SEISMICITY AT DEPTH AND SURFACE GEOMORPHOLOGY INDICATE A BROADER, ACTIVE MAACAMA FAULT ZONE, NORTHERN CALIFORNIA COAST RANGE

Best-fit planes through clusters of earthquake foci along the Maacama Fault Zone (MFZ) indicate multiple seismically active fault zones at depth. When projected to surface, many of these seismogenic zones correlate with previously mapped and/or trenched active faults (Sickler 2005, Upp 1989). However, several foci clusters yield fault planes that project to the surface where faults are unmapped or presumed inactive. This suggests a broader zone of active deformation, comprising a number of distinct active fault strands, rather than the relatively more simple fault trace that is generally shown on current maps.

A total of 12 distinct clusters have been identified between 39-39.75° N. The strikes of best-fit planes through these clusters at depth agree with the trends of corresponding mapped fault traces at the surface. Those best-fit plane projections that do not coincide with mapped faults either project within close proximity of geomorphic features that may be unmapped faults, or do so when amalgamated with adjacent clusters. Analysis of the earthquake foci beneath Little Lake Valley (LLV) suggests a best-fit fault plane with an orientation of 338,70° NE, which agrees with published focal mechanism analyses (Castillo and Ellsworth 1993, Warren et al 1985). The surface projection of the determined best-fit fault plane is essentially coincident with faults and fault related geomorphic features mapped by Pampeyan et al (1981) and Upp (1989) on the east side of LLV, suggesting that these faults are currently active. In addition, one of these geomorphically defined fault traces has been studied using shallow resisitivity cross-sections and a well bore transect, which are interpreted to show fault offsets of Pleistocene and possibly Holocene sediments.