MEDIA RELEASE
25 August 2014
___________________________________________________________
Deep Alpine Fault sensitive to nearby earthquakes
Victoria University of Wellington researchers have discovered that seismic waves produced by earthquakes happening
several hundred kilometres away trigger activity deep beneath the Alpine Fault.
Using a network of seismic recorders called the Southern Alps Microearthquake Borehole Array (SAMBA), set up during a
2009 study, PhD student Calum Chamberlain is exploring the range of seismicity occurring in the central section of the
Alpine Fault.
Calum is researching under the supervision of Associate Professor John Townend and Professor Tim Stern from the School
of Geography, Environment and Earth Sciences and in collaboration with Dr David Shelly of the United States Geological
Survey. This work was recently published online by the journalGeochemistry, Geophysics, Geosystems (G-Cubed).
The Alpine Fault runs along the western side of the Southern Alps and forms the boundary between the Pacific tectonic
plate (to the east) and the Australian plate (to the west). Calum’s PhD research involves regular helicopter trips into
the Southern Alps to download data and make sure the seismic recorders are operating reliably.
“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 focussed on much, much smaller earthquakes happening almost every day. We are
investigating how the fault is being loaded towards an eventual large earthquake,” says Dr Townend.
“Our group has recently detected very small earthquakes—smaller than magnitude one—occurring at depths of 15 to 40 km
near the Alpine Fault. These earthquakes have distinctive low-frequency characteristics that enable them to be detected
and monitored despite their small size, and provide one of the few signs we have of present-day faulting activity deep
inside the plate boundary.”
Analysing three years of data, Calum has found that low-frequency earthquakes happen almost continuously near the Alpine
Fault southwest of Aoraki Mt Cook. In addition, when a large earthquake happens elsewhere in the South Island, such as
the Dusky Sound earthquake of 2009 in Fiordland or the 2010 and 2011 Canterbury sequence of earthquakes, there’s a sharp
spike in the production of low-frequency earthquakes.
One example is the magnitude 6 earthquake near Christchurch in June 2011 that triggered many low-frequency earthquakes
on the Alpine Fault over the course of several days. “The Alpine Fault responds to earthquakes hundreds of kilometres
away, showing that it’s very sensitive to the small stresses associated with seismic waves,” says Calum.
The next stage of Calum’s study is focussed on understanding the conditions beneath the fault that make it extremely
sensitive to weak triggers. This work will also add to knowledge of the fault’s structure at depth.
“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,” says Dr Townend.
The research is funded by the Marsden Fund of the Royal Society of New Zealand and Victoria University of Wellington,
and viewable online athttp://doi.wiley.com/10.1002/2014GC005436