Ground Deformation from the South Napa Earthquake

NASA’s Jet Propulsion Laboratory, Pasadena, California, has been monitoring active earthquake faults in California with a number of remote sensing and ground-based techniques. One such technique is the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) instrument, which has been in use since 2009. UAVSAR is an L-band Interferometric Synthetic Aperture Radar instrument that flies mounted underneath a NASA C-20A Earth science research aircraft from NASA’s Armstrong Flight Research Center in Edwards, California. UAVSAR is able to detect minute changes in Earth’s surface that occur over time between flights of the instrument.

UAVSAR has monitored much of California’s seismically active regions including the Napa area about every six months since November 2009. The temporal history is key to identifying and understanding change when an event, such as an earthquake occurs. A comparison of data collected May 29, 2014, three months before the earthquake, and data collected August 29, 2014, five days after the magnitude 6.0 South Napa earthquake on August 24, 2014, determined that the earthquake surface rupture was more complex than originally anticipated with motion on multiple strands of the fault near the earthquake’s epicenter. The colors in the image represent the amount of ground motion between the two flights in the direction from a point on the ground to the instrument, which flies at an altitude of 41,000 feet (12,497 meters). Each colored contour, or fringe, of the image represents 4.7 inches (12 centimeters) of ground displacement toward or away from the instrument. These preliminary results indicate that several inches/centimeters of horizontal slip occurred on the various strands of the fault. In addition, linear discontinuities in the colored zones indicate locations where surface rupture is highly likely and these are of profound concern. The exquisite detail of the UAVSAR imagery provides local, state and federal agencies with the exact location of the fault traces that shifted during the earthquake and how they relate to levees, buildings, roads, and other vital infrastructure, as well as to help provide a fundamental understanding of earthquakes processes.

Further analyses of UAVSAR data will reveal how deep under Earth’s surface the faults slipped and the amount of the slip. Initial GPS analyses (yellow arrows), indicate an average slip of nearly 23.6 inches (60 centimeters) along a 9.3-mile-long (15-kilometer) fault, which is equivalent to a magnitude 6.1 earthquake, suggesting that additional quiet (non-shaking) slip occurred along the fault following the main earthquake.

Image credit: NASA/JPL-Caltech/ASI/Google Earth

Note: For more information, see PIA18798: NASA Analyses of Global Positioning System Data and Italian Radar Satellite Data Reveal Napa Quake Ground Deformation and NASA Research Aids Response to California Napa Quake.