California hit by rapid quake swarm

Northern California woke to a rapid sequence of earthquakes, raising questions about the region’s fault systems and future risk.

California hit by rapid quake swarm near the San Andreas

A sequence of earthquakes shook Northern California early on Thursday. US Geological Survey data show six tremors or a quake swarm, with magnitudes from 2.6 to 4.0. The first tremors hit at 1:51 am Eastern Time, and four quakes occurred within about two minutes. The quakes were centered roughly 72 miles north of San Francisco, near the San Andreas Fault. A separate 3.9 magnitude quake occurred on the Maacama Fault just north of Santa Rosa. A later tremor, magnitude 3.6, was detected in Southern California around 9:50 am.

Residents felt light shaking, but officials reported no major damage or injuries. Experts say a cluster can be a swarm or an aftershock sequence rather than a single event. A study by Michigan Tech University notes that quakes below magnitude 2.5 are generally not felt, while those between 2.5 and 5.4 are often felt but rarely cause serious harm. Scientists also note that the San Andreas Fault remains a focus of risk planning, as a future major quake could produce huge damage. Caltech researchers warn that the next Big One could be bigger than believed. A study of the Sagaing Fault in Myanmar used satellite imagery to map ground movements and found the fault slipped over a longer distance than anticipated, with a 310 mile section moving about 9.8 feet during the event.

The swarm highlights how California sits on multiple active faults and how clusters can emerge even during calm periods. It underlines the limits of simple forecasts and the need for robust preparedness. The Myanmar Sagaing study shows that historical records can understate what is possible when a fault ruptures over long distances.

For communities, the lesson is not panic but resilience. Cross fault research and better imaging techniques can improve risk assessment, but they cannot guarantee precise forecasts. Readiness and investment in infrastructure matter as much as maps and models.

Highlights

Science moves even as the ground shifts.