Southernmost and Oldest Record of Megathrust Earthquakes Along the Valdivia Segment of the Chilean Subduction Zone
Abstract
Aysén Fjord is located in southern Chile, about 200 kilometers east of the Peru-Chile subduction trench, one of the tectonically most active regions in the world. More specifically, it is located along the southern section of this subduction zone (Valdivia segment) that hosted the largest ever instrumentally recorded earthquake. The latter is known as the 1960 Great Chilean earthquake and reached a magnitude of 9.5. Seismic intensities reached up to VIII in the northern part of the segment, but were reported to be limited at the location of Aysén Fjord (i.e. IV). Higher levels of ground shaking are often achieved here by the occurrence of crustal earthquakes along the Liquiñe-Ofqui Fault Zone (LOFZ), of which several fault branches run across the fjord. Previous studies have shown that these earthquakes are capable of triggering large-scale mass transport deposits (MTDs) and turbidites that are preserved in the fjord's sedimentary infill. Several of these MTDs were identified on high-resolution seismic profiles and are associated with turbidites that can be found in a radiocarbon-dated twenty-one meters long sediment core.
Apart from the MTD-related turbidites, the studied sediment core contains additional turbidites that are usually thinner and strongly bioturbated. Therefore, they are often difficult to identify and medical CT scans in combination with end-member modelling on high-resolution grain-size analysis was required to capture all of them. Some of the turbidites are fairly recent and the available radiocarbon-based age model is thus not sufficiently detailed. Therefore, radionuclide dating is performed in combination with geochemical analysis of a tephra layer in the top of the core to confirm the exact age of the most recent deposits. Comparison of the age distributions for all turbidites in the core with previously established records of megathrust earthquakes in southern Chile shows that they are in very good agreement, especially in the last 2200 years BP. Therefore, megathrust earthquakes can be considered as a triggering mechanism for these deposits. Additional turbidites can be found throughout the complete length of the core, with events as old as 8000 BP. This allows us to establish the southernmost record of megathrust earthquakes in Chile that, for the first time, dates back up to the Mid-Holocene.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMOS53A..08W
- Keywords:
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- 3045 Seafloor morphology;
- geology;
- and geophysics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3070 Submarine landslides;
- MARINE GEOLOGY AND GEOPHYSICS;
- 4313 Extreme events;
- NATURAL HAZARDS;
- 7221 Paleoseismology;
- SEISMOLOGY