Sonali TiwaryAt 10:33 a.m. on September 3, seismic instruments recorded a magnitude 3.5 earthquake near Larkfield-Wikiup, close to Santa Rosa in Sonoma County. The quake occurred about 7.4 kilometers underground—roughly 4.6 miles deep.
People felt the shaking across a wide area. The shaking registered as Instrumental Intensity IV (Light) in Santa Rosa and Larkfield-Wikiup. That’s the category where you definitely notice movement, objects might shift on shelves, and people stop what they’re doing.
But here’s what matters: magnitude 3.5 is small. A magnitude 3.5 earthquake is generally unlikely to cause significant damage, although shaking can still be noticeable. This wasn’t a major earthquake. The public question isn’t “was this a big quake” but rather “how can a small earthquake shake so many people?”
The answer is simple. The epicenter was shallow—only 7.4 kilometers deep. Depth affects how seismic energy reaches the surface. Shallower earthquakes tend to produce stronger surface shaking in the area directly above the event, which is why a relatively modest magnitude can still be widely felt across a populated region.
The earthquake occurred along the active fault system that’s part of the broader San Andreas Fault zone. USGS describes the total slip rate on the Rodgers Creek Fault as approximately 9 mm per year, with creep in some sections estimated at up to 6 mm per year.
No surface rupture was reported. No structural damage was reported. No injuries resulted from the event.
One important clarification: a small earthquake by itself does not provide a reliable prediction that a larger earthquake is imminent. This is the misconception that causes the most concern after minor seismic events.
Seismologists have long sought reliable earthquake precursors—warning signs before major events. No broadly reliable precursor has been established. Small quakes and large quakes follow different patterns. Faults can accumulate strain over long periods and release it in larger earthquakes; minor tremors are not evidence that this buildup is being safely discharged.
The September 3 earthquake was newsworthy because it was widely felt. But it was a minor seismic event, and the Rodgers Creek Fault continues to be closely monitored as part of California’s broader seismic hazard management.
