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Abstract:
We have computed the cross-correlation functions (CCF) of the cultural noise to measure the temporal variation in seismic velocity for each station pair among the local Chihshang array in Taiwan, which was deployed in 2013-2020. A significant seasonal variation of asymmetric CCFs in the frequency band of 2-8 Hz, which is supposed to be the cultural noise, has been identified in this study. We further extract the velocity change (δv/v) by using the stretching method with the largest value up to 12% and found that the δv/v highly correlates with the nearby shallow groundwater level (GWL) fluctuation that usually occurs from September to November after rain season every year. On the other hand, the temporal variation of δv/v can only be observed for coda CCF with frequency higher than 3 Hz, which indicates that the source depth is within 30 m in the near surface layer based on its corresponding sensitivity kernel. The station pairs along the eastern side of Longitudinal Valley present smaller amplitude of δv/v, which may reflect a reduced GWL fluctuation in this region. Even though there is a clear 1-month lag between the rainfall and the GWL, there is no phase delay between the time series of the observed δv/v and the GWL. Our results suggest that we are capable of quantitatively monitoring the transient groundwater storage by a analyzing the cultural noise cross correlation functions in the Chihshang area, where is covered with a thick gravel bed in the southern Longitudinal Valley.
