Dynamic Earthquake Ruptures in the Presence of Material Discontinuities

A general feature of tectonic faults is the juxtaposition of
materials with dissimilar elastic properties in a variety of
contexts and scales. Normal and reverse faults offset vertical
stratifications, large strike-slip faults displace different
crustal blocks, oceanic and continental crusts at subduction
interfaces, and oceanic transforms juxtapose rocks of different
ages. Bimaterial interfaces associated with rock damage are present
with various degrees of sharpness in typical fault zone structures,
and failure along a bimaterial interface can be effective even on
microscopic scale of grain boundaries. A first order representation
of a geological fault for seismic events is a frictional interface
embedded in an elastic body. This study focusses on dynamic effects
in the presence of material discontinuities altering dynamics of
failure and dynamic rupture propagation on frictional interfaces.
When the medium surrounding a fault is heterogeneous, the symmetry
of stress is broken up and perturbations of normal stress
introduces additional instability potentially generating additional
propagation modes of rupture. This study presents three specific
numerical investigations of the aforementioned rupture phenomena
associated with material contrasts at the fault. A first numerical
study (a) investigates 2-D in-plane ruptures in a model consisting
of two different half-spaces separated by a low-velocity layer and
possible simultaneous slip along multiple faults. This study shows
that bimaterial frictional interfaces are attractive trajectories
of rupture propagation, and ruptures tend to migrate to material
interfaces and becoming self-sustained slip pulses for wide ranges
of conditions. In a second numerical study (b), the propagation of
a purely material contrast driven rupture mode, that is associated
with the so-calledWeertman or Adams-instable pulse, is shown to
exist also in the general 3-D case, where there is a mixing of
in-plane and anti-plane modes, the bimaterial mechanism acting in
the in-plane direction only. Finally, in a further numerical
investigation (c) it is demonstrated, that the rupture dynamics and
ground motion can be significantly influenced by bimaterial
mechanisms of rupture propagation for ranges of parameters. The
model studied here comprises heterogeneous initial shear stress on
a slip-weakening frictional interfaces separating two dissimilar
elastic bodies, a free surface. The discussion focusses on the
diversity of existing rupture propagation modes and ground motion.
The investigated models and obtained results are motivated and
discussed in the context of complementary numerical investigations,
theoretical studies of stability analysis, seismological vii
observations of earthquakes and aftershock sequences, geological
observations of fault zone structures, tomographic studies, and
geodetic observations.