In the last 15 years geodetic and geologic studies have documented that although the San Andreas fault system accounts for most of the total displacement between the Pacific and North American plates, a significant amount of dextral slip is occurring along a series of fault zones known as the Walker Lane or Eastern California shear zone. Geodetically measured displacements have been used to define a block between the San Andreas fault system and Walker Lane, the Sierra Nevada microplate, that is rotating counterclockwise about an Euler pole to the SSW. The northern and northeast margins of the Sierra Nevada microplate are not well defined seismically or geodetically.

This study at Eagle Lake examined evidence for continuation of right-lateral slip along the Walker Lane to the north of the inboard projection of the Mendocino Triple Junction. The northernmost faults of the Walker Lane in northeast California known to have Holocene and historic slip are the Honey Lake fault zone and the Mohawk Valley fault zone. In map view, it appears the Honey Lake fault zone either branches or steps right to a lineament extending northward through Eagle Lake. Previous mapping studies reveal that Eagle Lake is dominated by middle Miocene to late Quaternary basalts and mafic andesites and local dikes and vents appear to be fault controlled.

Aerial photo analysis, fieldwork and bathymetric data confirm the presence of northwest-striking faults, with the principle through-going fault extending 40 km across Eagle Lake and continuing northward. Sub-horizontal slickenlines (pitch <~20º) on blocky basalt faces were recorded on this principle fault and on a few of the sub-parallel minor faults. This principle fault is also expressed by northwest-trending fresh scarp talus slopes. Bathymetric data reveal a significant and consistent drop in depth, SW side down, along a line that trends into the principle fault. Analysis of lineament trends in conjuction with field data indicate a predominantly strike-slip geometry, with a lesser component of extension. These analyses show that dextral slip associated with Walker Lane continues north of Honey Lake and deformation associated with the Sierra Nevada microplate propagates east and north of the microplate boundary.