Seismic triggering on active tectonic continental margins: Implications for hazards plus turbidite and mass transport deposition

For over a decade in the Pacific Ocean off western North America, we have studied deposition of well-correlated Holocene turbidites over great spatial length and long time intervals along the continental margins of the Cascadia Subduction Zone and northern San Andreas Fault of northern California. The correlation of these turbidites, by stratigraphic markers (i.e. ash, biostratigraphy), high-resolution radiocarbon ages and physical property signatures (i.e. density, magnetic susceptibility), implies a process of synchronous triggering. Seismic triggering by great earthquakes (equal or greater than 8 Mw) is the best candidate to explain this synchronous phenomenon of turbidite sedimentation throughout multiple turbidite systems. The following important new observations about hazards, and turbidite and mass transport deposition can be attributed to seismic triggering: (1) On both of the tectonically active margins of Cascadia and California, triggering of Holocene turbidity currents is dominantly controlled by earthquakes versus other generating mechanisms (e.g. ignitive turbidity currents, hyperpycnal flow). (2) The paleoseismic turbidites have different average recurrence times of 550 years in northern Cascadia Basin and 200 years along the northern California margin; this difference in frequency of turbidites in a subduction zone compared to a transform fault margin points out the significant differences in earthquake activity and earthquake hazards in different tectonic settings. (3) Because of earthquake triggering, both tectonically active margins of Cascadia and California exhibit continuous turbidite deposition during the sea-level high-stand of Holocene time; however, the main deposition is restricted to channel floors compared to deposition of thick beds throughout the turbidite systems during low sea levels. (4) Along the Cascadia and California margins, turbidite systems are dominant and mass transport deposits are subordinate; we attribute this difference to frequent seismic triggering that causes seismic strengthening of sediment. The frequent shaking by great earthquakes dewaters and densifies sediment, which reduces sediment mobilization during sediment failures. As a result, maximum run-out distances of mass transport deposits across basin floors are approximately an order of magnitude greater in passive margin settings (maximums of 800 km) compared to active margin settings (maximums of 100 km). (5) Another observation is that mass transport deposits are less common and less intermixed with turbidite deposits in basin floor turbidite systems of active tectonic margins compared to passive margins. Turbidite depositional history on active tectonic continental margins shows the dominance of seismic triggering for turbidity currents, the variation of seismic hazards in different tectonic settings and the influence of great earthquakes on mass transport deposition.