Final answer:
The abrupt change in seismic wave velocity at the base of the crust is called the Moho discontinuity. Seismic data is used to determine Earth's interior structure and composition by analyzing changes in wave velocity.
Step-by-step explanation:
The abrupt change in seismic wave velocity that takes place at the base of the crust is called the Moho discontinuity. Also known as the Mohorovičić discontinuity, it is the boundary between the Earth's crust and the underlying mantle. The velocity of seismic waves increases significantly across this boundary.
Seismic data is commonly used to determine the structure and composition of Earth's interior. When seismic waves pass through different materials, such as rocks or sediments, they can experience changes in their velocity. By analyzing these changes, geologists can infer the properties of the different layers.