Permanent Deformation in Large Strike-Slip Earthquakes – How Do Single Earthquakes Build the Geologic Record of Faulting?

Abstract:

Single earthquakes are primarily an elastic process; yet, geologic structures and geomorphic markers document a component of the permanent deformation history of slip along faults. The link between these time scales is broadly elusive because elastic and finite strain characteristics of earthquakes are measured across disparate timescales. Recent advances in geodetic imaging, however, provide an opportunity to quantify the permanent deformation characteristics of single earthquakes through co-seismic surface strain analysis. We present a synthesis of co-seismic surface strain analyses from the 2019 Ridgecrest, California (M_w6.4 and 7.1), 2016 Kumamoto, Japan (M_w7.0), and 2013 Baluchistan, Pakistan (M_w7.7) strike-slip earthquakes. These surface strain analyses were derived from inversions of high-resolution optical, topographic, and lidar observations. We show that co-seismic dilatation (fault zone expansion) is a ubiquitous characteristic of these events, and that inelastic failure (where dilatation strains exceed 0.5%) occurs beyond the surface rupture itself. The spatial extent of the inelastic failure zone in these earthquakes is highly homogeneous within a single event, and the surface width of the failure zone (widths of 28-50 m meters in the Ridgecrest earthquakes, 100-120 m in the Baluchistan earthquake, 110-150 m in the Kumamoto earthquake) correlates to both the maturity of the ruptured fault and the variations of rupture velocity within the earthquake. These relationships indicate that inelastic surface deformation in an earthquake is constrained to the fault’s damage zone. Surface strain results from these earthquakes additionally show that inelastic failure is not correlated to the width of off-fault, distributed deformation – a proxy commonly invoked to describe inelastic deformation. Off-fault deformation – the zone over which peak fault offsets occurs - instead, is likely a manifestation of dynamic shaking and shallow fault slip magnitudes. Finally, we show that surface strain orientations of the Ridgecrest earthquakes produce an S-C fabric similar to those observed in mid- to lower-crustal mylonites, indicating that near surface deformation is dominated by an elasto-plastic rheology.