Mapping giant earthquake potential
Despite more than 120 years of scientists recording earthquake activity across the globe, prediction of them remains a complicated task. Although earthquakes tend to occur along sections of the Earth’s crust where tectonic plates meet, pinpointing when and exactly where they may occur is not yet possible.
But new research is narrowing that range and providing more information about where huge earthquakes might hit. Geophysicist Dr Wouter Schellart from Monash University in Melbourne and seismologist Prof Nick Rawlinson at the University of Aberdeen in Scotland, have created a global map identifying which subduction zones – the points at which one tectonic plate forces another one under it – are capable of generating earthquakes greater than magnitude 8.5.
“Giant earthquakes at subduction zones might have a recurrence interval of hundreds to thousands of years,” Wouter says. “As these events have only been recorded for about a century, it is impossible to determine the likelihood of a giant earthquake occurring at a particular subduction zone as they happen so infrequently.”
There are 23 active subduction zones around the world. The total length of all subduction zones on Earth is about 50,000km and only about 10 per cent of this total length has produced giant earthquakes in the past 100 years, Wouter says. “The question that we are trying to answer is whether all subduction zones are capable of generating giant earthquakes.”
Earthquake potential ranked
In this study, Wouter’s team divided the active subduction zones into 241 segments, each 200km long and assessed 24 key characteristics. Six of these were found to be important in predicting a zone’s potential to produce a giant earthquake.
Wouter’s team then ranked each segment according to these six characteristics and mapped them. The results indicated that giant earthquakes only occur at subduction zones with particular geometric characteristics and states of stress.
The edges of overriding tectonic plates have different levels of stress. Some experience compression, while others are more neutral or experience tension. The largest earthquakes happen where the stress is either compressive or neutral, the study showed.
The steepness of the fault angle is also a key indicator of its giant earthquake potential. The researchers found that giant earthquakes only occur where the angle of the fault is gentle, about 10-30 degrees.
“When the surface of the subduction zone interface is flat, it is easier to rupture,” Wouter says. “Otherwise, it would be stopped by a more dramatic curvature.”
Predicting giant earthquakes near Australia
For Australia, the Sunda subduction zone of Indonesia, spanning from the Andaman Islands to Sumba, and the Hikurangi subduction zone off the south-eastern coast of New Zealand’s North Island are of the greatest concern. According to traits assessed in Wouter’s study, these subduction zones are all capable of generating giant earthquakes.
“Earthquakes resulting from the Sunda and Hikurangi subduction zones do not present a direct threat to Australia, as they are located too far away, but the potential tsunamis generated by giant earthquakes do,” Wouter says. “These can travel many thousands of kilometres through the open ocean. For example, a giant earthquake at the Sunda subduction zone could produce a tsunami that could reach the Western Australian coastline.”
Since the 2004 earthquake in Sumatra, a plethora of tsunami monitors have been installed throughout the Indian Ocean. These would detect an approaching tsunami and give Western Australian cities time to respond.
The research was published in the journal Physics of the Earth and Planetary Interiors.