What the USGS recorded
A swarm of small earthquakes struck Northern California early this week, with several clusters appearing near the San Andreas Fault, according to U.S. Geological Survey data cited by Newsweek. The tremors were recorded between Sunday and Monday and ranged in magnitude from 0.5 to 2.8.
USGS mapping showed the largest concentration around The Geysers, a geothermal area in the Mayacamas Mountains where minor shaking is routine. A magnitude 2.7 quake there was among the stronger events, recorded about 2.8 kilometers (1.7 miles) below the surface. More than two dozen weaker quakes were logged in the same vicinity, the data showed.
Other clusters around the region
A separate burst of activity occurred east of Lower Lake in Lake County. The USGS map listed quakes of magnitude 1.5, 2.1, 2.2, 2.5 and 2.8 there, all within roughly an hour of one another on Monday.
Farther south, two small quakes were felt in Concord, in Contra Costa County. A magnitude 2.1 event on Sunday was followed on Monday by a slightly stronger magnitude 2.5. Both were located about 15 kilometers (9 miles) down, according to the USGS.
Two more minor tremors were detected east of Berkeley, and another was picked up along the fault off the San Francisco coast. More than half a dozen additional small quakes were recorded still farther south along the fault line, the agency's data showed. Newsweek reported that no major damage or injuries had been attributed to any of the events as of publication.
The fault behind the headlines
The San Andreas is a narrow, continuous fracture in the Earth's crust running from Northern California south to Cajon Pass near San Bernardino. The fault system extends for more than 800 miles and reaches depths of at least 10 miles, according to Newsweek.
Small quakes like these are common in California, and the week's events were minor. Even so, concern about the "big one," the term for a future great earthquake on the San Andreas, has long shaped how Californians think about seismic risk.
New research on accumulated stress
Newsweek tied the week's activity to a recent study in the Journal of Geophysical Research: Solid Earth. Researchers used geological records and a physics-based model to simulate roughly 1,000 years of stress buildup and release along the southern San Andreas and San Jacinto fault systems. They found stress on multiple segments at or above the highest levels seen in that period, which suggests the region could be capable of a large rupture involving both systems.
Lead author Liliane Burkhard told Newsweek: "The most important thing to understand is that this is not a distant, theoretical risk." She said stress had been building continuously since 1857 and that the model showed it had passed anything seen in a thousand years of earthquake history.
Burkhard cautioned that this does not mean a quake will strike tomorrow. She said, however, that the system is as loaded as it has ever been in the record and that "the question is not really if but when." For people in Southern California, she said, the findings are a reason to take preparedness seriously. She urged them to know evacuation routes, to make a communication plan with friends and family, and to keep an emergency kit with water, food and medication.
