The 4.8 earthquake that struck Wyoming's Yellowstone National Park Saturday is the strongest seen since 1980, according to the U.S. Geological Survey.
The quake did little damage and few visitors were in the park.
Experts warned a stronger quake has the potential to erupt the park's super volcano - a force 2,000 times the size of Mount St. Helens.
The earthquake came one day after a 5.1 temblor hit Los Angeles and was followed by hundreds of aftershocks.
"We have a lot of cosmetic damage to structures. We're trying to decipher what's cosmetic and structural. For hazard, we first go out and assess if there's a collapse or potential collapse and now were really trying to go out to get investigations," Fullerton Fire Battalion Chief John Stokes said.
Friday night's quake was centered in La Habra along the Puente Hills thrust fault, which stretches from Orange County to Hollywood.
Experts said a major quake of magnitude 7.5 or larger along the fault could kill thousands.