Researchers have found that few aftershocks take place where faults slipped a great deal during the initial earthquakes. The study supports the common assumption that areas of fault line are relatively safe for some time after an earthquake.
Study lead author Thorne Lay is a professor of Earth and Planetary Sciences at the University of California, Santa Cruz.
“This intuition has been challenged by statistical treatments of seismic data that indicate that, based on the clustering of earthquakes in space and time, the area that has just slipped is actually more likely to have another failure,” said Lay.
“The truth appears to be more nuanced. Yes, the area that slipped a lot is unlikely to slip again, as the residual stress on the fault has been lowered to well below the failure level, but the surrounding areas have been pushed toward failure in many cases, giving rise to aftershocks and the possibility of an adjacent large rupture sooner rather than later.”
The research team analyzed 101 major earthquakes that occurred along the Pacific Ring of Fire between 1990 and 2016.
When the experts investigated the locations of aftershocks, they found that few occurred along the regions of fault line that had just experienced a large amount of slip. Instead, most of the activity following the mainshock had taken place on the margins of areas that had slipped.
“This produces a halo of aftershocks surrounding the rupture and indicates that the large-slip zone is not likely to have immediate rerupture,” said Lay.
The results of the study suggest that the pressure relieved during a major earthquake leads to a prevalent reduction of stress over the large slip zone on the surface of the fault. The pressure will gradually build up again, but it is a slow process.
According to the study authors, regional clustering of earthquakes is likely to occur outside the main slip zone due to increased stress. The experts also emphasized, however, that intense aftershock activity within the high-slip zone means that another large earthquake is possible.
The study is published in the journal Science Advances.