See the context of this sign.

Slump or Landslide

Is the disturbed land before you a landslide - or a slump?

A landslide - a mass of soil and rock plunging down a
slope - can exceed 50 miles an hour. The earth churns
and mixes. At the bottom it may break like a wave and
surge up the other side.

Slumps are different. The soil moves slowly as a block,
often coming to rest temporarily with trees and other
plants still growing on it undisturbed. The movement
may be undetectable, even to an observer standing on
the block.

Slumps and landslides are alike, though. Both occur on
steep slopes and after rains lubricate the contact between the
soil and underlaying rock. Sometimes, when conditions
are right, the slightest vibration will trigger the journey

No, the answer: It is a slump.

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