Alpine Fault earthquakes

What is the pattern of Alpine Fault earthquakes over time?

Alpine fault earthquake layers at Hokuri Creek

Alpine fault earthquake layers at Hokuri Creek

Research published by scientists from GNS Science in 2012 documented an 8000 year-long record of 24 Alpine Fault earthquakes based on data collected near Lake McKerrow, northeast of Milford Sound. Based on the dates of each earthquake measured using radiocarbon analysis, the researchers calculated an average time between successive large earthquakes of 330 years. This sequence of earthquakes is remarkably regular by the standards of other large faults that have been studied in this way, but does not mean that the Alpine Fault ruptures like clockwork every 330 years. In fact, the intervals between the 24 successive earthquakes measured at Lake McKerrow varied between 140 years and 510 years.

Can we predict the next big Alpine Fault earthquake?

It is not possible to reliably predict the time, location, or size of individual earthquakes. However, using measurements of past earthquakes and knowledge of the frequencies of earthquakes of different sizes, it is possible to calculate the likelihood of an earthquake of a particular size occurring in a specific interval of time.

Using the record of pre-historic Alpine Fault earthquakes from Lake McKerrow, scientists have calculated the probability of a large (M8) earthquake in the next 50 years at 30%.

How will the next Alpine Fault earthquake compare to the M7.1 Darfield earthquake of 4 September 2010?

For every one unit increase in magnitude (e.g. from M4 to M5) there is about a 30-fold increase in energy release. This means, for instance, that an Alpine Fault earthquake of M8.1 would release about 30 times more energy that the Darfield earthquake of M7.1. An Alpine Fault earthquake will likely rupture a larger fault length (several hundreds of kilometres rather than several tens of kilometres) over a longer period of time (100s of seconds rather than tens of seconds) and affect a much larger area than the Darfield earthquake. Moreover, it is likely that the aftershock sequence following an Alpine Fault earthquake will involve earthquakes of as much as M7.

It is important to remember that the size of an earthquake is not the only factor determining its severity, as has been starkly illustrated by the 2010–2012 Canterbury earthquake sequence and the much greater effect of the M6.3 Christchurch earthquake than the M7.1 Darfield earthquake five months earlier.

How long would the impacts of a big Alpine Fault earthquake last?

Alpine Fault Rupture cartoon

Research conducted at the University of Otago and GNS Science in the last few years has revealed that a large Alpine Fault earthquake will trigger a cascade of environmental effects that could persist for up to 50 years after the next earthquake. Violent shaking along the entire length of the earthquake rupture will trigger large landslides in steep topography and weaken hillslopes making them more susceptible to landsliding in subsequent storms. As West Coast rivers and streams transport material produced by this landsliding down stream, alluvial fans and floodplains will aggrade rapidly causing rivers to change their course abruptly and more frequently. The cascade of impacts has the potential to chronically affect towns, road, communications and power infrastructure for decades after the earthquake. Additionally, aftershocks triggered by the main earthquake could be expected to be as large as M7 and to continue for many years. Thus, the effects of the next big Alpine Fault earthquake will extend well beyond the immediate period of damage and disruption.