AsianScientist (May 28, 2012) – Scientists have discovered the existence of slow, creeping earthquakes deep beneath New Zealand’s Alpine Fault, which is regarded as the country’s most hazardous fault line.
The apparent absence of earthquakes in the central section of the Alpine Fault, between Fox Glacier and Whataroa Valley 50 kilometers to the north, has puzzled scientists for decades.
But a study in the journal Geophysical Research Letters led by Victoria University Geophysics Professor Tim Stern has shown the area often experiences seismic tremors, or a series of slow, creeping earthquakes that each last up to 30 minutes.
According to Stern, seismic tremors are located at depths of 20 to 45 kilometers whereas regular earthquakes are mainly confined to the top 10 kilometers 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.
“Our research shows that between large earthquakes, the fault is still moving,” said Dr. Aaron Wech, a member of the team. “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.”
PhD candidate Carolin Boese led the field work which involved drilling holes up to 100 meters 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 kilometer-wide area under the Southern Alps, rather than on the Alpine Fault.
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 said.
The article can be found at: Wech AG et al. (2012) Tectonic tremor and deep slow slip on the Alpine Fault.
Source: Victoria University.
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