Elevated shear strength of sediments on active margins: Evidence for seismic strengthening

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. Here we show that within the uppermost 100 m 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.

Earthquake Impact on Active Margins: Tracing Surficial Remobilization and Seismic Strengthening in a Slope Sedimentary Sequence

“…The average S u at 9‐15 cm (~6 kPa) would only be expected at ~3‐ to 7‐m depth in a normally consolidated sequence. Also, the average normalized S u /σ′ v0 of ~12 is an order of magnitude higher than the range 0.4‐1.0 derived from a compilation of low‐resolution measurements on 100‐m‐long cores in active margins (Sawyer & DeVore, ). Therefore, we propose that seismic strengthening is most effective on the uppermost sediment and its impact decreases with burial depth and each successive earthquake.…”

Seismicity and sedimentation rate effects on submarine slope stability

“…The core probably spans 20 large Holocene earthquakes within the past 5000 yr (Lee et al, 2004). Undrained shear strength values measured in Ocean Drilling Program (ODP) cores from the Japan, Costa Rica, and Cascadia margins are significantly higher than those from the U.S. Atlantic and Amazon margins (Sawyer and DeVore, 2015).…”

“…However, the stronger correlation with earthquake recurrence and with sediment accumulation per earthquake cycle (Figs. 2B and 2C) suggests that the excess pore pressures generated by sedimentation typically have time to dissipate, so that at least normal consolidation prevails, and in sites of frequent earthquakes, the sediment may become overconsolidated, as shown in ODP sites from convergent margins (Sawyer and DeVore, 2015).…”

Sediment gravity flows triggered by remotely generated earthquake waves

“…However, the importance of assessing susceptibility and triggering of slope failure is evident in documented massive submarine landslides not only in active regions like the CSZ [ Goldfinger et al ., ] but also along tectonically stable margins like the U.S. Atlantic seaboard [ Heezen and Ewing , ; ten Brink et al ., ]. Seafloor observations of remotely generated seismic waves and their impacts provide new tools for such assessments and for understanding the continuous resurfacing of continental margins generally [ Sawyer and DeVore , ].…”

“…The primary observations we employ are seafloor temperature measurements, mostly those from the base of the continental slope where the signal of warm sediment‐laden flows clearly stands out from background variability, along with collocated measurements and analyses of pressures, seismic waves, bathymetry, and video images. Our study suggests a new approach to assessing susceptibility and triggering of submarine slope failure and new understanding the continuous resurfacing of continental margins generally [ Sawyer and DeVore , ], not only in active regions like the CSZ [ Goldfinger et al ., ] but also along tectonically stable margins like the US Atlantic seaboard [ Heezen and Ewing , ; ten Brink et al ., ].…”