Shallow geometry and displacements on the San Andreas Fault near Point Arena based on trenching and 3-D georadar surveying

Abstract

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.