20thto 21stCentury Relative Sea and Land LevelChanges in Northern California:Tectonic Land Level Changes and theirContribution to Sea-Level Rise, Humboldt BayRegion, Northern California

Abstract

Sea-level changes are modulated in coastal northern California by land-level changes due to the earthquake cycle along the Cascadia subduction zone, the San Andreas plate boundary fault system, and crustal faults. Sea-level rise (SLR) subjects ecological and anthropogenic infrastructure to increased vulnerability to changes in habitat and increased risk for physical damage. The degree to which each of these forcing factors drives this modulation is poorly resolved. We use NOAA tide gage data and ‘campaign’ tide gage deployments, Global Navigation Satellite System (GNSS) data, and National Geodetic Survey (NGS) first-order levelling data to calculate vertical land motion (VLM) rates in coastal northern California. Sea-level observations, highway level surveys, and GNSS data all confirm that land is subsiding in Humboldt Bay, in contrast to Crescent City where the land is rising. Subtracting absolute sea-level rate (~1.99 mm/year) from Crescent City (CC) and North Spit (NS) gage relative sea-level rates reveals that CC is uplifting at ~2.83 mm/year and NS is subsiding at ~3.21mm/year. GNSS vertical deformation reveals similar rates of ~2.60 mm/year of uplift at Crescent City. In coastal northern California, there is an E-W trending variation in vertical land motion that is primarily due to Cascadia megathrust fault seismogenic coupling. This interseismic subsidence also dominates the N-S variation in vertical land motion in most of the study region. There exists a second-order heterogeneous N-S trend in vertical land motion that we associate to crustal fault-related strain. There may be non-tectonic contributions to the observed VLM rates.

Crafting effective oversight for the long-term storage of spent nuclear fuel on sites at risk of climate and coastal hazards

“…Research participants recognized the basic need to update annual safety reports as new scientific information emerges. For example, PG&E's safety reporting for the HB ISFSI assumes tectonic uplift (PG&E, 2021), despite scientific consensus and research produced by some of our study participants that subsidence is causing Humboldt Bay to experience the highest rate of relative sea level rise in California (Patton et al, 2023). Additionally, although PG&E's current safety reports submitted to the Nuclear Regulatory Commission ("NRC") analyze risks to the stored fuel from a magnitude 8.8 earthquake (PG&E, 2021: 172, 216, 218), a geologist at Cal Poly Humboldt specialized in tectonics evaluated the probable risk to be higher: "So if you ask me, should we prepare for a 9.2 Cascadia Megathrust event?…”

“…Meanwhile, Humboldt Bay is experiencing the fastest rate of relative sea level rise in California (Anderson, 2018;Patton et al, 2023). It remains uncertain whether 37 tons of SNF can be safely stored on an erosive coastal bluff "in perpetuity" (California Coastal Development Permit, 2005;California Coastal Commission, 2011).…”