A swarm of 30 000 quakes shook the region of Harrat Lunayyir from May to June last year and opened an 8km long rift, the US Geological Survey (USGS) team reported.
A wary Saudi government evacuated 40 000 residents at the time but has since let them move back home.
But the residents should be ready to leave again if the ground starts to shake, the USGS team reported in the journal Nature Geoscience.
"This finding indicates that the region is at risk from significant geo-hazards," John Pallister of the USGS and colleagues at King Abdullah University of Science and Technology in Saudi Arabia and elsewhere wrote.
The area is known for its lava fields, called harrat in Arabic, as well as small volcanic cones and volcanic ash called tephra.
"The 2009 intrusive episode at Harrat Lunayyir, along with geomorphically young lava and tephra deposits are reminders that, although eruptions are not frequent, the harrat fields remain active and potentially hazardous," they wrote.