(a) Subject to the provisions of subdivisions (b) and (c), in the event of public necessity and to avoid impending peril to persons or property as a result of gradual earth movement, a local public entity is not liable for damages for injury to persons or property resulting from such impending peril or from any action taken to abate such peril providing the legislative body of the local public entity has, on the basis of expert opinion or other reasonable basis, done all of the following:

1. (1) On the basis of adequate evidence such as expert opinion or otherwise, found the existence of such impending peril.
2. (2) Determined appropriate remedial action to halt, stabilize, or abate such impending peril.

3. (3) Undertaken to implement such remedial action.

   As used in this chapter, “gradual earth movements” includes, but is not limited to, perceptible changes in the earth either in a subterranean area or at the surface, or both, which if not arrested or contained will over a gradual period of time result in damage to or destruction of underground or aboveground property or harm to persons. However, “gradual earth movement” does not include movement which is caused by activity undertaken by a local public entity for purposes other than the abatement of peril caused by gradual earth movement.

   As used in this chapter, “local public entity” has the meaning set forth in Section 900.4.