slats11, something else besides acceleration is worth considering, the rate of change of acceleration or "jerk". That is what whips the head around on thrill rides, for example. A person can set muscles and adapt to some levels of acceleration. But if it's that same acceleration limit and rates of onset well below a second the sudden change in acceleration is acting on a rag doll. Jerk is why railroads use transition curves on mainlines to minimize the onset rate of the curve's radial acceleration.
Then consider that the seats are not in any way designed to support the body safely in a sudden onset vertical acceleration the broken pelvis becomes believable modulo just how it broke. It's a strong structure. But it's not all THAT strong.
Meanwhile the back is to some degree supported by the seat back so pressure would tend to remain more or less straight down the backbone, particularly with a sudden stop from a mostly downwards and slightly forwards motion.
So I'd but invest too much in pelvis broken and backs not totally wiped out. The accident investigators have seen this sort of injury before and probably have a pretty good idea what causes it.