Floodplain and regional scale variation in earthquake effects on forests, Westland, New Zealand

Using existing and new tree age data, we assess spatial variation in earthquake effects in Westland at regional and floodplain scales. In Westland there have been four major episodes of forest cohort establishment, three of which appear to relate to Alpine Fault earthquakes in ad c. 1460, 1610–1620, and 1717, and the fourth of which coincides with a Fiordland earthquake in ad 1826. The ad 1826 event had its greatest effect in south Westland. In contrast, the impacts of the ad 1610–1620 and 1717 earthquakes were more extensive, with forest cohort initiation following both events occurring throughout Westland. However, these two events differed in their effects at the floodplain scale, with the ad 1610–1620 event triggering a larger aggradation event on floodplains than the ad 1717 event. Variation in earthquake effects at a regional scale most likely reflects variation in earthquake magnitude and the location of the epicentre. At a floodplain scale, variation in effect could reflect differences in both earthquake magnitude and shaking and the amount of sediment stored in upland catchments as a consequence of the time elapsed since the last event. © 2003 Journal of the Royal Society of New Zealand.