Paleotsunami record of the past 4300 years in the complex coastal lake system of Lake Cucao, Chiloé Island, south central Chile

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

“…Melnick et al (2017) explain such events by an elastic response of both the oceanic and continental plate to coseismic and post seismic fault slip that results in higher shear stresses and increased locking on adjacent fault sections, in this way increasing the strain accumulation rates and bringing these sections closer to failure. Proposed rupture modes for the last~2,000 years inferred from paleoseismic evidence in Lake Calafquén (LC; Moernaut et al, 2018), Lake Riñihue (LR; Moernaut et al, 2018), Maullín (M;Cisternas et al, 2005), Cocotué (C; Cisternas, Garrett, et al, 2017), Lake Huelde (LH; Kempf et al, 2017), Lake Cucao (LCu; Kempf et al, 2020), and Aysén Fjord (AF; this study). All observations are presented aligned with the inferred event age (either based on the historical record (bold), the mean age in Aysén Fjord (normal), or an approximation based on other records (italic) for events that have already been described or cannot be identified in Aysén Fjord) and rupture patterns are color coded according to Figure 10.…”

“…A new rupture mode for the Valdivia segment is invoked to explain the paleoseismic evidence observed at AD~590. Paleoseismic shaking evidence is recorded along the entire segment, but no conclusive tsunami evidence has been identified up to now: no tsunami deposit has been found in Maullín (Cisternas et al, 2005) or Lake Huelde (Kempf et al, 2017), and the only potential age-matching tsunami deposit in Lake Cucao (cE) has only a minor sedimentary imprint and also low confidence for attributing a tsunami origin (Kempf et al, 2020). Full ruptures unaccompanied by segment-wide tsunamis are unlikely, unless this was a full-segment rupture with mostly a deeper slip distribution that did not rupture the updip parts of the seismogenic zone (cf.…”

“…Apart from the 1960 earthquake, historical records document three significant megathrust earthquakes in south-central Chile with magnitudes exceeding 7.5, in AD 1575, 1737, and 1837 ( Figure 1a) (Lomnitz, 2004). Our current knowledge of prehistorical events is based on a compilation of sedimentary records covering the last 5,500 years with a focus on the last millennium, including coastal evidence for coseismic elevation changes and tsunamis (e.g., Cisternas et al, 2005;Ely et al, 2014;Garrett et al, 2015;Kempf et al, 2017Kempf et al, , 2020 and subaqueous turbidites in lakes and fjords (Moernaut et al, 2014(Moernaut et al, , 2018St-Onge et al, 2012). Although there are local differences between these records in terms of the number of recorded earthquakes and their recurrence intervals, they all point to a variable rupture mode along the Valdivia segment (Cisternas, Garrett, et al, 2017;Moernaut et al, 2014) including local, single-asperity ruptures with M w generally around 7-8 as well as multiasperity ruptures affecting large portions (>300 km) of the segment and reaching an M w of 8.6 or higher (Moernaut et al, 2018).…”

“…Age correlation of turbidites in Aysén Fjord with different types of megathrust earthquake (EQ) evidence (see legend) observed in several localities along the Valdivia segment. As there are several coastal records available, only a representative selection (i.e., containing all known events) is presented in this overview: Maullín (Cisternas et al, 2005), Cocotué (Cisternas, Garrett, et al, 2017), Lake Huelde (Kempf et al, 2017), and Lake Cucao (Kempf et al, 2020). Additionally, the lacustrine records of Calafquén and Riñihue (Moernaut et al, 2014(Moernaut et al, , 2018 and the only other fjord record available (Reloncaví; St-Onge et al, 2012) are shown.…”

“…The MD07-3117 core in Aysén Fjord contains 24 turbidites that are interpreted as the potential result of megathrust earthquakes during the last 9,000 years, of which 21 occurred in the last 5,500 years. We compare their age distributions to the ages of megathrust earthquakes identified by correlation between different paleoseismic sites along the Valdivia segment (Cisternas, Garrett, et al, 2017;Cisternas et al, 2005;Kempf et al, 2020Kempf et al, , 2017Moernaut et al, 2014Moernaut et al, , 2018St-Onge et al, 2012) (Figure 10). When the age range of a turbidite in Aysén Fjord overlaps with the age range of a known event-taking into account geological and stratigraphic plausibility-the previously established correlation (Moernaut et al, 2018) can thus be extended to the south.…”