Sea Floor Compliance

Knowledge of the structure of the basement below the ice shelf/water cavity system is necessary to model the ocean wave induced signal propagation across the RIS. The “compliance model” applied to broadband seafloor seismic data has been successful in the open ocean for inferring sub-seafloor rigidity anomalies, such as magma chambers (Crawford and Webb, 2000; Crawford et al., 1991; Webb and Crawford, 1999). The method assumes that, at low frequencies (0.002-0.050 Hz), the pressure on the seafloor, is an applied force and the resulting response of the seafloor (a deformation) is measured with an ocean bottom seismometer. The pressure fluctuations at the seafloor are caused by the variations in sea surface height associated with surface gravity waves. The ease with which the seafloor deforms under the applied pressure is called the “compliance”. Longer surface gravity wavelengths, having longer periods, are sensitive to the rigidity at deeper depths.

Now in the case of the Ross ice shelf, we can do a compliance calculation by assuming: 1) that the seismometer on the ice (RIS) measures sea surface elevation, and 2) that the seismometer on near-by land (e.g. Scott Base, SBA) is sufficiently close to the ice shelf that it gives the deformation of the seafloor under the ice. The agreement of the compliance calculation at frequencies below 0.02Hz for a simple half-space model (Figure 1) is remarkable. Equally remarkable is the similarity of the Antarctic compliance curve with the “S = ideal sediment-covered model” compliance curve from Crawford et al. (1991) (Figure 2).

What are the propagation characteristics of the gravity wave induced signals through, under, around the RIS? More data are needed to examine both the rich phenomenology of these interactions and to establish present-day baseline measurements. Can monitoring these signals help reveal seasonal and long term changes in RIS integrity? Compliance analysis of the linear portion of the proposed seismic array orthogonal to the ice front is anticipated to break down when the RIS becomes sufficiently rigid to no longer provide a reasonable estimate of sea surface elevation changes. Where (and if) that occurs will provide additional information on the mechanical properties of the RIS and its response to ocean forcing.

(1) The compliance for the Ross Ice Shelf (black line) using RIS2 and SBA spectral estimates. A uniform water depth of 600 m was assumed. The predicted compliance (red dashed line) for a half-space model at a typical continental margin agrees with the observed compliance at frequencies below 0.02Hz. The half-space model is more appropriate for deeper structure (5-10 km). This compliance methodology could be used to model the crustal structure beneath the RIS.
(2) Right: Compliance curves from Crawford et al. (1991; their Figure 3, curves S = “ideal model with a sediment layer”, H = “uniform half-space”) normalized by dividing by the wavenumber (1991) Left: Shear velocities of the models.