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 rupt ure at this location occurred between AD 1480 and 1645, with associated loc al river aggradation and terrace formation. A second trench 6 km farther no rth at Ahaura gives the same radiocarbon age for the last event. An enlarged record of radiocarbon ages for aggradation terraces and landsli des in central and north Westland has a group of dates which are a reasonab le match to the date range from the trenching. While this is compatible wit h regional aggradation and landsliding associated with an Alpine Fault eart hquake 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 sync hronous period of regeneration. Recent information of forest age in Westlan d indicates two such periods in the last 600 yr, and the most recent of the se reflects the earthquake rupture recorded in the trenches. This implies t he earthquake occurred at the young end of the trench date range at c. AD 1 600-1650.