On the morning of July 4, a magnitude 6.4 rocked Southern California, fracturing roads and sending people fleeing to safety. But that wasn’t all the Earth had in store: Less than a day and a half later, a powerful magnitude 7.1 temblor shook the region.

While earthquakes are not unexpected, the two most recent temblors are the largest that have struck this area in decades. And they promise to yield fresh clues about its complex geology.

The duo of quakes struck in what’s known as the Eastern California shear zone—an area east of the infamous San Andreas fault, where the Pacific Plate grinds against the North American Plate, creeping northwest at roughly two inches each year. The area