Widespread deformation of basin-plain sediments in Aysén fjord (Chile) due to impact by earthquake-triggered, onshore-generated mass movements

The 2007 Mw 6.2 earthquake in Aysén fjord caused widespread basin-plain deformation and has had prehistorical predecessors. Both superficial and buried deformed basin-plain deposits are mapped using multibeam bathymetry and seismic-reflection (sparker) profiling. The seismic signature of the sediment was ground-truthed with short cores on key locations. Deformed basin-plain deposits induced by the 2007 earthquake can be divided in frontally emergent and confined deposits, with both a deep and shallow basal shear surface. All deformed basin-plain deposits with a deep basal shear surface are induced by the weight and impact of a slope-adjacent mass-flow wedge. The frontally emergent – most mobile – basin-plain deformation is triggered by mass flows originating from onshore mass movements (i.e. debris flows, rock slides and avalanches) propagating into the fjord. This basin-plain deformation results in vertical seafloor offsets of up to 20 m. Therefore it might be even more important for far-field tsunami propagation than the impact of the onshore mass movements on the sea surface. In the depressions created by the basin-plain deformation, megaturbidites occur, while more distally, sandy density-flow deposits cover the seafloor. The data also indicates that these density flows propagate slower than the basin-plain deformation. Based on correlations with the two main eruptions of the Hudson Volcano, we hypothesize that during the Holocene three to four similar events have struck the fjord. The small variability of the structural characteristics of the Liquiñe-Ofqui Fault Zone in the northern Patagonian fjordland and historical seismic swarms in this area make us conclude that similar hazards should be taken into account for most of the fjords in this region.