Seasonal variations in crustal surface wave attenuation

Attenuation of seismic waves describes the loss of energy as waves travel through the medium. It is among others sensitive to fluids, making it a potentially important complementary observation in ambient noise monitoring. While seismic attenuation imaging with ambient noise yields convincing results at regional scale, time-dependent measurements of attenuation at this scale remain challenging. One of the challenges is the variability of ocean microseism sources, that leads to a variability of noise energy and measured seismic velocity in short-duration observations.In this contribution, we investigate changes in surface wave attenuation from ambient seismic noise recorded at the Swiss Digital Seismic Network. We observe seasonal variations in Rayleigh wave attenuation, with local averages matching previous time-independent studies. Following this finding, we pursue two goals: a) ensuring the robustness of the time-varying attenuation signal, and b) seeking an interpretation.In the Alpine region, energy from the secondary microseism drops noticeably during summer. Together with the geographical change in source distribution, this leads to seasonal variations in phase velocity estimates, which we account for when measuring attenuation; however, it may also lead to apparent changes in attenuation itself provided that source changes are not affecting the seismic stations uniformly. Using numerical modeling and an extended attenuation measurement, we test whether source distribution changes alone can account for the observed behavior.The observations show stronger attenuation in summer, than in winter. Given the relatively low frequency band targeted here, a shallow origin of the variations, such as ground temperature or soil moisture variations, does not seem plausible. If robustness can be confirmed, we will investigate the hypothesis of crustal attenuation changes due to seasonal loading by precipitation.