PERPLEXING RECORDS OF COSEISMIC SUBSIDENCE: EVALUATION OF SOURCES FOR COSEISMIC SUBSIDENCE IN THE SOUTHERN CASCADIA SUBDUCTION ZONE, NORTHERN CALIFORNIA

While the segmented recurrence of Cascadia subduction zone (CSZ) earthquakes is well resolved in the regions of Oregon, Washington, and Canada, there remains considerable uncertainty about this active seismic hazard in northern California (CA). The seismoturbidite chronology has been the key to unravelling previously unresolved earthquake correlations between terrestrial paleoseismology sites. Due to the high frequency of turbidite occurrence south of Smith River Canyon, estimating the recurrence for CSZ earthquakes here has been more indeterminate. The turbidite record south of the Smith River may record these more frequent, smaller magnitude earthquakes (either megathrust or intraplate). Records of coseismic subsidence in the Humboldt Bay region are not as easily attributed to the megathrust and may be related to faulting in the North America plate (e.g. Little Salmon, Mad River faults). This includes the 1700 AD CSZ earthquake.

Radiocarbon based earthquake chronology applied to stratigraphy in the Humboldt Bay/Eel River region spans several decades and includes a range of uncertainty from bulk ages calculated using conventional radiocarbon analyses to plant identifiable sample ages determined from accelerator mass spectrometry analyses. We use reported radiocarbon ages to build an OxCal age model and compare these results with the existing offshore chronology. We consider the possible scenarios to explain the stratigraphic evidence for coseismic subsidence given the radiocarbon chronology for earthquakes and tsunami in northern California.

We also incorporate secular geodetic observations (tide gage, GPS, level analyses) for a comparison. Land subsidence in and around Humboldt Bay, CA contributes to sea-level rise up to 2-3 times greater than anywhere else in California. This is in contrast to Crescent City, CA where the land is rising. Buried marsh horizons around the Humboldt Bay region have been interpreted to be the result of coseismic subsidence during interplate faulting on the CSZ, though these horizons are in regions that are currently subsiding. We re-interpret these buried horizons to be the result of a more complex deformational setting that may include active faulting in the upper plate as part of the local plate boundary deformation cycle.