CHANGES IN THE VOLUME OF ANAK KRAKATAU AND SURROUNDING BATHYMETRY POST THE 2018 ERUPTION

• Main Author: Ajeng Khusnul Hanifa, Rosi
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/68878
• Institution: Institut Teknologi Bandung

Mount Anak Krakatau which emerged from the remains of the Krakatoa collapse that erupted in 1883 shows active activity recently. Previously, Anak Krakatau was known to have erupted several times in 2007 – 2008. According to the Ministry of Energy and Mineral Resources, this eruption was of an explosive type, with a height of ash and other material ejected at an altitude of about 100-500 m. In addition, this volcano also erupted again in 2012. This eruption is said to be a strombolian type eruption, spewing incandescent material up to 200-300m. Then there was a fairly large eruption in 2018, during the eruption of Anak Krakatau on December 22, 2018, the side of the mountain was damaged so that some of the material collapsed and then entered the water body around Anak Krakatau. Satellite monitoring and field observations revealed that Anak Krakatau was at a stage of increasing activity throughout 2018. Aerial photos show a sharp and steep cut from around the crater to the southwest of the mountainside. One of them is the Sentinel 1A satellite, Sentinel 1 SAR imagery records the earth's surface using the C-band of the microwave spectrum. One of the important characteristics of the C-band is that the scattering signal from the Sentinel 1 SAR sensor can penetrate the upper vegetation canopy, but not penetrate deep enough to reach the middle or lower canopy layers. Due to its characteristics, it may not be possible to extract DEM using the interferometric method for areas with dense vegetation cover. The only possibility to extract DEM is using the InSAR method, this method uses a complex image or SLC (Single Look Complex). InSAR uses 2 radar images that have the same characteristics, with a cross product between the two image pixels to produce an image containing phase and amplitude information. InSAR technology utilizes the existing phase information in the SAR image to obtain the distance difference and change in distance from two SLC images on the same surface. The collapse of the side of the Anak Krakatau mountain that entered the body of water made the waters around Anak Krakatau silting up. To determine the presence of siltation in this area, water depth data is needed. Water depth data is obtained by conducting bathymetric surveys using underwater acoustic technology which records the bottom surface of the water by reflecting sound waves. Then this DEM and Bathymetry data will be used to find the value of the volume of the Anak Krakatau collapse and the volume of seabed changes that occurred on December 22, 2018. After data processing is carried out, the volume value in 2016 is around ± 0.356 km3, while in 2019 it is around ± 0.151. km3. So that the post-eruption volume difference is about ± 0.205 km3. So that the post-eruption volume difference is about ± 0.205 km3. While the value of the volume of material that enters the water body and causes silting on the caldera side is around ± 0.141 km3 and ± 0.022 km3 on the northside of Anak Krakatau island. The siltation that occurs in the waters southwest of Anak Krakatau is about ± 24-25 m. Bathymetry around the waters of the Krakatoa complex has an average depth of between ± 30 m to ± 90 m. However, the caldera section on the southwest side of Anak Krakatau in 2016 has a depth of between ± 230 m to ± 250 m, with the lowest area in this area being ± 250.3 m. Whereas in 2019, there was a change in bathymetry to ± 210 m to ± 220 m with the lowest depth of around ± 226.12 m. On the other hand, there is an increase in area and volume in the area outside Mount Anak Krakatau. The area of the island in 2019 became larger than the island in 2016, the volume obtained was around ± 0.116 km3 and in 2019 it increased to ± 0.135 km3. With this, the value added to the volume around the island of Anak Krakatau is obtained at ± 0.019 km3. Then the final difference in volume of ± 0.023 km.3