Marine Geohazard Identification (part-I)
Submarine Landslide
Hazard and Risk
Hazard (Potential Hazard) is the intrinsic properties of a substance, equipment or work process that can cause damage or harm to the surrounding. The potential hazard will remain a hazard without causing an impact or developing into an accident if there is contact (exposure) with humans.
DIS/ISO 45001 defines hazards as “sources or situations that have the potential to cause injury and illness” (clause 3.19).
In the context of marine hazards based on causal factors, hazards can be divided into 2 types, namely:
- Natural is a potential hazard caused by nature. The causes included in this natural factor are tectonics, gravity (slope), ocean dynamics, and erosion and accumulation.
- Man-made are potential hazards caused by human activities.
Hazards related to geological conditions are known as geological hazards or geohazards. One example of a geohazard is a submarine landslide.
Submarine Landslide
Submarine landslides can be defined as any material movement that makes up the slope, downward toward the continental shelf. James Jonathan Hance, B.S. (2003) analyzed 534 submarine landslide events and concluded that there were 14 triggering mechanisms. In the database, 366 events had reported triggers and the rest (168 events) were unknown. The distribution of events is shown in the following graph.
225 landslides (40%) were triggered by earthquake and fault activity, 25% by rapid sediment accumulation, 11% by gas and gas hydrate dissociation, 9% by erosion, and the rest by other triggers.
Only 399 of the 534 events in the database have information on the slope angle, and the slope angle data is plotted in Figure 4. Figure 4 shows that the slope angle with the most events is between 3 – 4 degrees, which is a gentle slope.
The cumulative frequency diagram shows that 80% of the 399 events had a slope angle of less than 10 degrees. Events occurring at gentle slope angles are not caused by gravity itself but by other mechanisms operating simultaneously. However, this does not eliminate the fact that submarine landslides can occur at gentle slope angles.
Submarine Landslide Cases
In some cases, submarine landslides can cause other types of hazards and result in casualties. Here are some examples of the cases:
- Flores (December 1992)
The 1992 Flores earthquake was an earthquake that occurred on December 12, 1992 with an earthquake magnitude of 7.5 Ms. Figure 6 shows a scarp left behind by a slumping event at Leworahang beach with an estimated height of up to 8 meters based on drowned coconut trees (Yeh, 1993). This event is one example of an earthquake-induced landslide.
Typical scenes of subaqueous slumps near Leworahang:
- Palu 2 (September 2018)
The tsunami in Palu in 2018 was a combination of tectonics and submarine landslides (Liu, 2020).
- Anak Krakatau (December 2018)
The tsunami that occurred in Banten in 2018 resulted from the collapse of the southwest flank of Mount Anak Krakatau (Ye et al, 2020).
- Maluku (June 2021)
The tsunami in Central Maluku in June 2021, according to BMKG, reached a height of 50 cm as a result of a 6.1 magnitude (M) earthquake due to active fault activity associated with the Kawa Fault Zone and is strongly suspected to have resulted in submarine landslides. This is what eventually caused the tsunami.
Technology for Geohazard Identification
Recent landslide activity will leave its footprint on the seafloor. Thus, the investigation of seafloor morphology using Multibeam Bathymetry and Side-Scan Sonar systems – is one of the most important aspects of evaluating submarine landslide hazards.
Geophysical methods are crucial to obtaining regional or local information on the sedimentation process that led to the landslide. Information on the stratigraphy of the landslide area was obtained utilizing Sub Bottom Profiler survey.
Reference
Bryn P, Berg K, Forsberg C F, Solheim A, Kvalstad T J (2005). Explaining the Storegga Slide. Marine and Petroleum Geology, 22(1–2): 11–19.
Chaytor, Jason D., Wayne E. Baldwin, Samuel J. Bentley, Melanie Damour, Douglas Jones, Jillian Maloney, Michael D. Miner, Jeff Obelcz, Kehui Xu. (2020). “Short- and long-term movement of mudflows of the Mississippi River Delta Front and their known and potential impacts on oil and gas infrastructure”, Subaqueous Mass Movements and their Consequences: Advances in Process Understanding, Monitoring and Hazard Assessments, A. Georgiopoulou, L. A. Amy, S. Benetti, J. D. Chaytor, M. A. Clare, D. Gamboa, P. D. W. Haughton, J. Moernaut, J. J. Mountjoy
Hance, B.S., (2003). Development of a Database and Assessment of Seafloor Slope Stability Based on Published
Literature (M.S. thesis). The University of Texas, Austin.
Liu, P.LF., Higuera, P., Husrin, S. et al. Coastal landslides in Palu Bay during 2018 Sulawesi earthquake and tsunami. Landslides 17, 2085–2098 (2020). https://doi.org/10.1007/s10346-020-01417-3
L. Ye, H. Kanamori, L. Rivera, T. Lay, Y. Zhou, D. Sianipar, K. Satake, The 22 December 2018 tsunami from flank collapse of Anak Krakatau volcano during eruption. Sci. Adv. 6, eaaz1377 (2020).
Tripsanas, Efthymios & Bryant, William & Phaneuf, Brett. (2004). Slope-instability processes caused by salt movements in a complex deep-water environment, Bryant Canyon area, northwest Gulf of Mexico. Aapg Bulletin – AAPG BULL. 88. 801-823. 10.1306/01260403106.
Yeh H, Imamura F, Synolakis C, Tsuji Y, Liu P, Shi S (1993). The Flores Island Tsunami. Eos, Transactions, American Geophysical Union, 74(33): 369–373.
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