Earthquake monitoring gets boost from a new satellite

John Elliott is a postdoctoral scientist at the University of Oxford. He works on modelling earthquakes using deformation data, and is co-funded by COMET and the Earthquakes without Frontiers consortium.
Understanding the August 2014 Napa Valley earthquake

For more than two decades, space-based radar satellites have been measuring how the ground moves with extraordinary precision and spatial resolution. Comparing ground heights from the same places at different times helps scientists to understand the dynamics of a variety of geophysical events including earthquakes.

On 24 August 2014, the San Francisco Bay area was shaken by a Mw = 6.0 earthquake, the region’s largest in 25 years. The tremors killed 1 person, injured around 200 and damaged buildings near the quake’s epicentre in the southern reaches of California’s Napa Valley.

It also set off a scientific scramble to measure the fault’s movement, and marked the dawn of a new age of earthquake monitoring thanks to the recent launch of Sentinel-1A.

By combining satellite data with GPS measurements made by our US colleagues on the ground, we were able to show that motion on the fault continued to slip in the weeks following the earthquake in a process called postseismic afterslip.

Sentinel-1A interferogram built by comparing scans near California’s San Pablo Bay from 7 August 2014 with those from 31 August 2014. The image shows ground displacement contours (changes in colour represent displacement of 2.8 centimetres) of motion toward and away from the satellite due to the 24 August South Napa earthquake. The satellite looks westward and down and therefore measures both horizontal motion along the fault and vertical motions at the ends of the fault.
Sentinel-1A interferogram built by comparing scans near California’s San Pablo Bay from 7 August 2014 with those from 31 August 2014. The image shows ground displacement contours (changes in colour represent displacement of 2.8 centimetres) of motion toward and away from the satellite due to the 24 August South Napa earthquake. The satellite looks westward and down and therefore measures both horizontal motion along the fault and vertical motions at the ends of the fault.

Using these regularly repeating observations, we found a whole range of different fault slip behaviour on the fault plane: from rapid shallow slip to slower, more prolonged, deeper slip.

Conceptual diagram of the time-sequence evolution of fault slip behaviour imaged on the Napa fault, from earthquake nucleation (1) through to deep afterslip (8), over the period of two months following the earthquake.
Conceptual diagram of the time-sequence evolution of fault slip behaviour imaged on the Napa fault, from earthquake nucleation (1) through to deep afterslip (8), over the period of two months following the earthquake.

Observations such as these are important for constraining types of fault slip behaviour and as a starting point to begin to understand the fault frictional behaviour.  This variability should be incorporated into seismic hazard models.

The research received extensive media coverage, including by the BBC:

Sentinel system pictures Napa quake

Sentinel radar satellite tracks continued Napa slip after quake

References

Floyd, M., Walters, R.J., Elliott, J.R., Funning, G., Svarc, J., Murray, J., Hooper, A., Larsen, Y., Marinkovic, P., Burgmann, R., Johanson., I., Wright, T.J. (in prep.) Afterslip evolution following the 2014 South Napa earthquake exposes variations in fault plane friction.

Elliott, J. R., Elliott, A., Hooper, A., Larsen, Y., Marinkovic, Wright, T.J. (2015) Earthquake Monitoring Gets Boost from a New Satellite, EOS 96. doi:10.1029/2015EO023967