  W49A is a giant molecular cloud which harbors some of the most luminous
embedded clusters in the Galaxy. However, the explanation for this
starburst-like phenomenon is still under debate. Methods. We investigated
large-scale Spitzer mid-infrared images together with a Galatic Ring Survey
13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J =
2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM
30m telescope. Two expanding shells have been identified in the mid-infrared
images, and confirmed in the position-velocity diagrams made from the 13CO J =
2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has
an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4
M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49
erg which is probably provided by a few massive stars, whose radiation pressure
and/or strong stellar winds drive the shells. The expanding shells are likely
to have a common origin close to the two ultracompact Hii regions (source O and
source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in
an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales,
remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data.
Both ejections seem to have the same center as the expanding shells with a
total energy of a few times 10^50 erg. The main driving mechanism for the gas
ejections is unclear, but likely related to the mechanism which triggers the
starburst in W49A.
