Research ArticleSeismology

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

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Science Advances  28 Jul 2017:
Vol. 3, no. 7, e1700525
DOI: 10.1126/sciadv.1700525
  • Fig. 1 Study area of the 2014 South Napa earthquake and vine row images.

    (A) Airborne light detection and ranging (LIDAR) base map of the study area. Buhman Avenue, the northern trench (T2); South Avenue, the southern trench (T1). Inset: Regional location map with major fault strands of the San Andreas Fault system. (B) Photograph of offset vine rows. Arrows indicate right-lateral sense of motion. (C) Map view examples of MLS point clouds from blocks 1 and 7. (D) Example vine row point cloud and elastic dislocation models. Inset: Same model curves as main figure, except with larger across-strike distance. (E) Cartoon illustrating the inversion schematic. S, fault slip; zl, lower dislocation limit; zu, upper dislocation limit; uy(x), fault-parallel surface displacement.

  • Fig. 2 Surface and near-surface expression of faulting.

    (A) Base map, optical image collected on 9 September 2014. Red lines, en echelon fractures mapped by hand. Blue line, location of trench in (D). (B and C) Field photos of surface expression of faulting. Yellow lines, en echelon fractures. (D) Trench log from the South Avenue trench. Orange lines, fractures. Black square, location of sample for mechanical tests. (E) Interpreted photomosaic of trench at Buhman Avenue. Brown lines, fractures. See Fig. 1A for location of trenches.

  • Fig. 3 Inversion results.

    (A to H) Results from the indicated vineyard block. Results are for the second-measurement epoch (28 to 30 September) for each vineyard block (see Fig. 1A for block locations and fig. S4 for similar plots for first-measurement epoch). For each block: Top, point cloud (gray points) of example row with optimal model (red line). Middle, probability distribution for slip S. The colors represent probability p. Black circles, estimate of fault-parallel surface displacement uy. Bottom, probability distribution for upper edge of dislocation zu. White dotted line, location of the example row.

  • Fig. 4 Comparison of shallow (this study) and deeper (3941) slip estimates.

    (A) Colored columns, stacked PDFs of S from each vineyard block for the second-measurement epoch (Fig. 3, middle panels). The width of the column represents the along-strike length-scale of the vineyard block. Black circles, hand measurements of fault-parallel surface displacement made immediately following the earthquake. Gray circles, alinement array measurements 60 days after the event (45). (B) Colored columns, stacked PDFs of zu from each vineyard block for the second-measurement epoch (Fig. 3, bottom panels). (C) Coseismic, kinematic slip model from Wei et al. (41). (D) Normalized cumulative slip from three coseismic slip models. W, the study by Wei et al. (41); J, the study by Ji et al. (36); M, the study by Melgar et al. (40). Colored rectangles in the shallowest portions are cumulative slip S from all the vineyard blocks in this study, normalized according to the maximum value of each of the kinematic models.

  • Fig. 5 Plot of the ratio of fault-parallel surface offset uy and slip S.

    Derived from the 1D screw dislocation solution (3) and plotted versus distance x normalized by upper edge depth zu.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/7/e1700525/DC1

    fig. S1. Surface displacement profiles of elastoplastic models for varying values of cohesion C.

    fig. S2. Results of running the elastic inversion on the elastoplastic models in fig. S1 expressed as a percentage of the true value.

    fig. S3. Example of estimation of shallow slip model (S and zu) and fault-parallel surface offset (uy) for vine rows.

    fig. S4. Inversion results for the first measurement epoch (1 to 2 September) for each vineyard block.

    fig. S5. Fault zone location for each vine row measurement.

    fig. S6. Comparison of estimates of near-surface slip S and fault-parallel surface displacement uy for all vineyard blocks.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Surface displacement profiles of elastoplastic models for varying values of cohesion C.
    • fig. S2. Results of running the elastic inversion on the elastoplastic models in fig. S1 expressed as a percentage of the true value.
    • fig. S3. Example of estimation of shallow slip model (S and zu) and fault-parallel surface offset (uy) for vine rows.
    • fig. S4. Inversion results for the first measurement epoch (1 to 2 September) for each vineyard block.
    • fig. S5. Fault zone location for each vine row measurement.
    • fig. S6. Comparison of estimates of near-surface slip S and fault-parallel surface displacement uy for all vineyard blocks.

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