SCEC Award Number 15009 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title The role of seismogenic depth on rupture across stepovers and damage zone thickness
Name Organization
Jean-Paul Ampuero California Institute of Technology
Other Participants
SCEC Priorities 3c, 4b, 4e SCEC Groups DRCV, Seismology, FARM
Report Due Date 03/15/2016 Date Report Submitted 11/14/2016
Project Abstract
The thickness of fault damage zones, a characteristic length of the cross-fault distribution of secondary fractures, significantly affects fault stress, earthquake rupture, ground motions and crustal fluid transport. Field observations indicate that damage zone thickness scales with accumulated fault displacement at short displacements, but saturates at few hundred meters for displacements larger than few kilometers. To explain this transition of scaling behavior, we conduct 3D numerical simulations of dynamic rupture with off-fault inelastic deformation on long strike-slip faults. We find that the distribution of coseismic inelastic strain is controlled by the transition from crack-like to pulse-like rupture propagation associated with saturation of the seismogenic depth. The yielding zone reaches its maximum thickness when the rupture becomes a stable pulse-like rupture. Considering fracture mechanics theory, we show that seismogenic depth controls the upper bound of damage zone thickness on mature faults by limiting the efficiency of stress concentration near earthquake rupture fronts. We obtain a quantitative relation between limiting damage zone thickness, background stress, dynamic fault strength, off-fault yield strength and seismogenic depth, which agrees with first-order field observations. Our results help linking dynamic rupture processes with field observations, and contribute to a fundamental understanding of damage zone properties. In particular, they shed light on the relative role of short-term and long-term damage processes in the evolution of fault zone structures.
Intellectual Merit This work addressed SCEC priorities 3c “formulation of theoretical and numerical models including interaction with damage-zone properties” and 4b “investigations of along-strike variations in damage perpendicular to the fault” by developing mechanical models for the scaling of fault zone and fault system thickness with fault depth and maturity. It also addressed priority 4e “use of earthquake modeling tools to quantify how large-scale fault system complexities govern the probabilities of large earthquakes and rupture sequences”, by proposing mechanical models to understand what fault zone properties control first-order earthquake source properties such as slip distribution and rupture speed. This work draws mechanical connection between short-term rupture dynamic processes and long-term fault zone evolution processes.
Broader Impacts The project contributed to the training of two graduate students and to international and cross-disciplinary collaborations. The relations found between fault structural properties inferred from fault maps and earthquake source properties are of potential benefit to society: they may serve as prior information for earthquake early warning systems.
Exemplary Figure Figure 1. Left: depth cross-section view of plastic strain generated by ruptures with different seismogenic depths, W=9, 12 and 15 km, respectively. Fault length is much larger in all cases. Right: cross-fault decay of plastic strain at 5 km depth for five cases with different W. Inset: Damage zone thickness H at middle depth (defined here by a threshold of plastic strain ∼0.001) increases linearly with W, asymptotically when W is large.