A friend recently moved to the Napa Valley in California, almost certainly motivated by the vast area given over to the grape and the quality of Napa wines. Shortly after the flit, in the middle of some minor refurbishment, he had quite a shock; a Magnitude 6.0 earthquake at 3:20 a.m. local time on 24 August, the worst in northern California for 25 years. My friend lives only 15 km from the epicentre in South Napa, but his house was undamaged. His confidence in the move remains unshaken, however, as there was no effect on this year’s grape harvest.
The event was monitored by the European Space Agency’s Sentinel-1A high-resolution radar satellite that entered orbit in April 2014. Sentinel revisits any area on the ground every 12 days, has all-weather/day-night imaging capacity, a 250 km-wide image swath with 10 m spatial resolution and is designed to analyse ground movements using interferometry between radar data before and after events. Interferometric radar imaging or InSAR relies on changes in the phase of radar waves between two dates of ‘illumination’ of the ground – radar images normally use only the amplitude of a radar wave, ignoring its phase – and potentially can measure shifts in ground elevation of the order of centimetres.
The image records ground movement in small steps of elevation that are assigned colours, the sequence blue-green-yellow-red-magenta spans a ground shift of about 3 cm. If several of these ‘fringes’ are closely spaced over relatively small areas this is due to significant motions locally. Broad areas with little change in colour have barely moved in the period between the dates of the two images.
The epicentre of the South Napa earthquake clearly shows up at the NE corner of the image, like half a ‘bull’s eye’. A closer look at the enlarged image (click on the image) shows two such features sharply bounded to the west by a line: that coincides with the West Napa Fault.
My friend lives to the west of the faults where the broad areas of colour signify much smaller motions than in the main affected area. He woke and left the building thinking this was a foreshock of a much more destructive event, and had an anxious few days. The United States Geological Survey estimated that during the main ‘quake 15,000 people experienced severe shaking, 106,000 people felt very strong shaking, 176,000 felt strong shaking, and 738,000 felt moderate shaking. But there was only one fatality and 13 hospitalised casualties.