The creeping movement of the ground across California’s Napa Valley following its 24 August earthquake has been observed from space.
Europe’s Sentinel-1a satellite has returned beautifully sharp images of this “afterslip” process.
The information will help assess future seismic hazards in the region.
Sentinel-1a was launched earlier this year to acquire frequent and systematic radar data on the world’s major tectonic hotspots.
As a consequence, researchers have been getting a snapshot of Napa every time the satellite goes overhead, every 12 days.
“What’s absolutely stunning is that we see the fault continuing to slowly creep at the surface – by up to 10 cm over a couple of months,” explained Prof Tim Wright from the UK’s Nerc Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET).
“Normally, we’d miss these kinds of processes because previous European Space Agency satellites only gave us images every 35 days, and other missions don’t acquire data systematically over fault zones.
“This new capability tells us about the frictional properties of the fault.
“That’s what we’re interested in – what actually causes some parts of faults to fail in a stick-slip fashion when other parts will gently creep past each other.
“And it seems the Napa fault can actually do both. The southern end in particular exhibits this slow afterslip.”
Prof Wright is presenting the Sentinel imagery here at the American Geophysical Union Famm Meeting in San Francisco.