4D seismic tomography unveiling velocity increase prior to seismic swarms at the Tatun volcano group of Taiwan.

The Tatun Volcano Group (TVG) is in close proximity to the Taipei metropolitan area, which is home to over 7 million residents. Although there has been no recorded volcanic eruption in the TVG throughout human history, recent seismic observations suggest that it may still be active. To investigate possible volcanic seismicity and activity in the TVG, we conducted a 4-dimensional seismic tomography study using abundant seismic data collected from 2014 to 2021. We obtained 3D seismic velocity structures to examine both the temporal and spatial variations in seismicity each year. Our results show that the dramatic increase in seismicity in 2019 followed an increase in P-wave seismic velocity in the Dayoukeng fumarole area and Mt. Chihsin in 2018. This increase in seismic velocity may have been caused by rocks or sediments subjected to higher pressure beneath the TVG, resembling a pressure cooker on a flame. Thus, the sequential rise in both seismic velocity and seismicity strongly suggests that careful monitoring of temporal velocity variations in the volcanic area might provide an early warning of potential seismic swarms or volcanic activity in the future.