Calibrating the turbidite paleoseismometer on the Hikurangi margin using the Kaikoura earthquake

Abstract

Subduction zones generate the largest and most damaging earthquakes and tsunami on Earth, but we do not fully understand the dynamics of these faults nor the hazard they poise due to the paucity of earthquake records that span millennia. Arguably the most comprehensive subduction zone earthquake records have been produced using turbidite paleoseismology, an approach that reconstructs earthquake timing and magnitude from the geographic distribution of turbidite deposits formed by seismically triggered sediment flows.

Here we use observations of turbidites deposited by the 2016 Mw 7.8 Kaikōura earthquake in New Zealand to settle one of the most vigorous and unresolved debates in earthquake science: whether or not marine turbidites can be used to reconstruct the longest and most spatially complete subduction zone earthquake records. The well-instrumented earthquake provides an ideal test for turbidite paleoseismology because for the first time the fault source, ground motions and turbidite deposition in discrete canyons are well-resolved. The Kaikōura earthquake triggered flows in eleven consecutive canyons along 200 km swath of the Hikurangi Subduction Margin (HSM) where a Peak Ground Velocity (PGV) exceeded a triggering threshold. The spatial distribution of turbidite deposition and their sedimentary structures validate the concepts used to reconstruct earthquake records from turbidites.

We also show that the spatial distribution of turbidites may resolve the rupture directivity, which is an important breakthrough because it is essential for accurately quantifying ground motion hazard but is notoriously hard to define for prehistoric earthquakes.

Furthermore, textural grading preserved within turbidites correlates with the velocity-time histories of earthquake ground motions, revealing a novel means to resolve the seismograms of pre-historic earthquakes. These new insights confirm that turbidites on active margins are sensitive natural seismometers that archive detailed information on earthquake occurrence, seismograms and rupture dynamics.