Seafloor geomorphology of convergent margins: Implications for Cascadia seismic hazard

Abstract

We compare the geomorphology of several convergent continental margins to constrain the seismic hazard of the Cascadia margin offshore Oregon, and present the possibility of a slow earthquake mechanism for a characteristic Cascadia event. The Cascadia seafloor has a very delicate bathymetry, with well‐preserved landslides and noneroded slopes approaching 20°, unusual for a margin that produces M∼9 earthquakes. Three‐dimensional seismic and multibeam bathymetry data from the Nankai Trough suggest ubiquitous erosion over the entire margin with a smooth lower slope, devoid of large landslides, as would be expected on an accretionary margin that produces M∼8.5 earthquakes. The accretionary Makran (Pakistan) and Kodiak (Alaska) margins have evidence of mass wasting and smooth lower slopes that lack large landslides. The nonaccreting Nicaraguan, Sanriku, and Aleutian margins have large, well‐preserved landslides that add roughness elements to the slope, and have produced anomalously large tsunami, suggesting a slow source mechanism. Quantitative analysis of the lower slope roughness suggests the Cascadia has a characteristic geomorphology substantially different than the other sedimented convergent margins. The geomorphology, heat flow, pore pressure regime, and accounts of the 1700 “megathrust” event suggest a possible characteristic earthquake with a slow source mechanism. Rupture velocity would be high enough to be tsunamigenic, but accelerations would be low such that downslope erosion is minimal. To make the distinction between a slow and rapid source mechanism is critical in planning for seismic hazard in the Pacific Northwest.

Late Oligocene–early Miocene olistostromes (sedimentary mélanges) as tectono-stratigraphic constraints to the geodynamic evolution of the exhumed Ligurian accretionary complex (Northern Apennines, NW Italy)

“…In this framework, mass-transport processes are leading agents in maintaining the dynamic equilibrium by reshaping the accretionary wedge topography and producing high concentration of small-to medium-scale submarine landslide accumulations (McAdoo et al 2004;Mosher et al 2008;Camerlenghi et al 2009;Moore et al 2009;Harders et al 2011;Strasser et al 2011) and megaslides (Moore et al 1976;Golfinger et al 2000;Cochonat et al 2002;von Huene et al 2004;Hühnerbach et al 2005;von Huene, 2008;Yamada et al 2010). Earthquake shaking, as well as long-term causal factors (i.e., sea level variations, rate of sedimentation, gas-hydrates instability, tectonic oversteepening), may exert a primary role in triggering slope failures (e.g., Keefer 1984;Hampton et al 1996;Cochonat et al 2002).…”

Submarine mass failures as tsunami sources: their climate control

“…Their size varies from 'super-scale' in Cascadia (Goldfinger et al 2000) to 'small' along the Sunda margin in the Indian Ocean (Tappin et al 2007). Off Japan, there are large SMFs on the upper parts of the Nankai accretionary prism (Kawamura et al 2009), but the highly eroded lower slopes show little evidence of large, well-preserved events (McAdoo et al 2004). Along the Makran and Kodiak accretionary margins there is evidence of mass wasting on the upper slopes, with the lower slopes lacking large SMFs.…”

“…SMFs have been mapped along many convergent margins (McAdoo et al 2004). Their size varies from 'super-scale' in Cascadia (Goldfinger et al 2000) to 'small' along the Sunda margin in the Indian Ocean (Tappin et al 2007).…”

Mass Transport Events and Their Tsunami Hazard

“…are small-scale and less of a hazard in sourcing destructive events (Tappin et al 2007). Submarine landslides have been mapped on many convergent margins (McAdoo et al, 2004). Their size varies from 'super-scale' in Cascadia (Goldfinger et al 2000) to 'small' along the Sunda margin in the Indian Ocean (Tappin et al 2007).…”