Temporal variation of ambient noise at the Grande Dixence reservoir recorded by a nodal deployment

Grande Dixence, the tallest gravity dam in the world, is located in the Swiss Alps on the Dixence River with a catchment area of 4 km² at a towering elevation of 2000m. The lake serves as a collecting point of melt water from 35 glaciers and reaches full capacity by late September, subsequently draining during winter and dropping to lowest levels in April. For a reservoir as large as the Grande Dixence, the variation in hydrological load can be expected to induce changes in crustal stress. The goal of this study was to harness the loading effect of the time-varying level of reservoir load as a source of known stress to investigate the variation in seismic velocity of the bedrock due to changes induced in crustal stress and strain rates. 22 seismic nodes were thus deployed along the banks of the reservoir which were operational from mid-August to mid-September, corresponding to the time period when the lake level reaches its maximum. Of the 22 nodes, 18 were deployed in closely spaced patches of six in order to carry out coherent stacking and to increase the signal-to-noise ratio, besides one group of three nodes and one single node. Measurement quality appears satisfactory: small local earthquakes are recorded well, and the probabilistic power spectral densities (PPSDs) computed for data quality validation evidence the ambient noise levels to be well within the global noise limits. However, the recorded noise is unexpectedly complex and, at periods shorter than 1 second, varies strongly by location. The 0.5--5s (0.2--2 Hz) period band at lakes generally records a diurnally varying noise level, often associated with lake generated microseism. Diurnal variations around 1 second of period are observed in our study as well. The amplitude of ambient noise level around 1 second of period is observed to be highest when the lake level changes, along with the prominent diurnal variation. A similar variation is observed in the seismic velocity variation (dv/v) computed from cross-correlated and auto-correlated ambient noise filtered between 0.5--1 Hz, with dv/v exhibiting a drop with rising lake level. These results provide preliminary evidence for possible change in crustal stress state with changing hydrological load. Future direction of this study consists of analytically modeling the results to quantify the influence of thermobarometric parameters on PPSDs and dv/v, and deconvolve it from the lake induced variations.