Active Faulting in the Region of the Mendocino Triple Junction: Field Investigations of the Lahsāséte Fault

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The Mendocino triple junction has overlapping tectonic regimes, with the San Andreas fault (SAF) to the south and Cascadia megathrust to the north. Multiple phases of tectonic deformation are observed that represent changes in relative plate motion during the Cenozoic. Late Pleistocene northward migration of the triple junction juxtaposes north-south convergence from Pacific-North America (PAC-NAM) plate motion at the northern on-shore termination of the SAF with E-W compression from Cascadia. Active N-NW striking faults like those in the N30W striking Mad River fault zone near Arcata represent deformation related Cascadia convergence. Structures striking ~E-W, like the Table Bluff fault south of Eureka and the Russ fault zone along the Eel River, represent deformation related to PAC-NAM convergence where slip is transferred from the termination of the SAF and strain is partitioned onto structures to the east. These crustal faults contribute to the seismic hazard in the region and may comprise a third of the tectonic strain in coastal northern California. We examine an E-W striking fault near the Eel River that is not currently incorporated in seismic hazard models for the region.

We locate a topographic scarp adjacent to the Russ fault zone that probably represents Holocene slip on a west striking reverse fault offsetting late Pleistocene to Holocene fluvial terraces. We identify this structure as the Lahsāséte fault, a south facing scarp crossing multiple terraces with increasing scarp heights on progressively older terraces. Using regionally derived incision rates as a proxy for terrace age, we use topographic swath profiles to measure scarp heights and calculate a late Pleistocene slip rate of about 0.75 mm/yr.

We utilize Ground Penetrating RADAR surveys to image the Lahsāséte fault in the subsurface. These data will help to determine the thickness of sediments overlying the terrace straths (for additional slip rate estimates) and to locate the fault in preparation for a paleoseismic investigation to constrain timing and slip in the most recent surface deforming event.

Presenting Author: Jason R. Patton

Authors