Magnitude
m

Maximum runup
Metres

Damage potential

-2

<0.3

 

-1.5

0.3 – 0.5

 

-1

0.5 – 0.75

Nil

-0.5

0.75 – 1.0

 

0

1.0 – 1.5

Very little damage

0.5

1.5 – 2.0

 

1

2 – 3

Shore and ship damage

1.5

3 – 4

 

2

4 – 6

Some inland damage and loss of life

2.5

6 – 8

 

3

8 – 12

Severe destruction over 400 km of coast

3.5

12 – 16

 

4

16 – 24

Severe destruction over 500 km of coast

4.5

24 – 32

 

5

>32

 

The tsunami magnitude scale above is commonly used. The magnitude and maximum run-up values are from Iida (1963); the damage potential discussions come from somewhere else. The scale below comes from Murty (1977) and is atributed to Imamura and Iida - note that some of the descriptions are for different magnitudes to those above.

Tsunami magnitude, m = log2 H where H is the maximum run-up height measured at a coast 10-300km from the tsunami source (Murty, 1977, p31).

Grade Scale

Significance

-1

Minor tsunami with max. wave height (h) less than ½m

0

h is of the order of 1m; no damage

1

h is of the order of 2m; house damage along coast; ships washed ashore

2

h is of the order of 4-6m; some destruction of houses; considerable loss of life

3

h is of the order of 10-20m; damaged area along coast about 400km

4

h is greater than 50m; damaged area along coast more than 500km

 

Soloviev (1970) pointed out the inappropriateness of the term 'tsunami magnitude' as used above.

"If seismological terminology is applied to description of tsunamis, the grades of the Imamura-Iida scale must be designated as the intensity of the tsunami and not its magnitude. This is because the latter value must characterise dynamically the processes in the source of the phenomenon and the first one must characterise it at some observational point...the nearest point to the source included."

The tsunami intensity scale below is from Soloviev (1978, p131; after Alexander, 1993, p32).

Intensity Run-up height (m) Description of tsunami

I

0.5

Very slight. Wave so weak as to be perceptible only on tide gauge records.

II

1

Slight. Waves noticed by people living along the shore and familiar with the sea. On very flat shores waves generally noticed.

III

1

Rather large. Generally noticed. Flooding of gently sloping coasts. Light sailing vessels carried away on shore. Slight damage to light structures situated near the coast. In estuaries, reversal of river flow for some distance upstream.

IV

4

Large. Flooding of the shore to some depth. Light scouring on made ground. Embankments and dykes damaged. Light structures near the coast damaged. Solid structures on the coast lightly damaged. Large sailing vessels and small ships swept inland or carried out to sea. Coasts littered with floating debris.

V

8

Very large. General flooding of the shore to some depth. Quays and other heavy structures near the sea damaged. Light structures destroyed. Severe scouring of cultivated land and littering of the coast with floating objects, fish and other sea animals. With the exception of large ships, all vessels carried inland or out to sea. Large bores in estuaries. Harbour works damaged. People drowned, waves accompanied by a strong roar.

£ VI

16

Disastrous. Partial or complete destruction of man-made structures for some distance from the shore. Flooding of coasts to great depths. Large ships severely damaged. Trees uprooted or broken by the waves. Many casualties.

References

Alexander D., 1993, Natural disasters, UCL Press, London, 632p.

Murty, T. S., 1977, Seismic sea waves, tsunamis, Canadian Department of Fisheries and the Environment, Bulletin 198, 337p.

Iida, K., 1963, Magnitude, energy and generation mechanisms of tsunamis and a catalogue of earthquakes associated with tsunasmis, International Union of Geodesy and Geophysics Monograph, vol 24, 7-17.

Soloviev S. L., 1970, Recurrence of tsunamis in the Pacific, in W. M. Adams (ed.), Tsunamis in the Pacific Ocean, East West Center Press, Honolulu.

Soloviev, V., 1978, Tsunamis, in The assessment and mitigation of earthquake risk, UNESCO, Paris.

Iida, K., 1956, Earthquakes accompanied by tsunamis occurring under the sea off the islands of Japan. J. Earth Sci. Nagoya University, 4, 1-43.