Recent Variations of the Seismic and Geochemical Observations of the Tatun Volcano Group

Recent Variations of the Seismic and Geochemical Observations of the Tatun Volcano Group

Among the volcanic activity in the northern Taiwan area, the geothermal and seismic activities of the Tatun Volcano Group (TVG) are still very strong. Recent studies over the past decade have suggested that the TVG can be considered as active volcanoes. The Taiwan Volcano Observatory at Tatun was established in 2011 to monitor volcanic activity and to evaluate the possibility of re-eruption in the TVG. Several different real-time observation systems including seismic observation, geochemical analyses, crustal deformation, and geothermal monitoring have been adopted for monitoring. Recent geochemical observations show that a significant increase in the amount of helium (He) has been found from gas sampled from most of the fumaroles since early 2019. However, the helium isotopic ratio (3He/4He ratio) has not changed significantly. Meanwhile, the Stotal/HCl ratio of the gases indicates that there was a significant increase in the HCl concentration in all fumaroles from the end of 2018 to the first half of 2019. In addition, as compared with background data, the CO2/Stotal ratio of the gas composition suggests that the system has trended more towards hydrothermal activities. Several earthquakes with magnitude larger than M3.0 have recently occurred in the Tatun volcanic area. First, one sequence with two events occurred on January 28, 2019, accompanied by numerous aftershocks located in the Bayen area. Following the February 9 sequence, two earthquakes were detected in the Shilin area, south of Mt. Cising. An earthquake sequence occurred on August 6, also located at Mt. Cising, similar to the previous sequence. Combining the geochemical indicators and seismicity, we suggest that the activities of the TVG during late 2018 to 2019 are mainly caused by the disturbances of the hydrothermal system. The strong ground shaking of the two earthquakes that occurred in Bayen might have broken the stability of the hydrothermal system and opened a pathway to the surface. An increased amount of helium gas escaped from the smoother pathway, and thus an increasing amount of helium is detectable. In addition, the variation of HCl concentration is associated with the movement of fluid at great depths. At present, no obvious magmatic activity is suggested from the observations of the TVG; however, the volcano monitoring must be continued to forecast possible disasters caused by hydrothermal activities, such as phreatic eruptions.