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