In the majority of cases, in reality, seismic loads are not a concern for underground piping. It is universally supported. There's no way for it to bend underground.

If you have thin pipe, I guess you could have the pipe ovalize due to weight of soil, and then attempt to de-ovalize due to sudden momentum... but that's not a CAESAR problem.

If you were to apply a uniform load to caesar's model of underground piping will mean that between each spring support, the pipe will appear to "sag" between supports, except in the horizontal plane, which does not reflect reality.

The reality is all of your underground piping, as well as the buried portions of your pipe supports, will suddenly displace X in/cm in Ts, and your above ground piping and supports won't due to their inertia. You can simulate this statically as though the ground were stationary and a force is applied to the pipes in the opposite direction. You could also simulate this dynamically as a displacement of all the underground piping, supports at the bases, etc.