A study on the contribution of satellite RADAR interferometry to analyse the activity of Aso volcano (Japan)

The aim of this work is to compare the use of SAR datasets acquired by different sensors and understand which one has the best performance estimating from small to large ground displacements in volcanic area, maintaining a good spatial information. Through the reconstruction of the deformations evolution, it is possible to analyse the behaviour of the volcanic edifice before, during and after eruptions. The study is focused on Aso volcano, in the central part of the Kyushu island (Japan), which stands out for its wide caldera (18 km x 25 km). Inside the rim is included the post-caldera central cones younger than 0.1 Ma. Among 17 cones, the only crater which has been active for 80 years is Nakadake, composed by seven craterlets. In the considered time span (2007-2018), no large eruptions occurred; during its unrest period, a prevalent subsidence has persisted simultaneously with the degassing activity. Although the low intensity activity, ground displacements, detectable through remote sensing techniques, can reflect the inflate-deflate cycles of the magma chamber, situated below one of the main inactive crater (Kusasenri) at a depth of 4-5 km. Using Small Baseline Subset (SBAS) InSAR technique, ALOS Palsar-1 from 2007 to 2011, Sentinel-1 from 2014 to 2018 and ALOS Palsar-2 from 2016 to 2018 SAR datasets have been calibrated through Global Navigation Satellite System (GNSS) measurements. With the employment of SARscape software, for each time span, velocity and displacements maps have been generated to obtain deformations time series to analyse and identify the motion due to volcanic activity. An important seismic event occurred during the investigated time period is the Mw 7.0 Kumamoto earthquake happened on April 16, 2016, 60 km far from the caldera rim. Both in SAR and GPS time series was important to exclude the coseismic effect to estimate the correct trend movement due to the volcano activity. In the displacement time series, three points in correspondence of GPS located within the caldera and few points in the post-caldera central cones have been examined for each time span. Analysing the displacements time series is necessary considered that deformations are affected by many factors as geodynamic, atmospheric effects, noise, type of images processing and earthquakes further the volcanic activity and the characteristics of the sensor used for the acquisitions. The final results show that only in case of short satellite revisiting time and lesser wavelength is possible to detect low intensity activities, but sometimes using SAR data with longer revisiting time and higher wavelength helps to obtain a better spatial information in vegetated area, as in the case of Aso caldera.