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Earthquake-triggered deposits in the subduction trench of the north Ecuador/south Colombia margin and their implication for paleoseismology

Abstract:

The north Ecuador/south Colombia convergent margin is affected by recurrent subduction earthquakes with magnitudes > 7.5, like the 1906, 1942, 1958, 1979 and 2016 events. The subduction trench is characterized by the construction of the Esmeraldas Turbidite System (ETS) fed by the large Esmeraldas Canyon that deeply incises the continental slope and that connects directly onshore with the Esmeraldas River. The detailed description of cores collected in the left-hand (western) proximal levee of the ETS and in two lobes allowed discriminating two types of coarse-grained deposits: (1) “classical” flood-generated turbidites are normally graded beds with structureless, laminated and cross-laminated intervals and high organic-matter content, while (2) earthquake-induced deposits consist of amalgamated normally-graded laminated/cross-laminated intervals separated by erosive surfaces. These latter are interpreted to be deposited by quasi-synchronous flows generated during a single earthquake. Organic matter is absent in such beds while ferromagnesian minerals and pumices are abundant, suggesting remobilization of the slope deposits. When two amalgamated beds are superimposed, the interbedded clayey interval is not bioturbated, suggesting a short time period between the beds deposition, and thus the impact of a major earthquake shock and following earthquakes on the triggering of landslides. Along the ETS, core-to-core correlation based on 210Pb excess revealed that 20th Century sedimentation occurred mainly in the proximal levee. There, a temporal relationship was established between the 1906, 1942, and 1979 earthquakes, and three coarse-grained beds showing features of earthquake-induced turbidites, suggesting the Esmeraldas Canyon was the main source for sediments to be remobilized during these earthquakes. The fining and thinning observed between the 1906, 1942 and 1979 turbidites correlate with the increasing distance of the rupture zone of each earthquake with the Esmeraldas Canyon. Earthquakes with magnitudes lower than 7 also affected the margin during the 20th Century but were not recorded in the trench sedimentation, suggesting that the turbidite levee acts as a natural filter so that potentially the highest the levee the strongest the earthquake magnitude recorded. At least ten earthquakes with the highest magnitudes were recorded on the turbidite levee within the last 800 years with a recurrence time ranging from about 268 years to 42–82 years, or less for the 20th Century earthquakes. The comparison of the main features of the 1906 turbidite with older earthquake-triggered turbidites identified in a core collected in the trench suggests that one or two earthquakes similar to the 1906 event might have occurred ~ 600 years ago.