SIX PAST EARTHQUAKES ON THE SOUTHERN ONSHORE ALPINE FAULT, NEW ZEALAND, REVEALED BY A RECORD OF LANDSCAPE CHANGE EVENTS IN A FAULT-ADJACENT WETLAND

We use evidence of landscape change events preserved in fault-adjacent wetland sediments to derive a record of past surface-rupturing earthquakes on the Alpine Fault. The Alpine Fault is a high slip rate (23-25 mm/yr) dextral strike-slip fault at the Pacific-Australia plate boundary along the west coast of the South Island, New Zealand. It last ruptured in a large-great earthquake nearly 300 years ago and there is a 30% probability of a similar rupture occurring in the next 50 years. At John O’Groats River, the upstream-facing, 15 m high scarp of the Alpine Fault impedes river drainage and a wetland exists on the south-eastern side of the fault.

Sediment cores collected from the John O’Groats wetland show decimetre-scale alternations of peat and silt. In our wetland depositional model peat accumulates during the stable, interseismic period. At the time of an earthquake, the river drainage is temporarily blocked by a reverse component of fault slip. A large pulse of sediment is released from the mountainous river catchment and the raised scarp results in deposition of silt and sand across the John O’Groats wetland. Under this model, the contact of silt over peat results from an earthquake on the Alpine Fault. Our stratigraphic and paleoenvironmental analysis, which includes diatom studies, classification of sediment facies and a ranking of the amount of change across a contact, indicates there is evidence for six earthquakes that ruptured the ground surface at this site in the last 2000 years. Alternative explanations including storm, landslide and flooding events do not adequately explain the ensemble of data.

Over 50 radiocarbon dates are used to constrain the age of the peat-silt contacts that represent earthquakes. We will integrate the earthquake chronology from the John O’Groats wetland with the long earthquake record (24 events over 8000 years) obtained from Hokuri Creek, 20 km northeast along fault strike. At Hokuri Creek evidence for the last 2-3 earthquakes is missing. The John O’Groats wetland site appears to fill this gap and record the most recent six surface-rupturing earthquakes on the southern onshore Alpine Fault.