Geodetic and Geologic Observations Along the Southern Cascadia Subduction Zone: Implications for Strain Accumulation in the North America Plate – the Lahsāséte Fault

Date: 4/24/2019

Time: 02:30 PM

Room: Puget Sound

Geodetic evidence for tectonic deformation in northern California associated with the overlapping Cascadia megathrust and northern San Andreas fault informs us of a direct impact upon local sea level rise and seismic hazard.

We analyse 3 independent geodetic data sets (tide gage, benchmark survey, GPS) to document 20^th to 21^st century rates of vertical land motion (VLM) along coastal northern California. VLM rates, corrected for glacial isostatic adjustment, range from about -5 mm/yr in southern Humboldt Bay to +2 mm/yr near Crescent City in the north. An East-West trending variation in VLM is primarily due to Cascadia subduction zone plate tectonics

We associate a second order heterogeneous North-South trend in VLM to crustal fault related strain. Discrete changes in VLM rates are localized to 5 known (and 2 previously unknown) Quaternary active crustal [MHH1] [JRP2] faults. We use these offsets to derive vertical separation and slip rates for these faults.

The faults we calculate vertical separation rates for include the newly described Lahsāséte fault 1.2-1.9 mm/yr, Table Bluff fault 3 mm/yr, Little Salmon fault 1.9-2.1 mm/yr, a newly described Eureka fault 1.3-1.6 mm/yr, Fickle Hill fault 1.3-1.6 mm/y, Trinidad fault 1.2-2.4 mm/yr, and Big Lagoon – Bald Mountain fault system 0.6-1.2 mm/yr.

The geodetic location of the Lahsāséte fault led to our discovery of a south facing topographic scarp crossing Quaternary fluvial terraces along the Eel River at Shively, CA with increasing scarp heights on progressively older terraces.

Using regional incision rates we estimate the terrace ages based on relative elevation. We use LiDAR measured scarp heights to derive a mean late Quaternary slip rate of about 0.35 mm/yr.

The Lahsāséte fault may be the southernmost expression of the south vergent imbricate thrust system related to the onshore portion of the Cascadia subduction zone. Alternatively, this structure may be evidence that the Russ fault may be a south vergent thrust fault, placing younger Yager Terrane stratigraphically above Franciscan Fm.

Presenting Author: Jason R. Patton

Authors