ESTIMATING COSEISMIC SUBSIDENCE FROM COASTAL MARSH STRATIGRAPHY: INCREASING THE MINIMUM

In the past 3,650 years (cal. yr. B.P.) evidence of coseismic subsidence was recorded five times in stratigraphy of bay margin deposits in southern Humboldt Bay, California. 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, the 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.

The largest subsidence estimates based on the biostratigraphic paleoelevation method are determined to be a minimum of 0.9 meters. This minimum estimate is increased by utilizing the relief of the upper contact for one buried soil. The relief of the upper contact was over two meters. Since the paleoecology of the soil was sub-aerial and freshwater pre-submergence, and the entire soil was coseismically buried by tsunami sands and then by tidal silts to clay-silts, the subsidence estimate is increased from less than one, to greater than two meters.