As part of a program to determine the location and geometry of the
San Andreas Fault (SAF) buried beneath shallow sediments near its
northern landward limit, three >20-m-long parallel trenches were
constructed at positions distributed over a distance of similar to
55 m. The majority of excavated material comprised unconsolidated
fluvial sediments deposited in a number of paleochannels. Single
zones of active faulting identified in each of the trenches were
initially interpreted in terms of a solitary strand of the SAF. To
map the SAF between and beyond the trenches and to detect other active
fault zones hidden by the young sedimentary cover, we collected a
dense ground-penetrating radar (georadar) data set across a 23.2
x 72 m area. The data were recorded using a semi-automated acquisition
system that included a conventional georadar unit coupled to a self-tracking
laser theodolite with automatic target recognition capabilities.
Since these data were plagued by system ringing as a result of the
moderate-to-high electrical conductivities of the surficial sediments,
an extensive data processing scheme was required to extract meaningful
subsurface information. The final processed georadar volume (cuboid)
contained numerous subhorizontal and trough-shaped reflections that
originated from the fluvial paleochannels. Using the geological interpretation
of the trench walls as a guide to pick semi-automatically the times
of the most important reflecting horizons, we discovered that alignments
of the nearly linear boundaries of these horizons defined two NW-SE
trending strands of the SAF within the survey area. The georadar
expression of the eastern SAF strand could only be traced over a
distance of similar to 38 m. It had been intersected in the northern
trench. In contrast, the western SAF strand extended over the entire
length of the georadar volume and had been intersected in the central
and southern trenches. Prominent reflections on georadar cross sections
were found to be vertically displaced by 0.2-0.3 m across both SAF
strands. A conspicuous linear-trending feature observed on horizontal
sections at 3.3-3.6 in depth was laterally offset by 4.5 - 5.5 m
along the eastern SAF strand. The interpreted vertical and horizontal
offsets could have been generated by the 1906 San Francisco earthquake
and/or earlier events. Undetermined amounts of aseismic slip may
also have occurred along the newly defined SAF strands.