The first fine-scale image of the shallow magma system beneath Sierra Negra is determined using attenuation tomographic methods. The t ∗ spectral decay method for P-wave phases was used to highlight regions of high Q p−1 which suggest the presence of magma melt. High Qp −1 anomalies, ranging from 0.005 − 0.04, are concentrated beneath the caldera from 0.5 − 10.5 km depths. The high attenuation is interpreted as a zone of magma accumulation. Modeling of caldera deformation indicates a magma sill or flat-topped diapir at 2.1 km depth of unknown thickness. The attenuation model supports the diapir model.