Seabed stability inferred from the 2019–2020 earthquake swarm under a volcanic cone field and slopes of Condor Seamount, Azores

Abstract

Knowledge of the strength of submarine volcaniclastic deposits is important for assessing the stability of slopes of such materials and their geohazards but is difficult to measure. An opportunity for an alternative evaluation has been presented by an earthquake swarm under a volcanic seamount in the Azores. Attenuation relationships applied to earthquake data suggest that a cone field and flanks of the seamount experienced horizontal accelerations of >0.3 g during the swarm. However, multibeam sonar data collected before and after the swarm suggest that no slope failures occurred. Furthermore, in backscatter data collected after the swarm, low intensities below slopes suggest that muddy aprons were undisturbed by landslide debris. The swarm overlies cones with slopes near typical repose angles of non-cohesive particles. During earthquake shaking, the direction of maximum acceleration deviates from that due to gravity alone. We show that cone slopes effectively experienced much steeper gradients than their repose angles during the swarm. As they survived the shaking without failing, they were effectively stronger than non-cohesive sediment. We use a pseudo-static analysis to investigate the implied sediment strength, finding a ratio of undrained shear strength to vertical stress of >0.4–0.5. This implies shear strength of >24–30 kPa at 10 m depth below seabed. We speculate that carbonate cements and/or compaction may be responsible. If shallow areas are more widely strengthened, slope failure may then be less likely during moderate (M_L ∼ 4.0 or less) seismic shaking and hence be less hazardous than if the slopes comprised wholly non-cohesive materials.