Slow earthquakes discovered deep beneath the Alpine Fault

Victoria University scientists have discovered that slow, creeping earthquakes take place deep beneath the Alpine Fault on the South Island’s West Coast, which is regarded as New Zealand’s most hazardous fault line.

Scientists have been puzzled for decades by an apparent absence of earthquakes in the central section of the Alpine Fault, between Fox Glacier and Whataroa Valley 50km to the north.

But a study led by Victoria University Geophysics Professor Tim Stern has shown the area often experiences seismic tremor, or a series of slow, creeping earthquakes that each last up to 30 minutes.

Professor Stern says that the seismic tremor is located at depths of 20 to 45 kilometres whereas regular earthquakes are mainly confined to the top 10 kilometres of the Earth’s crust.

It’s only the second time that this type of seismic activity has been recorded on a strike-slip fault, which are those with walls that move sideways rather than up or down. The other is the San Andreas Fault in California.

A member of the team, Dr Aaron Wech, who had previously researched seismic tremor in the United States, says these slow earthquakes don’t cause damage but knowing they are happening sheds new light on activity in the Alpine Fault.

"Our research shows that between large earthquakes, the fault is still moving. It’s not yet clear what this means for future earthquakes – it could be that constant tremor builds up stress and may trigger a major fault movement or, alternatively, the activity may decrease the likelihood of a major quake by acting as a release valve for stress.

"What’s important is that we find out more about these tremor events, such as where they happen and how often, so we can better predict the hazard the Alpine Fault poses."

Professor Stern says the research, which received a $700,000 Marsden grant, was carried out in one of New Zealand’s toughest environments.

PhD candidate Carolin Boese led the field work which involved drilling holes up to 100 metres deep and installing sensors in them which vibrate when an earthquake takes place.

An array of 11 stations, called the Southern Alps Microearthquake Borehole Array  - or SAMBA -  were installed in late 2009 and are still producing data. To date, SAMBA has recorded around 2,500 small earthquakes which are taking place in a 30 kilometre-wide area under the Southern Alps, rather than on the Alpine Fault.

The challenges faced by Carolin and her assistants in installing and maintaining the stations included rain, snow, extreme temperature variations and strong winds. They also had to cope with mountainous terrain – it took 12 hours to hike to and from one of the least remote sites – and constant interest in the equipment from native keas.

The research findings have been published today in a top-rated American journal, Geophysical Research Letters, which specialises in short papers on recent and important discoveries.

Professor Stern says the team hopes to expand SAMBA by adding new sites to record more small earthquakes and also to measure the seismic tremor over a longer period.

"A better understanding of these tremor events could provide vital clues in our understanding of both faults and earthquakes," he says