Derek Sawyer and his graduate student Joshua DeVore published a study in December 2015 in the journal Geophysical Research Letters that sheds light on a paradox: given that earthquakes are a primary trigger of underwater landslides, why do the most earthquake- active areas on Earth have fewer submarine landslides than passive margins?
Sawyer and DeVore provide an answer to this paradox by demonstrating a simple, but surprising, result: sediments are stronger on active margins and thus are less prone to landsliding. Their dataset contains over 1000 shear strength measurements made by the Ocean Drilling Program in a range of active and passive margins (Figure). They show that within the upper 100 meters below seafloor, which is where seafloor landslides originate, sediments on active margins have higher shear strength by a factor of 2-3 relative to the same interval on passive margins.
The fact that active margin sediments are stronger than passive margins is a surprising result in light of conventional understanding that seafloor sediments compact at similar rates with burial depth regardless of location (Athy’s Law). The more compaction, the higher the shear strength, which is the key parameter to withstand landsliding. This study suggests additional mechanisms other than progressive burial should be considered in compaction models along active margins.
The paper lends support to the conceptual model of ‘seismic strengthening’ in which it is proposed that the repeated exposure to shaking from small-magnitude earthquake events can physically dewater sediments and thus enhance compaction and strength. This model remains mostly untested however.
Sawyer has been awarded 2 years of funding from the American Chemical Society to investigate the mechanism behind the enhanced strengthening and results can have important implications for understanding the feedbacks between earthquakes and submarine landslides as well as for understanding the physical processes that govern mudrock properties early in burial history. Funding will support graduate student research beginning in September of this year.
More details can be found in the published article:
Sawyer, D. E., and J. R. DeVore (2015), Elevated shear strength of sediments on active margins: Evidence for seismic strengthening, Geophysical Research Letters, 42, doi:10.1002/2015GL066603.