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

Historical and paleoseismic records in south-central Chile indicate that giant earthquakes on the subduction megathrust - such as in AD1960 (M-w 9.5) - reoccur on average every similar to 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 M-w 8 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 +/- 69 yr). Moreover, distribution fitting and the coefficient of variation (CoV) of inter-event times indicate that the stronger earthquakes recur in a more periodic way (Coy: 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 similar to 300 km and can be assigned to M-w >= 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 M-w 7.7-8.5 earthquake during the next 110 years whereas the probability for a M-w >= 8.6 (AD1960-like) earthquake remains low in this period. (C) 2017 Elsevier B.V. All rights reserved.