Fault-zone damage promotes pulse-like rupture and rapid-tremor-reversals

Benjamin Idini, & Jean-Paul Ampuero

Published 2019, SCEC Contribution #10084

Damage zones are ubiquitous components of faults that may affect the nucleation, propagation and arrest of earthquake ruptures. Dynamic rupture simulations show that waves trapped in a fault damage zone can induce pulse-like rupture, a mode of earthquake rupture propagation that is a primary candidate for the origin of fault slip complexity and for the apparent weakness of major faults. However, the efficiency of the mechanism revealed in previous studies depends strongly on arbitrarily prescribed initial stresses. Here, we investigate the promotion of pulse-like rupture by damaged fault zones through numerical simulations of multiple earthquake cycles in which the distribution of initial stress before each rupture is a self-consistent result of the earthquake cycle. We consider a fault bisecting a homogeneous low-rigidity layer embedded in an intact medium. Using scaling arguments we show that pulse-like ruptures are expected to appear in a highly compliant fault zone after the rupture has grown larger than the fault zone thickness. We confirm this result by conducting quasi-dynamic earthquake cycle simulations on fault zones with varying degrees of damage and thickness. Over a wide range of fault zone properties, fault-zone effects produce pulse-like ruptures with shorter rise-times and flatter slip profiles than ruptures in an intact homogeneous medium. We also find complex rupture patterns involving back-propagating secondary fronts that emerge from the primary rupture front and propagate in the opposite direction. These complex slip patterns robustly persist over multiple earthquake cycles. While such patterns are challenging to resolve in current seismological observations of large earthquakes, slow-slip numerical models show similar slip complexity, suggesting a connection between a basic structural feature of faults and rapid-tremor-reversals observed during episodic tremor and slip in Cascadia and Japan.

Citation
Idini, B., & Ampuero, J. (2019). Fault-zone damage promotes pulse-like rupture and rapid-tremor-reversals. Nature Geoscience,. doi: 10.31223/osf.io/v8xr2.