A Twitter user took this photo that purports to show serious damage to a building in Napa - a city around three miles (5km) from the epicentre of the earthquake.
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Metropolitan Police Commissioner Cressida Dick said the bomb could have caused deaths if it had fully detonated.
A man and a woman have been charged with murder after a badly burned body was found in the back garden of a house in south-west London.
After a chilly start, there will be some warm sunshine across southern and eastern parts.