Triggered earthquakes shed new light on behavior of the Earth’s crust
Scientists from the Massachusetts Institute of Technology (MIT), Los Alamos National Laboratory and University of Tokyo have demonstrated that when an earthquake in one part of the world triggers another over 1 000 km away, widespread changes occur below the Earth's surface. The research is important as it provides new insight into behavior of the Earth's crust and sheds new light on how earthquakes are triggered, MIT reported on October 21, 2015.
When an earthquake happens the elastic crust of our planet can 'feel' fundamental changes up to 6 000 km (3 728 miles) away and its ability to hold stress alters for a couple of weeks.
Earthquakes happen when enough stress builds up along a tectonic fault, causing the two surfaces previously held together by friction to suddenly displace. During this process energy is being released in the form of seismic waves. Experts distinguish two types of seismic waves: body waves, which cause damaging movement felt during quakes, and surface waves capable of traveling thousands of kilometers beneath the surface.
While traveling through fault regions, surface waves change the balance between the frictional properties that keep the surfaces locked together and the stress state that can cause it to fail.
"When surface waves pass through, all of these properties rearrange and change. If a fault with high stress is ready to fail, it will accumulate more stresses in the fault, meaning an earthquake could occur at any time," Kevin Chao, MIT's postdoc explained.
In order to analyze these changes, researchers have conducted a study of the 2012 earthquake off the coast of North Sumatra in the Indian Ocean. The earthquake of a magnitude 8.3 has been followed by two more quakes in Japan of a magnitude exceeding 5.5.
When the researchers studied data from strain meter readings, GPS equipment, and information on seismicity in the region, as well as the migration of the earthquakes, they discovered that the two triggered quakes with a magnitude of greater than 5.5 were part of a cluster of activity in the area in the days after the Indian Ocean event.
"When the Indian Ocean earthquake occurred, the surface wave passed through the northeast of Japan, and the seismicity in the region was suddenly triggered. During that time of increased seismicity, there were three triggered earthquakes in the region with a magnitude of greater than 5.5," Chao said.
After the major Japanese earthquake in 2011 the region was already stressed, so the additional stress, brought on by the surface waves that were passing through the area, was enough to trigger another cluster of earthquakes.
The researchers also studied signs of compressive stress in Japan region, following the earthquake in Indian Ocean, because the earthquake, once it occurs, compresses the crust in the neighboring area. They discovered that cracks in the rocks under the Japanese mainland were closing in response to compressive stress.
"We still cannot say that there will definitely be another earthquake after the first one has struck, because although we know there will be changes, we do not know the existing stress conditions in every region, so we cannot predict anything with certainty. But one important thing we can say is that we know earthquakes do interact with each other, because surface waves can travel thousands of kilometers, and change the elasticity in another region," Chao said.
John Vidale, a professor of Earth and space sciences at the University of Washington regards the research as extremely interesting, and surprising.
"It is a surprise to see indications that the deformation might be pervasive rather than limited to the immediate vicinity of the Earth’s surface or a single fault," Vidale said.
The clear observation of an increase in rock stiffness, beginning at the time of the distant earthquake, is a new observation. “The weeks-long progression of small earthquake activity, material change, and motion are just the latest in a line of geophysical studies indicating that the Earth moves much more irregularly than we suspected just a decade or two ago,” Vidale concluded.
Research team published their discoveries in the Science Advances journal on October 16, 2015.
Reference:
- "Cascading elastic perturbation in Japan due to the 2012 Mw 8.6 Indian Ocean earthquake" – Andrew A. Delorey, Kevin Chao, Kazushige Obara, Paul A. Johnson – Science Advances (2015) – doi:10.1126/sciadv.1500468
Featured image: Artistic impression of an earthquake signal. Image credit: MIT
As an amateur EQ observer and anticipator, for many years, this is nothing new to me. When one watches the behaviours of quakes worldwide on a daily basis, combined with a large sprinkle of intuition, you can see that this is the case and nothing new. At least the report/study will help those who don’t understand the concept (mentioned in article) to understand their home planet a little better and raise awareness of natural processes happening under our feet.