Earthquake aftershock forecasting 'must be improved'
[SANTIAGO] The need to speed up work on a reliable system for predicting potential aftershocks in the days following a strong earthquake has become more urgent, say US scientists, after a rare quake earlier this year was found to have triggered many large, and potentially damaging, earthquakes around the world.
Writing in Nature last month (26 September), researchers said that the magnitude 8.6 earthquake that struck off the coast of Sumatra, Indonesia, on 11 April this year unleashed an unprecedented number of large events as far away as Japan and Mexico.
"The number of earthquakes worldwide of more than [magnitude 5.5] increased by a factor of five over a six-day period," Roland Burgmann, a professor in the Department of Earth and Planetary Science at the University of California, United States, and one of the authors of the report, told SciDev.Net.
"No other recorded earthquake has triggered as many large aftershocks around the world. We believe this was because it was the largest 'strike-slip' earthquake (where the two sides of a fault slip horizontally past each other) ever recorded, involving horizontal motions.
"Seismic waves from this type [of earthquake] are particularly strong and last long enough to affect distant fault zones," he said.
Last April's quake followed 6–12 days of exceptionally low global seismicity, which — coupled with the strength and duration of the shaking related to the strike-slip geometry of the fault — may have been behind the large jump in global seismicity.
John Vidale, a seismologist at the University of Washington, United States, told SciDev.Net: "Prior work documented that earthquakes can trigger more earthquakes up to a certain size, up to a certain distance. But why should only small earthquakes get triggered … And why should there be a distance beyond which triggering doesn't work?
"With just a single triggering 'megaquake', the conclusions will remain tentative until confirmed with follow-up studies, but our understanding of how earthquakes trigger one another has just gained a step up," Vidale added.
The strong and potentially damaging on-land shaking in Indonesia, Japan and Mexico caused by the Sumatra event has implications for the effect of a large earthquake on the global seismic hazard, the paper says.
According to Burgmann, earthquakes triggered at much larger distances should be included in the definition of aftershocks if they occur immediately or just within a few days of a major earthquake.
"It would make sense to include distant triggering of hazardous events in operational earthquake forecasting, that estimates the hazard of likely aftershocks following a large event," he added.