Progress in understanding the paleoseismicity of the central and northern Alpine Fault, Westland, New Zealand

Radiocarbon dates from trenching of the Alpine Fault trace at Crane Creek, between the Haupiri and Ahaura Rivers, demonstrate the last earthquake rupture at this location occurred between AD 1480 and 1645, with associated local river aggradation and terrace formation. A second trench 6 km farther north at Ahaura gives the same radiocarbon age for the last event. An enlarged record of radiocarbon ages for aggradation terraces and landslides in central and north Westland has a group of dates which are a reasonable match to the date range from the trenching. While this is compatible with regional aggradation and landsliding associated with an Alpine Fault earthquake during this period, this type of data cannot be used to demonstrate the dates were synchronous or coseismic. However, historical earthquakes have shown that, in steep forested terrain, an earthquake causes extensive forest mortality, with a corresponding synchronous period of regeneration. Recent information of forest age in Westland indicates two such periods in the last 600 yr, and the most recent of these reflects the earthquake rupture recorded in the trenches. This implies the earthquake occurred at the young end of the trench date range at c. AD 1600-1650.