Taiwan is among the activest neotectonic place in the World as it is the result of the rapid collision of both Eurasian and Philippine Sea Plates, with an annual average convergence rate of more than 9cm.y-1. This active tectonic shortening generates the Taiwan relief. The latter is characterized by two major mountain ranges: (1) the « backbone » metamorphic Central Range from Eurasian continental origin, and (2) the eastern Coastal Range corresponding to the outcropping Luzon volcanic arc (Philippine Sea Plate). In between both runs the Longitudinal Valley Fault (LVF) which is the Taiwan major crustal suture, 125km long and N020°E trending, which presents both inter-seismic motions (2 to 4 cm/y, see Champenois et al., 2013) and major earthquake seismic crisis. Effectively the LVF has been re-activated by 7 earthquakes (EQ) of magnitude larger than 5 within 43 days (in late 1951 - early 1952) : Oct.22, 1951 in Hualien M7.1-7.3 ; Nov25, 1951 in Chihshang M6.1 and in Yuli M7.3 ; Dec5, 1951 in Taitung M5.8, (see Central Weather Bureau, 1952). More recently some EQ reactivate again the LVF such as : May20 1986 M 6.2 in Hualien ; 2003 Mw6.8 in Chengkung ; 2006 Mw5.9 in Taitung ; 2013 Ruisui EQ ; and the Feb.6 2018 Mw 6.4 Hualien EQ. Consequently, the detailed study of this major active seismic plate suture zone is a major concern for any Taiwan citizens.
We herein settle an UAS survey above the Longitudinal Valley Fault zone and acquired 17483 high resolution photographs through 2 drones flying at 350 meters above ground level height, by total 23 flight missions, covering a total area of 195km2. After classical photogrammetric processing, we calculate the high resolution Digital Terrain Model (HR-DTM) on a 2,5kms buffer on each side of the LVF active fault line, (with a 11cm planimetric resolution and a 40cm vertical accuracy). This UAS HR-DTM enables us through classical morphostructural interpretation to map into much details the active tectonics structures of the Longitudinal Valley Fault that lead us to up-date pre-existing published works (e.g. CGS geological maps, Lin et al, 2009 ; Shyu et al, 2005, 2006, 2007, 2008). Moreover, we use and combine this morphostructural interpretation with levelings, GPS and PSInSAR datasets (Champenois et al, 2013) in order (1) to locate the LVF active tectonic structures; (2) to characterize those using also geological field studies; and (3) to quantify them with various complementary geodetic measurements (e.g. Yu et al, 1997; Lee et al., 2008; Hsu et al., 2009; Huang et al., 2010).
The comparison in between 58 years levelings and an interpolated average interseismic GPS rate may highlight a part of the LVF earthquake cycle (differenciate the coseismic component from the interseismic one). One may note also some directions and magnitudes discrepancies which arguments the tectonic heterogeneity and local inversions.
Moreover, several Sea cruises where settled North of the LVF and the Coastal Range in order to extend structures at sea. They highlights complexe structural deposits which may be interpreted simply in the lights of both this tectonic setting exposed to landslides processes.