The Frailty of Earthquake Faults: Cracks, Damage, and
Fault Interaction 

Elizabeth Cochran
Earth and Space Sciences
U.C. Los Angeles

ABSTRACT

Recent studies of fault structure and behavior indicate
that faults maintain a memory of slip and stress
different from that of the surrounding rock. This has
implications for how faults respond to stressing and
the resulting fault interaction. We better
characterize faults and their associated properties
using detailed studies across several faults in the
Mojave Desert, within the eastern California shear
zone (ECSZ).  

We examine the structure and evolution faults related
to the Landers and Hector Mine ruptures.  Following
both the Landers and Hector Mine earthquakes we used
trapped waves to delimit a 100-200 m wide zone of
highly damaged rock and found significant velocity and
shear modulus reduction within this zone to at least 5
km depth. In addition, we tracked the healing, or
velocity recovery of these fault zones. Our study of
the Landers fault zone showed a consistent increase in
velocity in the years following the mainshock until
the time of the Hector Mine earthquake. The Hector
Mine earthquake shook and re-damaged the Landers fault
zone resulting in a temporary reversal of healing.

InSAR and anisotropy studies complement the trapped
wave studies by providing a regionally extensive view
of the deformation field. Anisotropy studies show
rotation in microcrack orientation along strike of the
Hector Mine earthquake and also variable distribution
of crack density.  Post-seismic InSAR images indicate
poroelastic rebound is a major player in deformation
fields following both the Landers and Hector Mine
earthquakes. Localized zones of post-seismic
deformation detected by InSAR correspond to regions of
high crack density and velocity reduction observed by
anisotropy and trapped wave studies.

While we see dramatic recovery of the fault zones
within the first few years after major rupture the
long term weakness of faults is evident from coseismic
InSAR images during both the Landers and Hector Mine
earthquakes (Fialko et al., 2002). They see strain
localized on compliant zones of nearby unbroken
faults, i.e. Pinto Mountain, Calico and Rodman faults,
indicating that fault zones are weaker than the
surrounding country rock for many years. By combining
the short- and long- term behavior of faults we can
get a better picture of fault structure as well as how
faults respond to imposed stress.