On August 24, 2014, just south of Napa, California, a fault in the Earth suddenly slipped, violently shifting and splitting huge blocks of solid rock, 6 miles below the surface.
The underground upheaval generated severe shaking at the surface, lasting 10 to 20 seconds. When the shaking subsided, the magnitude 6.0 earthquake—the largest in the San Francisco Bay Area since 1989—left in its wake crumpled building facades, ruptured water mains, and fractured roadways.
But the earthquake wasn’t quite done. In a new report, scientists from MIT and elsewhere detail how, even after the earthquake’s main tremors and aftershocks died down, earth beneath the surface was still actively shifting and creeping—albeit much more slowly—for at least four weeks after the main event. This postquake activity, which is known to geologists as “afterslip,” caused certain sections of the main fault to shift by as much as 40 centimeters in the month following the main earthquake.
This seismic creep, the scientists say, may have posed additional infrastructure hazards to the region and changed the seismic picture of surrounding faults, easing stress along some faults while increasing pressure along others.
The scientists, led by Michael Floyd, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, found that sections of the main West Napa Fault continued to slip after the primary earthquake, depending on the lithology, or rock type, surrounding the fault. The fault tended to only shift during the main earthquake in places where it ran through solid rock, such as mountains and hills; in places with looser sediments, like mud and sand, the fault continued to slowly creep, for at least four weeks, at a rate of a few centimeters per day.