Ascending Volcanic Fluids Portended by Spatiotemporal Variations of the Earthquake Mechanisms in the Tatun Volcano Group in Northern Taiwan

To investigate the possible precursors of surface volcanism, we examined 2,473 earthquake focal mechanisms and the relative abundance of total sulfur (S t ) to carbon dioxide (CO 2 ) in the Tatun Volcano Group in the northern Taiwan, where various reports have indicated ongoing volcanic activities. We analyzed three time‐dependent observations: the occurrence rate of both shallower (0–2 km) reverse‐type earthquakes and deeper (2–4 km) normal‐type earthquakes, as well as the surface S t /CO 2 ratio. Among these observations, we found a series of similar variations, which progressed in a stepwise manner as the depth range of the observation decreased. To interpret the spatiotemporal variations of three observations, we construct a model involving the ascending volcanic fluids in the conduit and hydrothermal layer. The significant variations in deeper and shallower faulting behaviors portend a double warning of surface volcanism associated with the ascending fluids at ∼4 and ∼1 month(s) ago, respectively.