Abstract

Evidence of rapid seismic-induced subsidence at Humboldt Bay, California, is produced by analyses of annual growth rings of relict Sitka spruce [Picea sitchensis (Bong.) Carr.] roots and entombed herbaceous plants. These results add to previously reported evidence that an earthquake caused subsidence ∼300 yr ago at Mad River slough, California. Both types of remains are rooted in buried soils that stood at or above the high-tide level until the area subsided at least 0.5 m into the intertidal zone. Burial by intertidal muds took place quickly enough to preserve the herbs in the growth position. Analysis of the annual growth rings of the tree roots shows that all died within four growing seasons, but the time of root death varies even among roots of the same tree. With no central nervous system, tree cells do not die simultaneously throughout the organism. The 0.5 to 1.5 m of subsidence, as evidenced by stratigraphy and sedimentology, was not enough to kill all the trees even in one season. Although such gradual death could be due to rapid aseismic subsidence, the tree deaths and preserved herbs are much better explained by sudden coseismic subsidence.

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