Secular Deformation in Southern Cascadia: Elastic Modeling as Informed by Geodetic Observation

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Northwestern California is sheared over time by deformation along the Cascadia megathrust, the northern San Andreas fault, and subsidiary faults. We use geodetic data and modeling to shed light on the earthquake cycle and seismic hazard in this region. We synthesize tide gage, benchmark survey, and GPS data to document 20^th-21^st century rates of vertical land motion. These rates (corrected for glacial isostatic adjustment) range from -5 mm/yr in southern Humboldt Bay to +2 mm/yr near Crescent City. Subsidence in Humboldt Bay is apparent in all three datasets and may suggest downwarping of the surface above a local locked patch on the megathrust, motion on upper-plate faults, or possible anelastic/nontectonic processes.

We explore these possibilities using the new Slab2.0 Cascadia geometry and inverse methods that combine these uplift rates with horizontal GPS velocities corrected for locking on the San Andreas. We also examine how the earthquake cycle on these faults would affect the local land surface.

Uniform-slip models with the Slab2.0 geometry suggest a Cascadia earthquake rupturing from the trench to 35 km depth (located ~75 km inboard of the coast) would uplift Humboldt Bay by ~15% of the slip amount (e.g. ~2 m for 15 m slip), but an earthquake that only ruptured offshore (where existing interseismic models put the highest locking) would instead cause Humboldt Bay to subside by this amount; this is important for coastal infrastructure and tsunami hazard.

We also explore how postseismic viscoelastic relaxation in the mantle wedge and/or below the downgoing slab could modulate these land motions; we find that if this were to mainly occur below the slab, it would contribute modest uplift following the deep-rupture event but could contribute as much as ~200% of the coseismic subsidence following the all-offshore event (e.g. 4 m for 15 m of slip, for a total of 6 m). This may inform interpretations of coastal subsidence in past Cascadia earthquakes.

Presenting Author: Jason R. Patton

Authors