1
TSUNAMIS OF SEISMIC ORIGIN
-Science, Disasters and Mitigation-
N. SHUTO
Faculty of Policy Studies, Iwate Prefectural University
Iwate 020-0193, Japan
Abstract
Present knowledge of tsunamis are reviewed and discussed. The tsunami generation
mechanism is not yet fully understood. Theories are examined in relation to observed
tsunami phenomena. Conditions to be satisfied in numerical simulation are summarized. In
addition to loss of human lives, several disasters are tabulated. Defense works including
coastal structures, city planning and prevention systems are briefly introduced. Three
subjects in urgent needs are the deep-sea measurement of tsunamis, transfer of tsunami
knowledge and strengthening of coastal cities against tsunamis.
1. Introduction
Since 1970’s, tsunami science and technology have been developing remarkably, assisted
by the progress of seismology and computer science. In 1990’s, more than ten disastrous
tsunamis occurred in the Pacific. International cooperation is successfully established to
survey tsunami heights and to understand the tsunamis.
In the present paper, the knowledge commonly used in these works is reviewed and the
problems to be solved in the near future are discussed.
2. Science of Tsunamis
2.1 GENERATION OF TSUNAMIS
Since Thucydides, a Greek historian, recognized in 426 BC that a tsunami on Euboea
Island was the result of an earthquake, many tsunamis were generated and recorded in the
world. Causes are submarine earthquakes, landslides, and volcanic action.
The most of causes are submarine earthquakes. Not the ground shaking but the vertical
sea-bottom deformation generates a tsunami. The greater an earthquake is, the larger the
vertical displacement is and consequently the greater tsunami is generated. This rule is
applicable to the ordinary tsunamigenic earthquake with a serious exception, tsunamiearthquake. Since the Nicaraguan tsunami in September 1992, more than ten tsunamis gave
damages in the Pacific. Three of them were tsunami-earthquakes.
In the 1970’s, the method to determine the tsunami initial profile generated by
submarine earthquake was developed. In 1971, Mansinha and Smylie [1] proposed a
method to estimate sea bottom displacement caused by an earthquake if fault parameters
A. C. Yalçıner, E. Pelinovsky, E. Okal, C. E. Synolakis (eds.),
Submarine Landslides and Tsunamis 1-8.
@2003 Kluwer Academic Publishers. Printed in Netherlands
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were given. Later [2] gave a more complete set of analytic formulas to compute the surface
displacement due to a fault placed in an elastic homogeneous half-space. In 1974,
Kanamori and Ciper [3] opened the way to calculate fault parameters from seismic records
with the aid of the advancing high-speed computer, by using the 1960 Chilean earthquake
as an example. Combined these two, the final displacement caused by an earthquake, i.e.
the initial tsunami profile, can be estimated. This method has been most popularly used
since then. Sometimes, however, the tsunami initial profile thus determined cannot satisfy
the tsunami energy distribution measured along the shoreline. This difference may be due
to heterogeneity of fault movement, existence of sub-faults, dynamic movement of fault, or
so on.
There is only one example of the measured tsunami profile. Plafker [4] determined the
vertical displacement caused by the 1964 Great Alaska earthquake, by displacement on
islands and by comparison with a pre-earthquake topography. Along the direction of the
short axis of the deformation area, the tsunami initial wave is about 450 km long with one
trough and one crest. The wave height is 6 m. This long wave is considered to correlate
with the major energy of the earthquake and is, therefore, estimated from the fault
parameters. Near its crest, a sharp rise was found. This rise about 6 m high and 30 km wide
at its base, although its contribution to tsunami height is apparently important, can not be
estimated from seismic information. This rise may be caused by a sub-fault.
These short wave components are not important in case of a far-field tsunami.
Entrapped and scattered by islands and seamounts on the route of propagation, their
systematic wavy shape will be lost. Only long period components can arrive at distant
shores, not disturbed so much by topography along the path.
In case of a near-field tsunami, short period components that are not estimated from
seismic information are very important and are considered to be the major reason of the
discrepancy between the measured tsunami heights on the shore and the computed. It is
often noticed that the initial tsunami profile based upon fault parameters should be made
nearly double in order to explain the measured tsunami traces.
The mechanism of tsunami earthquake is not yet clearly understood. Fukao [5]
explained two tsunami earthquakes at the Kurile Trench by sub-faults in the thick
sedimentary wedge at the leading edge of the continental lithosphere. Tanioka et al. [6]
consider that tsunami earthquakes at the Japan Trench are the result of the horst and graben
structures of sea bottom, which gives the scattered contact zones in the subducted
sediments along the interplate boundary.
The generation mechanism by other causes is not yet clearly understood, although many
researchers have been studying.
2.2 PROPAGATION AND RUN-UP
A major tsunami in the ocean is several tens kilometres long and several meters high. It is
long and small compared to the ocean depth of several kilometres. The linear long- wave
theory is successfully applicable if the travel distance is not long.
For a long travel, e.g. over the Pacific Ocean, other consideration is required.
Firstly, the equations should be described with the spherical co-ordinate system,
because the earth is a sphere. Secondly, the Coriolis term should not be missed. Thirdly,
the frequency dispersion term of the first order approximation should be included, if the
Kajiura parameter relating to the dimension of the source, the travel distance and the water
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depth is smaller than 4 [7]. Under these conditions, the linear Boussinesq equation is often
used.
Approaching the shore, the wave height increases and the water depth decreases. Then,
non-linear characteristics of water motion require the shallow-water theory including the
bottom friction term, in which the amplitude dispersion term becomes important. The
boundary between the linear long-wave theory and the shallow-water theory is
approximately the water depth of 200 m.
Approaching further the shore, a tsunami shows many faces in relation to water depth,
bottom slope, wave height and wave length. If a tsunami is like a rapid tide without
breaking front, the shallow-water theory is successfully applied. If a tsunami with breaking
front comes nearly normally to the shore, the shallow-water theory is also applicable if the
tsunami runup height is the major concern. If a tsunami with breaking front runs along the
shore as an edge bore that follows sometimes the ordinary refraction law depending upon
the water depth but sometimes propagates neglecting the law, there is no theory applicable
at present. A hydraulic experiment revealed that a slight change in the side boundary
condition resulted in a big change in waveform [8]. For a tsunami with an undular-bore
front, higher order dispersion terms are required. The Boussinesq equation, Peregrine
equation or Goto equation should be used in accordance with the strength of non-linearity.
But accuracy and limit of these equations are not yet well determined.
Tsunamis finally run up and down the shore. Since equations in the Eulerian description
cannot express the front condition, approximate moving boundary conditions are
introduced. For ordinary topography on land where the slope is gentle, the shallow-water
theory in which the vertical acceleration is neglected is applicable. In the rare cases of very
steep slope, the vertical acceleration of water flow becomes non-negligible and other
equations than the shallow-water equations are required.
Tsunamis run up on land and leave sediments as a proof of their existence. Many
paleotsunamis were excavated. No work succeeded until now to explain the magnitude and
movement of paleotsunamis from sediment samples.
2.3 DESIGN OF NUMERICAL SIMULATION
Difference equations are not the original differential equations but are the approximate
equations of the latter. Numerical errors are inevitable. When difference equations are
solved, several conditions should be satisfied to ensure the accuracy of the results and the
stability of computation. In addition to the CFL condition that is required for the wave
equation, several other conditions should be satisfied.
In case of the leapfrog scheme used in FDM computation, more than 20 spatial grids are
required within one local wavelength. More than 50 spatial grids are necessary at the front
of runup waves, if an approximate moving boundary condition is used [9]. Grid lengths
should express well tsunami diffraction and refraction due to local topography. There is no
criterion to design spatial length in relation to topography, although several attempts are
made.
In case of other schemes, e.g. FEM, numerical inaccuracy due to truncation error should
be examined well.
There are two crucial factors in the due evaluation of the computed results. The first one
is the initial condition and the second the sea bottom topography. An initial profile
constructed from seismic data always requires, particularly in case of a near-field tsunami,
some adjustment as described in 2.1. The sea bottom topography that governs refraction
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then determines the path and convergence of tsunami, is not always precise data. The most
popular sea bottom topography is the chart, the main purpose of which is to ensure the
route of navigation. The area far from the route is sparsely measured and imprecisely
determined by interpolation. This method often misses sea topography important in
refraction.
TABLE 1. Kinds, types and causes of tsunami disaster
Human Lives
Drowned. Buried by sands. Injured hit by debris etc. Disease caused by swallowing alien substances
during drifting.
Houses
Washed away. Destroyed. Flooded. Burnt.
Coastal Structures
Toe erosion, displacement and overturning of sea walls, sea dikes, breakwaters and quay walls.
Scattering and subsidence of concrete blocks.
Traffics
Railway
Erosion of embankments.
Displacement of rails and bridges. Rails buried by sands.
Highway
Displacement and falling down of bridges.
Overturning of bridge abutment by scouring.
Erosion of embankment.
Closure of traffic by debris on roads.
Harbour
Change in water depth (erosion and deposition)
Closure of port area due to transported debris and cars.
Closure of port entrance by fishing gears washed-away.
Collision of ships in harbour.
Lifelines
Water supplies
Destruction of hydrants by collision of debris.
Electricity
Overturning and washed-away of electric poles.
Power plants flooded.
Telephone
Damage to telephone lines and poles.
Cut-off of underground telephone line at the junction to the aerial lines.
Submergence of telephone receivers.
Fishery
Damage to fishing boats.
Destruction and loss of rafts, fishes and shells in aquaculture.
Loss of fishing nets and other fishing gears.
Commerce
Depreciation of goods by submergence.
Agriculture
Physiological damage to crops due to submergence.
Damage to farms buried by sands.
Closure of irrigation channels filled by sands and debris.
Forest
Physical damage (breaking and overturning of trees). Soil erosion.
Physiological damage by seawater and sands.
Oil spill
Environmental pollution.
Spread of fires.
Fire (causes)
Kitchen fire. Heating. Engine room of fishing boats. Submerged batteries of fishing boats.
Collision to gasoline tanks. Electricity short circuit caused by seawater.
The validation of the computed results, i.e., tsunami height, is done by comparing them
with the tide records and tsunami trace data. Some type of tide gauge has hydraulic filtering
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effect that reduces short-period components [10]. Tsunami trace data are often biased
because many surveyors are attracted to measure only high values.
There is no attempt to validate the computed velocity, wave profile, wave forces and so
on, except one [11].
3. Disasters
3.1 KINDS OF DISASTER
Table 1 summarizes disasters caused by tsunamis, collected from documents in the past.
Detailed analysis is carried out in relation to tsunami intensity [12, 13].
Loss of human lives depends much upon the action of people on and near the shore
when an earthquake occurs. Once caught by a water flow even as thin as 70cm, a person
may be swallowed to drown, because the current velocity is quite strong.
Wooden houses are weak. On an average, a wooden house is completely destroyed if
the tsunami height above ground exceeds 2 meters. Reinforced concrete buildings are
strong. According to records, every reinforced-concrete buildings were not damaged except
for windows and gates and were resistant enough to protect weak wooden houses behind
them. Destructive force is not only the tsunami force but also impact of lumbers, fishing
boats and houses transported by tsunamis. Impact force of one lumber is formulated by
hydraulic experiments [14].
Damage to fishing boats moored and/or placed near the shoreline begins with the
tsunami height of 2 m. If a ship or a boat meet a tsunami on the sea deeper than 200 meters,
they are safe because of small tsunami height and gentle tsunami front slope. Many
fishermen want to bring their boats to the deep sea when a tsunami warning is issued. This
action may lead them to the very dangerous situation that boats get aground by tsunami ebb
near shore and get turned by the next flood before they arrive at the safe place.
Violent currents induced by tsunamis are the cause of erosion and deposition. Not only
local scouring at the toe of coastal structures but also topographical change of large scale
often occurs. No one has ever succeeded to numerically simulate these topographical
changes, partially due to the inaccuracy of computed currents and partially due to the lack
of knowledge about the movement of sediment under tsunami effect.
3.2 TSUNAMI, OIL AND FIRE
Even in olden days, tsunami-related fires occurred. At night on January 27, 1700, a tsunami
suddenly hit the village of Miyako, Sanriku Coast, Japan, with no precedent earthquake. A
fire started from overturned houses and about 40 houses were burnt. The tsunami was
generated at the Cascade subduction zone off the western coast of USA.
In the future, the most hazardous effect will be given to the coastal industrial areas by
the combination of tsunami, oil and fire. If an earthquake or a tsunami damages oil tanks,
and if the oil spread by the tsunami catches fire, or if the burning oil is transported by the
tsunami, the result is devastating. There were five examples: three in Alaska, USA, one in
California, USA, and one in Niigata, Japan. All of them occurred in 1964. Spread of oil can
be numerically simulated, if equations for movement of oil are simultaneously solved with
equations for tsunami [15]. An empirical formula is proposed to estimate the size of the
burnt area in terms of the volume of stored oil [16].
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4. Mitigation of Tsunami Hazards
4.1 COASTAL STRUCTURES
Tsunami countermeasures consist of three parts from hard wares to software: structures,
city planning and systems.
Sea walls and coastal dikes are the typical defense structures. They are effective if the
tsunami height is lower than their crown height, or a slightly higher (may be, 50 cm or 1
m). If the tsunami height is higher by more than 5 m, they do not work at all [17].
Tsunami breakwaters are constructed at the mouth of a bay where the water is deep, in
order to limit the water discharge into the bay. Since they are expensive, only a few are
constructed.
A tsunami gate at the river mouth stops the tsunami invasion into the river, in place of
heightening long river embankments.
Coastal forests are also one of defense structures. A well-designed coastal forest of the
Japanese red-pine trees works well to stop boats and other floated debris if the tsunami
height above ground is smaller then 3 meters [12].
4.2 CITY PLANNING
The major items in the city planning are movement of residences to “tsunami-free” high
ground and establishment of the tsunami-resistant building zone near the shoreline.
Since olden days, the movement of residence to high ground is one of the most effective
methods in tsunami defense work. The highest tsunami runup measured and/or computed
for the biggest tsunami in the past is usually used to determine the tsunami-flooded area.
Recently in Japan, tsunamis that may be generated by the largest earthquake expected from
seismo-tectonics are also taken into consideration. The tsunami-free high ground is outside
of the area thus determined.
In many documents and after-tsunami survey reports, it is recorded that reinforcedconcrete buildings are strong enough to resist tsunami force. Only one exception is a
lighthouse that was hit by the 1946 Aleutian tsunami 20 m high above ground. If the
tsunami height above ground is less than 5 m, all the buildings can withstand [12]. This fact
leads to the idea of the tsunami-resistant building zone that cannot perfectly stop the
tsunami water intrusion but is expected to stop more dangerous floated materials.
In the coastal industrial area, fishing harbor and leisure boats anchorage, storage tanks
of inflammable materials should be carefully located and protected against tsunami effects.
4.3 TSUNAMI PREVENTION SYSTEMS
In addition to structures and city planning, software should be taken into consideration to
complete the tsunami defense work. The tsunami prevention system consists of forecasting,
warning, evacuation, public education, drills, inheritance of disaster culture, and the relief
operation after disaster.
The last way to save human lives is an early evacuation based on the forecasting and
warning. An earthquake that makes you unable to stand by yourself on a beach is a natural
warning of a tsunami. Leave the beach as soon as possible and climb up to a ground higher
than 20 m. This is the rule to save the lives from the danger of the near-field tsunami
generated by tsunamigenic earthquake. Another rule is “a loud booming noise could mean a
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tsunami is coming.” This noise may be generated by the breaking front of a tsunami higher
than 2.5 meters [13].
Many countries have their own tsunami forecasting and warning systems. Most of them
use the empirical relationships between earthquake magnitude and tsunami magnitude for
tsunamigenic earthquakes. The forecasting system in French Polynesia uses the mantle
magnitude and is effective for a tsunami earthquake, too.
In April 1999, the Japan Meteorological Agency renewed its forecasting system based
upon numerical simulation for nearly 100,000 cases. This is also for tsunamigenic
earthquakes. Records of broadband seismographs are used to make the correction for a
tsunami earthquake.
The public education is the crucial key to save human lives. It is very rare for any
person to experience plural large tsunamis in his life. If a person has a simple knowledge
that a tsunami will come after an earthquake, and if he behaves wisely to climb up to high
ground, he will be saved. This knowledge should be transferred to the future generation and
to coastal residents in every tsunami-risk areas. The most difficult is to find an effective
way to continue this knowledge for several tens or hundreds years.
5. Concluding Remarks
The most urgent subject in research as well as in practical application is the improvement
of the method to determine tsunami initial profiles with seismic data alone. In order to
solve this problem, we need observation networks in the ocean. Ocean-bottom
seismographs near tsunami sources are indispensable to understand the details of fault
movements. Deep-ocean tsunami gauges will catch tsunamis at or just after their birth. New
technologies such as satellite photometry are desirable to obtain a plan of tsunami profile.
With these data, a further development in tsunami research becomes possible.
The second is the continuation of tsunami knowledge to future generation as well as the
transfer of it to those who live in tsunami-risk areas but have no clear tsunami history.
Every means such as public education, TV media and others should be used. But it is quite
difficult is to keep coastal residents’ continuous attention. In the Sanriku region, the most
tsunami-risky area in the world, special drills once a year have been carried out on the
Memorial Day of the past tsunamis with participators constantly reducing in number. To
find a way to break this situation is an urgent task.
Thirdly, keeping the fact in mind that disastrous tsunami occurs once per tens or
hundreds years, the coastal residents should make their town resistant to tsunamis at every
occasion when the town is changed and developed. The knowledge for this reinforcement
should be given in terms of building codes and /or manuals.
References
1. Mansinha, L. and D.E. Smylie (1971) The displacement field of inclined faults, Bulletin of the Seismological
Society of America, 61, 1433-1440.
2. Okada, Y. (1985) Surface deformation due to shear and tensile faults in a half-space, Bulletin of the
Seismological Society of America, 75, 1135-1154.
3. Kanamori, H. and J.J. Ciper (1974) Focal process of the Great Chilean Earthquake, May 22, 1960, Physics of
Earth and Planetary Interiors, 9, 128-136.
4. Plafker, G. (1965) Tectonic deformation associated with the 1964 Alaska Earthquake, Science, 148, 1675-1687.
5. Fukao, Y. (1979) Tsunami earthquakes and subduction processes near deep-sea trenches, J. Geophysical
Research, 84, 2303-2314
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6. Tanioka, Y., L. Ruff and K. Satake (1997) What controls the lateral variation of large earthquake occurrence
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No.357, II-3, 217-223 (in Japanese).
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