  Accurate models of fault zone geometry are important for scientific and
hazard applications. While seismicity can provide high-resolution point
measurements of fault geometry, extrapolating these measurements to volumes may
involve making strong assumptions. This is particularly problematic in
distributed fault zones, which are commonly observed in immature faulting
regions. In this study, we focus on characterizing the dip of fault zones in
Southern California with the goal of improving fault models. We introduce a
novel technique from spatial point process theory to quantify the orientation
of persistent surficial features in seismicity, even when embedded in wide
shear zones. The technique makes relatively mild assumptions about fault
geometry and is formulated with the goal of determining the dip of a fault zone
at depth. The method is applied to 11 prominent seismicity regions in Southern
California. Overall, the results compare favorably with the geometry models
provided by the SCEC Community Fault Model and other focused regional studies.
More specifically, we find evidence that the Southern San Andreas and San
Jacinto fault zones are both northeast dipping at seismogenic depths at the
length scales of 1.0-4.0 km. In addition, we find more limited evidence for
some depth dependent variations in dip that suggest a listric geometry. The
developed technique can provide an independent source of information from
seismicity to augment existing fault geometry models.
