ALBERT

Description: Temporal variation of shallow subsurface structure is monitored by the SPAC-based method using a seismic array on a landslide area. We deployed a seismic array that consists of 14 three-component seismometers with a natural frequency of 4.5Hz on flat ground with diameters of 28m x 75m in a landslide area in the western part of Shizuoka prefecture, Japan. We used the data from October 2020 to May 2022. We applied the SPAC method using ambient noise data. The SPAC is obtained for each day and temporal variations are investigated. We compared the daily SPAC with the reference SPAC, which is obtained by stacking all the daily SPAC. The results show a characteristic change in SPAC that includes weekly changes, rainfall-related changes, and seasonal changes. In the weekly changes, a systematic difference is found between weekends and weekdays. Regarding the rainfall-related change, the large vibration, which has a significant impact on SPAC analysis, by a stream near the array is often observed during/after heavy rain. We statistically estimated and removed the effect of these two changes to monitor the variation due to the subsurface structure. We found the SPAC in the high-frequency region decreases due to rainfall, which is regarded as being due to a rise in the groundwater level caused by rainfall. MMSPAC (multimode spatial autocorrelation) method (Asten.2006, Ikeda.2012) is used to estimate the most strongly affected depth ranges to compare the structure model using reference SPAC. The seasonal change may be associated with the state of the shallow groundwater.