A detailed analysis of one earthquake recorded by a dense array of seismographs on the surface of an alluvial valley shows two locally-generated waves which propagate down-valley. The faster travelling one is a Rayleigh wave, and the slower one is similar to a Love wave, but has a vertical component thought to arise from the need to meet lateral boundary conditions. These waves can mimic normal modes, and their interaction provides a basis for explaining directional resonances.