Perturbation Motion : The motion of a disturbance (usually but not necessarily assumed infinitesimal), as opposed to the motion of the steady state of the system on which the perturbation is superimposed
Intergovernmental Oceanographic Commission IOC technical series 37, Tsunami Glossary (A glossary of terms and acronyms used in the tsunami literature - English), , United Nations Educational, Scientific and Cultural Organization UNESCO, Paris, France 1991. http://unesdoc.unesco.org/images/0008/000885/08853 8eb.pdf, (ND/ Tsunami Sci - Brit) {590}