Long-term Deformation Of Cerro Azul Volcano Monitored By In SAR Time-Series

Interferometric Synthetic Aperture Radar（In SAR）is a modern geodetic technology which is commonly used in geodesy research especially in natural disasters such as volcanic eruptions.In SAR recovers small deformation information on a large scale with high spatial coverage and ground resolution,including stable revisited period and sufficient redundant observation data,which is particularly suitable for long-term monitoring of various natural disasters such as earthquakes,volcanic unrests and glacial migration.In SAR time series technology constructs two-dimensional surface deformation fields on time domain through revisited observations,displaying the time-varying regional surface displacement fields among long-term monitoring.Time series effetely inverse the time-varying surface deformation fields with highly precision,retaining the high spatial resolution in SAR scenes and reducing the phase residuals.Geodetic modeling inversed by sufficient surface observation data proposed the shape,location and magma volume changing of the underground sources in deformation events,interpreting the deformation modes which helped to improve the understand of volcanic deformation.Cerro Azul Volcano,which located in Galápagos archipelago,South America,is an active volcano that had undergone an unrest event in March 2017,resulting in significant surface deformations observed in the southern Isabela Island.No eruption occurred in this event,nor did the deformation leading to ruptures to the surface,leaving great coherence in In SAR satellite observations.Taking the long-term surface deformation monitoring before and after this unrest event of Cerro Azul volcano as the research target,here we collected Sentinel-1A descending SAR single-look complex（SLC）images at 98 days between December 13^th,2014 and June 19^th,2018.489 interferograms interfered by SLC images constructed the small baseline interferogram network.The distributed scatterers（DS）were extracted from the interferogram set and continuous displacements on distributed scatterers were calculated by small baseline time series method.The time-dependent displacement fields on the surface on line-of-sight（LOS）direction in 1185 days in Isabela Island,Galápagos archipelago were obtained under the long-term geodetic monitoring.The deformation fields characters and underground deformation sources model were calculated by geodetic observations in pre-,co-and post-unrest stages respectively,in which the observation data of surface interferometric deformation fields on LOS direction in time domain were monitored by long-term time series In SAR,and the volcanic activity stages were divided by the unrest event in March 2017 at Cerro Azul volcano.During the pre-unrest stage,geodetic data indicated that weak expansion occurred around the caldera with its average annual displacement rate of 0.22cm/y in the 817 days from the monitoring beginning to March 8^th,2017,before the unrest event.During the co-unrest stage the surface deformation reached a negative peak displacement of-32.9 cm in the caldera and a positive peak displacement of 41.8 cm on the south-east plain in the line-of-sight direction from March 8^thto April 1^st.After the unrest,recovering displacement appeared in the caldera and the south-eastern plain where the average annual displacement rate of the caldera reached to3.81cm/y in the post-unrest stage which was far exceeded than it in pre-unrest stage.We inverted the parameters of underground deformation models,which included parameters of location,shape and volume of magma flowing,during the monitoring period for the three stages by geodetic modeling method.Base on the deformation models,we explained the magma transport mechanism in the long-term monitoring under the Cerro Azul volcano.Geodetic modeling result suggested that a point source Mogi located about 5000m approximately below the Cerro Azul caldera,which stored magma in the long-term monitoring window.Pressure was given from the internal Mogi,bringing on the magma discharge of the Mogi source,and resulting in the overall contraction of the Cerro Azul caldera in the co-unrest stage.Meanwhile,the long rectangular platform-shaped Sill source was tensile,delivering magma from Mogi to south-eastern plain,causing extensive inflation on the surface.The Mogi accumulated magma once again after the unrest,resulting in significant expansion around the caldera in the post-unrest stage.The models corresponded with the time-varying surface deformation inversed by small-baseline time series,explaining the influence of the Cerro Azul underground deformation sources to surface observation fields during the long-term monitoring reasonably.