment exhibits east-dipping strata are overthrust onto flat-lying recent alluvial deposits. These two features not only explain a hinterland imbricate thrusting on the hanging-wall of the Fengyuan segment, but also explain the change in strike of the Chi-Chi surface ruptures at the northern end. The southern end of the 1999 Chi-Chi rupture is interpreted to be linked to a series of NW-trending strike-slip faults. In particular, we propose that the Luliao strike-slip fault served as the lateral ramp of the Chelungpu fault, and the Gukeng strike-slip fault acted as a barrier to end the southern propagation of the 1999 rupture. Geomorphic features and paleoseismological data indicate that the range-front Chelungpu fault has generated large earthquakes during the last several thousand years. Alternatively, in the Miaoli area to the north and the Chiayi area to south, historical earthquakes as well as active geomorphic features are not restricted on the range-front thrust faults. Instead, more complicated structures, including tightly spaced folds, duplex structures, and strike-slip faults are involved in seismogenic processes. A more detailed investigation of regional structural characteristics is needed for mitigation against the seismic hazards in the 300-km-long active fold belt in western Taiwan, where several damaging large earthquakes have been documented during the last century.

Four campaigns of dense GPS measurements have been conducted since 1995 to investigate crustal deformation patterns in SW Taiwan. The station velocities decrease westwards from 42.2 to 55.5 mm/yr along the azimuths from 247.1° to 272.5°. In central part of the study area, GPS stations move nearly toward the west, whereas in the Pingtung–Kaohsiung coastal area, displacement vectors demonstrate a clear counter-clockwise deviation toward the SW. The transtensional deformation and the along-strike variation of southward increase of extensional deformation is due to the low lateral confining conditions related to the Manila subduction zone as a free boundary or/and the presence of the prominent Peikang High as a rigid indentor. The Chishan Fault is dominated by right-lateral motion with a fault slip rate ∼7 mm/yr in a N50°W direction. The Kaoping Fault is dominated by left-lateral motion with a ∼4–8 mm/yr in a N–S direction. The significant right-lateral component of motion of ∼24–30 mm/yr is accommodated along the active structures west of the Chishan Fault. The right-lateral and left-lateral structures facilitate the southwestward extrusion. The velocity gradients of the GPS stations across the Chaochou Fault are not significant. This implies that the Chaochou Fault is locked along the fault plane.

We applied newly acquired high-resolution airborne LiDAR data to study a segment of the Hsincheng fault, a well-known active fault near a large industrial park in northern Taiwan. The Hsincheng fault has received much attention and study in the past; but high spatial resolution digital elevation models have not previously been applied to the study of the fault and its surrounding structures. We processed the acquired LiDAR data and produced 1 m digital elevation models (DEMs) to investigate the active fault adjacent to the densely populated and important infrastructure in Taiwan. Using the LiDAR DEMs, aerial photographs and topographic maps, we show highly detailed geomorphic characteristics around the study area of the Hsincheng fault. Three major characteristics of the study area are defined that include three very well preserved river terraces at varying levels, the fault/fold scarp of the Hsincheng fault, and meandering river systems. Using the LiDAR DEMs, we were able to detect with precision several fault/fold scarps and very gentle NE-trending folding of the river terraces. In general, the LiDAR DEMs have provided unprecedented clarity of landforms for one segment of the Hsincheng fault, which has helped the characterization of subtle but important geomorphic features.