Geoscience Reference
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Table 5.3 Soloviev's scale of tsunami magnitude
Tsunami magnitude
tsunami on the east coast of Japan shows the following
relationship (Abe 1983 ):
Mean run-up
height (m)
Maximum
run-up height (m)
M t ¼ log 10 H r þ log 10 R e þ 5 : 80
ð 5 : 5 Þ
-3.0
0.1
0.1
where
-2.0
0.2
0.2
-1.0
0.4
0.4
M t
= tsunami magnitude (dimensionless)
0.0
0.7
0.9
R e
= the shortest distance to the epicenter of a tsunami-
genic earthquake (km)
1.0
1.5
2.1
2.0
2.8
4.8
The constant in this equation is dependent upon the
source region. At present, few values have been calculated
beyond Japanese waters, so there is no universal value that
can be easily inserted into Eq. 5.5 . The Imamura-Iida scale
has been converted to a form similar to Eq. 5.5 as follows
(Hatori 1986 ):
2.5
4.0
7.9
3.0
5.7
13.4
3.5
8.0
22.9
4.0
11.3
40.3
4.5
16.0
73.9
Source Based on Horikawa and Shuto ( 1983 )
m II ¼ 2 : 7 log 10 H r þ log 10 R e
ð
Þ 4 : 3
ð 5 : 6 Þ
Finally, tsunami waves clearly carry quantitative informa-
tion about the details of earthquake-induced deformation of
the seabed in the source region. Knowing the tsunami
magnitude, M t , it is possible to calculate the amount of
seabed
maximum tsunami run-up heights are summarized in
Table 5.3 . Neither the Imamura-Iida nor the Soloviev-
Imamura scales relate transparently to earthquake magni-
tude. For example, both tsunami scales contain negative
numbers and peak around a value of 4.0. Most tsunami are
also generated by earthquakes over a narrow range of
magnitudes, whereas the two tsunami scales described
above span a broader range. Several attempts have been
made to construct a more identifiable tsunami magnitude
scale. Abe ( 1983 ) established one of the more widely used
of these scales as follows:
involved
in
its
generation
using
the
following
formula:
M t ¼ log 10 S t þ 3 : 9
ð 5 : 7 Þ
= area of seabed generating a tsunami (m 2 )
where S t
There is excellent agreement between the tsunami magni-
tudes calculated using Eqs. 5.4 and 5.7 .
M t ¼ log 10 H r þ 9 : 1 þ DC
ð 5 : 4 Þ
where
M t
= tsunami magnitude at a coast
5.3
How Earthquakes Generate Tsunami
DC
= a small correction on tsunami magnitude dependent
on source region
5.3.1
Types of Faults
Average DC corrections for Hilo, California, and Japan are
-0.3, 0.2, and 0.0 respectively, irrespective of the source
region of the tsunamigenic earthquake. There have been 17
historical events in the Pacific Ocean with a tsunami mag-
nitude greater than 8.5. The largest of these was the May 22,
1960 Chilean Tsunami with an M t value of 9.4. All Pacific-
wide events have had a tsunami magnitude greater than 8.5.
Ten of these events occurred in the twentieth century.
The tsunami magnitude scale has the advantage of being
closely equated to the magnitude of earthquakes near their
source because the average value of M t for a coastline is set
equal to the average M w value of source earthquakes.
Recently, emphasis has been placed upon near-field earth-
quakes and the M t scale has been reformulated to include the
exact distance between a coast and the epicenter of a tsun-
amigenic
Rupturing along active fault lines where two sections of the
Earth's crust are moving opposite each other causes tsun-
amigenic earthquakes. Only three types of faults (Fig. 5.2 )
can generate a tsunami: a strike-slip earthquake on a vertical
fault, a dip-slip earthquake on a vertical fault, and a thrusting
earthquake on a dipping plane (Wiegel 1970 ; Okal 1988 ;
Geist 1997 ). In each case, rupturing can occur at any point
along a fault line deep in the Earth's crust. This location is
known as the focal depth of the epicenter. It is a crucial
parameter in determining whether or not an earthquake will
generate a tsunami, especially in subduction zones that
include a land component. Tsunami potential grows the
more an epicenter lies under the seabed. The dip-slip and
thrust fault line configurations are better at producing tsu-
nami than the strike-slip pattern. From a depth of 30 km
earthquake.
For
example,
research
on
many
 
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