Imaging Magma Under St. Helens

The Ambient Noise Tomography (SMT) method provides a powerful tool for sampling the Earth’s shear-wave-velocity structure (Vs) (e.g., Campillo and Paul 2003; Shapiro et al. 2005). Correlation of noise between pairs of stations at tens of km spacing, stacked over many months, provide Green’s functions of wave propagation between the stations, largely surface waves. These signals are most robust at 5-20 s periods where fundamental-mode Rayleigh waves sample the crust and uppermost mantle, allowing 3D imaging at these depths (e.g., Harmon et al. 2008; Bensen et al. 2009). ANT complements active-source tomography, both by measuring variations in Vs rather than Vp, and by having high sensitivity to absolute velocities of the lower crust. Vs tends to be more sensitive than Vp to presence of melt, water, and to some compositional variations associated with magmatic differentiation. Frequency-domain methods (Ekstrom et al. 2009) provide a relatively robust measurements capable of reliably measuring phase velocities to path lengths as small as one wavelength (Calkins et al. 2011). For an array of the aperture proposed here (Fig. 1) we will be able to image to derive some absolute velocity information for the upper mantle, and detailed 3D variations in the middle to lower crust. Work with the nearby CAFE array (Fig. 5; Calkins et al. 2011) shows a strong Moho and sharp upper-plate velocity variations beneath the Coast Range, but gradational velocities beneath Mt Rainier, indicating potential to reveal variability in composition. From the closely-spaced stations near the volcanic edifice, we will analyze higher frequency signals to image Vs around the shallow magma plumbing system and perhaps, if signals are strong, temporal changes as magma inflates (e.g., Brenguier et al. 2008). As previously, we can use existing ballistic surface wave measurements as long-period constraints on the models (Calkins et al., 2011).