Sedimentary processes and deposits around the Azores volcanic islands and implications for hazard assessment

Abstract

In this project, interdisciplinary methods are applied to study abundant submarine landslide valleys, sediment wave trains and volcaniclastic deposits in submarine parts of Azores volcanic islands to assess the sedimentary processes, characteristics of deposits and their hazard implications. Marine geophysical data help to characterise geomorphological features of submarine topography. Sedimentary structures, morphometric parameters, geochemical compositions of volcanic particles and bulk sediments, and chronostratigraphy are derived from marine sediments. Statistical methods are used to find relationships between data sets. Numerical calculations help to assess the sedimentary processes induced by external forces (e.g., ground motion, wave-induced stress and oceanic current). Four key findings are found. 1. More than 1200 submarine slope valleys have been found around the central Azores, mostly caused by landslides. Thirteen landslides would likely have generated tsunamis with heights of 1–7 m at source, hence potentially hazardous. Higher landslide volumes of submarine slopes in Terceira and São Jorge Islands compared to Faial and Pico Islands may result from more frequent large earthquakes beneath Faial and Pico. 2. Sediment wave trains are twice as abundant on northern island slopes compared with their southern slopes. This asymmetry is associated with greater wave energy arising from the northwest, leading to greater coastal erosion and wave-induced bed stress, encouraging sediment suspension and redeposition on the slopes and eventually initiating sediment gravity flows. (3) A wide range of information is integrated to discriminate tephra fallout and primary and secondary volcaniclastic turbidites in four gravity cores collected nearby the Azores islands. Sediment type discrimination suggests that two-thirds of volcaniclastic beds originate from eruptions and only one-third are from submarine landslides. (4) Modelling of turbidite volumes in the basins suggests only sediments from the largest landslides and eruptions have been deposited in the basins. Age-depth models built from 14C dates of foraminifera from hemipelagic intervals and tephra bed correlation suggest the emplacement ages and the frequencies of large submarine landslides and volcanic eruptions are both > 1 ky.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Neil Mitchell (Supervisor) & Bart Van Dongen (Supervisor)