Topographic disequilibrium, landscape dynamics and active tectonics: an example from the Bhutan Himalaya

International audienceThe quantification of active tectonics from geomorphological and morphometric approaches commonly implies that erosion and tectonics have reached a certain balance. Such equilibrium conditions are however rare in nature, as questioned and documented by recent theoretical studies indicating that drainage basins may be perpetually rearranging even though tectonic and climatic conditions remain constant. Here, we document these drainage dynamics in the Bhutan Himalaya, where evidence for out-of-equilibrium morphologies have for long been noticed, from major (> 1 km high) river knickpoints and from high-altitude low-relief regions in the mountain hinterland. To further characterize these morphologies and their dynamics, we perform field observations and a detailed quantitative morphometric analysis using χ plots and Gilbert metrics of drainages over various spatial scales, from major Himalayan rivers to their tributaries draining the low-relief regions. We first find that the river network is highly dynamic and unstable, with much evidence of divide migration and river captures. The landscape response to these dynamics is relatively rapid. Our results do not support the idea of a general wave of incision propagating upstream, as expected from most previous interpretations. Also, the specific spatial organization in which all major knickpoints and low-relief regions are located along a longitudinal band in the Bhutan hinterland, whatever their spatial scale and the dimensions of the associated drainage basins, calls for a common local supporting mechanism most probably related to active tectonic uplift. From there, we discuss possible interpretations of the observed landscape in Bhutan. Our results emphasize the need for a precise documentation of landscape dynamics and disequilibrium over various spatial scales as a first step in morpho-tectonic studies of active landscapes

Seismic analysis of the detachment and impact phases of a rockfall and application for estimating rockfall volume and free‐fall height

International audienceWe analyzed 21 rockfalls that occurred in limestone cliffs of the Chartreuse Massif (French Alps). These rockfalls were detected both by Terrestrial Laser Scanning or photogrammetry and by a local seismological network. The combination of these methods allowed us to study relations between rockfall properties (location of detachment and impacts areas, volume, geometry, propagation) and the induced seismic signal. We observed events with different propagation modes (sliding, mass flow, free-fall) that could be determined from Digital Elevation Models. We focused on events that experienced a free-fall after their detachment. We analyzed the first parts of the seismic signals corresponding to the detachment phase and to the first impact. The detachment phase has a smaller amplitude than the impact phase, and its amplitude and duration increasewith rockfall volume. By measuring the time delay between the detachment phase and the first impact, we can estimate the free-fall height. We found a relation Es=aEpbbetween the potential energy of a rockfall Ep and the seismic energy Esgenerated during the first impact, with parameters a=10-8and b=1.55 and with a correlation coefficient R²=0.98. We can thus estimate both the potential energy of a block and its free-fall height from the seismic signals. By combining these results, we obtain an accurate estimate of the rockfall volume. This relation was then tested on different geological settings and for larger volumes using Yosemite and Mount Granier rockfalls. We also compared our results with a data set of controlled releases of single blocks (Hibert et al., 2017) in order to investigate smaller volumes

Activation volontaire réduite et inhibition intracorticale accrue à l’exercice chez le patient atteint de syndrome d’apnée obstructive du sommeil

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The 2019 Le Teil surface-rupturing earthquake along the La Rouvière Fault within the Cévennes fault system (France): What does paleoseismology reveal?

International audienceThe 2019-11-1, Mw4.9 Le Teil earthquake occurred within the NE termination of the Cévennes faults system (CFS) in southern France, along the La Rouvière fault (LRF), an Oligocene normal fault which was not known to be potentially active. This shallow moderate magnitude reversefaulting event produced a 5 km-long surface rupture and strong ground shaking. No evidence of previous quaternary activity was observed in the morphology, raising the question whether the fault had been reactivated for the first time since the Oligocene or had broken the surface in the past without being detected in the morphology. To address this issue, we carried out paleoseismological investigations to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. We discovered that at least one event prior 2019, occurred between 13.5 and 3.3 ka within the central part of the fault segment that broke in 2019, and that a possible earlier surfacerupturing event occurred within the northern part of this segment during the 16th century. Further investigations coupling sub-surface geophysical investigations and trenching are now carried out within the southern and northern segments of the LRF as well as along the other fault segments of the CFS

Potentially large post-1505 AD earthquakes in western Nepal revealed by a lake sediment record

International audienceAccording to paleoseismological studies, the last earthquake that ruptured the Main Frontal Thrust in western Nepal occurred in 1505 AD. No evidence of large earthquakes has been documented since, giving rise to the concept of a seismic gap in the central Himalaya. Here, we report on a new record of earthquake-triggered turbidites from Lake Rara, western Nepal. Our lake-sediment record contains eight possibly moderate-to-large earthquake-triggered turbidites during the last 800 years, three of which overlap in age with previously reported Mw ≥ 7 events in western Nepal. Shaking intensity modelling, together with instrumental records, suggests that near-field earthquakes (≤15 km) should have a minimum Mw 5.6, and regional earthquakes (≤80 km) a Mw > 6.5, to trigger turbidites. We present a likely scenario that western Nepal may be as seismically active as central Nepal; however, more data are needed to revaluate the seismic risk in the central Himalaya