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Buried shallow fault slip from the South Napa earthquake revealed by near-field geodesy.

  • Author(s): Brooks, Benjamin
  • Minson, Sarah
  • Glennie, Craig
  • Nevitt, Johanna
  • Dawson, Tim
  • Rubin, Ron
  • Ericksen, Todd
  • Lockner, David
  • Hudnut, Kenneth
  • Langenheim, Victoria
  • Lutz, Andrew
  • Mareschal, Maxime
  • Murray, Jessica
  • Schwartz, David
  • Zaccone, Dana
  • et al.
Abstract

Earthquake-related fault slip in the upper hundreds of meters of Earths surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests.

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