Dynamics of the 2021 unrest at Changbaishan Tianchi volcano from ALOS-2/PALSAR-2 and seismic data

The Changbaishan Tianchi intraplate volcano is one of the most active and hazardous volcanoes of NE Asia,
characterized by a summit caldera formed after the 946 CE ‘Millennium’ Plinian eruption. From December 2020
to June 2021, the frequency and magnitude of earthquakes at Tianchi were significantly higher than during
background periods, with hundreds of earthquakes (46 events per month in average) and reaching a local
magnitude of ML 3.1. This study reports a comprehensive deformation analysis and geophysical inversion scheme
aimed to unveil the dynamics of this period. Multi-temporal InSAR analysis results of 32 ALOS-2 images from
2018 to 2022 show that the surface deformation is a combination of seasonal fluctuations (± 25 mm in average,
with a maximum ± 45 mm) and a long-term positive component. The least squares linear regression of the
deformation time series and temperature data, isolates the seasonal fluctuations, revealing a clear upliftsubsidence
process from June 2020 to July 2021 in the caldera area. To constrain the Tianchi plumbing system
dynamics, a combined inversion scheme consisting of three deformation sources is designed. The inversion
results and the seismic records indicate that Tianchi volcano has experienced a low-level unrest episode from
December 2020 to June 2021. The shallower plumbing system, located at about 5–9 km depth and modelled by
pressurized spheroids, underwent a cumulative volume increase of 26 × 106 m3 from November 2018 to April
2021, followed by a volume decrease of 9 × 106 m3 from April to July 2021. This suggests magma uprising from
the 14 km deep storage zone to the shallower plumbing system, followed by depressurization of the plumbing
system due to the escape of fluids. This research provides a comprehensive understanding of the magma and fluid
migration dynamics within the Tianchi multi-level plumbing system for the first time.