Far-off quakes affect Alpine Fault

DATA COLLECTION: Victoria University PhD student Calum Chamberlain at one of the seismic recording sites in the Southern Alps where small earthquakes on the Alpine Fault have been measured.
DATA COLLECTION: Victoria University PhD student Calum Chamberlain at one of the seismic recording sites in the Southern Alps where small earthquakes on the Alpine Fault have been measured.

Earthquakes several hundred kilometres away trigger activity deep beneath New Zealand's Alpine Fault, Victoria University researchers have found.

PhD student Calum Chamberlain collated three years of data from a range of recording stations across the Southern Alps.

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He found very small earthquakes, smaller than magnitude one, occurring 15 to 40km near the Alpine Fault.

The earthquakes' distinctive, low-frequency characteristics enabled them to be detected despite their small magnitude.

Across the three years, Chamberlain found these low-frequency earthquakes were occurring almost continuously near the Alpine Fault, southwest of Aoraki Mt Cook.

Associate Professor John Townend said the Alpine Fault was known to "produce large earthquakes every few hundred years, most recently in 1717AD".

"But this research is focused on much, much smaller earthquakes happening almost every day. We are investigating how the fault is being loaded."

Townend said the earthquakes provided "one of the few signs we have of present-day faulting activity deep inside the plate boundary".

Chamberlain's data came from the Southern Alps Microearthquake Borehole Array (Samba), an array of 11 recording stations that were installed in 2009.
The seismic recorders also caught evidence of the effects of nearby earthquakes.

The Dusky Sound earthquake in 2009 and the 2010 and 2011 sequence of earthquakes in Canterbury caused a sharp spike in the production of low-frequency earthquakes.

A magnitude 6 earthquake near Christchurch in June 2011 triggered many of these low-frequency quakes over the course of several days.

"The Alpine Fault responds to earthquakes hundreds of kilometres away," Chamberlain said, which showed it was sensitive to small stresses associated with seismic waves.

The next stage of his research will focus on the conditions beneath the fault that make it sensitive to weak triggers.

Townend said understanding the low-frequency earthquakes would assist in better understanding the Alpine Fault and the hazard it poses.

The Samba recorders have already contributed other knowledge about the Alpine Fault. In 2012 research was published documenting slow-moving quakes lasting up to 30 minutes each in an area between Fox Glacier and Whataroa.

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