Late Holocene coseismic subsidence and coincident tsunamis, Southern Cascadia Subduction Zone, Hookton Slough, Wigi (Humboldt Bay), California

Thesis (M.S.)--Humboldt State University, Geology: Environmental Systems, 2004I investigate evidence of coseismic subsidence in southern Humboldt Bay and the relation of this subsidence to Cascadia subduction zone earthquakes and to earthquakes on the Little Salmon fault, an active upper plate thrust fault that borders Humboldt Bay. Coseismic subsidence at Hookton Slough is correlated with paleo earthquakes at Swiss Hall, ca. 1 kilometer to the east. I identified five buried marsh soils over a 1-kilometer long transect along Hookton Slough, a tidal channel tributary in Humboldt Bay. Using the lateral extent of burial, the abrupt upper contacts to the soils, and the diatom biostratigraphy, soils subsided coseismically and those soil burials were accompanied by abrupt rises in relative sea level. I also infer that tsunami-transported sand, observed in the stratigraphy from Hookton Slough, was deposited directly on two soils at the time of subsidence. Buried soils at Hookton Slough are best explained by coseismic subsidence during Cascadia subduction zone earthquakes. Radiocarbon age estimates constrain timing of subsidence and allow me to estimate a recurrence interval of Cascadia subduction zone earthquakes in the Humboldt Bay region. A recurrence interval for these large earthquakes ranges from 650 to 720 years for the last 2,400 years. Three of the buried soils correlate to similar buried soils found at other sites around Humboldt Bay, and timing of subduction zone earthquakes at Hookton Slough overlaps with timing of earthquakes on the Little Salmon fault.Pacific Gas & Electric\ud U.S. Geological Survey National Earthquake Hazards Reduction Program, Award No. 01HQGR0125\ud HSU, Department of Geolog