FAULT DAMAGE ZONES ENHANCE EARTHQUAKE RUPTURE COMPLEXITY OVER MULTIPLE CYCLES

Abstract:

Faults consist of principal slip zones surrounded by damage zones with reduced wave speeds relative to intact rock. Previous modeling studies found that damage zones affect earthquakes by promoting complex rupture patterns, but were based either on single-earthquake simulations, which strongly depend on arbitrarily prescribed initial stresses, or on quasi-dynamic cycle models, which do not account completely for wave propagation effects. Here we show that the presence of a damage zone enhances earthquake rupture complexity in fully-dynamic earthquake cycle models in which the stress before earthquakes is consistent with the previous slip activity on the fault, and the dynamic effects of fault zone waves are fully accounted for. Our analysis considers values of fault zone parameters observed in nature: damage zone thicknesses much smaller than fault length and realistic levels of damage. We found two persistent manifestations of rupture complexity: back-propagating fronts and slip-rate oscillations. Back-propagating fronts emerge near stress concentrations left by previous earthquakes and generate sub-events in the Source Time Function that are potentially observable in the far-field. Slip-rate oscillations are induced by fault zone trapped waves, have dominant frequencies close to the fundamental resonance frequency of the damage zone and could be observable in the near-field. Our computational study confirms that fault zone structure may have strong effects on earthquakes, and identifies seismological observations that can help constrain such effects.