Ancient rock structures guided rupture pathway in Australian quake

A recent study finds that ancient rock structures in Australia that formed over 500 million years ago determined the pathway of a 2016 earthquake.
 

By Helen Santoro, freelance science journalist, (@helenwsantoro)
 

Citation: Santoro, H., 2020, Ancient rock structures guided rupture pathway in Australian quake, Temblor, http://doi.org/10.32858/temblor.104
 

Early one May morning in 2016, a magnitude-6.0 earthquake ripped through central Australia, causing a 13-mile (21-kilometer) stretch of land to shift upwards by up to three feet (approximately one meter). Typically, an earthquake of this magnitude results in far larger ground displacements and jagged ruptures — but this rupture was long and smooth, puzzling scientists at the University of Melbourne.

 

 

Generally, ruptures that break the surface are rough and curved, says Januka Attanayake, a seismologist at the university and lead author on the study. “This one was different.”

 

A history of larger ground displacements

Although Australia is located in the middle of the Indo-Australian tectonic plate, far from a plate boundary where large quakes typically occur, the continent still has a history of destructive earthquakes. A magnitude-5.6 earthquake in 1989, for example, hit the harbor city of Newcastle and caused 13 deaths and $4 billion in damage — making it one of Australia’s worst natural disasters. Luckily, most earthquakes of this magnitude happen in remote areas far away from cities and towns.

The majority of these moderate earthquakes also occur on faults that don’t generate clear surface ruptures. If an earthquake generates a surface offset, or “fault scarp,” researchers get a chance to make direct observations of the fault surface and can better understand the processes behind the rupture. Any fault scarp needs to be documented immediately following an earthquake, before the forces of nature — wind, water, animal, etc. — work to erase any clues that could give scientists valuable insight into the rupture process.

 

Uncovering the foundation of the smooth rupture

As luck may have it, the 2016 earthquake created a clear surface rupture — providing a perfect opportunity for scientists from the University of Melbourne to study the tremor. The earthquake originated near the Petermann Ranges, a mountain range that extends almost 200 miles (320 kilometers) across central Australia that was formed around 550 million years ago.

After the earthquake, the team trekked out into the field to create a detailed map of the fault scarp. They used satellite-based global positioning system (GPS) data to map the feature from above and found a relatively smooth and straight 13-mile (21-kilometer) scarp. To see the fault underground, the group used data from seismometers — instruments that record ground motion — to detect and locate aftershocks. These smaller quakes result from the redistribution of stress following a larger shock and tend to cluster along the fault surface that ruptured in the main quake. They therefore can be used to map the extent of the fault below the surface.

 

 

Attanyake and his team discovered that the pattern of aftershocks followed along a known subsurface rock structure, suggesting that the surface that ruptured during the quake was related to this feature. In fact, the orientation of the structure seemed to control the path of the rupture.

 

Old rocks dictate modern earthquakes

Around 550 million years ago India slammed into Western Australia, causing the Petermann mountains to form. The grinding together of these land masses at extreme pressure and temperature deep within the Earth’s crust caused weak zones of rock to form. Over time, as Earth’s surface was slowly eroded away, these zones made their way closer to the surface.

 

 

Stress that builds in the crust through time causes rocks to break through the path of least resistance — in the case of the 2016 earthquake, one of these weak zones.

“We don’t know why this particular weak layer ruptured, but that layer is what caused the long, straight line,” Attanayake explained. “The weak mechanics of the rocks allowed it to easily react to the earthquake,” meaning that earthquake essentially took advantage of the presence of this weak layer.

Earthquakes like this that occur far from plate boundaries are rare, says John Paul Platt, a professor of geology at the University of Southern California. But he adds, they “can be particularly dangerous because they affect areas where buildings are not constructed to withstand earthquakes.” Understanding where these types of ruptures may occur could be vital for disaster preparation. This latest study suggests that in some cases, the rocks at the surface and at depth could give scientists clues about where a future quake could occur.

 

Further Reading

Attanayake, J., T. R. King, M. C. Quigley, G. Gibson, D. Clark, A. Jones, S. L. Brennand, and M. Sandiford (2020). Rupture Characteristics and Bedrock Structural Control of the 2016 Mw 6.0 Intraplate Earthquake in the Petermann Ranges, Australia, Bull. Seismol. Soc. Am. 110, 1037–1045, doi: 10.1785/ 0120190266

Salleh, Anna (2009). Mystery mountain range explained. Retrieved July 1, 2020, from https://www.abc.net.au/science/articles/2009/12/10/2765285.htm

Verdouw, E. (2018, September 02). On this day: Newcastle earthquake strikes. Retrieved June 19, 2020, from https://www.australiangeographic.com.au/blogs/on-this-day/2013/11/on-this-day-newcastle-earthquake-strikes/