Mapping, modeling, and assessing volcanic hazards on Bioko Island (Equatorial Guinea)

Jacob Brauner

doi:10.17918/00010874

Bioko Island (Equatorial Guinea) is a volcanic island, located in central Africa, which consists of three coalescent shield volcanoes. The last eruption on Bioko occurred in 1923, and current emissions of CO₂ indicate the presence of magma at depth and the potential for future activity. Despite the fact that that the entire population of Bioko lives in the proximity of the volcanoes, primary volcanic hazards such as lava flows have not been assessed, yet. The volcanic environment in combination with a humid tropical climate lead to an increased hazard of landslides, which has not been assessed yet either. I use a TanDEM-X digital elevation model (DEM) with a resolution of 12 m to create an inventory of 436 vents and monogenetic cones, over 1330 structural elements and lineaments, and 6 lava flows. Furthermore, I identify 505 areas of soil erosion, with 275 confirmed as landslides, utilizing 106 satellite scenes from the Sentinel 2, and Landsat 8/9 missions between January 2015 and June 2024. A key aspect that governs the distribution of hazards on Bioko is the complex interplay between tectonic and volcanic processes controlling the geomorphology. Hence, I describe the general geomorphology of Bioko Island and discuss possible volcano-tectonic and regional tectonic contributors, such as the Cameroon Volcanic Line and intersecting fracture zones, as well as gravity-tectonic processes. Based on vent density and lineament mapping, I am able to identify volcanic rift zones, aligning with the principal NE-SW axis of Pico Basilé in the northern part of the island, as well as E-W oriented rifting along the saddle between Pico Biaó and Gran Caldera de Luba in the southern part of the island. To assess the lava flow hazards on Bioko, I create a series of lava hazard maps based on the TanDEM-X DEM using the probabilistic Q-LavHA model. I constrain the lava flow length parameter based on the mapped lava flows and a reconstruction of the 1923 lava flow to 2 to 12 km. I constrain the flow elevation corrective factors through field observations to 2 to 8 m. Based on the 436 mapped vents, I calculate a kernel density estimation (KDE) and use it to weight the probability of vents opening in a given location. The highest vent opening probabilities coincide with increased lava inundation probabilities and are located along the volcanoes' main rift zones and adjacent flanks. However, simulated lava flows are often redirected by structural features such as the summit graben at Pico Basilé, highlighting the strong impact of aforementioned volcano-tectonic mechanisms. Furthermore, simulated lava pathways often follow erosional incisions, with a channeling effect that becomes less pronounced with increasing flow thickness. Important infrastructure, including the capital city of Malabo, are mostly located in coastal areas with relatively low lava inundation probabilities. To assess the landslide susceptibility on Bioko, I create a landslide susceptibility map using the bi-variate statistical Frequency Ratio approach. I show that steep slopes and proximity to major faults are the most influential factors to increase a given area's susceptibility to landslides. High precipitation rates are associated with being the main trigger of landslides. I find that 93 landslide scars clustered along the steep caldera walls of Gran Caldera de Luba, while additional 88 landslides were found at the headscarps of erosional amphitheaters at the flanks of Pico Basilé, and 59 near fault structures at Pico Biaó. Hence, I classify the caldera walls of Gran Caldera de Luba, the eastern flank of Pico Biaó, and the flanks of Pico Basilé, as highly susceptible to landslides. Furthermore, the spatial distribution of landslides and the downhill tapering of erosional amphitheaters at Pico Basilé suggests regressive erosion starting at the topographical knickpoints, caused by directional spreading as the main driver for the erosional amphitheater formation on Bioko.

Mapping, modeling, and assessing volcanic hazards on Bioko Island (Equatorial Guinea)

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Mapping, modeling, and assessing volcanic hazards on Bioko Island (Equatorial Guinea)

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