Evidence on the Effect of Weather Seasons on Soil Response

Date: 4/25/2019

Time: 01:45 PM

Room: Pine

We analyze ground acceleration time histories of the ARGONET (Argostoli, Cephalonia Isl., Greece) vertical array with interferometry by deconvolution aiming to studying the relation between changes in soil moisture due to rainfall and shear-wave velocity, V_S, variations. We identify a clear, cycling pattern in the variation of V_S at shallow depths, which correlates negatively to rainfall height and a soil moisture proxy. Lower V_S values are observed during the raining season and highest values during the dry summer, the overall variation being of the order of 20-25% within the shallowest examined depth interval (0-5.6m) and estimated up to 40% within the top 2m. Interferometry results are verified by an independent surface waves dispersion analysis, using ambient vibration data collected by a temporary, small-aperture array of 6 seismometers that was installed for several months at the ARGONET site. Phase velocity at high frequencies (>13 Hz) was found to be significantly different when data from rainy and dry seasons were examined. Further comparison of standard spectral ratios based on synthetic and actual earthquake data suggests that the seasonal variation of V_S values is capable of causing differences in the level of high frequency ground motion between dry and rainy seasons. During our analysis we also identified sporadic V_S values that deviated from the cycling pattern toward even lower values than those observed during the rainy seasons. We attributed these values to nonlinear response of the soil formations, i.e., providing evidence for deviation from the linear stress-strain relationship at shaking levels lower than what is commonly believed (circa 30 cm/s²). Such deviations seem to appear systematically at the studied site but are often “masked” by the seasonal cycling pattern of V_S, which differs during the replenishment and depletion phases of soil moisture.

Presenting Author: Philippe Guéguen

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