  Solar extreme-ultraviolet (EUV) waves generally refer to large-scale
disturbances propagating outward from sites of solar eruptions in EUV imaging
observations. Using the recent observations from the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO), we report a
quasi-periodic wave train propagating outward at an average speed of $\sim$308
km s$^{-1}$. At least five wavefronts can be clearly identified with the period
being $\sim$120 s. These wavefronts originate from the coronal loop expansion,
which propagates with an apparent speed of $\sim$95 km s$^{-1}$, about 3 times
slower than the wave train. In the absence of a strong lateral expansion, these
observational results might be explained by the theoretical model of Chen et
al. (2002), which predicted that EUV waves may have two components: a faster
component that is a fast-mode magnetoacoustic wave or shock wave and a slower
apparent front formed as a result of successive stretching of closed magnetic
field lines. In this scenario, the wave train and the successive loop expansion
we observed likely correspond to the fast and slow components in the model,
respectively.
