Images of asteroid (596) Scheila have been acquired at various dates after the detection of the 2010 outburst. Assuming a short-duration event scenario, as suggested by the quick vanishing of the dust tail brightness with time, and integrating numerically the equation of motion of individual particles ejected from the surface, we have developed a tail model from which we estimate the parameters associated to the geometry of the ejection, the size distribution, and the velocity distribution of the ejected particles, as well as the total mass ejected. We found a weak inverse power-law dependence of ejection velocity versus particle radius, with velocities ranging from 50 to 80 m s-1 for particle radii in the range 5 cm to 8×10-5 cm, respectively. These velocities are very different fromthose expected from ice sublimation at the asteroid heliocentric distance ( 3 AU), and suggest a collision scenario as a likely cause of the outburst. We found that the ejected particles are distributed in size following a power law of index -3, and, based on the ejecta mass and scaling laws, the impactor size is estimated at 30-90 m in radius, assuming an impact velocity of 5 km s-1, and the same density (1500 kg m-3) for the asteroid as for the projectile. We have inferred an asymmetry in the ejecta along the axis normal to the asteroid orbit plane, a likely indicator of an oblique impact. The impact is estimated to have occurred on November 27th, with an accuracy not better than ±3 days.