Disable U1, U2, etc for the buried pipe. They're not applicable, because the buried piping gets moved in bulk with the earth.

CAESAR doesn't actually know the pipe is buried - it doesn't know anything beyond the pipe being weightless and held by springs.

As the buried piping actually shifts north during a seismic event, the movement of the unburied pipe, relative to the buried pipe, is to the south. And that's what you're simulating.