A new Earth-observation satellite called Sentinel-1A has been used for the first time to map the ground movements caused by the magnitude 6 earthquake that shook up California’s wine-producing Napa Valley on 24 August 2014 (European Space Agency). The interferogram of the rupture clearly confirms that part of the West Napa Fault system was responsible for the earthquake (BBC News Article on the satellite interferogram and earthquake). This fault had not been identified as being particularly hazardous prior to the event. The images are being used by scientists (such as such as field teams from the University of California Davis) on the ground to help them map the surface rupture.
The interferogram was created by COMET collaborators Dr Yngvar Larsen, from the research institute Norut in Norway, and Dr Petar Marinkovic, from PPO.labs in the Netherlands. 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 acquired on 31 August, it is possible to create a map of the surface deformation caused by the magnitude 6.0 earthquake by looking at the difference in time it takes the signal to reach the satellite.
The earthquake occurred north of the Bay Area, California, 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 40 km 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, although this most recent earthquake was many times smaller.
The extent of the ground deformation in interferogram shows that the fault slip which occurred in this earthquake continues further north than the extent of the mapped rupture at the surface. The interferogram 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 interferorgam 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.
The small surface displacements measured in the interferogram agree with the small offsets measured in the field by geologists surveying the fault rupture, who find displacements in roads and kerb stones of about 10-20 cm.
The interferogram also reveals other portions of the fault system that have moved slightly in this event. 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.
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 & EwF researchers to analyse the surface deformation more quickly following earthquakes.