Sediment gravity flows triggered by remotely generated earthquake waves

What is it about?

Recent giant subduction zone earthquakes and tsunamis around the world, such as the 2011 magnitude (Mw) 9.0 Tohoku, Japan and 2010 Mw8.8 Maule, Chile earthquakes and ensuing tsunamis, have heightened awareness of the inevitability of similar events occurring within the Cascadia Subduction Zone in the U.S Pacific Northwest and Southwestern British Columbia, Canada. It has been 317 years since the last giant Mw9 Cascadia earthquake and tsunami, raising concerns about when the next one may strike. Estimates of the likelihood of the next ‘big one’ depend strongly on knowledge of how often previous megathrust subduction zone earthquakes have occurred, based on evidence they leave behind in the geologic record. One such geological recording of past giant earthquakes are widespread submarine slope failures (landslides) of rock debris and sediments that flow and settle on the seafloor as turbidite deposits shaken loose by the earthquakes’ seismic waves. These landslides originate along the steep slopes of the continental margin, shelf and submarine canyons. It has always been assumed that earthquake-triggered turbidites result from shaking radiated from nearby giant earthquakes. However, new results from the analysis of 4 years of ocean bottom sensor deployments in Cascadia, off the Washington, Oregon, and California coasts from 2011 to 2015, have shown that the seismic waves from a 2012 Mw8.6 earthquake near Sumatra, 13,000 km distant, likely caused slope failures and sediment flows on the Cascadia margin. Numerous studies have shown that seismic waves generated from large but distant earthquakes have caused faults to fail, producing sympathetic earthquakes that occur well after the initial distant triggering event, but this is the first study documenting their potential to trigger submarine slope failures and sediment flows.

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Why is it important?

The new recognition that seismic waves from distant earthquakes can destabilize submarine sediment slopes, not just by shaking from local earthquakes, is likely to provide an additional mechanism for semi-regular reshaping of the continental margin and should be considered in assessment of seismic hazards that use turbidites to constrain the reoccurrence interval of past giant earthquakes. In addition, the potential to observe these processes on the seafloor as they happen, as in the new Cascadia study, suggests novel opportunities to study submarine slope stability. Submarine slope failures and sediment flows have broken and continue to threaten submarine communications cables. Tsunami-generating slope failures occurring on distant continental margins is evident in documented massive submarine landslides that take place not only in tectonically active regions like the Cascadia subduction zone, but similar events are also possible along seismically-stable margins like the U.S. Atlantic seaboard.