On 24 August 2014 (03:20 local time), a magnitude 6.0 earthquake struck the Napa Valley, California, just south of the city of Napa (population 77,000). This is the first earthquake for which the surface deformation has been measured by ESA’s Sentinel-1 satellite.  Read more below and see also the related articles by BBC News and the European Space Agency (ESA). 

ESA’s new Earth observation satellite, Sentinel-1A, has been used to map ground movements caused by the magnitude 6.0 earthquake that shook up California’s wine-producing Napa Valley on 24 August 2014. The resulting interferogram confirms that part of the West Napa Fault system, which had not been identified as being particularly hazardous prior to the event, was responsible for the earthquake. The images are now being used by scientists including field teams from the University of California Davis on the ground to help them map the surface rupture.

The interferogram above is available to download from insarap.org.

Sentinel-1A was launched on 3 April 2014, but it only reached its final operational orbit on 7 August. The pre-earthquake image was acquired on that day. By comparing it with an image from 31 August, it was possible to create a map of the surface deformation caused by the magnitude 6.0 earthquake by looking at the difference in time it took the signal to reach the satellite.

This picture shows fault damage in the suburbs west of Napa. The fault rupture ran through homes and across roads, buckling the tarmac surface and pavements, resulting in damage expected to cost hundreds of millions of dollars.

The earthquake occurred north of the Bay Area, in a part of the fault system which runs down the length of California and includes the San Andreas Fault. However, this particular earthquake occurred on one of the parallel fault systems 40km east of the San Andreas. This earthquake is the biggest to hit the San Francisco Bay Area since the Mw 6.9 Loma Prieta earthquake in1989.

This is the fault map (red lines) of the region around the Napa Valley earthquake, California. The fault rupture (mapped by UC Davis scientists) resulting from the August 24 earthquake is shown by the black line, south-west of Napa. The mainshock and smaller aftershocks are denoted by the yellow circles. Earthquake locations and existing fault locations are sourced from USGS. The dashed line denotes the regions shown in the other figures.

 The interferogram shows that the fault slip continues further north than the extent of the mapped rupture at the surface. It also shows that the southeast-side of the rupture has moved towards the satellite by about 10cm, whilst the northern portion has moved away by 10cm.

Whilst most of the motion in a strike-slip earthquake is horizontal in the direction of the fault, the ground motion at the end of these fault ruptures is a combination of fault perpendicular and vertical. This interferogram shows up-down and to a lesser extent east-west motions.

On the east side of the fault, these motions are in the same direction, either both towards or both away from the satellite, resulting in a large signal. However on the western side of the fault, the east-west and vertical motions are in the opposite sense, cancelling each other out and explaining the asymmetry seen in the deformation pattern across the fault.

Above left is the Sentinel-1A interferogram of the ground deformation from the Napa earthquake. The contours show the ground motion towards and away from the satellite. The black line denotes the surface rupture mapped in the field by scientists from UC Davis. Above right is the same interferogram processed to show the total motion towards and away from the satellite.

South of the town of Napa, the ground has moved towards the satellite by up to 10 cm (red colours), whereas to the north it has moved away by 10 cm (blue colors). Despite being a strike-slip earthquake in which most motion is horizontal, the satellite measures mainly vertical and east-west motion and sees the ends of the rupture bulge up (and eastwards) at one end (towards the satellite in the south) and down (and west) at the other. The mainshock and smaller aftershocks are denoted by the yellow circles. Earthquake locations and existing fault locations (red lines) are sourced from the USGS.

The small surface displacements measured in the interferogram agree with the small offsets measured in the field by geologists surveying the fault rupture, who found displacements in roads and kerbstones of about 10-20 cm.

The surface rupture offset kerbstones to the right by about 10-20 cm. This relative motion is consistent with the right-lateral strike-slip faulting in this earthquake. This photo is taken at the northern end of the fault rupture, on the West side of Napa.

The interferogram also reveals other portions of the fault system that have moved slightly. Sharp lines in the interferogram known as discontinuities show minor movements on other faults, such as the part of the West Napa Fault system that crosses Napa airport.

(Top) This satellite image shows Napa County Airport, five miles south of Napa (GoogleEarth) and the southern extent of the mapped surface rupture (shown in white, UC Davis). (Bottom) This Sentinel-1A interferogram shows the deformation field around the fault. A small discontinuity running north-south is also visible in the interferogram (marked by white triangles) and is due to minor rupture of a parallel fault strand across the airport runways.

When the Sentinel-1 constellation is fully operational (with the launch next year of the identical Sentinel-1B satellite), the average time delay between an earthquake and a radar acquisition will only be a few days. This will enable COMET researchers to analyse the surface deformation much more quickly.