Larger earthquakes recur more periodically: New insights in the megathrust earthquake cycle from lacustrine turbidite records in south-central Chile

• Published in: Earth and planetary science letters Vol. 481; pp. 9 - 19
• Main Authors: Moernaut, J., Van Daele, M., Fontijn, K., Heirman, K., Kempf, P., Pino, M., Valdebenito, G., Urrutia, R., Strasser, M., De Batist, M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-01-2018
• Subjects: limnogeology; megathrust earthquake; seismic cycle; south-central Chile; turbidite paleoseismology; south-central Chile; seismic cycle; turbidite paleoseismology; limnogeology; megathrust earthquake
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2017.10.016

Historical and paleoseismic records in south-central Chile indicate that giant earthquakes on the subduction megathrust – such as in AD1960 (Mw9.5) – reoccur on average every ∼300 yr. Based on geodetic calculations of the interseismic moment accumulation since AD1960, it was postulated that the area already has the potential for a Mw8 earthquake. However, to estimate the probability of such a great earthquake to take place in the short term, one needs to frame this hypothesis within the long-term recurrence pattern of megathrust earthquakes in south-central Chile. Here we present two long lacustrine records, comprising up to 35 earthquake-triggered turbidites over the last 4800 yr. Calibration of turbidite extent with historical earthquake intensity reveals a different macroseismic intensity threshold (≥VII1/2 vs. ≥VI1/2) for the generation of turbidites at the coring sites. The strongest earthquakes (≥VII1/2) have longer recurrence intervals (292 ±93 yrs) than earthquakes with intensity of ≥VI1/2 (139±69yr). Moreover, distribution fitting and the coefficient of variation (CoV) of inter-event times indicate that the stronger earthquakes recur in a more periodic way (CoV: 0.32 vs. 0.5). Regional correlation of our multi-threshold shaking records with coastal paleoseismic data of complementary nature (tsunami, coseismic subsidence) suggests that the intensity ≥VII1/2 events repeatedly ruptured the same part of the megathrust over a distance of at least ∼300 km and can be assigned to Mw≥8.6. We hypothesize that a zone of high plate locking – identified by geodetic studies and large slip in AD 1960 – acts as a dominant regional asperity, on which elastic strain builds up over several centuries and mostly gets released in quasi-periodic great and giant earthquakes. Our paleo-records indicate that Poissonian recurrence models are inadequate to describe large megathrust earthquake recurrence in south-central Chile. Moreover, they show an enhanced probability for a Mw 7.7–8.5 earthquake during the next 110 years whereas the probability for a Mw≥8.6 (AD1960-like) earthquake remains low in this period.
•Multi-threshold turbidite paleoseismic records in south-central Chilean lakes.•Probability estimates for Mw≥7.7 and Mw≥8.6 megathrust earthquakes.•Quasi-periodicity suggests a dominant role for a large megathrust asperity.