magnitude A number assigned to a quantity by means of which the quantity may be compared with other quantities of the same class.

mareogram or marigram 1. Record made by a marigraph.  2. Any graphic representation of the rise and fall of the sea level, with time as a bscissa and height as ordinate, usually used to measured tides, may also show tsunamis. Mareograms of tsunami signals measured by an underwater gauge located 50 km outside the entrance to Tokyo Bay in about 50 m of water (upper trace), and another gauge located at the shore (lower trace). The tsunami is detected on the outside gauge about 40 minutes before it reaches shore (arrows). The offshore gauge was developed by Japan’s Port and Harbours Research Institute.

marigram or mareogram 1. Record made by a marigraph.  2. Any graphic representation of the rise and fall of the sea level, with time as a bscissa and height as ordinate, usually used to measured tides, may also show tsunamis. Mareograms of tsunami signals measured by an underwater gauge located 50 km outside the entrance to Tokyo Bay in about 50 m of water (upper trace), and another gauge located at the shore (lower trace). The tsunami is detected on the outside gauge about 40 minutes before it reaches shore (arrows). The offshore gauge was developed by Japan’s Port and Harbours Research Institute.

mareograph A recording tide gauge.

Master Plan The principal plan which outlines the methods and procedures that need to be followed in order to accomplish the long term goals of a program. The first edition of the ICG/ITSU Master Plan was released in 1989. The second edition was released in 2000.

maximum inundation  Maximum horizontal penetration of the tsunami from the shoreline. A maximum inundation is measured for each different coast or harbor affected by the tsunami.

maximum run-up  Maximum difference between the elevation at the maximum tsunami penetration (the inundation line) and sea-level at the time of the tsunami attack. A maximum run-up is measured for each different coast or harbor affected by the tsunami.

mean height Average height of a tsunami measured from the trough to the crest after removing the tidal variation.

mean sea level  The average height of the sea surface, based upon hourly observation of tide height on the open coast or in adjacent waters which have free access to the sea. These observations are to have been made over a "considerable" period of time. In the United States, mean sea level is defined as the average height of the surface of the sea for all stages of the tide over a nineteen-year period. Selected values of mean sea level serve as the sea level datum for all elevation surveys in the United States. Along with mean high water, mean low water, and mean lower low water, mean sea level is a type of tidal datum.

microtsunami  A tsunami of such small amplitude that it must be observed instrumentally and is not easily detected visually.