California region was hit by a massive earthquake measuring 6 magnitude on Richter scale at 3:30 am on Sunday morning, injuring about 100 residents, igniting fires and damaging many buildings in the vicinity of Napa area, said initial reports.
Many residents were jolted out of their sleep and forced to rush on to the streets, said witnesses. The earthquake struck Napa in California, about 10 miles northwest of American Canyon, said the U.S. Geological Survey. The agency said the depth of the earthquake was just less than seven miles, and numerous small aftershocks have occurred in the Napa wine country.
Laguna Niguel resident and structural engineer Narsing Rao said over phone that it was a major quake to have hit the region in the recent past and may huge buildings would have been damaged or weakened by the jolts caused by it. “Though it did not shake violently, it did shake initially and began to grow making it one of the worst and lengthy ones,” he said.
Randy Baldwin, a geophysicist with the U.S. Geological Survey in Golden, Colorado told AP that it was a sizeable one in the area. “The 6.0 is a sizeable quake for this area. It’s a shallow quake. It’s about 6 miles deep. We received hundreds of reports on our website from people that felt it in the surrounding area,” he said.
In Napa area, glass windows of many shops and stores were seen broken and water mains were broken, said residents of the area. Some residents have complained that the streets in one area were flooded with the broken water pipes. Other reports said emergency vehicles were seen rushing injured residents to hospitals in Napa and Sonoma counties.
Tonight’s earthquake lies within a 70-km-wide (44 miles) set of major faults of the San Andreas Fault system bordering the Pacific and North American tectonic plates, caused by the persistent northwestward movement of the Pacific plate relative to North America.
It mainly causes right-lateral slip across the major faults, triggering deformation between the major faults. It is located at the eastern shore of San Pablo Bay between two major active fault systems: the Hayward-Rodgers Creek Fault system on the west and the Concord-Green Valley Fault system on the east.
The earthquake occurred near the West Napa Fault, and the Carneros-Franklin Faults, which juxtapose different suites of rocks. Although there are several faults in the region, only the West Napa Fault has displaced Holocene-age sediment — which is positive evidence of surface fault rupture in the last 11,000 years.
Historically, shaking during the earthquake had damaged structures at Mare Island during the 6.8 magnitude 1868 Hayward Fault earthquake, the M7.8 1906 San Andreas Fault earthquake, and even during the M6.3 1898 Mare Island earthquake.
In addition, the epicentral region of this earthquake is depicted on the USGS National Seismic Hazard Maps to have a high probability of strong shaking in the future. Several aftershocks were also felt tonight, reports said.
Tonight’s earthquake occurred near the north shore of San Pablo Bay, in the San Francisco Bay region, which is prone mudslides causing greater damage to buildings during the earthquakes. “Such damage is caused by soil failure in the fills and amplification of ground shaking by the soft bay mud,” said USGS on its website.