Combined ScS-S differential attenuation and travel-time studies, with data from temporary and permanent arrays, produce quality factor, Q_μ(D″), estimates for D″ beneath Central America. Similarities between S and ScS ray paths through the upper mantle reduce the upper mantle elastic and anelastic contributions to differential studies. Differential attenuation correlates with ScS attenuation but not S attenuation, indicating that the attenuation primarily stems from lower mantle interactions. We image a ∼250 km wide high-attenuation low-velocity anomaly within a ∼600 km wide low-attenuation high-velocity region at the core-mantle boundary (CMB). The observed inverse exponential relationship between attenuation and velocity is consistent with a 200 ± 50 km thermal anomaly, but does not preclude chemical, phase, or structural variations. Observation of a small-scale thermal anomaly within D″ may provide significant evidence in favor one kind of plume formation at the CMB.