Distant tremors 'loading' major quake on New Zealand fault line

One of New Zealand's major fault lines is being "loaded" towards a major earthquake by tremors happening hundreds of kilometers away, researchers said Monday.
Research on the Alpine Fault, which runs along the western side of the Southern Alps, in the South Island, showed the number of low-frequency tremors spiked after larger quakes elsewhere in the South Island, Victoria University Associate Professor John Townend said in a statement.
The study, in collaboration with the U.S. Geological Survey, looked at three years of data from a network of seismic recorders on the central section of the Alpine Fault, which forms the boundary between the Pacific tectonic plate to the east and the Australian plate to the west.
"The Alpine Fault is known from geological measurements to produce large earthquakes every few hundred years, most recently in 1717 AD, but this research is focused on much, much smaller earthquakes happening almost every day. We are investigating how the fault is being loaded towards an eventual large earthquake," said Townend.
"Our group has recently detected very small earthquakes -- smaller than magnitude 1 -- occurring at depths of 15 to 40 km near the Alpine Fault."
When a large earthquake happened elsewhere in the South Island, such as the 2010 and 2011 Canterbury sequence of earthquakes, the number of low-frequency earthquakes spiked.
"The Alpine Fault responds to earthquakes hundreds of kilometers away, showing that it's very sensitive to the small stresses associated with seismic waves," researcher Calum Chamberlain said in the statement.
The researchers were trying to understand the conditions beneath the fault that made it extremely sensitive to weak triggers.
"Documenting how often these low-frequency earthquakes occur and how they're triggered by very small perturbations in stress is important for determining how the Alpine Fault is loaded by tectonic processes and better understanding the hazard it poses," said Townend.