Impact of nested moisture cycles on cliff coast failure revealed by multi-seasonal seismic and topographic surveys

16 Cliff failure is a fundamental process shaping many coastlines worldwide. Improved in17 sight into direct links between cliff failure and forcing mechanisms requires precise in18 formation on the timing of individual failures, which is difficult to obtain with conven19 tional observation methods for longer stretches of coastline. Here we use seismic records 20 and auxiliary data spanning 25 months to precisely identify and locate 81 failure events 21 along the 8.6-km long chalk cliff coast of Jasmund, on Germany’s largest island, Rügen. 22 The sub-minute precision of event timing allows the linkage of individual failures to trig23 gers over a wide range of relevant time scales. We show that during the monitoring in24 terval, marine processes were negligible as a trigger of cliff failure, although still being 25 important for the removal of resulting deposits. Instead, cliff failure was associated with 26 terrestrial controls on rock moisture. Most failures occurred when water caused a state 27 transition of the cliff forming chalk, from solid to liquid. Water content was modulated 28 by: i) subsurface flow towards the cliff, ii) rain onto the cliff, and iii) condensation of at29 mospheric moisture, leading to clustered failures preferentially during the night. Seasonal 30 water availability, controlled by plant activity, imposed an annual cycle of cliff failure, 31 and wetter and drier than average years imposed a month-long legacy effect on cliff fail32 ure dynamics. Similar terrestrial control mechanisms may also be relevant for other coastal 33 chalk cliffs, in addition to already investigated marine triggers. 34 Plain Language Summary 35 Cliffs line many coastlines worldwide. They are eroded by cliff falls, with conse36 quences for human safety, land loss, ecosystem dynamics and availability of sediment along 37 the coast. The discrepancy between rapid, short-lived failure processes and episodic ob38 servation techniques does not allow for a full analysis of the causes and drivers of cliff 39 erosion. Combining measurements from a seismometer network on Germany’s largest is40 land, Rügen, with 3D models from drone surveys and weather station data, we detected, 41 located and timed 81 cliff failures in two years, and analysed the circumstances that gave 42 rise to their occurrence. These events were predominantly associated with the presence 43 of water, which turns the solid, cliff-building chalk into a failure-prone slurry. Water avail44 ability is modulated at different time scales by rain on the cliff and moisture condensa45 tion, soil water flow, vegetation water uptake, and possibly the lunar cycle. Our find46 ings sharpen the picture of when and why cliffs fail, and offer a better prediction of the 47 impact of global change on cliff coasts. 48