The Dynamics of Unlikely Slip: 3D Modeling of Low-angle Normal Fault
Rupture at the Mai’iu Fault, Papua New Guinea
Abstract
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. 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.