Crustal Shear Structure and Seafloor Compliance in the Shallow and Deep Ocean

Abstract:

We use recently collected ocean-bottom seismometer (OBS) data from four deployments to investigate crustal shear properties via seafloor compliance. Very-long period (greater than 30s) ocean waves create pressure fields on the seafloor, and the displacement resulting from these fields depends on the density and elastic parameters of the oceanic crust; seafloor compliance is defined as the transfer function between the ground acceleration and the pressure. Compliance has been used as a geophysical prospecting tool for several decades, and still holds significant potential to explore shallow layers where other seismic data fail to provide the necessary resolution; in particular, seafloor compliance is a superior tool to investigate shear in ocean sediments.

Our first data set comes from several deployments of the ADDOSS (Autonomously Deployed Deep-Ocean Seismic System) experiment in the San Diego Trough and the nearby deeper ocean. We analyze data from two different sites: one at 1000m depth (deployed for two weeks during the summer) and one at 4000m depth (deployed for 2.5 months during the winter). Our second data set is based on records from the 2005-2007 PLUME (Plume-Lithosphere Undersea Mantle Experiment) OBS deployment to investigate subsurface structure at nearly 70 stations between depths of 4500 and 5800 meters. The two PLUME deployments operated for one year each, with a station spacing of 75 and 200 kilometers respectively.

We perform extensive forward modeling to demonstrate the sensing capabilities and limitations of the method in our regions, and we provide results from our data analysis in the San Diego Trough and in the adjacent deep ocean, as well as around Hawaii. We pay particular attention to the implications of seasonal variability on compliance measurements. Finally, we provide a brief overview of how we can expand our compliance work into new and less well understood regions of the ocean.