Very-long-period seismic signals at the Tatun Volcano Group, northern Taiwan

Very-long-period (VLP) seismic events have been detected in the Tatun Volcano Group (TVG), located around the border between Taipei City and New Taipei City in northern Taiwan, in which has ~ 7 million residents. By using both analyses of particle motions and travel-time delay, one VLP volcanic earthquake's source is estimated to be at a shallow depth (~ 800 m) beneath Mt. Chihsin, which is the highest and youngest volcano in the TVG. The significant variation of seismic energy at different azimuths provides strong evidence to distinguish a crack source from other kinds of sources, such as a sphere, vertical pipe or even double-couple source. This is further confirmed by synthetic modeling of the seismograms recorded at two stations as well as the compressional first-motion at three seismic stations. Thus, a deeper plumping system with high-pressure fluid is required to generate the VLP signals and other volcanic earthquakes in the TVG. Combining this result with those of previous studies, we conclude that the TVG might not be totally extinct and some further investigations must be carried out to improve understanding of the volcanic characteristics of the TVG.