Earthquake Science Center Seminars
Rethinking turbidite paleoseismology at Cascadia
Stratigraphic variability complicates deep-sea turbidites as guides to earthquake and tsunami hazards along the Cascadia Subduction Zone. The variability affects a confluence test that compares counts of Holocene turbidites above a stratigraphic marker upstream and downstream of a deep-sea channel junction. A quarter century ago, similarity in the turbidite counts at seven core sites provided initial evidence that turbidity currents from different submarine canyons off the Washington coast reached the channel junction around the same time, as expected of widespread seismic triggering. A fuller set of legacy cores fails to replicate this test result and casts doubt on the test design. Turbidite variability also affects recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates presuppose that pulses of seismic shaking during an earthquake yield similarly pulsed deposition from turbidity currents, despite observed and expected differences in turbidite stratigraphy and facies. The recurrence estimates require turbidites to correspond with megathrust earthquakes more dependably than they do off northern Washington, where turbidite frequency appears governed less by seismic shaking than by sediment supply. These complications add to the challenge of extracting reliable earthquake history from deep-sea deposits along the Cascadia Subduction Zone.
Brian Atwater, USGS ESC Seattle
June 11, 2014