Tsunami of 26 December 2004: Observations on Kerala Coast

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0016-7622/2005-65-2-239/$ 1.00 © GEOL. SOC. INDIA
JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.65, February 2005, pp.239-246
Tsunami of 26 December 2004: Observations on Kerala Coast
A. C. NARAYANA1, R. TATAVART I2 and MUDRIKA SHAKDWIPE1
1Department of Marine Geology and Geophysics, School of Marine Sciences, Cochin University of Science
and Technology, Lakeside Campus, Fine Arts Avenue, Cochin - 682 016
2Naval Physical and Oceanographic Laboratory, Defence Research and Development Organisation, Cochin - 682 021
Email: a_c_narayana@yahoo.com
Earthquakes and Tsunamis
Large vertical movements of the earth’s crust can occur
at plate boundaries. Plates interact along these boundaries
called faults. Although, an earthquake can be caused by
volcanic activity, movements along fault zones associated
with the plate boundaries generate most of the earthquakes.
Around the margins of the Indian Ocean, for example, denser
oceanic plates slip under continental plates in a process
known as subduction. The recent tsunami was reported to
have generated at the subduction zone at the Indonesian
plate boundary and near the Java trench. Most strong
earthquakes, representing 80% of the total energy
released worldwide by earthquakes, occur in subduction
zones where an oceanic plate slides under a continental
plate or another younger oceanic plate.
Tsunamis can be generated when the sea floor abruptly
deforms and vertically displaces the overlying water.
Tectonic earthquakes are a particular kind of earthquake
that are associated with the earth’s crustal deformation; when
these earthquakes occur beneath the sea, the water above
the deformed area is displaced from its equilibrium position.
Waves are formed as the displaced water mass, which acts
under the influence of gravity attempts to regain its
equilibrium. When large areas of the sea floor elevate or
subside, a tsunami can be created.
Not all earthquakes generate tsunamis. To generate a
tsunami, the fault where the earthquake occurs must be
underneath or near the ocean, and cause vertical movement
of the seafloor (up to several meters) over a large area (up
to a hundred thousand square kilometers). Shallow focus
earthquakes (depth < 70 km) along subduction zones are
responsible for most destructive tsunamis. The amount of
vertical and horizontal motion of the sea floor, the area over
which it occurs, the simultaneous occurrence of slumping
of underwater sediments due to the shaking, and the
Fig.1. Schematic sketch showing the concept of plate tectonics
(UNESCO, Intergovernmental Oceanographic Commission
Brochure, 2002).
Why Earthquakes Occur?
The dynamics of earth can be explained by the theory of
plate tectonics. Plate tectonic theory is based on an earth
model characterized by a small number of lithospheric plates
(there are 7 major plates and many smaller ones), 70 to 250
km thick, that float on a viscous under-layer called the
asthenosphere. These plates, which cover the entire surface
of the earth and contain both the continents and seafloor,
move relative to each other at rates of up to ten cm/year.
The region where two plates come in contact is called a
plate boundary, and the way in which one plate moves
relative to another determines the type of boundary:
spreading, where the two plates move away from each other;
subduction, where the two plates move toward each other
and one slides beneath the other; and transform, where the
two plates slide horizontally past each other. Deep ocean
trenches characterize subduction zones. Most of the volcanic
islands or volcanic mountain chains are generally associated
with the many subduction zones in the world oceans.

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
240 A. C. NARAYANA AND OTHERS
efficiency with which energy is transferred from the earth’s
crust to the ocean water are all part of the tsunami generation
mechanism.
Tsunamis are unlike wind-generated waves, which many
of us may have observed at a coastal beach, in that they are
characterized as shallow-water waves, with long periods and
wavelengths. The wind-generated swell one sees at a beach,
for example, spawned by a storm out in the Bay of Bengal
and rhythmically rolling in, one wave after another, might
have a period of about 10 seconds and a wave length of
150 m. A tsunami, on the other hand, can have a wavelength
in excess of 100 km and period on the order of one hour. As
a result of their long wavelengths, tsunamis behave as
shallow-water waves.
A wave becomes a shallow-water wave when the ratio
between the water depth and its wavelength gets very small.
Shallow-water waves move at a speed that is equal to the
square root of the product of the acceleration of gravity
(9.8 m/s2) and the water depth - let’s see what this implies:
In the Indian Ocean, where the typical water depth of is
about 2500 m, a tsunami travels at about 160 m/s, or
approximately 575 km/hr. Because the rate at which a wave
loses its energy is inversely related to its wavelength,
tsunamis not only propagate at high speeds, they can
also travel great distances with limited energy losses.
What Happens to a Tsunami as it Approaches Land?
As a tsunami leaves the deep water of the open ocean
and travels into the shallower water near the coast, it
transforms. A tsunami travels at a speed that is related to
the water depth. Hence, as the water depth decreases, the
tsunami slows down. The tsunami’s energy flux, which is
dependent on both its wave speed and wave height, remains
nearly constant. Consequently, as the tsunami’s speed
diminishes as it travels into shallow water, its height grows.
Because of this shoaling effect, a tsunami, imperceptible at
sea, may grow to be several meters or more in height near
the coast. When it finally reaches the coast, a tsunami may
appear as a rapidly rising or falling tide, a series of breaking
waves, or even a bore.
In the open ocean a tsunami is less than a few tens of a
centimeter high at the surface, but its wave height increases
rapidly in shallow waters. Tsunami wave energy extends
from the surface to the bottom even in the deepest waters.
As tsunami attacks the coastline the wave energy is
compressed into a much shorter distance and a much
shallower depth thus churning up the entire water column
and creating life-threatening waves.
What Happens When a Tsunami Encounters Land?
As a tsunami approaches shore, it begins to slow and
grow in height. Just like other water waves, tsunamis begin
to lose energy as they rush onshore - part of the wave energy
is reflected offshore, while the shoreward-propagating wave
energy is dissipated through bottom friction and turbulence.
Despite these losses, tsunamis still reach the coast with
tremendous amounts of energy. Tsunamis therefore can
travel thousands of kilometers and still have devastating
effects on land. The tsunamis have great erosional potential,
stripping beaches of sand that may have taken years to
accumulate and undermining trees and other coastal
vegetation. Capable of inundating, or flooding, hundreds of
meters inland past the typical high-water level, the fast-
moving water associated with the inundating tsunami can
crush homes and other coastal structures. Tsunamis may
reach a maximum vertical height onshore above sea level,
often called a run up height, of 10, 20, and even 30 m. The
first wave of tsunami is not likely to be the biggest.
Wave amplitudes are relatively large shoreward of
submarine ridges. In general as tsunami obeys the shallow
water wave dynamics, the ocean floor or bathymetry governs
the direction of propagation, once a tsunami is generated by
a seismic disturbance. Because wave velocity is a function
of water depth only in shallow waters, variations in depth
can focus or defocus wave energy reaching the shore. They
are relatively low shoreward of submarine valleys, provided
the features extend into deep water. Wave amplitudes are
decreased by the presence of coral reefs bordering the coast.
Waves can bend around circular islands without great loss
of energy, but become considerably smaller on the backsides
of elongated angular islands and continents. The reason why
specific coasts are battered by tsunamis while neighboring
regions are not affected is because of the focusing and
de-focusing of wave energy dictated by the bottom
topography. During the recent earthquake this phenomenon
was evident when tsunami struck the Kerala Coast which
Fig.2. Schematic sketch of tsunami wave characteristics as it
approaches the coast.
Depth Velocity Wavelength
(m) (km/h) (km)
7000 943 282
4000 713 213
2000 504 151
200 159 48
50 79 23
10 36 10.6
3 5.4 1.59

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TSUNAMI OF 26 DECEMBER 2004: OBSERVATIONS ON KERALA COAST 241
is on the lee side of travel direction. Some bays like the Bay
of Bengal have a funneling effect.
How Fast Can Tsunami Travel?
Where the ocean is over 6,000 m deep, unnoticed
tsunami waves can travel at the speed of a commercial jet
plane, over 800 km per hour. They can move from one side
of the Ocean to the other in less than a day. This great speed
makes it important to be aware of the tsunami as soon as it
is generated. Scientists can predict when a tsunami will arrive
at various places by knowing the source characteristics of
the earthquake that generated the tsunami and the
characteristics of the seafloor along the paths to those places.
How Big Can Tsunamis Grow?
Offshore and coastal features can determine the size
and impact of tsunami waves. Reefs, Bays, entrances to
rivers, undersea features and the slope of the beach all
help to modify the tsunami as it attacks the coastline. When
the tsunami reaches the coast and moves inland, the water
level can rise many meters. In extreme cases, water level
has risen to more than 15 m (50 ft) for tsunamis of distant
origin and over 30 m (100 ft) for tsunami waves generated
near the earthquake’s epicenter. The first wave may not be
the largest in the series of waves. One coastal community
may see no damaging wave activity while in another
nearby community destructive waves can be large and
violent. The flooding can extend inland by 300 m (~1000 ft)
or more, covering large expanses of land with water and
debris.
How Frequently Can Tsunamis Recur?
Since scientists cannot predict when earthquakes will
occur, they cannot determine exactly when a tsunami will
be generated. However, by looking at past historical
tsunamis, scientists know where tsunamis are most likely to
be generated. Past tsunami height measurements are useful
in predicting future tsunami impact and flooding limits at
specific coastal locations and communities. Historical
tsunami research may prove helpful in analyzing the
frequency of occurrence of tsunamis.
Tsunami Warning Centers in the Pacific Rim Countries
The Richard H. Hagemeyer Pacific Tsunami Warning
Center (PTWC) serves as the international warning center
for tsunamis that pose a Pacific-wide threat. This
international warning effort became a formal arrangement
in 1965 when PTWC assumed responsibility as the
operational center for the Tsunami Warning System in the
Pacific (TWSP). The ICG/ITSU, a subsidiary body of the
IOC comprised of 25 international Member States, oversees
TWSP operations and facilitates coordination and
cooperation in all other international tsunami mitigation
activities. The initial objective of PTWC is to detect, locate
and determine the seismic parameters of potentially
tsunamigenic earthquakes occurring in the Pacific Basin or
its immediate margins. To accomplish this, it continuously
receives seismographic data from more than 150 stations
around the Pacific through cooperative data exchanges with
the U.S. Geological Survey, Incorporated Research
Institutions for Seismology, International Deployment of
Accelerometers, GEO-SCOPE, the U.S. West Coast/Alaska
Tsunami Warning Center (WC/ATWC), and other
international agencies running seismographic stations and
networks.
If the earthquake location, depth, and magnitude criteria
needed to generate a tsunami are met, a tsunami warning is
issued to warn of an imminent tsunami hazard. Initial
warnings apply only to areas the tsunami could reach within
a few hours and bulletins include the predicted tsunami
arrival times at selected coastal communities within those
areas. Communities located outside those areas are put into
alert. Warning center scientists then monitor incoming sea
level data to determine whether a tsunami has occurred. If a
significant tsunami with long-range destructive potential is
detected, the tsunami warning is extended to the entire
Pacific Basin. PTWC receives sea level data from more than
100 stations through cooperative data exchanges with the
U.S. National Ocean Service, WC/ATWC, the University
of Hawaii Sea Level Center, Chile, Australia, Japan, Russia,
and other international sources. Tsunami warnings, watches,
and information bulletins are disseminated to appropriate
emergency officials and the general public by a variety of
communication methods.
In addition, individual countries may operate National
or Regional Warning Centers to provide warning information
on regional or local tsunami threats. The Japan
Meteorological Agency provides tsunami warnings to Japan
and additionally to Korea and Russia for events occurring
in the Sea of Japan or East Sea. The Centre Polynesien de
Prevention des Tsunamis provides warnings in French
Polynesia, and Chile (Sistema Nacional de Alarma de
Maremotos) and Russia (Russian Hydrometeorological
Service) operate national warning systems. In the United
States, WC/ATWC provides tsunami warnings to the U.S.
West Coast and Canada, and PTWC provides tsunami
warnings to Hawaii and to all other U.S. interests in the
Pacific. Other countries, including Australia, Colombia,
Nicaragua, Peru, and Korea, are also developing warning
capabilities.

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
242 A. C. NARAYANA AND OTHERS
Tsunami Research Activities
With the broad availability of relatively inexpensive yet
powerful computers and desktop workstations, there is
growing interest and activity in tsunami research. Using
the latest in computer technology, scientists are able to
numerically model tsunami generation, open ocean
propagation, and coastal run up. Ocean-bottom pressure
sensors, able to measure tsunamis in the open ocean, are
providing important data on the propagation of tsunamis
in deep water, and satellite communications have enabled
these data to be used in real time to detect and confirm
that a tsunami has been generated in the deep ocean.
NOAA’s Pacific Marine Environmental Laboratory has
pioneered the development of these tsunami detection
buoys, and presently seven DART buoys are in operation
in the northern and eastern Pacific and available for use
by the tsunami warning centers. Better equipment and
numerical modeling methods are helping scientists to better
understand the mechanism of tsunami generation.
Seismologists, studying the dynamics of earthquakes with
broadband seismometers (20 to 0.003 Hertz), are
formulating new methods to analyze earthquake motion and
the amount of energy released. Where the traditional
Richter (surface wave) magnitude of earthquakes is not
accurate above 7.5, the seismic moment and the source
duration are now used to better define the amount of
energy released and the tsunami generation potential. Real-
time determination of the depth of the earthquake, type of
faulting, and extent of slippage will significantly improve
the warning centers’ ability to identify the likelihood of a
threatening tsunami.
Tsunami generation is initiated by three-dimensional
deformation of the ocean bottom due to movement of the
fault. Better characterizations of the earthquake fault
mechanism will produce more realistic numerical models
of propagation, run up, and inundation.
During post-tsunami field surveys, inundation and run
up measurements are taken to describe the tsunami effects.
Inundation is defined as the maximum horizontal distance
inland that a tsunami penetrates. Run up is the maximum
vertical height above mean sea level that the sea surface
attains during a tsunami. Actual tsunami wave heights can
be measured from the amplitude of the wave signals seen
on sea level or tide gauge instruments.
Currently, numerical models of propagation generally
use an implicit-in-time finite difference method. Tsunami
inundation models, defining the extent of coastal flooding,
are an integral aspect of tsunami hazard and preparedness
planning. Using worst-case inundation scenarios, these
models are critical to defining evacuation zones and routes
so that coastal communities can be evacuated quickly when
a tsunami warning has been issued.
The December 26th Earthquake and the Consequent
Tsunami
At 0628hrs IST on the morning of 26th December, 2004,
a shallow earthquake with focus at a depth of 10 km, and a
magnitude of 8.9 on the Richter scale occurred, with
epicenter located at 3.298oN-95.779oE off Sumatra,
INDIAN
OCEAN
Fig.3. Ocean floor map of north Indian Ocean with tsunami generation location and travel directions.

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
TSUNAMI OF 26 DECEMBER 2004: OBSERVATIONS ON KERALA COAST 243
Indonesia. The earthquake’s epicenter was located at
257 km SSE of Banda Aceh, Sumatra, Indonesia; 990 km
SSE of Port Blair, South Andaman Sea, India; 1806 km
ESE of Colombo, Sri Lanka; and 2028 km SE of Chennai,
India. Thereafter, ten earthquakes (4 in Sumatra region,
including the first Killer earthquake; and 4 in Andaman
Islands and 3 in Nicobar Islands) were recorded between
0628 and 0951hrs. Most of the earthquakes in Andaman &
Nicobar Islands had a magnitude of 6; one that occurred at
9.51 hrs in Nicobar Islands had a magnitude of 7.3.
As a consequence to this seismic activity tsunamis were
generated which hit different coasts in the Indian Ocean
with devastating effects, to both human life and property.
Figure 3 shows the location of the earthquake and the
resultant tsunamis’ travel directions.
The tsunami waves travelling with high speeds were
topographically (bathymetry controlled) steered to different
locations of the Asian and African countries as depicted in
Fig.3. Because of the characteristically long wave nature of
tsunami, the earth’s rotation (Coriolis force) also played a
significant role in steering them towards the right side of
the travel direction. Hence, the Coriolis force and the
bottom topography of the ocean were primarily responsible
for the specific locations of onslaught of the tsunamis
along the coastlines of different Asian and African
countries.
The Kerala Coast – Pre-Tsunami Scenario
Figure 4 shows the satellite image covering a part of the
Kerala coast (bounded by 9° N and 9°20' N latitudes and
76° 20' E and 76°35' E longitudes), captured during January
2004 by the LISS III sensor of IRS 1D satellite. This
region was chosen to study the after effects of tsunami on
the coast.
During November to January the coastal currents
generally have a counter-clockwise direction due to the
orientation of the coast and the prevailing wind conditions.
The significant wave heights are approximately 0.5 m, with
periods ranging from 10 to 12 seconds. Two groins were
constructed recently on either side of the Kayamkulam
estuary as part of an on going activity for establishing a
fishing harbor inside the estuary. As part of coastal erosion
mitigation efforts a seawall comprising of granite boulders
was constructed.
Kerala Coast: Tsunami Effects
Although the entire Kerala coast experienced the effects
of tsunami waves, a stretch of 10 km along the coast, off
Azhikkal (9°2' N to 9° 9.5' N), was the most affected in
terms of inundation, run up, and erosion (Fig.5). On this
Fig.4. Satellite imagery showing the tsunami affected coastal
Kerala. The image is obtained from LISS III sensor of the
IRS 1D satellite during January 2004. The maximum
affected area is marked along the coast with arrows.
Fig.5. Satellite imagery showing the tsunami affected coastal
Kerala. The image is obtained from LISS III sensor of the
IRS P6 satellite on December 27, 2004. The maximum
affected area is marked along the coast with arrows.
Note the discoloration (light green) in the Kayamkulam
estuary.
Azhikkal
Valiyazhikkal
Kayamkulam
Estuary
K
E
R
A
L
A
Karunagapalli
9°20
76° 20' E
76° 35' E
N
N
Thottappalli
TS Canal
Azhikkal
Valiyazhikkal
Kayamkulam
Estuary
K
E
R
A
L
A
Karunagapalli
9°20' N
76° 20' E
76° 35' E
9° N
N
Thottappalli
TS Canal

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
244 A. C. NARAYANA AND OTHERS
coastal stretch, the maximum devastation occurred along
the Kayamkulam estuary (i.e., 4 km south and 2 km north of
Kayamkulam estuary, where fishing harbor is under
construction). Coastal inundation was rampant along this
stretch with run up extending up to 1.5 km eastward from
the shoreline, with the influx brought by tsunami through
the estuary into the TS canal being primarily responsible
for the inundation. As can be seen from Fig.5 that the entire
water body is churned up which manifests as light blue to
light green colour of the sea in the satellite image, suggesting
turbid sediment-laden water.
At Azhikkal, the first tsunami hit the coast at around
1230 hrs, followed by a series of waves with 10 - 15 minute
intervals. After hitting the coast, the sea receded
progressively revealing the sea bed up to1km from the
shore at around 1300 hrs. The largest wave then followed
immediately and hit the coast at 1310 hrs with devastating
fury with an approximate height of 5 m, causing vast
inundation along the coast. The run up of largest wave
rushed a distance of 1.5 km from the shoreline along the
10 km stretch. It was observed that predominant run up
occurred along the course of the closed and open inlets,
suggesting a natural course selection for the run up. The
maximum damage occurred across the shore, between
beach and TS canal. Most of the concrete houses, property,
fishing vessels and automobiles on the road running parallel
to the coast in this affected area, were uprooted and thrown
to distances of 100 to 200 m (Fig.6).
Most of the boulders (having an average size of 1 m) of
the seawall, which was constructed to protect the coast
from erosion were thrown ashore up to a distance of 150 m
by the rushing tsunami (Fig.7). At some places, the uprooted
boulders, fishing vessels and trucks got struck in the coconut
groves parallel to the coast, while some were washed into
the neighboring flooded TS canal. The sea that gushed on
to the land made huge erosion pits along the roadside and
also at the basement of the houses (Figs. 8, 9 and 10).
Extensive quantities of black sands also were transported
by tsunami blanketing the area between sea wall and TS
canal (Figs. 11 and 12). In the Azhikkal area, the black sand
was of 2 feet thick for a stretch of 2 km of the road. The
black sand deposit was more extensive in Valiya Azhikkal
area (i.e., northern side of the estuary), where the coastal
road was buried by a blanket of 4 ft thick black sands.
(Fig. 13), suggesting that the tsunami hit the coast from a
south west direction resulting in sediment transport towards
north east. We suspect that this may be the artifact of the
alignment of the coast, the predominant tsunami wave
direction and the fact that more erosion results on the
leeside of an inlet due to waves and associated currents
thus transporting the middle ground shoals located at the
entrance of the estuary along with the churned up sediments
along the sea bed on to the northern side of the estuary
(Figs. 9 and 13). A large amount of black sand entered the
houses and also deposited in porticos and it was piled up
into heaps in front of the houses.
Implications to Coastal Zone Management
After witnessing the devastating power of the sea in
destruction of concrete structures as well as seawalls
constructed for protecting the coast, it would be prudent
to leave a safe buffer zone from the shoreline. It would be
wise to adhere to the coastal regulation zone act, which
Fig.6. The picture shows the coastal road which runs parallel to
the coast 50 m from the shoreline and a mini truck which
was uplifted and dumped away from the road. The debris
caused by the tsunami is also visible.
Fig.7. The picture shows the transported boulders from the
seawall. Also seen are the remnants of the foundation of a
completely destroyed home.

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
TSUNAMI OF 26 DECEMBER 2004: OBSERVATIONS ON KERALA COAST 245
Fig.8. The picture shows the remnants of a ravaged home. The
roof of the flattened house is visible on the ground.
Fig.9. The picture shows the huge load of sand deposited on the
northern side of the Kayamkulam estuary, after the tsunami
hit this coast. The displaced boulders from the sea wall are
visible in the foreground.
Fig.10. A flattened concrete structure amidst other debris.
Fig.11. Concentrated patches of black sand deposits and the
destroyed sea wall.
Fig.12. The dump of a black sand deposit on the shore face at
Valiya Azhikkal.
Fig.13. The coastal road blanketed with 4 ft thick black sand
deposits at Valiya Azhikkal.

JOUR.GEOL.SOC.INDIA, VOL.65, FEB. 2005
246 A. C. NARAYANA AND OTHERS
envisages a buffer zone of 500 m from the shoreline. The
present tsunami effects have amply demonstrated the lesson,
which nature teaches time and again, that seawalls are not
the panacea for coastal erosion problems.
The need of the hour is to have capabilities to predict
tsunami occurrence. To forecast tsunamis, tsunami
measurements from the deep ocean are required. The idea
of measuring tsunamis in the deep ocean and actually
reporting such data in real time is scientifically challenging
but feasible. India has the means and technology to establish
tsunami warning systems once the political will is in place.
At present, there is no completely satisfactory
explanation for the occurrence of disproportionately large
tsunamis, but it is an area that will require further research
by tsunami scientists. As brought out in the recommendations
of the March 2004 workshop on “Seismo-Acoustic
Applications in Marine Geology and Geophysics”, at the
Woods Hole Oceanographic Institution, USA; it is important
to look at the role of seismo-acoustics (T-phases) in
understanding the dynamics of earthquakes and tsunamis.
Another area that needs research attention is the role of
resonance amplification in explaining why along the
coastlines of bays and gulfs the tsunami amplitudes are so
large while at other nearby locations the amplitudes are
considerably smaller.
Therefore scientific emphasis should be on better
predictions of earthquake epicenter and intensity, the
physics of tsunamis, and the consequent run up and
inundation patterns. It pays to prepare detailed run up and
inundation maps to understand the consequences of
tsunamis. The same information is vital for flooding and
storm surge eventualities. Increasing the public awareness,
and dissemination of run up and inundation patterns to
civic administration for evacuation management, would
certainly curtail the intensity of devastation of life and
property.
Acknowledgements: Authors thank Dr. B.P. Radhakrishna,
President, Geological Society of India for inviting to write
this article.
References
TSUNAMIS: The Great Waves. Information Brochure, published by
UNESCO Inter-Governmental Oceanographic Commission,
Paris, France, May 2002.
ODM, R.I. and STEPHEN, R.A. (2004) Report on Proceedings
‘Seismo-acoustic applications in marine geology and
geophysics. Workshop - WHOI, 24-26th March 2004. Technical
Report (No. APL-UWTR 0406), Applied Physics Laboratory,
University of Washington; July 2004, 61p.
For this short review information has also been compiled from
various web sources.