Interplay between geometry and brittle deformation of bedrock fault scarps

Billy James Andrews , Constanza Rodriguez Piceda, David C.P. Peacock, Zoe K Mildon 

Energy built up during interseismic phases is released during earthquakes as seismic energy and dissipated within fault zones. Seismological observations indicate spatial variations in earthquake behavior across a fault (e.g., rupture speed, stress drop, wave directivity). However, how this relates to energy dissipation through brittle fracturing in the fault zone is difficult to quantify from seismological observations. Spatial patterns of on-fault fractures across two bedrock normal fault scarps in Malta are investigated, with fracture network properties related to identified cross-faults and changes in fault-plane geometry. Cross-faults (i.e., faults that offset the main plane) are common across both faults, with throws from 0.2 to 14 m. On-fault fractures are commonly en echelon and contribute to a connected fracture network. The orientation, intensity and connectivity of on-fault fractures display spatial changes associated both with proximity to cross-faults and changes in fault-plane geometry (e.g., corrugations). On-fault fracture and corrugation trends are block rotated by 10˚ to 40˚ by cross-fault zones, indicating corrugations and some on-fault fractures pre-date cross-faults. The elevated fracture intensity and connectivity adjacent to cross-faults, and the sharp contrast in fracture properties across them, indicate that on-fault fractures continued to develop during or after cross-fault formation. This suggests cross-faults are an energy sink during earthquake slip, reducing radiation efficiency and possibly promoting rupture arrest. This observation indicates interaction between the cross-faults and the main fault at surface, and suggests cross-faults play a significant role in earthquake energy dissipation in the shallow sub-surface.