Seismic tomography of magmatic systems

Seismic inversion for three-dimensional variations of velocity and attenuation are often used to delineate magma bodies in the crust and upper mantle. Problems related to spatial resolution and data noise can obscure details relevant to investigating magma chambers, and the introduction of smoothing constraints, or damping, causes blurring. Tomographic inversions for P- and S-wave velocity/attenuation are summarized including large calderas, rift zones and smaller scale subduction zone volcanoes. While results vary considerably from place to place, most anomalies are found to be in the range of ± 10% perturbation, a range often controlled by the method of smoothing or regularization imposed during analysis. At many volcanoes high velocity anomalies are observed in the shallow regions below active areas where conduits, dykes or sills are expected to be present. At other locations low velocity perturbations are seen and interpreted as magma accumulation. Resolution limitations and regularization play a significant role in determining the level of perturbation observed in tomographic studies, although there may be regions where diffuse accumulations of magma do not exhibit strong anomalies and their identification will be elusive.