Earthquake-Generated Tsunami - Tsunami - page 83

Table 5.1 Disparity between the surface wave magnitude, M s , and the moment magnitude, M w , of recent earthquakes illustrating tsunami

earthquakes
Event Date Seismic magnitude M s Moment magnitude M w Maximum run-up (m)

Sanriku 15 June 1896 7.2 8.0 38.2

Unimak Island, Alaska 1 April 1946 7.4 8.2 35.0

Peru 20 November 1960 6.8 7.6 9.0

Kurile Islands 20 October 1963 6.9 7.8 15.0

Kurile Islands 10 June 1975 7.0 7.5 5.5

Nicaragua 2 September 1992 7.2 7.7 10.7

Java 2 June 1994 7.2 7.7 13.9

Source Based on Geist ( 1997 ) and Intergovernmental Oceanographic Commission ( 1999a , b )

Table 5.2 Earthquake magnitude, tsunami magnitude, and tsunami
run-up heights in Japan
Earthquake magnitude

to trigger a substantive response using the M s scale based
upon surface-wave detection algorithms. Instead, a potential
tsunami earthquake can be detected better using the moment
magnitude, M w (Geist 1997 ). Technically, tsunami earth-
quakes are ones that occur in the ocean where the difference
between the M s and M w magnitudes is significantly large.
Table 5.1 illustrates this difference for some modern tsu-
nami. Tsunami earthquakes happen with two conditions:
where thick, accretional prisms develop at the junction of
two crustal plates and wherever sediments are being sub-
ducted. Earthquakes under the former setting generated the
1896 Meiji, Sanriku and 1946 Unimak Island, Alaskan
Tsunami. In contrast, the Peru, Kuril Islands, and Nicaragua
Tsunami listed in Table 5.1 were generated beneath sub-
duction zones. The mechanisms of tsunami earthquake
generation will be discussed in more detail later.

Tsunami magnitude Maximum

run-up (m)

6.0 -2 \0.3

6.5 -1 0.5-0.75

7.0 0 1.0-1.5

7.5 1 2.0-3.0

8.0 2 4.0-6.0

8.3 3 8.0-12.0

8.5 4 16.0-24.0

8.8 5 [32.0

Source Based on Iida ( 1963 )

10 % of the total energy of the source earthquake (Abe
1979 ). The relationship between the moment magnitude of
an earthquake and the Imamura-Iida scale is presented in
Table 5.2 . Only earthquakes of magnitude 7.0 or greater are
responsible for significant tsunami waves in Japan with run-
up heights in excess of 1 m. However, as an earthquake's
magnitude rises above 8.0, the run-up height and destructive
energy of the wave dramatically increases. A magnitude 8.0
earthquake can produce a tsunami wave of between 4 m and
6 m in height. Magnitudes approaching a value of 9.0 are
required to generate the largest Japanese tsunami.
The Imamura-Iida magnitude scale has now acquired
worldwide usage. However, because the maximum run-up
height of a tsunami can be so variable along a coast, Sol-
oviev ( 1970 ) proposed a more general scale as follows:

5.2.3 Tsunami Magnitude Scales

Historically, the first scale proposed for measuring a tsunami
was the Sieberg scale based on the destructive effect of a
tsunami (Gusiakov 2008 ). It did not contain any measurement
of tsunami wave height. The latter parameter was incorpo-
rated in the Imamura-Iida scale using approximately a hun-
dred Japanese tsunami between 1700 and 1960 (Iida 1963 ):

m II ¼ log 2 H rmax

ð 5 : 2 Þ

where

m II = Imamura-Iida's tsunami magnitude scale

(dimensionless) i s ¼ log 2 1 : 4 H r

ð Þ ð 5 : 3 Þ

H rmax = maximum tsunami run-up height—Eqs. ( 2.11,

2.12, 2.13 )

where

i s = Soloviev's tsunami magnitude (dimensionless)

On the Imamura-Iida scale, the biggest tsunami in Japan—
the T ¯hoku 2011 Tsunami with a run-up height of 38.9 m—
had a magnitude of 5.3. The Meiji Great Sanriku Tsunami of
1896 along the same coast, with a run-up height of 38.2 m,
was comparable. Japanese tsunami have between 1 and

H r

= mean tsunami run-up height along a stretch of
coast (m)

This scale—now known as the Soloviev-Imamura scale
(Gusiakov 2008 )—and its relationship to both mean and