A 5.1-magnitude earthquake rattled the San Francisco Bay Area, including San Jose and Silicon Valley, on Tuesday, triggering alerts on tens of thousands of cellphones.
Despite widespread shaking reported in a region that's home to nearly 5 million people, there were no immediate reports of injuries or significant damage.
Lucy Jones, the retired U.S. Geological Survey seismologist, said it was the largest earthquake in the Bay Area in 15 years. She told NBC Bay Area it was followed by aftershocks that measured 3.1 and 2.2.
The quake struck the foothills at 11:42 a.m. 12 miles east of San Jose, the Bay Area's largest city, the USGS reported. Its depth was measured at about 4 miles.
An estimated 95,000 people who signed up for an earthquake warning system received a ShakeAlert message, most getting it on their cellphones seconds before the ground shook, California Office of Emergency Management and USGS officials said.
The number represents the wireless providers that had reported their users' alert figures so far Tuesday, officials said. The figure was likely to increase significantly as other providers weigh in, said Robert de Groot, the USGS' ShakeAlert outreach coordinator.
"People had several seconds of alert time before shaking began at some locations," he said.
In San Francisco, roughly 80 miles north, some cellphone users had 18 seconds between the alert and shaking, state emergency officials said.
Warning time depends on distance, with those closest to a temblor getting little or no warning and those living farthest away getting alerts that can outrun an earthquake's wave by as long as two minutes.
Because the quake surpassed a threshold of magnitude 5.0, alerts went out through the federally managed Wireless Emergency Alerts system.
The ShakeAlert system is fully operational in California, Oregon and Washington, and it can reach more than 50 million people, De Groot said.
Californians as far away as Sacramento and Fresno reported feeling Tuesday's temblor.
State emergency officials were coordinating with Bay Area first responders to assess injuries and damage, the office said. The San Jose Fire Department said it received no emergency calls for help or service related to the shaker.
"Following SJFD's #Earthquake Policy, firefighters are checking on personnel, surveying their immediate response area & inspecting stations and apparatus to ensure they're ready to respond to any emergencies," the fire department tweeted.
The Valley Transportation Authority, serving Santa Clara County, where the earthquake was located, said car and track inspections caused a brief five-minute delay in train service. No injuries or damage were reported, and service was back on schedule, it said.
Bay Area Rapid Transit, serving San Francisco, the East Bay and other communities, implemented its own five-minute lag to facilitate inspections but warned that "major delays" were also possible.
Jones said Tuesday's temblor likely took place along the Calaveras Fault, which was home to the last major quake in the region, the Alum Rock earthquake on Oct. 30, 2007, which measured 5.4 and caused a 3-mile surface rupture.
The Calaveras Fault is part of a system of clashing plates ruled by the state's most feared geological feature, the San Andreas Fault, which runs parallel to the coast along most of California and is estimated to be capable of producing an 8.2-magnitude shaker.
Jones said, however, that the Calaveras Fault is best known for producing moderate quakes like Tuesday's.
"The Calaveras Fault is one that tends to have smaller earthquakes," said Jones, a visiting associate professor at the California Institute of Technology. "It’s something that pops off with 5s more often than the other faults. It’s not accumulating much slip [energy], but it does have the capacity for larger."