Estimates of minimum shaking intensity required to induce liquefaction and sediment redistribution in southern Cascadia forearc lakes

Abstract:

Sedimentary sequences from several southern Cascadia forearc lakes near the California/Oregon border contain anomalous deposits suspected to have formed as the result of strong ground motions in great earthquakes. We compared the sedimentary sequences from two pairs of lakes that lie ~30 km above the subduction interface, to determine the impact of shaking on deposit characteristics. We find that disturbance event deposit characteristics depend on the geomorphologic and geologic setting of each lake, and thick minerogenic layers occur in lakes with deltas or slides through which source water flows.

Proximal lake pair (~140 km from the trench): These lakes are cirque lakes ~20 km from one another. One lake has visible silty clay layers, and the other only has slight changes in density and mineral content not visually apparent. Deposits may be preceded by, or coincident with, a layer of coarse plant macrofossils.

Distal lake pair (~185 km from the trench): These lakes were created by the same landslide and contain thick disturbance event deposits with a high percentage of minerogenic sediment. The smaller lake contains a record of pseudo-annual flood deposits that have been interpreted as a time series of erosion magnitudes (see poster by Gavin et al. also in this session). The thickest of these events fall above a strongly linear relationship, suggesting a separate process (such as earthquakes). The thick deposits grade from organic-rich to mineral-rich, and are capped by a thin layer of fine-grained silty clay. A sequence of progressively thinner deposits follows each thick layer, and may reflect post-earthquake erosional events. The larger of the two landslide-dammed lakes contains thick minerogenic deposits with normal grading, and appear to be coeval with the thickest layers in the smaller lake. The upper portion of this record is well-dated, and likely contains a deposit resulting from the 1700AD earthquake.

We hypothesize that strong ground motions cause liquefaction of the coarse, unconsilidated sediments of the deltas or slides, causing a release of fine sediment into the lake forming the thick minerogenic layers interpreted to be seismogenic deposits. We test the hypothesis using delta composition and structure data combined with current ground motion equations for different Cascadia earthquake magnitudes.