Two Volcanic eruptions were analyzed, the 2017 Bezimianny volcano (Done by my lab partner, Yi-Ni, Chen) and the 2018 Krakatoa volcano (Done by me). We calculated the seismic wave velocity difference based on (Snieder et al., 2004) with two different algorithms calculating the time variations, the MWCT (Clarke et al., 2011) and Stretching (Sens-SchÖnfelder and Wegler, 2006). 

The precursor signals were well detected as the coherence of the seismic waves declined.

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The seismic wave velocity and the coherence between stations were strongly fluctuated after the eruption for the Krataoa eruption. Since the volcanic eruption was so serious that part of the mountain on the island collapsed, it might refer to the unstableness of the underlying structures. The coherence and seismic wave velocity difference gradually restored to the normal state with some wiggles observed a few months after. Although it is still unclear what led to such anomaly, the major eruption event was clearly captured by such method and data.

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On the other hand, the Bezimianny volcano had a relatively distributed velocity difference spatially. The velocity difference was detected before the eruption and the coherence between stations restored quickly within one month and kept relatively stable. We speculated that it might have a larger and more complicated magma reservoir around the area. However, the eruption was not as severe as the Krakatoa's, thus the coherence retained quickly and stabled.