Assessing coseismic slip variation with terrestrial lidar scans of the 4 April 2010 El Mayor-Cucapah surface rupture

We assess the slip distribution at four sites along the 4 April 2010 El Mayor-Cucapah (EMC) earthquake surface rupture in northern Baja-California with cm-resolution terrestrial lidar (TLS). Slip distributions based on field measurements typically show abrupt variation (0.5-2 m) over short along-strike distances (10 to 100 m), implying large along-fault strains (>10-2). While some of this variation may be real, the degree to which it can be attributed to measurement uncertainty is not well known because accurately assessing uncertainties in the field is difficult. We quantify this uncertainty by evaluating the repeatability of virtual field measurements using TLS datasets in an immersive 3D-cave environment. The TLS scans, collected 12 to 18 days following the EMC earthquake, capture fault displacements in a range of geologic and geomorphic settings. Where slip is concentrated on to a single fault plane, we build slip distributions by measuring individual displaced landforms, and where slip is more widely distributed, by summing offsets along strike-perpendicular profiles. For comparison, we also measure orientations of free-face striations where observable in the scans. At two sites where slip is highly concentrated, we measured each suite of offsets 15 times, with the range in individual offset measurements defining uncertainties of 5% to 25% of the total displacement. These uncertainty measurements confirm that true fault slip is less variable than commonly portrayed by slip distribution curves. We also find that the displacement curve may not be accurately represented by the envelope of the displacement maxima, as is the common practice. Continuing analyses will further assess measurement uncertainty and the efficiency of slip transfer in zones where distributed slip is shared among several faults. In addition, repeat TLS scans collected ~1 year after the EMC earthquake expand on the initial scans and will enable future expansion of the slip distribution curves, as well as quantification of post-event surface change by comparing datasets.