Date of Award

2020

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Geology (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

Simon A. Carn

Advisor 2

Gianluca Groppelli

Committee Member 1

Roohollah R. Askari

Abstract

The study was focused on the Anak Krakatau sector collapse that occurred on 22 December 2018 in the Sunda Strait (Indonesia). The goal of the study was to monitor and analyze changes of the volcanic edifice and to try to understand causes that may have predisposed and triggered the sector collapse.

The use of different remote sensing techniques allowed the acquisition of thermal data, SO2 emission data, structural data and the identification and analysis of the eruptive events that occurred on Anak Krakatau in the period from 1° January 2016 to 28 February 2019. The acquisition of 1221 thermal data and 1156 SO2 emission data was performed using MODIS and OMI. Anak Krakatau began a new and intense activity phase on 30 June 2018 which continued in the following six months preceding the lateral collapse of the SW flank of the volcanic edifice, reaching its climax in September. The activity subsequently followed a decreasing trend, ending shortly after the collapse. The thermal data and the use of PlanetScope images allowed the identification of 8 lava flows, 7 of which developed between July and November 2018, exactly in the months that preceded the collapse. Almost all the lava flows affected the SW, S-SW and S slopes of the volcanic cone. These events led to an increase of the lithostatic load on the area subsequently collapsed. Precisely, their volume is equal to 6.8x106 m3. The volumes of all the lava flows were calculated considering an average thickness of 10m, obtaining a total volume equal to 8.2x106 m3. Further analysis of the satellite images highlighted a shift of the summit crater mainly towards the area subsequently collapsed and the generation of different curvilinear fractures which can be grouped in three main strikes’ groups: NW-SE; E-W and NNE-SSW. The structures with NW-SE strike agree with regional tectonics while those with E-W and NNE-SSW direction delimit the subsequent collapse. What has been observed indicates that there were precursory signs of structural instability. Finally, based on the collected data, bibliographic information regarding the extent of caldera-forming ignimbrite and deep surveys, it was possible to create the first pre-collapse 3D model representing the area present before the 2018 collapse. This model is a key element for a possible simulation of the sector collapse that occurred on Anak Krakatau.

The model allows to test three hypotheses of possible predisposing causes in future studies: 1) Increase of the lithostatic load on the SW side of the volcano generated by the deposition of all products erupted during the last phase of activity, considering also its high inclination (> 20 °); 2) Dike intrusion into the shallow portion of the volcanic edifice; 3) External factors which are common predisposing causes that influence the stability of the volcanic slopes such as a failure plane, zones of weakness, hydrothermal activity and mechanical weakening by alteration.

The study shows that the collapse of Anak Krakatau could have been anticipated through continuous monitoring of the volcano and its activity using different remote sensing techniques. Finally, we believe that the combination of 3D models, representing areas with signs of instability, and remote sensing techniques is an important method for predicting potentially disastrous events, and Anak Krakatau is an example.

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