Damage zones are important to the rupture dynamics, evolution and fluid
coupling of earthquakes. However, information about the damage zone at
depth is limited. It is unclear if damage zones increase or decrease in
intensity with depth. Here we use marine 3-D seismic surveys and modern
fault detection methods to address the depth-dependent structure of
damage zones. We use two overlapping legacy industry seismic volumes
collected offshore of Los Angeles span approximately 20 km of the Palos
Verdes strike-slip fault. The data here allows visibility of the damage
zone in the sedimentary formations to 2,200 meters depth, which is
comparable to the constraints provided by SAFOD and other studies. Using
both interpreted mapped primary fault strands and seismic attributes to
identify subsidiary faults, we map and quantify spatial variations in
damage zone size and intensity. The damage zone consists of subsidiary
faults, or linked discontinuities in the seismics selected within
assigned ranges of geometries to the primary strands. Damage was
identified using a variation of the seismic attribute semblance, or
multi-trace similarity. This method allows interrogation of damage zone
in response to changes sedimentary lithology and fault geometry.
Subsidiary faults delineate the damage zone to approximately 1 km in
width and fracture density decays with distance from the primary fault
strands for all sedimentary lithologies in the study area. The damage
zone narrows with depth, but fracture density increases because the
intensity of fracturing more than compensates for the decreased width.
In the thickest formation we find that fracture density increases as
Z1.8, where Z is depth in meters. These results are then compared to
resolution changes with depth. The damage intensity increase and
localization potentially provides a strong constraint for efforts to
determine an appropriate rheology for producing damage zones and
studying their effects.