But interestingly, we are clearly some way (about 50 kilometres) to the east of the San Andreas Fault, which is popularly considered to be ‘the’ plate boundary in California. The Pacific plate is moving northwest relative to the North American plate at around 4 centimetres a year, and the focal mechanism for Sunday’s quake is consistent with right lateral strike slip on a northwest-southeast oriented fault, which is what we’d predict for a fault at this plate boundary. There have already been some excellent write-ups of the geology of the earthquake by Mika Mckinnon, Dana Hunter and Garry Hayes: in brief, the root cause of all earthquakes in California is its location on the boundary between two rigid but independently mobile bits of the Earth’s crust: the Pacific and North American plates. But in the grand scheme of things, a magnitude 6 earthquake is not particularly large: it may have been strong enough to wake people up in San Francisco (as data from fitness trackers has rather cleverly been used to illustrate), but it didn’t pack enough of a punch to do any real damage there. An earthquake in wine country produces dramatic visuals of wine bottles thrown from the shelves and leaking into the street, and there was strong shaking and significant damage in the region close the the epicentre.
In the early hours of Sunday morning, the Napa Valley region north of San Francisco was shaken by a magnitude 6 earthquake, the largest to hit this region since the magnitude 6.9 Loma Prieta earthquake in 1989.
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