This year’s largest earthquake, a magnitude 8.6 temblor on April 11 centered in the East Indian Ocean off Sumatra, did little damage, but it triggered quakes around the world for at least a week, according to a new analysis by seismologists from the University of California, Berkeley, and the U.S. Geological Survey (USGS) which will be published in Nature next week.
The April 11 quake was unusually large — the tenth largest in the last 100 years and, similar to a few other recent large quakes, triggered small quakes during the three hours it took for seismic waves to travel through Earth’s crust. The new study shows, however, that some faults weren’t rattled enough by the seismic waves to fail immediately, but were primed to break up to six days later.
One possible mechanism for the delayed action is that the East Indian Ocean quake triggered a cascade of smaller, undetectable quakes on these faults that led to larger ruptures later on.
Alternatively, large quakes could trigger nearly undetectable tremors or microquakes that are a sign of slow slip underground.
The 2012 East Indian Ocean quake involved lateral movement — referred to as strike-slip, the same type of movement that occurs along California’s San Andreas Fault — and was the largest strike-slip quake ever recorded. However, most large quakes take place at subduction zones, where the ocean bottom sinks below another tectonic plate. This was the origin of the Sumatra-Andaman quake, which produced a record tsunami that took more than 200,000 lives.
The findings are a warning to those living in seismically active regions worldwide that the risk from a large earthquake could persist — even on the opposite side of the globe — for more than a few hours.