Investigating the source of the c. AD1620 West Coast earthquake: implications for seismic hazards

A c. AD1620 West Coast earthquake has been attributed to the northern and central segments of the Alpine Fault but does not match well with paleoseismic indicators or the currently understood recurrence interval of Alpine Fault ruptures. We model the landsliding and resulting river aggradation from a M-w 7.6 earthquake on a modelled reverse fault located between the central Alpine Fault and the main divide of the Southern Alps. The Waiho-Callery and Whataroa catchments are significantly more impacted by coseismic landsliding from this Southern Alps earthquake than from an Alpine Fault earthquake. The modelling suggests that the c. AD 1620 earthquake was not an Alpine Fault event, and that large earthquakes on subsidiary faults (or fault systems) can have larger localised impacts on river behaviour west of the Southern Alps than an Alpine Fault earthquake.