Fault scarps as evidence of historical co-seismic slip - a study of postseismic scarp degradation following the 2016 Norcia earthquake

Abstract

In immature near-surface normal fault zones, co-seismic slip on a main fault plane will be variably partitioned onto a primary fault scarp and ancillary hanging wall structures and will be subject to ongoing processes of deformation and erosion. The extent to which such processes affect the evidence of visible surface features over time is uncertain, particularly in the first few postseismic years. Using differential repeat Terrestrial Laser Scans (TLS) we investigate continuing postseismic deformation of near-fault areas and degradation in the Monte Vettore region in the Apennines, Central Italy where extensive surface ruptures formed as part of the M_w 6.6 30th October 2016 Norcia earthquake, during the Central Italy Earthquake Sequence (“CIES”), with widely distributed Off Fault Deformation (“OFD”). We concentrate here on one ancillary antithetic structure, the San Lorenzo fault, and the evolution of its scarp over three years following the Norcia earthquake.

The principal causes of postseismic alteration or degradation of fault scarps are expected to be tectonic-related after-slip and/or erosion. Combining careful alignment of repeat TLS, use of an ICP (Iterative Closest Point) algorithm, recursive filtering and detrending techniques, we characterise postseismic surface deformation at ~centimetre scale. We show that afterslip and erosion both play significant roles in the evolution of this fault scarp and the near-fault areas even within the first few postseismic years. Although variable along strike, vertical and horizontal postseismic displacements adjacent to the scarp are ~5–10 % of co-seismic values. Evidence of co-seismic slip associated with such ancillary structures will likely disappear or be significantly degraded quickly relative to the typical earthquake recurrence intervals in the Apennines region, even if the primary fault scarp remains visible. Where fault scarps are used as evidence of previous slip history, particularly in immature fault zones, those factors must be considered to avoid possible misinterpretation of that evidence.