B. Fruneau

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 total of 6,400 points) gives a continuous overview of the inter-seismic shallow deformation. The structural geometry combined with the mean velocity map (MT-InSAR) reveals two clear active faults situated above the scarps of the Pingting terraces and responsible for up to 7 and 20 mm/yr velocity offset along the radar line of sight. A temporal analysis of the deformation is performed with one measurement at each SAR acquisition date, giving major improvements in the characterization and quantification of the Longitudinal Valley active Fault trace.

Shallow geological structures triggered during the Mw 6.4 Meinong earthquake, southwestern Taiwan

The Meinong earthquake generated up to ~10 cm surface displacement located 10-35 km W of the epicenter and monitored by InSAR and GPS. In addition to coseismic deformation related to the deep earthquake source, InSAR revealed three sharp surface displacement gradients. One of them is extensional and is inconsistent with the westward interseismic shortening of ~45 mm/yr in this region. The gradient sharpness suggests slip triggering on shallow structures, some of which were not well documented before. To characterize these shallow structures, we investigated potential surface ruptures in the field. Sets of ~NS tension cracks distributed over 25-300 m width, with cumulative extension in the same order as InSAR observations, were found along 5.5 km distance along the extensional gradient and are interpreted as surface rupture. We build two EW regional balanced cross-sections, based on surface geology, subsurface data, and coseismic and interseismic geodetic data. From the Coastal Plain to the E edge of the coseismic deformation area, we propose a series of three active W-dipping back-thrusts: the Houchiali fault, the Napalin-Pitou backthrust, and the Lungchuan back-thrust. They all root on the 3.5-4.0 km deep Tainan detachment located near the base of the 3-km-thick Gutingkeng mudstone. Further E, the detachment would ramp down to ~7 km depth. Coseismic surface deformation measurements suggest that, in addition to the deeper (15-20 km) main rupture plane, mostly the ramp, the Lungchuan back-thrust, and the Tainan detachment were activated during or right after the earthquake. Local extension is considered as transient deformation at the W edge of the shallow main slip zone. Article history: Received 2 November 2016 Revised 13 March 2017 Accepted 20 March 2017

Geological mapping in the zone of Chotts, Tunisia, using ALOS sensors

The three sensors onboard the Advanced Land Observing Satellite (ALOS) are the Phased Array L-band Synthetic Aperture Radar (PALSAR), the Panchromatic Remote-Sensing Instrument for Stereo Mapping (PRISM), and the Advanced Visible and Near-Infrared Radiometer type 2 (AVNIR-2). Each of them has been evaluated for the geological mapping of the Zone of Chotts, Tunisia. Both the endoreic basins and the surrounding ranges have been studied. The 10 m-resolution AVNIR-2 sensor is a good trade-off for geological mapping. The 2.5 m-resolution PRISM sensor with its stereoscopic capability is very useful for the detailed study of sedimentary layers. Copolarized radar data are relevant for the study of the flat itself but the C-band is more accurate for sensing the roughness of the finer sediments (sand veneers, evaporites, etc.). The HV polarization is suitable for identifying the presence of halophytic plants that fringe the upper part of the flat.