The dynamics of strike-slip stepovers with linking dip-slip faults

David D. Oglesby
Department of Earth Sciences
University of California, Riverside

March 11, 2005

ABSTRACT

Fault stepovers with linking dip-slip faults are common features on
long strike-slip fault systems worldwide. It has been noted by various
researchers that under some circumstances, earthquakes can jump across
fault stepovers to cascade into large events, while under other
circumstances rupture is arrested at stepovers. There is also evidence
that fault stepovers may be preferential locations for earthquake
nucleation. The present work uses the 3D finite element method to model
the dynamics of strike-slip fault systems with stepovers and linking
dip-slip faults. I find that the presence of a linking normal or thrust
fault greatly increases the ability of earthquake rupture to propagate
across the stepover, leading to a larger event. However, the stress
interaction between the strike-slip and linking normal and thrust
faults causes a qualitative difference in the behavior of the
dilational and compressional stepovers, respectively: Slip on the
strike-slip faults reduces normal stress on the linking normal fault in
a dilational stepover, while such slip increases normal stress on the
linking thrust fault in a compressional stepover. This difference in
the sign of the normal stress increment causes dilational stepovers to
be much more prone to through-going rupture than otherwise equivalent
compressional stepovers. Even in cases for which both dilational and
compressional stepovers can experience through-going rupture,
dilational stepovers typically experience higher slip on both the
linking normal fault and on the strike-slip segments. In the
compressional case, rupture nucleation on the linking thrust fault
increases the likelihood of through-going rupture. Near the
intersections between the fault segments, the stress interaction
between the fault segments also causes a significant rotation of rake
away from that which would be inferred from the regional stress
field. The results help to emphasize the importance of two-way
interactions between nearby fault segments during the earthquake
rupture process. The results also may have implications for the
probability of large earthquakes along geometrically complex
strike-slip fault systems, and may help explain why stepovers sometimes
act as barriers and other times as nucleation locations for large
earthquakes.