Benoît Deffontaines

HighlightsUAS & InSAR Earthquake Monitoring1. Integrated Rapid Response: Establishes a high-resolution UAS workflow to optimize civil security, damage inventory, and remediation prioritization immediately after major seismic events.2. Multi-Platform Synergy: Leverages the complementarity of regional Fixed-Wing surveys and localized Multicopter monitoring for multi-scale tectonic analysis.3. Structural Characterization: Utilizes high-resolution orthophotos to identify co-seismic features, including joints, faults, and liquefaction-induced seismites, essential for structural interpretation.4. Synergistic Geodesy: Combines UAS morphostructural mapping with D-InSAR LoS displacement to quantify regional deformation and constrain active tectonic models.5. Operational Advocacy: Proposes a specialized "UAS Earthquake Group" to facilitate authorized, near-real-time mapping of disaster-affected areas.

The monitoring of coastal evolution (coastline and associated geomorphological features) caused by episodic and persistent processes associated with climatic and anthropic activities is required for coastal management decisions. The availability of open access, remotely sensed data with increasing spatial, temporal, and spectral resolutions, is promising in this context. The coastline of Northern Tunisia is currently showing geomorphic process, such as increasing erosion associated with lateral sedimentation. This study aims to investigate the potential of time-series optical data, namely Landsat (from 1985–2019) and Google Earth^® satellite imagery (from 2007 to 2023), to analyze shoreline changes and morphosedimentary and geomorphological processes between Cape Serrat and Kef Abbed, Northern Tunisia. The Digital Shoreline Analysis System (DSAS) was used to quantify the multitemporal rates of shoreline using two metrics: the net shoreline movement (NSM) and the end-point rate (EPR). Erosion was observed around the tombolo and near river mouths, exacerbated by the presence of surrounding dams, where the NSM is up to −8.31 m/year. Despite a total NSM of −15 m, seasonal dynamics revealed a maximum erosion in winter (71% negative NSM) and accretion in spring (57% positive NSM). The effects of currents, winds, and dams on dune dynamics were studied using historical images of Google Earth^®. In the period from 1994 to 2023, the area is marked by dune face retreat and removal in more than 40% of the site, showing the increasing erosion. At finer spatial resolution and according to the synergy of field observations and photointerpretation, four key geomorphic processes shaping the coastline were identified: wave/tide action, wind transport, pedogenesis, and deposition. Given the frequent changes in coastal areas, this method facilitates the maintenance and updating of coastline databases, which are essential for analyzing the impacts of the sea level rise in the southern Mediterranean region. Furthermore, the developed approach could be implemented with a range of forecast scenarios to simulate the impacts of a higher future sea-level enhanced climate change.

Taiwan result in the active collision of both Eurasian and Philippine Sea Plates characterized by an annual average convergence rate close to 10 cm.y-1. The Longitudinal Valley is parallel and eastward of the Central (Backbone) Range which is made of metamorphic rocks, and is also situated to the west of the Coastal Range (of volcanic affinity). In between both, lay the Longitudinal Valley (125km long and N020°E trending) which behave as the active crustal suture zone. The latter presents both inter-seismic creeping displacement (Champenois et al., 2013, Deffontaines et al., 2018) and was hit by 7 major earthquakes of magnitudes larger than 5 during the last 70 years which highlights its high seismic hazards.We combine herein a preseismic UAS survey (May 20, 2015) with one done immediately after the last large earthquake on the eastern Central Range (Oct 07, 2022). We therefore study both (1) the differences from a quantitative point of view; and (2) from a morpho-structural qualitative analysis point of view.We acquired so many high-resolution photographs using several drones flying at 350 meters above the ground. After photogrammetric processing, we calculate both (1) a high-resolution Digital Elevation Model (UAS-HR-DSM) that takes into account buildings and vegetations, and deduce (2) a Digital Terrain Model (UAS-HR-DSM) corresponding to the ground. Our ground validation (GCP’s) leads us to get a 7cm planimetric resolution (X, Y) and below 40cm vertical accuracy.This UAS-HR-DSM combined with field work and the preliminary PSInSAR (PALSAR-JAXA) processing led us to better characterize the active tectonic features through a detailed morphostructural analysis. It also permit us to map into much details the active structures and consequently to up-date the pre-existing geological mappings (e.g. CGS geological maps, Lin et al., 2009; Shyu et al., 2005, 2006, 2007, 2008). Then we up-date and combined our new structural scheme with geodetic data (levelings, GPS…) and PALSAR PSInSAR results acquired during the same monitoring time period to locate, characterize and quantify the active tectonic structures, taking into account previous works (e.g. Yu et al., 1997; Lee et al., 2008; Hsu et al., 2009; Huang et al., 2010…). We then precise structural geometries and some geological processes as well as the location of active folds and active faults during the PSInSAR monitoring time-period.This may lead us to better constrain the seismic hazards and the earthquake cycles of the place.

El Hairech structure represents one of the two exposed basement outcrops in the Tell orogen of northern Tunisia. The El Hairech anticline is bounded to the south by E-W trending faults and to the northwest by a NE-SW major fault. On the edge of these two faults, the Chemtou earthquake was recorded on August 6, 1985 (MW=3.5). In order to highlight the effect of seismotectonics on the morphology and recent tectonics of the El Hairech anticline, the aim is to study the geomorphometric response of the drainage network and their associated watersheds. To achieve this, we digitized the topographic isocontours from the Ghardimaou topographic map at a scale of 1/50,000 to create a Digital Terrain Model (DTM) with a ground resolution of 10m. Several geomorphometric indices, including the Hypsometric Integral (Hi), Hypsometric curve, and Topographic Roughness (STD), were calculated based on this DTM. Through morphometric analysis of the El Hairech anticline, we established the relationship between morphology, lithology, and active tectonics. Our analysis allowed us to classify over 105 sub-basins into high, moderate, and low clusters based on incision rate and tectonic activity. The results showed that the overall topography of El Hairech, characterized by a radial outward drainage pattern, is strongly influenced by the reactivation of NE-SW and E-W basement tectonic faults.

Taiwan island is the result of the active rapid collision of both Eurasian and Philippine Sea Plates with an annual average convergence rate close to 10 cm.y-1. The relief of Taiwan is composed of the metamorphic slate belt of the Backbone Range (also called Central Range) and to the east the Coastal Range mainly characterized by volcanic affinity. In between those, lay the Longitudinal Valley (125km long and N020°E trending) which is the active crustal suture zone. The latter presents both inter-seismic creeping displacement (Champenois et al., 2013) and was hit by 7 major earthquakes of magnitudes larger than 5 during the last 70 years. It highliths the geohazards importance of this area for any Taiwan citizens. In order to better constrain the seismic hazards and the earthquake cycles of the place, we settled several years ago numerous UAS surveys above the Coastal Range and the Longitudinal Valley (E. Taiwan) and acquired so many high-resolution photographs using several drones flying at 350 meters above the ground. After photogrammetric processing, we calculate both (1) a high-resolution Digital Elevation Model (UAS-HR-DEM) that takes into account buildings and vegetations, and (2) a Digital Terrain Model (UAS-HR-DTM) corresponding to the ground. Our ground validation (GCP’s) leads us to get 7cm planimetric resolution (X, Y) and below 40cm vertical accuracy. This UAS-HR-DTM combined with field work and a detailed morphostructural analysis permit us to map into much details the structures and consequently to up-date the pre-existing geological mappings (e.g. CGS geological maps, Lin et al., 2009 ; Shyu et al., 2005, 2006, 2007, 2008). Then we combined our new structural scheme with various geodetic data (levelings, GPS…) and PSInSAR results (Champenois 2011, and Champenois et al., 2013) to locate, characterize and quantify the active tectonic structures, taking into account previous works (e.g. Yu et al., 1997; Lee et al., 2008; Hsu et al., 2009; Huang et al., 2010…). We then precise structural geometries and some geological processes as well as the location of active folds and active faults during the PSInSAR monitoring time-period.

Volcanic areas in the World are often difficult to map especially in a structural point of view as (1) fault planes are generally covered and filled by more recent lava flows and (2) volcanic rocks have very few tectonic striations. Kuei-Shan Tao (11km from Ilan Plain – NE Taiwan) is a volcanic island, located at the soutwestern tip of the South Okinawa trough (SWOT). Two incompatible geological maps had been already published both lacking faults and structural features (Hsu, 1963 and Chiu et al., 2010). We propose herein not only to up-date the Kuei-Shan Tao geological map with our high resolution dataset, but also to create the Kuei-Shan Tao structural scheme in order to better understand its geological and tectonic history.Consequently, we first acquired aerial photographs from our UAS survey and get our new UAS high resolution DTM (HR UAS-DTM hereafter) with a ground resolution <10cm processed through classical photogrammetric methods. Taking into account common sense geomorphic and structural interpretation and reasoning deduced form our HR UAS-DTM, and the outcropping lithologies situated all along the shoreline, we have up-dated the Kuei-Shan Tao geological mapping and its major structures. To conclude, the lithologies (andesitic lava flows and pyroclastic falls) and the new structural scheme lead us to propose a scenario for both the construction as well as the dismantling of Kuei-Shan Tao which are keys for both geology and geodynamics of the SWOT.

The ground deformation phenomena are the result of the application of natural stress (tectonic) or anthropogenic stress (overexploitation of groundwater, embankment) on the soil. The land deformation may cause several impacts on the environment and human life. A good understanding of the deformation behaviour of the soil is essential to mitigate risks on the infrastructure and environment. In this study, a radar interferometry is carried out to analyse the mobility of an urban and suburban area of Greater Tunis region. The interferometric synthetic aperture radar (SAR) technique has been proven to be an effective methodology for detecting and monitoring soil displacement with millimetre accuracy and also, improving our understanding of the current deformations in the study area. Indeed, the application of differential SAR interferometry made it possible to determine the origin of the surface deformations related to natural or anthropic phenomena. In this paper, the interferometric method of small baseline subset (SBAS) developed by Berardino et al. 2002has been chosen, in order to monitor the spatial and temporal pattern of the deformation phenomena in Greater Tunis region. The analysis of Envisat ASAR (2003–2007, descending satellite orbit) and Sentinel 1B (2016–2018, ascending satellite orbit) SAR data allowed us to create deformation velocity maps associated with line of sight displacement time series. The results obtained by this method showed the existence of subsidence phenomenon in the urban areas (Tunis City) as well as in the rural areas (Mornag plain). In the first site (Tunis City), the subsidence rate reaches up to 13.98 mm/year and 19 mm/year during 2003–2007 and 2016–2018, respectively. The ground deformation that is detected around the region of Tunis Lake and Sebkhet Essijoumi, is probably due to the nature of the highly compressible and thickness alluvial deposits and whose substratum depth sometimes reaches more than 60 m. In the second studied site, ground subsidence was identified in the whole plain of Mornag. Indeed, the comparison between piezometric data and time series deformations shows that the trend of soil subsidence is coherent with the dynamic change in groundwater levels. Furthermore, the analysis of velocity deformation, geological data and hydrogeological information allows us to associate all the detected settlement patterns to groundwater overexploitation and compressible alluvium all over the plain. This overexploitation of groundwater in Mornag plain is mainly caused by the expansion of agricultural and industrial activities and the decrease in annual rainfall in recent years (from 2003 to 2018).

Characterizing active faults and quantifying their activity are major concerns in Taiwan, especially following the major Chichi earthquake on 21 September 1999. Among the targets that still remain poorly understood in terms of active tectonics are the Hengchun and Kenting faults (Southern Taiwan). From a geodynamic point of view, the faults affect the outcropping top of the Manila accretionary prism of the Manila subduction zone that runs from Luzon (northern Philippines) to Taiwan. In order to better locate and quantify the location and quantify the activity of the Hengchun Fault, we start from existing geological maps, which we update thanks to the use of two products derived from unmanned aircraft system acquisitions: (1) a very high precision (< 50 cm) and resolution (< 10 cm) digital surface model (DSM) and (2) a georeferenced aerial photograph mosaic of the studied area. Moreover, the superimposition of the resulting structural sketch map with new Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) results obtained from PALSAR ALOS images, validated by Global Positioning System (GPS) and leveling data, allows the characterization and quantification of the surface displacements during the monitoring period (2007–2011). We confirm herein the geometry, characterization and quantification of the active Hengchun Fault deformation, which acts as an active left-lateral transpressive fault. As the Hengchun ridge was the location of one of the last major earthquakes in Taiwan (26 December 2006, depth: 44 km, M_L = 7.0), Hengchun Peninsula active tectonics must be better constrained in order if possible to prevent major destructions in the near future.