In the first months of 2007, the Aysén region in southern Chile was affected by a crustal seismic swarm. Its largest earthquake (Mw 6.2) occurred in April and had its epicenter in Aysén Fjord. Seismic intensities became so high that hundreds of onshore mass movements were triggered, several of which entered into the fjord, resulting in mass transport deposits (MTDs) preserved at the fjord bottom. Here we present a Holocene record of paleo‐earthquakes in the previously unstudied Patagonian fjordland based on MTD stratigraphy. High‐resolution seismic data retrieved using two different seismic systems (sparker and TOPAS) reveal multiple older MTDs on different stratigraphic levels. Correlation of the seismic stratigraphy with sedimentological data obtained from a long Calypso core (MD07‐3117) allows conclusion on the seismic origin of these deposits. Additionally, radiocarbon dating permits constructing an age model, validated by tephrochronology, providing an age for the different MTD levels. We thus present a highly detailed paleoseismological history of the Aysén region, including at least six major Holocene earthquakes, one of which is likely related to a known megathrust earthquake. Other earthquakes are related to activity of the Liquiñe‐Ofqui Fault Zone (LOFZ), forming the main source of seismic hazard in the area. We can infer a general average recurrence time for LOFZ earthquakes of ~2,100 years in the vicinity of Aysén Fjord with clustered events during the early and late Holocene. Finally, we argue that cascading events (causal link between volcanic and seismic events) may be a frequent phenomenon along the LOFZ.

Contemporaneous mass‐transport deposits (MTDs) recorded in lake and fjord sediments provide evidence of past seismic shaking. However, because they are usually not connected to a fault rupture, assessment of the earthquake source remains difficult. Based on observed coseismic mass wasting and associated seismic shaking, previous studies assigned minimum intensities required to trigger them. Attempts to infer their earthquake source relied on methods developed to estimate the location and magnitude of historical earthquakes using intensity prediction equations, but considered these thresholds as actual intensities. Here we develop a probabilistic method to infer the most likely earthquake sources from the spatial distribution (or absence) of MTDs. This approach simultaneously allows the triggering intensity to exceed the assumed threshold and takes into account intensity prediction equation uncertainties, two shortcomings of existing methods. Additionally, we consider known active faults rather than a grid of possible epicenters. We apply this method to Aysén Fjord (southern Chile), which is intersected by the Liquiñe‐Ofqui Fault Zone. In 2007, an MW = 6.2 earthquake hit the fjord with intensities of VIII+, causing major landslides entering the fjord. Seismic‐reflection profiles show that its sedimentary fill contains nine prehistoric MTD levels. Following a sensitivity analysis, application of the method to the MTD record allows identifying the most likely fault sections and magnitude range for most events, confirming that they are mainly attributed to crustal earthquakes on the Liquiñe‐Ofqui Fault Zone. We conclude that the method has good potential to constrain the size and location of paleoearthquakes for which only shaking evidence is available.

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Grainsize analysis and end-member modeling of a long sediment core from Aysén 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 (V½-VI½), 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. Aysén 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 Aysén 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~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.

Earthquake doublets have been described in fault systems around the world but have not yet been confidently resolved in paleoseismic records. Our current knowledge is limited to historical occurrences, preventing researchers from uncovering potential patterns or recognizing common fault behavior. Identification of prehistoric doublets is thus of crucial importance for adequate seismic hazard assessment and risk mitigation. We developed a new methodology to reveal the sedimentary imprint of earthquake doublets in lacustrine paleoseismic records based on flow direction analysis in multipulsed turbidites, because the delayed arrival of turbidity currents originating from the same source location demonstrates the occurrence of individual triggering mechanisms. As grains tend to align in the presence of a flow, we analyzed flow directions by determining the dominant orientation of elongated grains using a combination of grain size, paleomagnetism, and high-resolution X-ray computed tomography. This methodology was applied to a turbidite deposited by the 2007 CE earthquakes in West Sumatra (Mw 6.4 and 6.3, 2 h apart), and it provides the first unmistakable sedimentary evidence for an earthquake doublet. We argue that this methodology has great potential to be applied to multipulsed turbidites in various subaquatic paleoseismic records and can reveal the occurrence of unknown earthquake sequences.