IDENTIFICATION OF COSEISMIC RUPTURING IN THE ABSENCE OF A “FREE-FACE” BASED ON TEXTURAL EVIDENCE WITHIN NON-LITHIFIED DEPOSITS

Paleoseismological studies often yield rates of active faulting and seismic hazards that are lower than those estimates based on seismological or geodetic data. A number of causes have been postulated for this phenomenon. An additional problem might lie within the intrinsic definition of what constitutes an active or potentially active fault. Paleoseismological criteria to identify a fault as active or potentially active require the presence of a free-face, i.e., a discrete offset of the earth’s surface across a fault. Small faults or the ends of large rupture segments — which yield more distributed deformation — are typically excluded from seismic hazard assessments. Debates are raging over whether deformational features constitute coseismic-rupture-related damage or were due to non-seismic mechanisms such as creep or permafrost. Evans and Bradbury (Geology, 2007) suggested the mapping of textural evidence in near-surface deposits as an urgently needed tool for paleoseismologists and structural geologists. Paleoseismological studies of smaller-offset faults may provide such opportunities. Based on results by Kübler et al. (Int. J. Earth Sci., 2018), I further analysed the damage pattern developed in a non-lithified Holocene coarse-clastic deposit above Devonian basement across a gentle surface scarp (intracontinental Europe), which had been under severe critique by reviewers. Analysis was based on the relationship between inter- and intraclast-fractures and statistically significant populations of rotated clasts. Analytical mapping yields textural and structural evidence for near-surface seismogenic rupturing: hundreds of systematically rotated, then fractured gravels in a 30-m-wide damage zone indicate particulate to cataclastic flow mechanisms, and deformation at high strain-rates consistent with the regional tectonic stress field. Results indicate that the clasts were first rotated then fractured. The sense of gravel rotation invokes an elastic rebound mechanism, which affected the underlying basement rocks. Coseismic strain may have been transmitted across the basement-sediment interface kinematically, and it then was effectively dissipated in the sediment by fracture branching. The thicker the sediment-cover above basement, the more effective the energy-dissipation by fracturing. Mixed-gravel textures furnish a high-resolution paleoseismological inventory to extend mappable limits of active faults, which will result in improved earthquake magnitude, hazard, and zoning estimates.