Probability and consequences of the next Alpine Fault earthquake

Technical Abstract

We have carried out a paleoseismic investigation of the central and northern sections of the Alpine Fault. Trenching of the fault at five locations between Hokitika and the Ahaura River indicates the last rupture south of the Haupiri River occurred after 1660 AD and probably between 1700 and 1750 AD. An earlier event in the central section at around 1600 AD is the most recent event at the more northern locations.  An updated record of landslide and aggradation terrace ages is consistent with two earthquakes in the last 500 years but does not significantly refine the date estimates.

Analysis of forest age in Westland from the Paringa River to Springs Junction reveals two periods of synchronous regional forest disturbance and re-establishment at 1625 ± 15 AD and 1715 ± 15 AD which we infer was the result of the two most recent earthquakes. Analysis of tree rings in trees which have survived these earthquakes allow estimates of event timing to be narrowed to 1620 ± 10 AD for the penultimate event. The most recent event occurred in 1717 AD and appears to have been asynchronous rupture from Milford Sound to the Haupiri River, a distance of approximately 375 kilometres.

The paleoseismic history can be used in conjunction with typical recurrence data from other plate boundary faults to predict the probability of the next earthquake using the method ofNishenko & Buland (1987). This indicates a probability over the next fifty years of 65 ± 15 % increasing to 85 ± 10 % over the next 100 years.

Based on the previous events the next rupture is likely to produce an earthquake of Magnitude 8 ± 0.25 with an epicentral area displaced slightly east of the fault trace. Very strong shaking will occur close to the epicentral area and for most locations the next Alpine Fault earthquake will be larger than any previous earthquake in the last 100 years. Landslides and liquefaction are likely to cause the greatest immediate impact but longer term the increased sediment loads will cause strong aggradation in major rivers with increased channel avulsion and flooding.