Showing papers by "Central Geological Survey, MOEA published in 2017"

Active interseismic shallow deformation of the Pingting terraces (Longitudinal Valley – Eastern Taiwan) from UAV high-resolution topographic data combined with InSAR time series

Abstract: We focus herein on the location, characterization and the quantification of the most active structural feature of Taiwan: the Longitudinal Valley Fault that corresponds to the suture in between the Philippine and Eurasian Plates. In order to determine and monitor its present inter-seismic deformation, we focus on the Pingting Terraces area, situated in the South Longitudinal Valley (Eastern Taiwan). We first determine the structural geometry issued from both photo-interpretation deduced from new unmanned aerial vehicle (UAV) high-resolution Digital Terrain Model data that we acquired (34.78 km2 with 7.73 cm ground sampling distance), combined with geological field work. In order to characterize and quantify the present deformational patterns over the Pingting terraces, we used an InSAR time series Interferometry algorithm (MT-InSAR) applied to nine L-band SAR images from ALOS satellite acquired over the period 2007–2010. The unprecedented density of measurements (about 120 points per km2 for a tot...

Adopting the I 3 – R 24 rainfall index and landslide susceptibility for the establishment of an early warning model for rainfall-induced shallow landslides

Abstract: . Rainfall-induced landslides number among the most devastating natural hazards in the world and early warning models are urgently needed to reduce losses and fatalities. Most landslide early warning systems are based on rainfall thresholds defined on the regional scale, regardless of the different landslide susceptibilities of various slopes. Here we divided slope units in southern Taiwan into three categories (high, moderate and low) according to their susceptibility. For each category, we established separate rainfall thresholds so as to provide differentiated thresholds for different degrees of susceptibility. Logistic regression (LR) analysis was performed to evaluate landslide susceptibility by using event-based landslide inventories and predisposing factors. Analysis of rainfall patterns of 941 landslide cases gathered from field investigation led to the recognition that 3 h mean rainfall intensity (I3) is a key rainfall index for rainfall of short duration but high intensity; in contrast, 24 h accumulated rainfall (R24) was recognized as a key rainfall index for rainfall of long duration but low intensity. Thus, the I3–R24 rainfall index was used to establish rainfall thresholds in this study. Finally, an early warning model is proposed by setting alert levels including yellow (advisory), orange (watch) and red (warning) according to a hazard matrix. These differentiated thresholds and alert levels can provide essential information for local governments to use in deciding whether to evacuate residents.

Geomorphological evolution of landslides near an active normal fault in northern Taiwan, as revealed by lidar and unmanned aircraft system data

Abstract: . 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.

Deep-Seated Landslide Mapping and Geomorphic Characteristic Using High Resolution DTM in Northern Taiwan

Abstract: Open image in new window Torrential rainfall and local geologic structure play principal roles in triggering deep-seated landslide around mountainous area. Typhoon Morakot (2009), the most destructive event occurred in 2009, battered southern Taiwan and caused severe casualties in Siaolin Village. For the purpose of reducing the damage resulted from geological disaster, this study adopted high resolution topographic data which extracted from airborne LiDAR scanning to map the recent and ancient deep-seated landslides in northern Taiwan. In this study, a visualization technique of sky-view factor was performed to generate quasi-3D relief image map by overlapping slope gradient, topographic openness, and multiple direction hill-shading maps, allowing us to interpret precisely the feature of landslide zonation. The study area covers an area of 5502 km2. The landslide micro-topography interpretation were presented with main scarp and landslide mass in polygon pattern, it shows more than 200 deep-seated landslides located on Hsuehshan Range, Tatun volcano group, and Western foothills in northern Taiwan. The spatial distribution of landslide relates highly to the regional strike of formation and local drainage pattern. Additionally, the detrimental geomorphic and geologic factors are extracted to evaluate the landslide evolution. For high scarp ratio slope, it usually represents the landform of ancient landslide characterized with apparent slope deformations. This work also explores the relationship between geomorphic evolutions of landslide and susceptibility. The paper describes expert mapping method combing with topography enhancement technique and provides competent authority to assess landslide magnitude and reduce disaster risk.