OBSERVATIONS ON DIATOM PRESERVATION IN POST-EARTHQUAKE MUDDY DEPOSITS, HUMBOLDT BAY, CALIFORNIA

Evidence for coseismic subsidence associated with the 1700 Cascadia subduction zone earthquake is observed along northern Humboldt Bay, California. Unlike locations in the Pacific Northwest that show evidence for tsunami deposits associated with this event, in the interior of northern Humboldt Bay there are no coarse-grained deposits present to suggest tsunami inundation. However, preliminary observations on diatom assemblages in muddy sediment deposited after the earthquake suggest there may be variability in degree of preservation in post-earthquake deposits. To quantify this, we examined diatoms at different stratigraphic interval in 3 gouge cores collected from marshes bordering Humboldt Bay. In each core, pennate diatoms found in sediment associated with subsidence during the 1700 event were counted and categorized based on degree of fragmentation. These data are compared with observations for diatom assemblages from control samples not associated with the 1700 event and assumed to have accumulated under natural estuarine conditions. Based on the results of Q-mode cluster analysis, preliminary results suggest that there may be more fragmentation of diatoms in sediment deposited immediately after the earthquake compared to sediment that accumulated some time later.