EVIDENCE FOR SEISMIC STRENGTHENING FROM UNDRAINED SHEAR STRENGTH MEASUREMENTS

Earthquakes are a primary trigger of submarine (and subaerial) landslides yet some of the most seismically active areas on Earth show a surprisingly low frequency of submarine landslides. Here we show that within the uppermost 100 meters below seafloor (mbsf) in previously unfailed 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 lithology and stress state. The enhanced shear strength is highest within the uppermost 10 mbsf. These results indicate that large areas of modern day slopes on active margins have enhanced slope stability, which may explain the relative paucity of landslides. These findings lend support to the seismic strengthening hypothesis that the repeated exposure to earthquake energy gradually increases shear strength by shear-induced compaction. The fact that active margin sediments are stronger than passive margins is a surprising result in light of conventional understanding that sediments compact at similar rates with burial depth regardless of location (Athy’s Law). This study suggests additional mechanisms other than progressive burial should be considered in compaction models along active margins.