NHRP / Hazard themes / Geological Hazards / Seismology / Seismology Highlights 2015-16

Seismology Highlights 2015-16

Past Large Earthquakes on the Alpine Fault

A future large earthquake on the Alpine Fault is inevitable. Now modern techniques are enabling the development of long earthquake records over the previous millennia. This ability to obtain the ages of past earthquakes means we can better forecast the timing of the next one and encourage appropriate preparations to ‘Get Ready, Get Thru’.

◊ The Significance of Long Earthquake Records

NHRP Ursula & Kate 300

Ursula Cochran (left) and Kate Clark (right) examine a geological core with sedimentary evidence of Alpine Fault rupture in 1717 AD. The core was collected from the John O'Groats River valley in a wetland adjacent to the Alpine Fault. The slices that have been removed from the core were samples for radiocarbon dating to determine earthquake ages. Photo: Margaret Low, GNS Science

New Zealand’s longest earthquake record consists of 24 earthquakes occurring over 8,000 years on the South-Westland section of the Alpine Fault at Hokuri Creek. This record provides a reasonable number of earthquakes on which to base hazard calculations and reveals that this part of the fault ruptures relatively regularly, with an average time between earthquakes of about 330 years.

However, the last thousand years of sediment is missing from Hokuri Creek, so the long record relies on comparisons with past findings from Haast, 100 km away. This possible weakness led us in search of more clues from the nearby John O’Groats River site.

◊ Coring a Wetland

We went to John O’Groats River to look for data that could provide a more complete record of activity over the last thousand years and  improve our estimates of earthquake frequency for the South-Westland section of the Alpine Fault. We collected eight core samples to a depth of seven metres from the wetland immediately adjacent to the fault.  By examining these cores, we found evidence of past earthquakes in the form of peat-silt couplets. Peat layers represent the wetland under stable conditions and silt layers represent deposition triggered by earthquakes. Radiocarbon dating of these layers provided ages for seven earthquakes occurring within the last 2000 years.

◊ Improved Long Earthquake Record

We compared the John O’Groats wetland record with that from Hokuri Creek, and found that the John O’Groats site preserved evidence of earthquakes that were missing from the Hokuri Creek record.

Our new findings, based on the combined Hokuri Creek – John O’Groats records, suggest there have been 27 previous earthquakes, with a recurrence interval of about 300 years (slightly shorter than the previous estimate of 330 years). We have greater confidence in this revised record because we have added to the Hokuri Creek data with a site in closer proximity. The new data will be fed into updated seismic hazard estimates that take account of the wide variability in earthquake recurrence in this record.

◊ Size and Extent of Quakes

We know these past earthquakes were large (magnitude 8 or greater) because they ruptured the ground surface, and we have found examples of large movements across the fault from a single event; 7.5 metres horizontally and 1 metre vertically.

However, it is important to note that these results are just from the South-Westland section of the Alpine Fault. While many of the earthquakes recorded at these two sites are likely to have ruptured the central and possibly North-Westland sections of the fault as well, we don’t know the length of rupture from this research. Long earthquake records from lakes along the length of the Alpine Fault are proving very useful for determining which sections of the fault ruptured in which earthquakes.

◊ Improve Our Understanding of the Hazards

The last major earthquake on the Alpine Fault occurred in 1717.  With an average of about 300 years between earthquakes, our findings suggest that we are due for another Alpine Fault earthquake in the near future. Simple actions, such as storing food and water, and securing large and heavy items, go a long way to prepare for such events, but also consider your neighbours, businesses, and wider community in your action plan. 

To learn more about what you can do to prepare for an earthquake, please visit ‘Get Ready, Get Thru.’  http://getthru.govt.nz/

By Ursula Cochran & Kate Clark, GNS Science

Additional research on the Alpine Fault

◊ Members of the MBIE-funded 'Economics of Resilient Infrastructure' programme have worked with roading and rail providers to examine expected disruption as a result of an Alpine Fault earthquake.  Their report is located here:

  • Robinson, T. R; Buxton, R.; Wilson, T. M.; Cousins, W. J.; Christophersen, A. M. 2015. Multiple infrastructure failures and restoration estimates from an Alpine Fault earthquake: Capturing modelling information for MERIT, ERI Research Report 2015/04. 80 p.

◊ In research funded by Contest 2015, Rob Langridge is doing field work that involves trench excavation at Springs Junction in the South Island to examine features of the Alpine Fault.

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Last updated 10 Aug 2016