Seismic imaging has depicted a sharp Lithosphere-Asthenosphere Boundary (LAB) worldwide, including beneath the Juan de Fuca (JdF) plate offshore Cascadia. It is widely assumed that the LAB is melt rich and hence has a low viscosity, but there has not been deformation evidence to support this assumption. Here we report our finding of a weak LAB causing diagnostic deformation signatures following large subduction earthquakes. Specifically, coastal areas outside the rupture zone show postseismic enhanced landward motion (ELM) over several years. Conventional viscoelastic models can explain other aspects of the postseismic deformation but not the observed ELM. Using three-dimensional viscoelastic finite element models, we show that a thin and weak LAB can decouple the slab from the asthenosphere during postseismic stress relaxation, allowing faster landward motion over a wide strike extent to cause the ELM. Model tests suggest a limited trade-off between LAB thickness and viscosity. Assuming a thickness of 10 km, an LAB viscosity of 5e16 Pa s, which is substantially lower than typical asthenospheric viscosities, can explain key characteristics of the observed ELM. A thinner (thicker) LAB with a lower (higher) viscosity can also explain the deformation, but a large thickness (>20 km) is less preferred because the results are inconsistent with existing, albeit limited, near-field vertical deformation observations. Our study raises important new geodynamic questions. For example, how does the seismically imaged LAB beneath the JDF plate govern viscoelastic stress transfer between closely spaced plate boundaries such as from the spreading JdF Ridge to the locked Cascadia megathrust?