Earthquake-Generated Tsunami - Tsunami - page 82

distance of 100 km from an earthquake's epicenter (Bolt
1978 ). This scale saturates around a value of seven. A more
useful scale measures the largest magnitude of seismic waves
at the surface at a period of 20 s. This yields the surface wave
magnitude, M s . The M s scale is so well recognized that it is
commonly used to describe the size of tsunamigenic earth-
quakes. It has already been used in this text. An M s magnitude
earthquake of 8.0 occurs about twice per year, but only 10 %
of these occur under an ocean with movement along a fault
that is favorable for the generation of a tsunami. Earthquake-
generated tsunami are associated with seismic events having
an M s magnitude of 7.0 or greater. Around the coast of Japan
any shallow submarine earthquake with a magnitude greater
than 7.3 will generate a tsunami. The tsunami period is also
proportion to the magnitude of uplift. Small earthquakes tend
to produce short tsunami wavelengths. Most tsunami-gener-
ating earthquakes are shallow and occur at depths in the
Earth's crust between 0 and 40 km.
Unfortunately, the M s scale also saturates, this time
around a magnitude of eight, precisely at the point where
significant tsunami begin to form. A better measure of the
size of an earthquake is its seismic moment, M o , measured
in Newton meters (Nm) and based upon the forces acting
along a fault line (Okal et al. 1991 ; Schindelé et al. 1995 ).
From this a moment magnitude, M w , can be determined
from long period surface waves of more than 250 s using
the following formula (Geist 1997 ; Geist 2012 ):

Hokkaido
12 July 1993

15

10

Nicaragua
2 September 1992

5

0

100

0 50

Time (s)

Fig. 5.1 Comparison of the rate of seismic force between normal
(Hokkaido) and slow (Nicaraguan) tsunamigenic earthquakes. Based
on Kikuchi and Kanamori ( 1995 )

however, it is now known that many earthquakes with small
and moderate seismic moments can produce large, devas-
tating tsunami. The Great Meiji Sanriku earthquake of 1896
and the Alaskan earthquake of April 1, 1946 were of this
type (Okal 1988 ). The Sanriku earthquake was not felt
widely along the adjacent coastline, yet the tsunami that
arrived 30 min afterwards produced run-ups that exceeded
30 m in places and killed 27,132 people. These types of
events are known as tsunami earthquakes (Kanamori and
Kikuchi 1993 ; Satake 1995 ). Besides the Sanriku and
Alaskan events, significant tsunami earthquakes also
occurred in the Kuril Islands on October 20, 1963, off
Nicaragua on September 2, 1992, and off Java on June 2,
1994, with maximum run-ups of 15 m, 10.7 m, and 13.9 m
respectively (Okal 1988 , 1993 ; Geist 1997 ). Submarine
landslides are thought to be one of the reasons why some
small earthquakes can generate large tsunami, but this
explanation has not been proven conclusively. Submarine
landslides as a cause of tsunami will be treated in more
detail in Chap. 7 . Presently, it is believed that slow rup-
turing along fault lines causes tsunami earthquakes. Only
broadband seismometers, sensitive to low-frequency waves
with wave periods greater than 100 s, can detect slow
earthquakes that spawn silent, killing tsunami. Figure 5.1
illustrates the difference between a tsunami earthquake and
an ordinary one (Kanamori and Kikuchi 1993 ). The Hok-
kaido 1993 Tsunami was an ordinary event. The earthquake
that generated it lasted for about 80 s and consisted of five
large and two minor shock waves. The earthquake was
regionally felt along the northwest coast of Japan. It pro-
duced a deadly tsunami. In contrast, the Nicaraguan Tsu-
nami of 1992 had no distinct peak in seismic wave activity.
Rather, the movement along the fault line occurred as a
moderate disturbance, for at least 80 s, tapering off over the
next half minute. The earthquake was hardly felt along the
nearby coast, yet it produced a killer tsunami. Both of these
events will be described in detail later in this chapter.
The energy released by slow earthquakes can be mea-
sured accurately; however, the change is not rapid enough

M w ¼ 0 : 67 log 10 M o 10 : 73 ð 5 : 1 Þ

where

M w = moment magnitude scale (dimensionless)

M o = seismic moment

The moment magnitude does not saturate and gives a
consistent measure across the complete span of earthquake
sizes. Only large earthquakes, with a moment magnitude,
M w, greater than 8.6, can generate destructive teleseismic
tsunami, ones that impact across an ocean basin. Tsunami
cannot be generated, without associated marine landslides,
below a value of 6.3 because the energy released is not
sufficient to generate long waves (Roger and Gunnell 2011 ).
It should also be noted there is only a weak correlation in
historical records between tsunami intensity and the mag-
nitude of the source earthquake (Gusiakov 2008 ). This fact
dictates that the prediction of a tsunami's characteristics for
any coastline based solely on seismic data is a difficult task.

5.2.2 Tsunami Earthquakes

The preceding scales imply that the size of a tsunami should
increase as the magnitude of the earthquake increases. This
is true for most teleseismic tsunami in the Pacific Ocean;