California: As alarm clocks go, it is one of the cruellest that Mother Nature holds in her arsenal: the residents of San Francisco and northern California's Napa Valley were jolted awake at 3.20am, local time, by an earthquake with a magnitude of 6.0.
The quake has left almost 100 people injured, caused about 50 gas main breaks and 30 water main leaks and left roughly 42,000 homes without power. There have been no reports of fatalities.
Large quake hits California
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Large quake hits California
A 6.0 earthquake hits California's wine country, Napa Valley, breaking gas lines and bursting water mains.
The quake is significant, as both the largest to hit northern California since 1989, and a major seismic event in a tranche of American geography which is, by all accounts, long overdue for "The Big One".
More than 10 aftershocks struck the city within an hour.
Memories of the 1989 earthquake are particularly resonant for San Francisco residents; that quake killed 63 people and injured almost 4000. It also collapsed a section of the San Francisco-Oakland Bay Bridge.
Napa's fire department said yesterday morning's quake caused six significant fires.
A number of freeways and overpasses were closed as a precaution, but they slowly reopened.
The epicentre – the point on the surface directly above the earthquake – was about nine kilometres south-west of Napa, California's iconic wine region.
The hypocentre – the actual location of the earthquake itself – was located about 10.8 kilometres underground.
"A quake of that size in a populated area is, of course, widely felt throughout that region," a spokesman for the US Geological Survey (USGS) said. "The 6.0 is a sizeable quake for this area. It's a shallow quake."
However, it is not the event itself that most disturbs people, but the uncertainty that follows.
The USGS says the area will be peppered with "aftershocks" for at least the next week.
Aftershocks are typically weaker – the aftershocks striking the affected area are presently about the two-to-three magnitude range – but some aftershocks can match the original quake for magnitude and impact.
The USGS said aftershocks could reach as high as 5.0.
The earthquake magnitude scale – also known as the Richter scale – assigns a score of between 1 and 10 to a seismic event; however each full digit increase represents a tenfold increase on the measurable impact of the preceding value.
So, in the case of the Napa quake, a 6.0 magnitude quake is 10 times more impactful than a 5.0 quake, and a 7.0 quake would be 10 times greater again.
According to the USGS, the earthquake occurred within 70 kilometres of a group of major faults in the San Andreas fault system.
That fault – perhaps the most famous in the world – forms the boundary between the Pacific and North American tectonic plates. It runs roughly 1300 kilometres through California, and is split into three segments: the northern, central and southern faults.
Since 2010, four quakes measuring greater than 6.0 magnitude have struck California: 2010's Cape Mendocino (6.5) and Baja (7.2), 2012's Catalina quake (6.3) and 2014's Cape Mendocino quake (6.8).
Those events sit like an uncomfortable spectre over California, where the picturesque countryside is riddled with fault lines and the threat of earthquake has loomed large for more than a century.
San Francisco's most famous quake – 1906 – claimed almost 3000 lives and devastated the city.
More recent quakes claimed fewer lives – just 115 in 1933's Long Beach quake, 65 in 1971's San Fernando quake, 63 in 1989's Santa Cruz quake and 60 in Northridge in 1994.
But the lingering uncertainty for California is the widely held belief that the region is "overdue" for a major seismic event.
According to a study in 2011, massive quakes have occurred on the San Andreas faultline roughly every 45 to 144 years.
However, the last massive quake was in 1857, measuring 7.9 magnitude and affecting more than 350 kilometres of the San Andreas faultline. It was, according to historical accounts, felt throughout California.
According to the study, the region is now "overdue for the type of catastrophic quake" often referred to as "The Big One".