To study the mechanics of distributed fault zone deformation we excavated two parallel trenches across a sag pond in the San Andreas fault near San Andreas Dam, San Mateo County, Calif. The trenches are parallel to, and on either side of, a fence line offset 0.7 meters in the 1906 San Francisco earthquake. The trenches exposed massive gravelly-clay in the center of the sag pond with interfingering clayey-gravel collu vium derived from the eastern hillslope and Franciscan serpentinite exposed in the western end.
We identified four, and possibly five, types of tectonic deformation. Most pronounced is a zone 0.75 m wide composed of mixed clays with anastomosing and lenticular clay bodies. There is only a weak planar fault trace within this zone and the zone aligns with the 1906 offset. Projected between the trenches the zones do not coincide suggesting a small (0.5 m) right-step between the trenches. The second style of deformation was fissuring in the eastern gravel colluvium. Some fissures showed small vertical separations of a few centi meters. It is not clear if the majority of these represent tectonic offsets or ground cracking in earthquakes. A third style of deformation was the distributed deformation between the pronounced fault zone and the western end of the trenches. This deformation penetrated the serpentinite exposed in the trench and could be correlated with offsets in the bedrock surface and small scarplets on the ground surface. The fourth style of deformation is a zone in the middle of the north trench. Gravelly colluvial layers are disrupted and gravel filled fissures are seen penetrating downward into the clay. Although this disruption is not seen in the underlying tan-brown sandy clay, the sandy clay does show the fifth, and least understood, feature: vertical veins filled with dark clay similar to that overlying it. The origin of these veins is not understood and they may be unrelated to tectonic deformation.
One question is whether grain-scale deformation of the rock or sediment can be identified using paleomagnetic methods. Preliminary results for the anisotropy of magnetic susceptibility (AMS) show an increase in the susceptibility of the clay in the pronounced fault zone with an accompanying increase in the scatter between neighboring samples but with a suggestion of a lower susceptibility region in the center of the zone with higher susceptibility at the edges. There is no apparent increase in the AMS of the samples, but again the scatter increases within the pronounced fault zone. Additional analysis is required to distinguish this as a tectonic effect from the fault motion or a sedimentological effect of mixing different clays. The serpentinite shows a strong and consistent AMS with its foliation parallel to the local trend of the fault.