  According to Biot's paper in 1956, by using the Lagrangian equations in
classical mechanics, we consider a problem of the filtration of a liquid in
porous elastic-deformation media whose mechanical behavior is described by the
Lam'e system coupled with a hyperbolic equation. Assuming the null surface
displacement on the whole boundary, we discuss an inverse source problem of
determining a body force only by observation of surface traction on a suitable
subdomain along a sufficiently large time interval. Our main result is a
H\"older stability estimate for the inverse source problem, which is proved by
a new Carleman estimat for Biot's system.
