FAST FRIENDS AND SLOW FAULTS: SEISMIC HAZARD STUDIES IN NORTHERN CALIFORNIA WITH RALPH KLINGER (Invited Presentation)

Geodetically derived microplate motions in the western Cordillera show that several millimeters of shear are expected between the Sierran (S) and Oregon Coast Block (OCB) microplates in a N70W orientation. Kinematic inversions of earthquake focal mechanisms suggest that this differential block motion may be accommodated in part by a combination of dextral transpression in the Northern Sacramento Valley and transcurrent shear strain in the southern Klamath Mountains. Previous Reclamation scientists identified 6 broad groups of lineaments and geomorphic features of possibly tectonic origin spatially related to faults identified in the subsurface and bedrock.

Here, we describe the results of mapping and trenching to define characteristics of 2 of these possibly active faults as well as reconnaissance observations of a newly identified lineament, all of which may accommodate a portion of the predicted deformation between the OCB and S. At the Dry Creek fault (DCF) near Redding, we observe an irregular but aligned change in surface texture that is coincident with previously recognized lineaments in vegetation and slope as well as faults interpreted in seismic reflection data. Mapping along the DCF suggests the change in texture is not coincident with bedrock contacts but is a change in geomorphic surface characteristics. Trenches excavated across DCF lineaments revealed a bedrock fault in one trench and Quaternary(?) age deposits that may be deformed. At the Bear Creek fault, we observed slope and vegetation lineaments, a large wind gap, and back tilted surfaces coincident with faults interpreted in seismic imaging. In trenches, we found tilted deposits that were dated to a minimum of ~0.8-1 Ma using detrital zircons. Trenches across both faults include ambiguity in deformation and timing. Our current interpretation is that these faults may be slowly deforming blind faults, or have not generated earthquakes in the Holocene. We recently identified linear changes in slope near the Hoadley fault that appear to displace Quaternary surfaces in the southern Klamath Mountains. By estimating the age of the surface and displacement, this fault may be moving in the tenths of mm/yr. These faults may all be important in understanding how deformation is accommodated between the OCB and S microplates.