Major earthquakes may trigger aftershocks in nearby faults

A major earthquake on one fault can trigger large aftershocks on separate faults within just a few minutes, suggests a new study.
Researchers at the University of California, San Diego (UC San Diego) in the US, said these findings have important implications for earthquake hazard prone regions where ruptures on complex fault systems may cascade and lead to mega-earthquakes.
They discovered 48 previously unidentified large aftershocks from 2004 to 2015 that occurred within seconds to minutes after earthquakes of magnitudes seven to eight on faults adjacent to the mainshock ruptures.
In one instance along the Sundra arc subduction zone, where the magnitude nine Sumatra-Andaman mega-earthquake occurred off the coast of Indonesia in 2004, a magnitude seven quake triggered two large aftershocks over 200 kilometres away.

These aftershocks miles away unveil that stress can be transferred almost instantaneously by the passing seismic waves from one fault to another within the earthquake fault system.
"The results are particularly important because of their seismic hazard implications for complex fault systems, like California," said Wenyuan Fan, from UC San Diego.

"By studying this type of triggering, we might be able to forecast hosting faults for large earthquakes," Fan said.
Large earthquakes often cause aftershock sequences that can last for months.
Scientists generally believe that most aftershocks are triggered by stress changes caused by the permanent movement of the fault during the main seismic event, and mainly occur near the mainshock rupture where these stress changes are largest.

The new findings show that large early aftershocks can also be triggered by seismic wave transients, where the locations of the main quake and the aftershock may not be directly connected.
"Multiple fault system interactions are not fully considered in seismic hazard analyses, and this study might motivate future modelling efforts to account for these effects," said Peter Shearer, from UC San Diego.
The study appears in the journal Science.