Seismo-Turbidites in Aysen Fjord (Southern Chile) Reveal a Complex Pattern of Rupture Modes Along the 1960 Megathrust Earthquake Segment

Grainsize analysis and end-member modeling of a long sediment core from Aysen Fjord (southern Chile) allows to identify over 25 seismo-turbidites in the last 9,000 years. Considering the shaking intensities required to trigger these turbidites (V1/2-VI1/2), the majority can be related to megathrust earthquakes. Multiple studies in south-central Chile have aimed at finding traces of giant, tsunamigenic megathrust earthquakes leading to the current 5,500-year-long paleoseismological record of the Valdivia segment. However, none of these cover the southern third of the segment. Aysen Fjord allows to fill this data gap and presents the first, crucial paleoseismic data to demonstrate that the 1960 event was not unique for the Valdivia segment, yielding a recurrence rate of 321 116 years in the last two millennia. Moreover, the oldest identified events in Aysen Fjord date back to 9,000 cal years BP and, thus, also extend the regional paleoseismological record in time. We infer a large temporal variability in rupture modes, with successions of full-segment ruptures alternating with partial and cascading ruptures. The latter seems to significantly postpone the occurrence of another full rupture when consecutively occurring in different parts of the segment. Additionally, one outstanding period of seismic quiescence-during which no megathrust earthquake evidence has been found at any paleoseismic site-occurred after a full rupture in AD similar to 745 that presents an unusual uplift/subsidence pattern. Such variability makes it highly speculative to anticipate the rupture mode of the next megathrust earthquake along the Valdivia segment.