Posted 20 April 2021

Likelihood of an Alpine Fault earthquake increases

New research on the Alpine Fault has revealed more information about how the fault has behaved in the past, and improved our understanding of the likelihood of a major earthquake in the next 50 years.

A study led by Te Herenga Waka – Victoria University of Wellington senior lecturer Dr Jamie Howarth indicates that there is a 75% probability of an Alpine Fault earthquake occurring in the next 50 years. Until now, it had been thought to be about 30%, based on evidence found in northern Fiordland.

These findings do not change the likely impacts of an Alpine Fault earthquake for South Island communities. However, they do reinforce how important it is for all of us to improve our understanding and awareness of the Alpine Fault so we can support each other to be better prepared for future major earthquakes.

Sediment core samples are collected at Lake Paringa (Source: Howarth, 2021)

The Alpine Fault

The Alpine Fault is a long plate boundary fault between the Pacific and Australian tectonic plates that moves horizontally during major earthquakes (strike-slip, including some vertical motion). Total movement on the Alpine Fault is about 480km. This movement has been achieved by the fault producing major earthquakes for millions of years.

In about 1717 the fault ruptured in what scientists estimate to be a magnitude 8+ earthquake. Earthquakes of this magnitude typically rupture several hundred kilometres of the fault, creating the potential for widespread damage and destruction in populated areas.

Although the Alpine Fault is a relatively straight fault when we look down from above, under the ground there are a few major bends and changes in the fault’s angle (dip). These changes along the fault are the basis for defining four sections of the fault, including the Central and South Westland sections.

“From space, the fault appears like a straight line on the western side of the Alps. But in reality it has massive variations in direction and slip rate and is split up into different segments” Dr Jamie Howarth

Based on international research, these changes along the fault are places that can act as barriers to a full fault rupture, and therefore reduce the rupture length and the magnitude of the earthquake. This means that not all earthquakes on the Alpine Fault will be the same size as the 1717 event, and this has important implications for people living in the South Island.

Where did this new earthquake evidence come from?

Most of the new information on earthquake activity has been discovered in sediment from lakes near the Alpine Fault. Four lakes on the Central Section of the fault were investigated to find evidence of earthquakes, including Lake Kaniere, Lake Mapourika, Lake Paringa and Lake Ellery.

To collect this evidence the scientists collect samples of the sediment called ‘cores’, which enable them to see the layers of history preserved overtime on the lake bed.

Earthquakes produce distinctive types of sediment deposited in lakes and careful analysis of the core samples, including radiocarbon dating, provides a record of previous large earthquakes on the Alpine Fault.

In this study these four lakes have revealed evidence for 15 earthquakes over the past ~4,000 years of time, and can be matched across all four lake sequences over a 2,500 year period.

Research lead, Dr Jamie Howarth with a sediment core sample (Source: Howarth, 2021)

What does the evidence tell us?

The findings from the lake sediments have been used to calculate the average time between Alpine Fault earthquakes as 249 years with an uncertainty +/- 58 years. The earthquakes are noted to occur quite regularly through time, making the Alpine Fault one of the most periodic faults on Earth.

Using all the evidence of Alpine Fault earthquakes – combining these new findings and those from previous research – the scientists have used computer simulations of thousands of synthetic Alpine Fault earthquakes to test how the section boundaries can act as barriers to earthquake propagation between the Central and South Westland sections of the fault.

These simulations suggest the section boundaries along the Alpine Fault can act as barriers to the propagation of some Alpine Fault earthquakes. Where earthquakes stop at these section boundaries, they are generally smaller (in the magnitude 7-8 range), compared to those that continue past the barrier and become magnitude 8+ earthquakes.

Research lead, Dr Jamie Howarth with a sediment core sample (Source: Howarth, 2021)

Key findings of this new research

The new earthquake evidence and computer simulations indicates that there is a 75% probability of a rupture on the central section of the Alpine Fault over the next 50 years. This means the next event is likely to happen within current planning horizons and the lives of many New Zealanders alive today.

The findings also suggest that the next Alpine Fault earthquake has an 82% likelihood of crossing the section boundaries. This means there is a 4 out of 5 probability that the next Alpine Fault earthquake will be a magnitude 8+ event.

“This finding doesn’t change the fact the Alpine Fault has always been hazardous. But now we can say this is an event that is more than likely to happen, so we should probably be moving from just thinking about response, on the off chance it happens, to thinking about how we actually build resilient communities and infrastructure.” Dr Jamie Howarth

How to be better prepared

These new findings and updated percentages can be overwhelming to think about, and that is 100% OK. However, there is lots we can do help each other be better prepared, talking with family and friends is a great place to start.

Here in New Zealand we are incredible at coming together after an emergency event – pandemics, earthquakes, floods and fires to name a few have shown us how great we are at supporting each other when things get a bit uncomfortable.

It’s even easier to have those conversations now, we don’t have to wait until it happens to come together – talking about it, making a plan, sharing resources – these are all things we can do now and together, so that we are better prepared for any event that might come our way.

1. Make a plan with your family/flatmates/friends. Think about the things you need every day such as food, water, power, communication and work out what you would do if you didn’t have them.

2. Know your neighbours. Your own neighbourhood is your first and best source of support. If you haven’t already connected with your neighbours, be sure to swap details in case of an emergency.

3. Work out what supplies you need. You probably have most of the things you need already need at home, but it can be useful to put them all in the same place. Don’t forget that you’ll need water – make sure you have at least nine litres of water per person for several days.

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