[S-VC32] Mitigation of volcanic disasters - basic and applied researches

[SVC32-P01] Characteristics and Emplacement History of Volcaniclastic Turbidites, Insights into Hazard Assessments on Volcanic Islands, the Central Azores

*Yu-Chun Chang^1,2, Neil Mitchell², Thor Hansteen³, Rui Quartau^4,5, Julie Schindlbeck-Belo³, Armin Freundt³, Christian Hübscher⁶ (1.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology, Japan, 2.Department of Earth and Environmental Sciences, University of Manchester, Manchester, UK, 3.GEOMAR, Helmholtz Centre for Ocean Research Kiel, Germany, 4.Divisão de Geologia Marinha, Instituto Hidrográfico, Lisboa, Portugal, 5.Universidade de Lisboa, Faculdade de Ciências, Instituto Dom Luiz, Lisboa, Portugal, 6.Institute of Geophysics, Center for Earth System Research and Sustainability, University of Hamburg, Germany)

Keywords:volcanic hazard assessment, volcanic eruption history, tephrostratigraphy, age-depth modeling, volcaniclastics emplacement mechanism, turbidite modelling

Natural hazards arising from volcanic and non-volcanic processes have been observed on many insular volcanic islands. However, limited research material and remote locations often hinder scientists from completely understanding the types, sizes and frequencies of different hazards occurring there. In the central Azores volcanic islands, more than 1200 submarine slope valleys were documented from the bathymetric data, and 41 volcaniclastic turbidites were preserved in four sediment cores, suggesting active sedimentary processes and volcanic activity.
The sediment cores were analyzed to discriminate the emplacement mechanism of volcaniclastic beds between tephra fallout, pyroclastic flows and submarine landsliding by integrating information from sedimentary structures, glass shard geochemistry and morphometrics, bulk composition and organic geochemistry. From the results, 2/3 of the volcaniclastic beds originated directly from erupting volcanoes, whereas only 1/3 involved slope remobilization such as landsliding. The low incidence of landslide-generated beds could be explained either by infrequent landslide-generated sediment flows reaching the basin floors and/or more frequent eruptions creating beds.
Turbidite volumes preserved in the nearby basins are modelled by multiplying basin areas with bed thickness, also considering different thinning rates and sediment input directions. These modelled landslide-generated turbidite volumes are only comparable with the size of the largest landslide valleys on the adjacent upper slope, indicating that such turbidites in the cores likely originate from these largest slope failures.
The emplacement history of turbidites reconstructed by radiocarbon datings and tephrostratigraphic correlations reveal that all types of turbidity currents have reached the core sites at a modest frequency of a few thousand years since the Last Glacial Maximum. Additionally, the hemipelagic sedimentation rates and the submarine landslide frequencies in cores all have gradually increased since the last 50 kyr.. The high sedimentation rates are interpreted to be associated with the rising sea level, enhancing sea-cliff erosion at the coastal areas and bioproduction on the shelves. The increased landslide frequencies are likely related to high sediment input toward the shelf edges, leading to thicker accumulations of unstable sediments before failure between major earthquakes. Emplacement frequencies of volcanic eruptions from individual cores did not follow the same trend though the overall trend shows a higher value during the Holocene than in the past a few ten thousand years. Our study provides accessible research approaches to comprehensively assess the natural hazards on the volcanic islands.