Impact‐Induced Seafloor Deformation From Submarine Landslides: Diagnostic of Slide Velocity?

Submarine landslides shape continental margins, transfer massive amounts of sediment downslope, and can generate deadly and destructive tsunamis. Submarine landslides are common globally, yet constraining hazard potential of future events is limited by a short historical record and a wide range of possible slide dynamics. We test a novel approach to investigate slide dynamics using properties of the deformation zone induced by a large submarine landslide along the Cascadia margin, offshore Oregon. We use a simple model of a line load on a poroelastic half space to show the deformation zone size required rapid transport and deceleration. We argue that the slide moved at high speeds, aided by low dynamic frictional resistance, suggesting this event could have generated a tsunami. This method is applicable where slide‐induced impact zones are observed. Plain Language Summary The Cascadia margin is susceptible to underwater landslides and tsunami hazards due to the active subduction zone that can produce large magnitude earthquakes. However, determining whether future submarine landslides will be tsunamigenic is challenging. While past landslide deposits can easily be identified and in many cases age‐dated, the dynamic process including the initial acceleration and velocity that created the deposit is almost never known. We present a novel approach to back‐analyze slide velocity of past landslides by using the characteristics of the deformation zone that occurred when the landslide deposit came to rest on the seafloor. Our models suggest that the slide hydroplaned while moving at high‐speeds (up to 60 m/s) with a run‐out distance of 10‐km from the source, which match the observations. While our method does not model tsunamis explicitly, the high speeds are consistent with known tsunamigenic slides. This approach provides critically important constraints on the slide velocity and therefore the hazards of submarine landslides that can occur at Cascadia. Key Points A broad zone of impact‐induced deformation is observed immediately adjacent to the massive 44‐N Slide offshore Oregon The deformation of seafloor sediments observed at the slide terminus requires slide deceleration, implying catastrophic emplacement Modeled impact forces require a slide velocity of up to 60 m/s, which may have been sufficient to generate a tsunami