Coseismic slip variation assessed from terrestrial lidar scans of the El Mayor-Cucapah earthquake surface rupture

Peter O. Gold, Michael E. Oskin, Austin J. Elliott, Alejandro Hinojosa, Michael H. Taylor, Oliver Kreylos, & Eric S. Cowgill

Published March 15, 2013, SCEC Contribution #1673

We analyze high-resolution (>104 points/m2) terrestrial lidar surveys of the 4 April 2010 El Mayor-Cucapah earthquake rupture (Baja California, Mexico), collected at three sites 12 to18 days after the event. Using point cloud-based tools in an immersive, virtual-reality environment, we quantify coseismic fault slip for hundreds of meters along strike and construct densely constrained along-strike slip distributions from measurements of offset landforms. Uncertainty bounds for each offset, determined empirically by repeatedly measuring offsets at each site sequentially, illuminate epistemic (interpretive) uncertainties that are difficult to quantify in the field. These uncertainties are used to define length scales over which variability in slip distributions may be assumed to reflect either recognizable earthquake mechanisms or measurement noise. At two sites, concentrated right-oblique slip ranges from 2 to 3.5 m, and individual offset measurements lie within +/-12% (2 sigma) of the individual mean. At these sites, each encompassing ~200 m along strike, a linear slip gradient satisfies all measurement distributions, and implies an along-fault strain of ~10-3. Conversely, following the common practice of defining the slip curve by the local slip maxima overestimates actual fault slip by ~30% and along-fault strain by over an order of magnitude by favoring measurements with large, positive, epistemic errors. At a third site, diffuse normal slip summed along fault-perpendicular profiles ranges from 1 to 2.5 m, and per-profile summed throw measurements lie within +/-17% (2 sigma) of the per-profile mean. Here, a low order polynomial fit through the measurement averages best approximates surface slip. However independent measurements of off-fault strain accommodated by hangingwall flexure suggest that over the ~250 m length of this site, a linear interpolation through the average values for the slip maxima at either end of this site most accurately represents slip at depth. In aggregate, these datasets show that the true slip distribution at each site is likely to be smoother than that implied by a single set of field- or virtual reality-based measurements, thus suggesting that the relatively smooth slip distributions we obtain over order 10^2 m distances reflect real physical phenomena, whereas short wavelength variability over order 10^0 to 10^1 distances can be attributed to measurement uncertainty. In addition, these datasets reveal correlations between slip sense, number of faults, consolidation of surface materials, and how the slip curve is defined that may be useful for more confidently predicting the shape of slip distribution curves in the absence of dense measurements.

Citation
Gold, P. O., Oskin, M. E., Elliott, A. J., Hinojosa, A., Taylor, M. H., Kreylos, O., & Cowgill, E. S. (2013). Coseismic slip variation assessed from terrestrial lidar scans of the El Mayor-Cucapah earthquake surface rupture. Earth and Planetary Science Letters, 366, 151-162. doi: 10.1016/j.epsl.2013.01.040.