  Fault zones are associated with multi-scale heterogeneities of rock
properties. Large scale variations may be imaged with conventional seismic
reflection methods that detect offsets in geological units, and tomographic
techniques that provide average seismic velocities in resolved volumes.
However, characterizing elementary localized inhomogeneities of fault zones,
such as cracks and fractures, constitutes a challenge for conventional
techniques. Resolving these small-scale heterogeneities can provide detailed
information for structural and mechanical models of fault zones. Recently, the
reflection matrix approach utilizing body wave reflections in ambient noise
cross-correlations was extended with the introduction of aberration corrections
to handle the actual lateral velocity variations in the fault zone [Touma et
al., Geophys. J. Int. 226, 780-794, 2021]. Here this method is applied further
to analyze the distribution of scatterers in the first few kilometers of the
crust in the San Jacinto Fault Zone at the Sage Brush Flat (SGB) site,
southeast of Anza, California. The matrix approach allows us to image not only
specular reflectors but also to resolve the presence, location and intensity of
scatterers of seismic waves starting with a simple homogeneous background
velocity model of the medium. The derived three-dimensional image of the fault
zone resolves lateral variations of scattering properties in the region within
and around the surface fault traces, as well as differences between the
Northwest (NW) and the Southeast (SE) parts of the study area. A localized
intense damage zone at depth is observed in the SE section, suggesting that a
geometrical complexity of the fault zone at depth induces ongoing generation of
rock damage.
