Van Tuyl Lecture: Derek Sawyer, Ohio State University
February 14 @ 4:00 pm - 5:00 pm
Derek Sawyer, Ohio State University, School of Earth Sciences
241 Berthoud, 4-5PM
Topic: Subaqueous Debris Flow Processes and Deposits
Abstract: Earthquakes are a primary trigger of submarine landslides yet some of the most seismically active areas on Earth show a surprisingly low frequency of submarine landslides. Our recent work has explored this apparent paradox using shear strength measurements from 50 years of scientific ocean drilling over the world’s oceans. We find that within the uppermost 100 meters below seafloor sediment, active margins have elevated shear strength by a factor of 2-3 relative to the same interval on passive margins. The elevated shear strength is seen in a global survey of undrained shear strength with depth, as well as a normalized analysis that accounts for lithological and effective stress differences. These results indicate that large areas of modern day slopes on earthquake-prone margins have enhanced slope stability. This may help explain the relative paucity of landslides observed on active margins. The mechanisms that lead to the observed strengthening are not fully clear but this observation is consistent with the seismic strengthening hypothesis: repeated exposure to earthquake energy over time gradually increases shear strength by shear-induced compaction. This observation of higher shear strength on active margins holds true in locations typified by relatively low sedimentation rates and hydrostatic pore pressure conditions. However, a different result is observed in a high-sedimentation rate and high-seismicity setting: the southern Alaskan offshore margin. The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examined recent shear strength measurements acquired by Integrated Ocean Drilling Program Expedition 341 on the continental slope and Surveyor Fan. These data reveal lower-than-expected sediment strength. Contrary to other active margins where seismic strengthening enhances slope stability, the high-sedimentation submarine fan environment behaves like a passive margin from a shear strength perspective. We interpret that seismic strengthening occurs but is offset by the high sedimentation rates and fluid overpressure within the slope and Surveyor Fan. This is supported because shear strength follows an active margin profile outside of the fan, where slower background sedimentation rates occur. More broadly, this suggests that seismically active margins with wet-based glaciers near the coast should be considered highly susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking.