Complex Slip Triggered by Complex Geology for 2014 South Napa Earthquake

Researchers: Michael A. Floyd, Massachusetts Institute of Technology; Richard J. Walters, Durham University; John R. Elliott, Andy J. Hooper, and Tim J. Wright, University of Leeds; Gareth J. Funning, University of California, Riverside; Jerry L. Svarc and Jessica R. Murray, United States Geological Survey, Menlo Park; Yngvar Larsen, Norut, Tromsø, Norway; Petar Marinkovic, PPO.labs, Den Haag, Netherlands; Roland Bürgmann, University of California, Berkeley; and Ingrid A. Johanson, Hawaiian Volcano Observatory.

Written by Linda Rowan
7 December 2016

Summary

Using GPS and Sentinel 1 radar imagery, the slip along the West Napa fault zone (WNFZ) during and after the Mw6.1 2014 South Napa earthquake was measured. The complexity of the slip and afterslip suggests that hard rock and soft sediment heterogeneity control the style of friction on the fault plane. The post-earthquake deformation caused damage and revealed the fault properties; these are important details to consider for estimating earthquake risks and understanding fault processes.

Observations

The South Napa earthquake occurred in a region with about six continuous and permanent GPS stations within 25 kilometers of the surface rupture. These permanent stations are operated by the Bay Area Regional Deformation (BARD) group and UNAVCO's Plate Boundary Observatory (PBO), a component of EarthScope. These networks are not close enough or dense enough to measure in detail the earthquake slip and the post-earthquake slip, so previous survey GPS sites (about 26) were re-established for a few weeks to capture more information about the fault properties. Many of the survey sites were originally measured by the California Spatial Reference Center (CSRC) and subsequently measured regularly by collaborators from the University of California, Riverside (UCR) and the Massachusetts Institute of Technology (MIT). Other sites were set up by the U.S. Geological Survey (USGS) and measured repeatedly by the USGS and UCR-MIT. USGS and UCR-MIT re-occupied the sites within 48 hours of the event and continued the survey for between 7 to 25 days after the event. The previous survey was completed just seven weeks before the earthquake, so the geoscientists were able to gather exceptional measurements of the fault motion.

In addition to the GPS measurements, the authors processed synthetic aperture radar (SAR) data from the Sentinel-1 satellite, a relatively new space mission from the European Space Agency (ESA). Six interferograms (InSAR), one encompassing the earthquake and five from different times after the earthquake, were acquired to measure deformation during and after the event. Airborne Lidar data of the Napa watershed were acquired by the National Center for Airborne Laser Mapping (NCALM) and accessed through OpenTopography (OT). UNAVCO helps to manage OT through an NSF grant with several partners.