Detecting seismic velocity change by noise-based direct surface wave

Recently, advances in the ambient noise analysis provide new ways to detect the velocity changes in the volcanic region by measuring the time delay of the daily cross-correlation functions (CCFs). Despite abundant studies on coda waves, studies examining the direct surface waves are relatively rare because of the influence of passive noise sources. However, direct surface waves have stronger energy and carry depth information, which can be obtained by the dispersion inversion. The direct surface waves' propagation direction along the great circle path is also beneficial for conducting tomography by finding a stable passive noise source, which is key to extracting depth-dependent velocity changes. Here, we used direct surface wave to detect velocity change caused by 2018 KIlauea volcano eruption and 2019 Ridgecrest earthquake. The results show that the noise-based direct surface wave is a powerful tool to study the change of earth medium caused by geological hazards, and can accurately obtain the information of the lateral position and depth position of the change.