The locations of where the world's largest earthquakes are most likely to take place have been pinpointed with greater accuracy than ever before, by researchers from the University of Sydney.
"Subduction zones, where one plate slips under another, have long been known to harbour very powerful earthquakes but our research suggests that regions where fracture zones on the seafloor meet subduction zones are at much higher risk," said Professor Dietmar Müller, from the University's School of Geosciences.
"The advantage of our new method is that it picks up many of the regions prone to recurring powerful earthquakes over long time periods, including some where no large earthquakes have occurred in the last 100 or so years. Our results could contribute to much-needed improvements of long-term seismic hazard maps."
Dr Müller is lead author of the research published in Solid Earth, a journal of the European Geosciences Union. Dr Thomas Landgrebe, also from the School of Geosciences, is the other author on the paper.
The Pacific 'ring of fire', an area of high earthquake and volcanic activity, and other regions where two tectonic plates converge, are sites for some of the world's largest earthquakes, also known as great earthquakes.
The new research shows that regions where subduction zones meet oceanic fracture zones, are at substantially elevated risk of earthquakes 8.4 magnitude or higher. Fracture zones are like railroad tracks on the sea floor, tracking the history of plate motions and often associated with enormous submarine ridges elevated by up to three kilometres above the surrounding abyssal plains.
"We found that 87 percent of the 15 largest (8.6 magnitude or higher) and half of the 50 largest (8.4 magnitude or higher) earthquakes of the past century are associated with areas of intersection between oceanic fracture zones and subduction zones," said Professor Müller. The connection is less striking for smaller earthquakes.
The coasts of Southern Chile and Peru, Indonesia's Sumatra Island, and several regions along the eastern Eurasian coastline, are some of the regions prone to great earthquakes.
The researchers considered about 1500 earthquakes in their study. They used geophysical data, mapping fracture zones and subduction zones, and a database of significant post-1900 events. They analysed the information by applying a recently developed data mining method previously only used to match internet users to consumer goods.
The scientists do not yet have a complete understanding of why great earthquakes prefer the intersection areas.
Their suggestion is it is due to the physical properties of fracture zone ridges which allow the regions where they intersect with subduction zones to accumulate stress over a long time period until the energy is released in a major quake.
"Our research highlights those tectonic environments which are statistically more prone to this accumulation of stress and to great earthquake supercycles, where powerful earthquakes recur every few centuries or millennia. These supercyles are not usually picked up as risk areas by seismic hazard maps, as these mainly use data collected after 1900," Professor Müller said.
An example is the area of the 2011 Tohoku-Oki earthquake, which had no record of large earthquakes over the past century and was not predicted to be of significant risk by previous hazard maps.
"Even though we don't fully understand the physics of long earthquake cycles, any improvements that can be made using statistical data analysis should be considered as they can help reduce earthquake damage and loss of life," Professor Müller said.