Mechanics of shallow slip in low-angle normal fault earthquakes: insight from 3D dynamic rupture models constrained by multi-timescale observations

Despite decades-long debate over the mechanics of low-angle normal faults dipping less than 30°, many questions about their strength, stress, and slip remain unresolved. Recent geologic and geophysical observations have confirmed that gently-dipping detachment faults can slip at such shallow dips and host moderate-to-large earthquakes. Here, we analyze the first 3D dynamic rupture models to assess how different stress and strength conditions affect rupture characteristics of low-angle normal fault earthquakes. We model observationally constrained spontaneous rupture under different loading conditions on the active Mai’iu fault in Papua New Guinea, which dips 16-24° at the surface and accommodates ~8 mm/yr of horizontal extension. We analyze four distinct fault-local stress scenarios: 1) Andersonian extension, as inferred in the hanging wall; 2) back-rotated principal stresses inferred paleopiezometrically from the exhumed footwall; 3) favorably rotated principal stresses well-aligned for low-angle normal-sense slip; and 4) Andersonian extension derived from depth-variable static fault friction decreasing towards the surface. Our modeling suggests that subcritically stressed detachment faults can host moderate earthquakes within purely Andersonian stress fields. Near-surface rupture is impeded by free-surface stress interactions and dynamic effects of the gently-dipping geometry and frictionally stable gouges of the shallowest portion of the fault. Although favorably-inclined principal stresses have been proposed for some detachments, these conditions are not necessary for seismic slip on these faults. Finally, we explore how off-fault damage and slip on steeper splay faults in the hanging wall of a detachment fault influences shallow rupture patterns and coseismic surface displacement during large earthquakes. We present a new suite of models with synthetic or antithetic splay faults dipping 45°, 60°, or 75° that incorporate off-fault plastic failure for different host rock strengths. Coseismic splay fault reactivation limits shallow slip on the detachment and localizes surface displacements outboard of the detachment trace, most strongly when synthetic shallowly-dipping splay faults are present. Our results demonstrate how integrated geophysical and geologic observations can constrain dynamic rupture model parameters to develop realistic rupture scenarios of active faults that may pose significant seismic and tsunami hazards to nearby communities.