California was rattled by three earthquakes on Monday, led by a magnitude 4.7 quake that struck about 52 kilometers west of Petrolia at about 3:41 a.m. The quake originated at a depth of 13.6 kilometers and was registered by the USGS at Level III intensity, indicating light shaking.
Nine people submitted felt reports on the USGS website, and ShakeAlert warnings were issued within seconds of the quake's origin, though the first estimates came in between magnitude 3.6 and 3.8. Cities exposed to the shaking included Ferndale, Rio Dell, Fortuna, Hydesville, Pine Hills, Humboldt Hill, Eureka, Arcata, Ukiah and McKinleyville, and the USGS said no damage was reported in those locations.
The morning sequence did not stop with the first jolt. Two smaller earthquakes followed, measuring magnitude 3.2 and 2.5, and the USGS has since recorded five aftershocks ranging from magnitude 3.6 to 2.3. That matters because the agency's forecast says there is a 16 percent chance of two or more aftershocks at magnitude 3 or higher in the coming week.
The same forecast puts the odds of an aftershock at magnitude 4 or above at 2 percent, while the chance of a harmful magnitude 5 or higher event is less than 1 percent. There are no chances of large quakes of magnitude 6 or 7, a reminder that the worst likely follow-up activity remains limited even after a pre-dawn jolt that woke a stretch of Northern California.
California's earthquake activity is driven by movement within the San Andreas Fault zone, where the Pacific plate and the North American plate pass each other and produce frequent shaking across the state. The warning system is built for that reality: it is meant to buy seconds, not certainty, and on Monday it did exactly that while delivering an early estimate that came in below the final magnitude.
Two smaller quakes also struck elsewhere, including a magnitude 3.2 earthquake southeast of Johannesburg and a magnitude 2.5 quake south-southwest of Maricopa, both of them relatively weak and shallow enough that the USGS said they did not cause significant vibrations to be felt. In California, though, the concern now is not whether the ground will move again, but how much more the fault system has left to give in the days ahead.