Earthquake-Generated Tsunami - Tsunami - page 87

5.3.3 Anomalies

they occurred underwater. In Western Europe, earthquakes
are shallow and the effects of vertical motion are indepen-
dent of earthquake magnitude. In Greece, earthquakes with
magnitudes as low as 4.2 have generated vertical motion of
8 cm, enough to cause substantial building destruction
(Carydis 2004 ). While this amplitude appears small, similar
uplift of the seabed in confined bodies of water could
generate tsunami that can not only cause noticeable effects,
but also explain some of the reports of tsunami following
small earthquakes. The possibility of small earthquakes
generating significant tsunami has been underestimated.

There are two anomalies that have virtually been ignored in
the tsunami literature. First, while it is generally perceived
that tsunami can only be generated by earthquakes with an
M s magnitude of 7.0 or greater, reality is far different. One
of the most definitive databases on tsunami is maintained by
NOAA's National Geophysical Data Center ( 2013 ). It
contains a record of 1684 earthquake-generated tsunami
from 2000 BC to the present. Surprisingly, 50 % of earth-
quakes with a magnitude below 7.1 have generated identi-
fiable tsunami. Fifty-three earthquakes with a magnitude of
5.8 or less have produced tsunami. Measurements exist for
only 33 of these events. Despite the paucity of records,
some of these small earthquakes have produced destructive
tsunami. Twenty-five of the events generated tsunami that
reached more than 1.0 m above sea level. Two earthquakes
with magnitudes of 5.2 and 5.6 produced tsunami reaching
3.1 and 6.0 m respectively above sea level. The first
occurred in southern California on August 31, 1930, while
the latter occurred on the South Island of New Zealand on
May 17, 1947. In the same category is the historical April 6,
1580 earthquake off Dover in the English Channel (Haslett
and Bryant 2008 ). Based upon damage to churches and
castles, the earthquake had an M w magnitude of only 5.5
(Roger and Gunnell 2011 ). Large waves were reported
reaching the shores of Kent and Calais. A boat passenger
reported his vessel touching the seabed five times and
estimating a wave height of approximately 9 m. At least
120 deaths were recorded at Dover—with more in France—
and 165 ships were sunk. Modeling work has confirmed
these historical observations as being related to an earth-
quake-generated tsunami but at a higher magnitude of M w
equal to 6.9 (Roger and Gunnell 2011 ). Otherwise, land-
slides into the English Channel had to be involved. Earth-
quake-triggered landslides may also explain many of the
other anomalies in the NOAA database.
Second, vertical ground motions caused mainly by the
passage of Rayleigh seismic waves can generate tsunami-
like effects. The tsunamigenic potential of such movements
is poorly researched. There are numerous accounts of sei-
ching or sloshing of water in small enclosed bodies of water
or narrow channels such as estuaries and rivers following
earthquakes with M s magnitudes of 6 or less. When ground
displacement linked to faulting is filtered out, the magnitude
of vertical ground motions can be substantial. For example,
vertical ground motions exceeded 23.5 cm for the 6.7 M w
magnitude Northridge earthquake of January 17, 1994
(Ellsworth et al. 2004 ) and 100 cm for the M w 7.9 magni-
tude Denali earthquake of November 3, 2002 in Alaska
(Porter and Leeds 2000 ). These ground motions are large
enough that they would have generated sizable tsunami had

5.3.4 Linking Tsunami Run-Up to Earthquake

Magnitude

For warning purposes, it is better to be able to predict the
height of a tsunami along a coast given the magnitude of its
source earthquake. Some tsunami approaching coasts tend
to have a height that is consistent over long stretches of
coastline. This certainly holds true along the east coast of
Japan and the west coast of the United States. This fact can
then be used to calculate the run-up height of a tsunami at
various locations even if the slope varies—Eqs. ( 2.11, 2.12,
2.13 ). Figure 5.4 shows the relationships between the
moment magnitude, M w , of earthquakes and the amplitude
of mean tsunami height recorded on tide gauges for the east
coast of Japan and Papeete, Tahiti in the middle of the
South Pacific Ocean (Okal 1988 ; Kajiura 1983 ). The data
sets take into account both near field and distant earth-
quakes,

and have the following linear relationships

respectively:

Japan : log 10 H tmax ¼ 0 : 5M w 3 : 3 ð 5 : 8 Þ

Tahiti : log 10 H tmax ¼ 1 : 3M w 11 : 5 ð 5 : 9 Þ

where
H tmax

= mean maximum tsunami wave height (m) along
a coast

The Japanese pattern characterizes dispersive tsunami
propagating from point-like sources. Figure 5.4 indicates
that tsunamigenic earthquakes generate tsunami that have
less of an effect on Tahiti than they do on the east coast of
Japan. The reason for this has already been discussed in
Chap. 2 . Further research has shown that tsunami arrival in
the Pacific Ocean at a distant coast encompasses multiple
reflections, scattering, nearshore response and any harbor
resonance where most tide gauges are located (Geist 2012 ).
This recent analysis is also shown on Fig. 5.4 for both
tsunami earthquakes and inter-plate thrust earthquakes. All
of the lines plotted in Fig. 5.4 are statistically significant.