Seismic wave amplitudes have tremendous sensitivity to subduction structure; however, they are affected by attenuation, scattering and focusing, and have therefore been sparsely used compared with traveltimes. We measure and model teleseismic body wave amplitudes recorded at a dense broadband array in the Washington Cascades (iMUSH). These data show anomalous amplitude variations with complex azimuthal dependence at the low frequency of 0.05 Hz, accompanied by significant multipathing. We demonstrate using spectral-element numerical simulations that focusing of the teleseismic wavefield by the Juan de Fuca slab is responsible for some of the amplitude anomalies. The focusing effects can contaminate the apparent differential attenuation measurements and produce at least 20% of the inferred attenuation signal. The focusing results in complex azimuthal patterns that produce different phase and amplitude variations than does intrinsic attenuation, which should allow separation of elastic (focusing) and anelastic effects. Our results indicate that the amplitudes are sensitive to the subducting slab geometry and subduction structure, and can be used to refine seismic images. Ubiquitous focusing effects are observed along the arc, suggesting a continuous Juan de Fuca slab from Canada to northern California.