A Monte Carlo-Based Approach to Image Active 3D Fault Systems from Relocated Hypocenters

Despite the generally accepted concept that most earthquakes occur along pre-existing faults, the complex 3D geometries of seismically active fault systems at depth often remain unresolved. However, earthquake nucleation and migration processes are heavily influenced by the geometries and properties of such pre-existing structures, which limits our general understanding of earthquake nucleation and fault interactions.

Under the assumption that faults are reactivated at spatially and temporally different localities, previous studies have attempted to derive fault geometries from hypocenter locations, but were usually limited by the precision of relocation techniques. Enabled by the recent advances in hypocenter relocation techniques, we present a novel Monte Carlo-based method that uses relatively relocated hypocenters and their uncertainties to image geometries, stress states and kinematics of seismically active fault systems. The application of the developed Python toolbox on a natural earthquake sequence along the Rhone-Simplon fault zone in the northern Valais (Swiss Alps) reveals active strike-slip faults with a contractional stepover. Performed stress analyses indicate varying stress states along the fault system, which has direct implications for fault properties such as the reactivation potential or the fluid transmissivity. Overall, we document the migration of an earthquake swarm across a complex strike-slip fault system at an unprecedented spatiotemporal resolution.

Our toolbox can be applied to high-precision hypocenter catalogs of natural earthquake sequences or hydraulic stimulation experiments, which could help to improve our understanding of the role of pre-existing faults on earthquake nucleation and migration processes at various scales.