Martijn van den Ende , Jean Paul Ampuero , Françoise Courboulex, Bertrand Delouis, Maxime Godano, Christophe Larroque, Anthony Sladen

Recent developments in Distributed Acoustic Sensing (DAS) have greatly expanded our capabilities for dense geophysical instrumentation by tapping into existing (but unused) fibre-optic telecommunication networks. Leveraging these so-called "dark fibres" permits an extremely rapid deployment of thousands of vibration sensors over distances of several tens of kilometres, which is ideal for rapid postseismic response efforts. Here we report on the use of dark-fibre DAS for monitoring of the aftershock sequence of the 2019-11-11 Mw 4.9 Le Teil, France earthquake. Through comparison with the local seismometer network, we assess the capabilities of the DAS array to detect and locate small-magnitude seismic events. Likely owing to cable deployment and DAS sensing characteristics, we find that the DAS noise floor is up to 3 orders of magnitude higher than that of nearby seismometers, which greatly inhibits the detection and analysis of the low-energy events. However, locating a selected aftershock with DAS yields an accuracy and precision that is comparable to that of the seismic network, even though the DAS array has a relatively unfavourable geometry. Based on these observations we provide a number of recommendations for routinely incorporating DAS into postseismic response protocols, and for optimal use of DAS alongside conventional seismic instrumentation.