<img alt=“shake map ind quake map.jpg” src=“https://blogs.nature.com/news/thegreatbeyond/shake%20map%20ind%20quake%20map.jpg” width=“385” height=“383” align=“right” border=0 hspace=“10px”/>Yesterday’s earthquakes in the Pacific and Indian Ocean have caused death and destruction in Samoa and Indonesia. In Samoa, a tsunami triggered by a magnitude 8.3 quake off Tonga killed at least 114 people and left thousands homeless. The death toll of the magnitude 7.6 earthquake which hit just a few hours later off the western coast of the Indonesian province of Sumatra may exceed 4,000, local authorities fear. Nature asks whether the double disaster was coincidence.
Is there a link between the Tonga and Sumatra quakes?
Although both quakes occurred on the boundaries of the Australian Plate, there is no known causal link between the two events. Normal physical interaction between earthquake zones doesn’t apply at that range. The ruptures also had quite different seismic characteristics, which is why the Tonga quake generated a tsunami but the Sumatra quake luckily didn’t.
However, a paper in Nature today suggests that seismic waves from earthquakes might indeed have an effect on distant fault lines, increasing the risk of earthquakes far away. Whether this long distance-effect was involved in yesterday’s events is not known.
Why didn’t both quakes generate tsunamis?
The Samoan earthquake was caused by normal faulting: one slab sliding under gravity down the inclined side of another slab, says Kevin McCue, president of the Australian Earthquake Engineering Society. As in the great 2004 earthquake off Sumatra that triggered the devastating Indian Ocean tsunami, the deadly waves resulted from sudden vertical displacement of the seafloor.
But yesterday’s quakes off Sumatra began about 80 km below the sea floor – too deep to trigger a tsunami.
Is there a link between the Sumatran earthquake and the December 2004 quake?
Here’s where things get speculative.
Seismologists do believe that recent quakes off Sumatra belong to a ‘domino’ sequence, or quake super-cycle, which occur in more or less regular intervals.
Along the so-called Sunda subduction fault, where the Indian and Australian plates descend beneath the Indonesian archipelago, earthquakes seem to ‘unzip’ the plate every 100 to 200 years (see paper). This is because the plates don’t slide past each other smoothly. Instead, they grind and get stuck so that pressure builds up that is released every so often through a series of powerful quakes. When all the stress is released the ‘dominos’ slowly start building up again.
However, the characteristics of yesterday’s quake and aftershock don’t quite match that expected from the domino theory. Rather than occurring at the interface of two plates, the main quake seems to have taken place internally in a subducting slab – an ‘intra-plate’ quake. So while it is possible that the five strong quakes since 2004 made yesterday’s earthquake more likely, it was more likely an anomaly.
What does that mean for future earthquake risk in the region?
It is, unfortunately, not good news. Yesterday’s quake has probably released none of the tectonic stress that is still awaiting release there. Indonesia has been warned that the worst quake is likely still to come. For example, seismologists fear that a section of the Sunda trench, which has not ruptured since 1797, might break lose very soon (see paper).
“This was not the ‘big one’ we’re all worried about,” says John McCloskey, an earthquake expert at the University of Ulster in Coleraine, Northern Ireland. “All the strain is still there.”
Earthquake prediction is loaded with uncertainty. Nonetheless, there are hints that the biggest earthquake in the current cycle, and possibly its conclusion, will be a full magnitude stronger than yesterday’s quake, with the worrisome potential of generating a major tsunami.
“I fear that what we’ve seen yesterday was not a relief, but an addition to the problems of the people of Sumatra,” says McCloskey. “In all likelihood it had almost no effect on the risk of another very strong quake in the region.”
Image: USGS shake map of the Indonesian earthquake, key below.
Posted on behalf of Quirin Schiermeier