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Evaluating Turbidite Correlations for Paleoseismology

Description: 

Marine turbidite paleoseismology relies on inferring synchronous triggering of turbidity currents by earthquake shaking. Such inference commonly relies on uncertain age dating and correlation of physical stratigraphy over a wide area. We assess the subjectivity of lithostratigraphic correlations based on existing core photographs, X-Ray computed tomography (CT) images, and high-resolution magnetic susceptibility (MS) logs, which underpin current understanding of earthquake recurrence along the Cascadia Subduction Zone. We highlight the importance of considering depositional processes associated with turbidity currents when attempting to correlate their deposits, by visually evaluating the similarity of turbidite facies at mm scale in photos and CT images in these contexts. Results confirm established ideas about alteration of signals from source to sink by complex sedimentologic processes, and which imply not all deposits should be correlated. Even when correlating seems appropriate, not every correlation is meaningful. Thus, we use dynamic time warping (DTW) to correlate MS logs objectively and repeatably, and we quantify significance ­by comparing observed DTW correlation coefficients and p-values to a distribution derived from randomly generated turbidite sequences. All our analyses show that turbidite correlation metrics degrade markedly with distance between cores >10 km apart, and analyses illuminate missing turbidites in some cores, from the previously inferred sequence. Lastly, our analyses reveal a promising paleoseismic turbidite signature, in the form of uniquely thick turbiditic mud caps in some cores. Thinner mud caps that are more characteristic of most turbidites are found in channels and at the base of slope, whereas the thickest mud caps are preserved in abyssal plain and slope basin cores. These thick mud caps suggest ponding of flows and might be indicative of anomalously large turbidity currents that may have been mobilized by great earthquakes.

Session: Constraining Seismic Hazard in the Cascadia Subduction Zone

Type: Oral

Date: 4/20/2023

Presentation Time: 02:30 PM (local time)

Presenting Author: Joan Gomberg

Student Presenter: No

Invited Presentation: 

Authors