Shear Velocity Structure of Abyssal Plain Sediments in Cascadia

Abstract

The sediments that form the upper layer of the oceanic crust vary significantly in thickness. As the crust ages, it accumulates more and more sediment, and when it is near a continental shelf, it can receive large deposits of continental sediment deposited by turbidity currents forming abyssal plains. Because of their soft, unconsolidated nature, the uppermost sediments tend to have very low shear velocities. These low velocities create significant delays in the arrival times of seismic waves moving through them, and these delay times can provide a significant source of error for body‐wave tomography. For surface‐wave tomography, these soft sediments can also provide a source of error by disproportionately slowing short‐period waves in a layer too thin to independently resolve. Correcting for sediments can significantly improve both body‐ and surface‐wave tomography. A deployment of ocean‐bottom seismometer (OBS) stations around the Juan de Fuca plate, the Cascadia Initiative is being deployed in four phases (Fig. 1; Toomey et al. , 2014). Year one focused on the northern portion of Juan de Fuca plate, and year two focused on the southern portion of the Juan de Fuca plate, including Gorda. Year two was coupled with the Blanco experiment, an independent National Science Foundation‐funded experiment, five stations of which have generously been made available to us by principal investigator John Nabelek. Year‐three stations largely reoccupied year‐one sites. Year‐four stations are currently deployed and their data are not yet available for analysis. Figure 1. A contour map of delay times. For each station identified as sitting on topography rising above the abyssal plains, the delay time was set at zero, and tie points with zero delay times were placed along the ridges. The triangles represent stations where we measured a sediment thickness, squares represent nonabyssal plain stations where we could not measure a sediment thickness, and …