Slowing moving quakes last 30 minutes

Slow, creeping earthquakes lasting up to 30 minutes are taking place deep beneath the Alpine Fault on the South Island's West Coast, 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 found the area often experienced seismic tremor, or a series of slow, creeping earthquakes that each last up to 30 minutes.

It was not clear what the discovery meant for future earthquakes. He said it was not possible to accurately measure the small quakes for magnitude.

Dr Aaron Wech, a member of the Victoria University team, said it could be that constant tremors build 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.

It was important to find out more about the tremor events, such as where they happened and how often, so the hazard posed by the Alpine Fault could be better predicted.

Wech, who previously researched seismic tremor in the United States, said the slow earthquakes did not cause damage, but knowing they were happening shed new light on activity in the Alpine Fault.

Stern said it was only the second time this type of seismic activity has been recorded on a strike-slip fault - those with walls that move sideways rather than up or down. The other was the San Andreas Fault in California.

Findings of the research, which received a $700,000 Marsden grant, were published today in American journal Geophysical Research Letters.

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 - was installed in late 2009 and is still producing data.

To date, SAMBA has recorded around 2500 small earthquakes which are taking place in a 30km-wide area under the Southern Alps, rather than on the Alpine Fault.

Conditions facing Boese and her assistants in installing and maintaining the stations included rain, snow, extreme temperature variations and strong winds.

Mountainous terrain was another challenge - with a 12-hour hike to and from one of the most remote sites, while kea showed constant interest in the equipment.

Stern said they hoped to expand SAMBA by adding new sites to record more small earthquakes and also to measure the seismic tremor over a longer period.