Rayleigh Wave Tomography of Noise-Removed Cascadia Initiative Data

Abstract:

A deployment of ocean bottom seismometers (OBSs) in the area of the Juan de Fuca and Gorda plates, the Cascadia Initiative (CI) provides a useful laboratory to probe the tectonics of ocean plates. With intermediate spreading rates, the Juan de Fuca and Gorda ridges provide a contrast with the rapidly spreading East Pacific Rise, where the MELT experiment, the only other detailed OBS study of a spreading ridge, was conducted. One key question we explore is whether migration in the hotspot reference frame drives an asymmetry in ridge structure. On the westward-migrating East Pacific Rise, more abundant seamounts and lower seismic velocities are observed on the west side. On the Juan de Fuca ridge, which is also migrating westward, a similar seamount asymmetry has been observed, and preliminary results from Rayleigh wave tomography using the first year of data suggested a corresponding low velocity anomaly. The second and third year of data from the CI will allow us to test the asymmetry of the Gorda ridge and improve the resolution of the Juan de Fuca ridge. Another key question we explore is the validity of the conductive cooling model of oceanic plates. In the MELT experiment at the East Pacific Rise, significant deviations from the conductive cooling model were found, attributed to the combined effects of melt and dehydration of the uppermost mantle. At the Juan de Fuca and Gorda ridges, we assess whether the conductive cooling model is more applicable at intermediate spreading rates. Finally, we explore whether there are any structural differences between the Juan de Fuca and Gorda plates. Unlike the Juan de Fuca ridge, there is a pronounced median valley at the Gorda axis and there is much intraplate seismicity within the deforming Gorda plate. To explore these questions, we present Rayleigh wave tomography of the Cascadia region. Because OBS data have high noise at long periods, and long periods are necessary for probing greater depths, noise complicates exploring the lower lithosphere and upper mantle with Rayleigh wave tomography. There are two primary sources of noise, tilt noise and compliance noise, which we have removed using a method pioneered by Crawford and Webb (2000). With noise removed, we can analyze data at periods of 125 to 150 s, giving us good control on lithospheric and asthenospheric structure.