The Use of Sentinel-1 in Response to Sequences of Natural Disasters in Indonesia: the South Kalimantan Floods and the Mamuju-Majene Earthquake

Abstract A series of floods occurred in the early year of 2021 are the first major South Kalimantan flood in the past fifty years. The major regions which were affected by them are Banjarmasin, Central Hulu Sungai Regency, and Tanah Laut. There are 211,670 people severely affected by flooding and some of them need to be evacuated to higher zones. Not only floods as a concern of Indonesia National Disaster Management Authority (BNPB) but also earthquake that struck the Mamuju and Majene Regency, in the province of West Sulawesi on 14 and 15 January 2021. Damage was reported in 4 regencies which more than 6,000 buildings were destroyed, more than 3,300 were injured, and 105 people were confirmed dead due to this earthquake. Quick monitoring of two events must be conducted in order to support spatial information for BNPB mitigating the disasters. A geodetic ground measurement could be the best result in terms of accuracy and a large-scale map. However, a lack of pre-event measurement and a difficulty to access the primary roads and bridges in the affected regions makes ground survey complicated to be conducted. An alternative solution is using a space-borne remote sensing which its sensor regularly passes the affected regions. We used Synthetic Aperture Radar (SAR) data from Sentinel-1 to regain spatial information before and after disasters in the provinces of South Kalimantan and West Sulawesi such as inundation and deformation areas, respectively. Using an interval of 12 days and the SAR backscatter analysis, we found inundated areas mostly located near to the Barito River and in sub-districts along its tributaries for a case study of South Kalimantan floods. Meanwhile, we detected deformation about -5 cm in the southern part of West Sulawesi using the Differential Interferometric SAR (DInSAR) technique. Due to high decorrelation with the range of coherence values between 0 and 0.2 in the epicenter of earthquake area, the generated interferograms can not confirm the surface displacement caused by the sequence series of foreshocks, mainshocks, and aftershocks in this area.