Deep Alpine Fault sensitive to nearby earthquakes

(Phys.org) —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 Chamberlainis 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 journal Geochemistry, 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.