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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. [ABSTRACT FROM AUTHOR]

Slow-moving landslides can be found worldwide, exhibiting gradual movement over years to decades, with velocities ranging from millimeters to meters per year. However, slow-moving landslides can undergo transformation into rapid and catastrophic events, leading to severe damage to human society. Given the presence of >2500 pre-existing slow-moving landslides identified and cataloged using LiDAR bare ground data in Taiwan, it is crucial to efficiently assess their activity prior to the typhoon and heavy rainfall seasons. Therefore, a key research objective is to enhance the ability to detect landslide velocities and movements over large areas spanning tens of thousands of square kilometers. This study aims to establish a workflow that employs the multitemporal interferometric synthetic aperture radar (MTInSAR) technique for the detection and monitoring of multiple slow-moving landslides across wide regions. Specifically, we propose an optimized analysis package called "multi-snap2stamps," based on the existing snap2stamps method, to generate interferograms for multiple regions simultaneously. Focusing on the analysis of nine pre-existing slow-moving landslides, with velocities <1.6 m/year (classified as very slow or extremely slow landslides according to established criteria), distributed across Taiwan with diverse terrain and geological conditions, this study investigates the characteristics of these deep-seated landslides using the proposed MTInSAR analysis workflow in both spatial and temporal domains. The results reveal that two slow-moving landslide sites exhibit significant seasonal signals in the time series, while seven sites do not display such signals. Additionally, the MTInSAR time series effectively captures accelerated movement in the Shiding-T001 landslide, which corresponds well with in situ inclinometer monitoring data. This study demonstrates the effective analysis of velocity distribution and movement characteristics of slow-moving landslides using the proposed MTInSAR workflow. Furthermore, it highlights the potential of this workflow for improving landslide motion detection over vast areas of interest in the future. • Simultaneous generation of SAR interferograms from multiple regions of landslides. • Optimizing an MTInSAR workflow for analysis of multiple slow-moving landslides. • Detecting accelerated movements of slow-moving landslides by time series analysis. • Implication of seasonal fluctuation signals from the MTInSAR time series. [ABSTRACT FROM AUTHOR]

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 40 Ar/ 39 Ar 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. [ABSTRACT FROM AUTHOR]

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 40 Ar/ 39 Ar 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. [ABSTRACT FROM AUTHOR]

Abstract: We analyzed continuous GPS (CGPS) data recorded on 15 stations from 2002 to 2009 and examined the CGPS-derived strain along with local seismicity to characterize the current crustal deformation at the plate junction around the Hualien area in Taiwan. By examining the CGPS time series in detail, we discovered abnormal variations in the CGPS horizontal displacements and an annual cycle with a peak-to-peak difference of more than 20mm. Most stations move in the ESE direction during May to October, and move in the opposite direction during November to April every year. We found that the average semi-annual velocity of each CGPS station is generally parallel to the direction of convergence between the Eurasian and Philippine Sea plates, and that the CGPS temporal areal strain is strongly related to the occurrence of larger local earthquakes, while the strain reverses from contraction to extension. The CGPS displacement is well known to have been influenced by seasonal changes or loadings from several environmental factors. We tested these perceptions with the newly acquired CGPS data and seismicity, and found that the environmental factors are unlikely to explain the patterns of surface motion in the study area. We also compared our results with previously reported cases and found distinctive patterns in the temporal and spatial distributions of the CGPS data and seismic behavior. The geodetic and seismic observations should provide motion constraints for further studies of the plate junction kinematics from collision to subduction around the Hualien area in Taiwan. [Copyright &y& Elsevier]

Terra Nova, 23, 85-91, 2011 Abstract Vitrinite reflectance and Raman spectroscopy of carbonaceous material data are used to better resolve the thermal history of the Hsuehshan Range, which is accreted between the foreland fold-thrust belt and bulldozer hinterland units in the Taiwan mountain belt. The observed thermal data indicate that the strata in the northern Hsuehshan Range underwent dynamic metamorphism during the Neogene orogeny, while the strata in the southern Hsuehshan Range may have predominantly experienced burial metamorphism during Palaeogene sedimentation. Based on the thermal constraints, the Hsuehshan Range is interpreted to consist of nappe stacks, originating from the rifted Eurasian continental margin. This interpretation is consistent with well-documented cases in the European Alps and the Himalayas and is also shown in physical modelling and thermo-kinematic studies invoking underplating and erosion processes. [ABSTRACT FROM AUTHOR]

Abstract: The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations. [Copyright &y& Elsevier]

Abstract: 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 1m 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The 1999 Chi-Chi earthquake triggered the catastrophic Tsaoling landslide in central Taiwan. We mapped the landslide area and estimated the landslide volume, using high-resolution digital elevation model from airborne LiDAR (Light Detection and Ranging), satellite images, aerial photographs and topographic maps. The comparison between cut and fill volumes, about 0.126 and 0.150 km3, respectively, suggests a volume increase of 19% due to decompaction during landsliding. In April 2002, the cut and fill volumes were about 0.137 and 0.116 km3, respectively. These estimates suggest that 2.5 years after the event, the volume of landslide debris removed by river erosion was nearly 0.045 km3. Such a large value highlights the importance of landslide processes for erosion and long-term denudation in the Taiwan mountain belt. To cite this article: R.-F. Chen et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution [sup 40]Ar/[sup 39]Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S[sub 2], S[sub 3], and S[sub 4]) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S[sub 3] and S[sub 4] are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative [sup 40]Ar/[sup 39]Ar geochronology. Mean [sup 40]Ar/[sup 39]Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S[sub 3] and 345.5 ± 1.7 Myr for S[sub 4]. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S[sub 3] and S[sub 4], these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region. [ABSTRACT FROM AUTHOR]

A slow-moving landslide on the Huafan University campus, which is located on a dip slope in northern Taiwan, has been observed since 1990. However, reliable monitoring data are difficult to acquire after 2018 due to the lack of continuous maintenance of the field measurement equipment. In this study, the multitemporal interferometry (MTI) technique is applied with Sentinel-1 SAR images to monitor the slow-moving landslide from 2014–2019. The slow-moving areas detected by persistent scatterer (PS) pixels are consistent with the range of previous studies, which are based on in situ monitoring data and field surveys. According to the time series of the PS pixels, a long period gravity-induced deformation of the slow-moving landslide can be clearly observed. Moreover, a short period seasonal surface fluctuation of the slow-moving landslide, which has seldom been discussed before, can also be detected in this study. The seasonal surface fluctuation is in-phase with precipitation, which is inferred to be related to the geological and hydrological conditions of the study area. The MTI technique can compensate for the lack of surface displacement data, in this case, the Huafan University campus, and provide information for evaluating and monitoring slow-moving landslides for possible landslide early warning in the future. [ABSTRACT FROM AUTHOR]

Reports on the results of a study done on the active Taiwan air-continent collision. Effects of normal faulting; Results of a GPS survey of Southern Taiwan.

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. • 17-year surficial monitoring distinguishes movement behavior of different parts of dip-slope landslides. • A model of movement process of landslides having listric sliding surface(s) is proposed to reveal intermittent movements. • A strain pattern obtained of the slope surface evidences a tectonic fault passing through the landslide block. • Economic and reliable study procedures are applicable to unstable slopes for research and disaster prevention. [ABSTRACT FROM AUTHOR]

Geodetic evidence for slow slip recurrence changed by stress perturbations was rare, especially from afterslip following a nearby large earthquake. The first observed slow slip events in the southernmost Ryukyu subduction had occurred in 2005, 2009, and 2015 following the nearby 2002 March 31 Mw 7.1 Hualien offshore earthquake. To investigate whether the Mw 7.1 earthquake had influenced the occurrence times of the slow slip, we calculated the coseismic slip distribution and afterslip distribution using the surface displacements from onshore Global Navigation Satellite System observations. The stress perturbation on the slow-slip regions caused by the coseismic slip was quantified using the Coulomb failure criteria. We also examined the aftershock distribution and the evolution with time to clarify the stress perturbations from the afterslip offshore. Our results show that the primary afterslip distribution may have overlapped the 2009 and 2015 slow-slip patch at the downdip of the earthquake. The coseismic stress perturbation may have influenced the SSEs area directly by a Coulomb stress increase of probably 0.10 MPa. However, the 2005 SSE patch in the updip depths had only a little coseismic slip and afterslip with the Coulomb stress increase of approximately 0.06 MPa. We find that most of the aftershocks had occurred in the 2009 and 2015 slow-slip region that evolved into a typical aftershock sequence at least 2.5 years after the earthquake. The surface geodetic displacements reveal that the afterslip may have lasted longer than 4.5 years after the earthquake. The evidence for the afterslip proves that the 2009 and 2015 slow-slip patch was influenced by the afterslip directly for years. We suggest that the ongoing afterslip may have modulated the coseismic stress perturbation. It may explain the delay of the 2009 slow slip occurrence compared with the recurrence interval between the 2009 and 2015 slow slip events. [ABSTRACT FROM AUTHOR]

Catastrophic landslides often cause serious disasters, and such incidents are often the focus of our attention. The well-known Tasoling Landslides occurred repeatedly, which were triggered by earthquakes, typhoons, and torrential rains in 1862, 1898, 1941, 1942, 1951, 1979, and 1999. Among these, the landslides in 1862, 1941, and 1999 were triggered by earthquakes. The Tsaoling area geologically belongs to a dip-slope terrain in interbedded Pliocene sandstone, mudstone, and shales. The volume of the rock debris accumulated from previous landslide events was about 0.026 - 0.126 km3, which formed a landslide dam with a height of 70-170 m and blocked the Qingshui river. The sudden failures of these landslide dams often resulted in additional catastrophes in the downstream areas, three of which occurred years after the landslide event. With the progress of surveying technology, high-resolution DEMs data could be used for important topics, such as landslides and geomorphic change. In this study, we presented multi-temporal remote sensing datasets for geomorphic changes in the Tsaoling landslide area before and after the Chichi earthquake and provided an effective DEM analysis technique to better estimate volume changes and surface movements. We found that multi-temporal data comparison could show the geomorphic changes of the Tsaoling landslide area directly at different times by using aerial photogrammetry and LiDAR DEM analysis. The observed results of geomorphic changes in the shorter period of 1- 3 years from 2011 to 2012, 2012 to 2016, 2016 to 2017 and over a long period time of about 9 years from 2002 to 2011, including the location of the landslide accumulation area, the change of cliff retreat, and the volume change of the debris after the landslide event. By means of terrain profile comparison, elevation change calculation and Particle image velocimetry (PIV) analysis from image and DEM data during 2016-2017, we found some surface displacement located in the lower half of the Tsaoling landslide area and distributed in the Chinshui shale area. Within the range of about 80 hectares, there was a clear trend of surface displacement, indicating that this range of the Tsaoling area may have slipped, which was similar to the scale as the 1942 landslide event. This study shows that the integrated use of aerial photogrammetry and LiDAR DEM data analysis method with multi-temporal topographic data collection and field surveys in the landslide area, and could resolve the surface changes and possible impacts of this large landslide and river geomorphological changes in the future. [ABSTRACT FROM AUTHOR]

The most commonly used 1:50,000 geological maps, which are generally in low resolution, provide only conceptual drawing for geological features. For advanced applications in science and engineering, the traditional geological maps no longer satisfy the requirements of precise 3D geological structures and accurate lithological distribution, particularly for engineering purposes. In order to solve the problem of insufficient accuracy of geological maps, we must apply a feasible set of techniques to further develop high-precision geological maps. Although various types of space measurement technology have made great progress in recent years, their application in producing geological maps is still seriously stagnant. This may be related to difficulty in obtaining comprehensive high-precision terrain models, lack of three-dimensional visual mapping environments, and especially failure to effectively develop techniques or modules needed to produce high-precision geological maps. In this study we used existing high-resolution LiDAR data and the latest computerized 3D environments to conduct and explore high-precision geological mapping. We developed computational modules for geological map production and explore various geological features in three dimensions for overcoming difficulties in traditional mapping. We mapped the terrain of the sedimentary rocks in the fold-and-thrust belt of western Taiwan, which contains typical strata from the Oligocene to Miocene. The study provides a new three-dimensional concept of geological mapping with a finished case example from the Shuanghsi quadrangle map in northern Taiwan. This research is expected to modify traditional geological mapping concepts and provides preliminary mapping techniques to enhance the foundation and strength of three-dimensional geological mapping for the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 décollement below the eastern Central Range is deeper than previously proposed and may extend below the brittle–ductile transition. [Copyright &y& Elsevier]

Abstract: The Jiufengershan rock avalanche was one of the largest and most damaging landslides triggered by the Chi-Chi earthquake (M L =7.3, M W =7.6) of 21 September 1999. The rock avalanche transported a mass of sedimentary rock 50 m thick and 1.5 km long, located on the western limb of the Taanshan syncline. The surface of rupture coincides with the bedding plane and dips moderately toward the Jiutsaihu valley. This paper is mainly devoted to the study of post-seismic surface processes that affected the sliding surface as well as the debris deposit, from September 1999 to February 2003. Large fractured blocks and a debris layer observed on the surface of rupture were subjected to mass wasting processes and denudation. The quantification of erosion was made using two different approaches. First, the subpixel correlation method was used to determine the horizontal displacement field from aerial photographs taken, respectively, 2 and 3.5 months after the earthquake. Displacements ranging from 1 to 6 m were observed around unstable blocks located at the western flank of the surface of rupture. Second, the co-seismic and post-seismic volume distributions in the sliding zone were determined from three digital elevation models, including a LiDAR image taken in 2002. Post-seismic erosion and deposition from September 1999 to April 2002 were mainly associated with mass wasting and denudation at the surface of rupture, deposition in small basins and lakes located in the debris deposit, and evacuation of materials from the debris deposit along natural and artificial drainage channels. The vertical compaction is 1% of the initial height of the deposit. [Copyright &y& Elsevier]

Abstract: The Jiufengershan rock and soil avalanche is one of the largest landslides triggered by the Chi-Chi earthquake Taiwan 1999. The landslide destabilized the western limb of the Taanshan syncline along a weak stratigraphic layer. It involved a flatiron remnant, which was almost entirely mobilized during the earthquake. The avalanche was slowed down by NS trending ridges located downstream along the Jiutsaihu creek. The landslide affected a 60 m thick and 1.5 km long sedimentary pile composed of shales and sandstones, which dip ∼22°SE toward a transverse valley. The triggering mechanism and the sliding process were analyzed by means of geological and morphological data from aerial photographs and observed in the field. A high-resolution airborne Light Detection and Ranging (LiDAR) image taken 2.5 years after the landslide allows the identification of morphological structures along the sliding surface and the landslide accumulation. The sliding surface shows several deformation structures such as fault scarps and folds. These structures are interpreted in terms of basal shear stresses created during the avalanche. Three major joint sets were identified at the sliding surface. The isopach map of the landslide was calculated from the comparison between elevation models before and after the earthquake. The coseismic volume of mobilized material and landslide deposit data are 42×106 m3 and 50×106 m3, respectively. The geometry of the landslide accumulation in the field has an irregular star shape. The morphology of the deposit area shows a sequence of smooth reliefs and depressions that contrast with the neighboring ridges. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The LiDAR sensor mounted on a UAV becomes a new powerful tool for geoinfomatic technology. We integrate autonomous unmanned helicopter Pulse Aerospace Vapor 55, carrying Riegl VUX-1 UAV LiDAR with Trimble Applanix AP20 for surveying missions. In this study, we applied the instruments for three regions for different purposes, including bore sight and lever arm parameter determination, and for multiple flight missions tests. Based on the drone and instrument capacity, adapted with the terrain landform and topographic elevation changes, the optimal drone mission planning and scanning parameters are thus assigned, thus capable to acquire dense point clouds higher than 159 points/m^2 in nadir direction within single fly line. To access the dataset, several software packages are used, including: the Trimble Applanix POSPac Mobile Mapping Suite software, GNSS-Aided Inertial post-processing for georeferencing data collected from UAS LiDAR. The Riegl RiPROCESS designed for managing, processing, analyzing, and visualizing and data export for the data acquired based on Riegl Laser Scanners. And finally, access and evaluate the dataset by means of Riegl RiPROCESS and TerraMatch softwares for managing, processing, especially for fly-line adjustment and classification the UAS LiDAR point clouds, so as to compare the UAS dataset with the airborne's. The study tries to evaluate the parameters for fully-automatic point cloud classification by TerraScan, which is used regularly in Taiwan. This study analyzes the influence and efficiency of different parameters for point cloud classification, to separates the ground point from the all point so as to construct the digital elevation model. Finally, the averaged point clouds density of the all and the ground point are higher than 400 and 25 points/m2, respectively. Thus the spatial resolution of digital elevation model (DEM) is about 20 by 20cm. Compared with the data points measured from site surveying, ground control points and check points by means of e-GNSS, RTK (real time kinematic GPS survey) and total station etc., the elevation errors is less than five centimeters; thus the high resolution and high precision digital terrain models (DTMs) are capable to construct. UAS LiDAR point cloud after instrument calibration and flight trajectory adjustment, the surveying data acquisition can achieved as centimetric precision. According to the results, the technology of UAS LiDAR is capable and suitable for high resolution geoinfomatic data acquisition and for multi-disciplinary applications. [ABSTRACT FROM AUTHOR]

Shallow slow slip events accompanied by seismicity has been increasingly reported in subduction zones during the last decade; however, the relationship between shallow slip events and seismicity is unclear. We report episodic slow slip events and seismicity at the southernmost Ryukyu Trench. The inversion results from GPS‐derived cumulative displacements indicate that the slow slip events occur offshore in northeastern Taiwan on the shallow subduction interface, where high seismic VP/VS ratios applies. Seismicity is observed above the subduction interface for locations of significant variations in the VP/VS ratio and does not follow the migration of peak slip along the subduction interface. We calculated the temporal evolution between the seismicity and the propagation of slow slip events and determined that seismicity is not primarily driven by slip propagation. The spatiotemporal relationship, coupled with the VP/VS ratio, suggests that high‐pressure fluid activity may be crucial to episodes of slow slip events and seismicity. Plain Language Summary: Slow slip events are a member of the slow‐earthquake family observed by geodetic measurements in global subduction zones in the past 15 years. Shallow slow slip events are considered to be important for the megathrust earthquake cycle of the shallowest locked zone. Recently, shallow slow slip events accompanied by different seismic behaviors has been increasingly reported. This paper reports episodic, shallow slow slip events accompanied by overlying plate seismicity at the southernmost Ryukyu Trench and analyzes their relationship. We discover that the spatiotemporal relationship between the propagation of slow slip and seismicity is weak, which infers that they may not have a causal relationship. We suggest that the initiation of shallow slow slip events and subsequent seismicity was probably caused by high‐pressure fluid activity. The interpretation may reveal the cause of many shallow slow slip events as a broad phenomenon. Key Points: Relationship between shallow slow slip events and seismicity is reported at the southernmost Ryukyu Trench for the first timeSpatiotemporal seismicity does not adequately correlate with propagation of slow slip but is dominated by mainshock‐aftershock sequencesHigh‐pressure fluid activity likely has a key role in the initiation of episodic slow slip events and subsequent seismicity [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: In the village of Tsaoling (in Yunlin County, Taiwan), a major landslide was triggered by the Chi-Chi earthquake in 1999 with more than 125×106 m3 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. [Copyright &y& Elsevier]

Rapid urbanization and population growth in Hanoi city raise the demand for groundwater and the risk of land subsidence. This study is the first to conduct a long-term interferometric synthetic aperture radar (InSAR) analysis with different SAR sensors including ALOS, COSMO-SkyMed and Sentinel-1 to evaluate the spatiotemporal evolution of subsidence in Hanoi between 2007 and 2018. The results show that subsidence in Central Hanoi had been diminishing during these 12 years, while the southern and western outskirts (Ha Dong and Hoai Duc district) had become hot spots with peak rates of ~50 mm/yr. We further model the subsidence time-series with 22 years of piezometric records to estimate hydrogeological properties in the area. The modeling yields a mean consolidation coefficient of 0.56 m2/yr, a value typical for aquitards composed of >90% fine-grain materials. The model also predicts another 0.5–1.3 m of subsidence in Ha Dong by the end of this century. Based on these results, we propose zoning for subsidence susceptibility as the basis for future groundwater management in Hanoi. From a regional perspective, Hanoi is relatively less affected by land subsidence compared to other South East Asian cities due to geotechnical factors including small consolidation coefficients, as well as social factors such as lower population density. • The first long-term InSAR analysis of subsidence in Hanoi. • 1D coupled model yields consolidation coefficient values for the basin in Hanoi. • The model predicts 0.5–1.3 m of future subsidence by the end of this century. • Hanoi is more resistant to subsidence compared to other SEA cities. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 55.5mm/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 ∼7mm/yr in a N50°W direction. The Kaoping Fault is dominated by left-lateral motion with a ∼4–8mm/yr in a N–S direction. The significant right-lateral component of motion of ∼24–30mm/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. [Copyright &y& Elsevier]

Abstract: The 1999 Chi–Chi earthquake triggered the catastrophic Tsaoling landslide in central Taiwan. We mapped the landslide area and estimated the landslide volume, using a high-resolution digital elevation model from airborne LiDAR (Light Detection And Ranging), aerial photographs and topographic maps. The comparison between scar and deposit volumes, about 0.126 km3 and 0.150 km3 respectively, suggests a coseismic volume increase of 19% due to decompaction during landsliding. In July 2003, the scar and deposit volumes were about 0.125 km3 and 0.110 km3 respectively. These estimates suggest that 4 years after the event, the volume of landslide debris removed by river erosion was nearly 0.040 km3. These determinations are confirmed by direct comparison between the most accurate topographic models of the post-landslide period, indicating a very high erosion rate at the local scale (0.01 km3/year) for the deposit area of the landslide. Such a large value highlights the importance of landslide processes for erosion and long-term denudation in the Taiwan mountain belt. [Copyright &y& Elsevier]

A vertical exposure across the principal thrust scarp of the 1999 Mw 7.6 earthquake allows quantification of fault slip. The exposure is located on the active Chelungpu fault near Wufeng, along the range front of the fold-and-thrust belt in western Taiwan. The 1999 surface ruptures at the Wufeng site are characterized by a west-facing 2 to 3 m high principal thrust scarp and an east-facing lesser backthrust scarp. We mapped a 15 m-long, 5 m-deep exposure across the principal thrust scarp and characterized complex deformation structures, which include a main basal thrust fault, a wedge thrust, and a pop-up anticlinal fold with two secondary opposing thrust faults. The vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall. The average vertical displacement is 2.2±0.1 m, and the maximum displacement is 2.5 m, at the crest of the small pop-up fold. Horizontal displacement estimates were determined using line- and area-balancing methods. With line-length methods we estimated a horizontal displacement of 3.3±0.3 m across the principal scarp for four sedimentary horizons. For area balancing, first we selected three horizontal soil/sand deposits with a total thickness of about 0.5 m. The estimate yields an average horizontal displacement of 4.8±1.0 m. Using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations. Second, we used the 3 m-thick overbank soil/sand and the lower part of fluvial pebble/cobble to calculate a horizontal displacement of 2.6±0.2 m with the area-balancing technique. According to the geometry of the dip angle (35–40°) of the basal thrust, the line-length measurement and the 3 m-thick package area balancing both provided reasonable results of horizontal displacement. By comparing the different deposits applied to the line- and area-balancing methods, we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits. Due to lesser confining pressure at the surface, additional deformation occurred in the upper 1–2 m thick overbank deposits. This additional deformation yielded further vertical uplift of 0.3–0.5 m and horizontal displacement of 0.2–0.8 m around the core of the pop-up fold. Our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late Quaternary deposits. [Copyright &y& Elsevier]

Surface ruptures associated with the 1999 Mw=7.6 Chi-Chi earthquake in central western Taiwan have been characterised by mapping along the northern fault-segment. The earthquake occurred on the reactivated Chelungpu fault in the frontal portion of the thin-skinned Taiwan fold-and-thrust belt. The N–S trending Chelungpu fault is a 90-km-long major west-verging thrust, which principally slips within, and parallel to, bedding of the Pliocene Chinshui Shale. In the northern segment of the earthquake fault trace, that we name the Shihkang–Shangchi fault zone, the surface ruptures turn to an E–W strike and produce a series of thrust-and-backthrust pop-ups, about 15 km long, forming several discontinuous subsegments distributed within a broad regional Pliocene syncline.The northern fault segments activated during the Chi-Chi earthquake, in the area where the displacement is largest, not only display anomalous trends and a variety of mechanisms, but also raise a major problem of structural inheritance. Detailed field investigation and kinematic analysis indicate that the surface ruptures in the Shihkang–Shangchi fault zone are the result of dip-slip thrusting, occasionally with a minor strike-slip component. The surface ruptures emerge at the surface from bedding-parallel thrusts on both limbs of the regional south-plunging syncline. In the middle part of the syncline, bedding-parallel thrusts are connected by thrusts that cross-cut beds. The surface ruptures also reactivate a NE–SW trending anticline (Diaoshenshan), with west-vergent thrust on the backlimb and east-vergent backthrust on the forelimb. This anticline is undergoing uplift by breaking through the regional N–S trending syncline.Combining GPS measurements, seismological data, and geological analyses, we propose a kinematic model with a 3-D fault surface for the 1999 earthquake in the area of the northern termination. We highlight the influence of both the local and regional structures (bedding parallel slip, pre-existing faults and folds) on the development of the earthquake rupture, and hence the role of the structural inheritance. We conclude that the south-plunging regional Pliocene syncline in fact acts as a slip/strain guide for the northern termination of the Chelungpu thrust. The Pliocene Chinshui Shale, as a major source of weakness within the syncline, has strongly influenced the pattern of slip surface during the 1999 earthquake rupture. We interpret the large vertical displacement along the northern segment as the fault rupture occurring over a surface whose radius of curvature tightens toward the north around the core of the syncline. [Copyright &y& Elsevier]