To characterize buried portions of the San Andreas Fault (SAF) near
its northern extremity in California, three trenches were excavated
perpendicular to it and a 3-D georadar data set was acquired. Similar
sequences of unconsolidated fluvial sediments, Franciscan serpentinite
and active fault zones were observed in the three trenches. Fault
traces in the trenches were initially assumed to be part of a single
SAF strand. However, the georadar data demonstrated the existence
of two SAF strands, one of which was exposed in the central and southern
trenches and the other in the northern trench. Offset of a linear-trending
georadar feature (possibly a fluvial paleochannel) suggests that
4.5-5.5 m of horizontal displacement occurred across the eastern
SAF strand, either during the 1906 San Francisco earthquake or earlier.
Georadar reflections from flat-lying and trough-shaped sedimentary
boundaries are vertically offset by 0.2-0.3 m along both fault strands.