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The Science of Tsunamis
The Sumatra-Andaman earthquake of 2004 produced the deadliest tsunami on record,
alerting the world to the destructive power of this phenomenon. In studying this
tsunami, scientists are using new tools that provide unprecedented insight into the
causes and effects of these events. The knowledge gained from their work will help
improve early-warning systems, mitigating the consequences of future occurrences.
Tim Bunting
Kuala Lumpur, Malaysia
Chris Chapman
Phil Christie
Cambridge, England
Satish C. Singh
University of Cambridge
Cambridge, England
Jim Sledzik
Gatwick, England
For help in preparation of this article, thanks to Eric Geist,
United States Geological Survey, Menlo Park, California,
USA; and Robert Stewart, Texas A&M University, College
Station, USA.
Q-Marine is a mark of Schlumberger. DART is a registered
trademark of the US National Oceanic and Atmospheric
Administration (NOAA).
1. “Indian Ocean Earthquake & Tsunami Emergency
Update December 29, 2005,” Center of Excellence in
Disaster Management & Humanitarian Assistance,
http://www.coe-dmha.org/Tsunami/Tsu122905.htm
(accessed September 27, 2007).
2. Nirupama N, Murty TS, Nistor I and Rao AD: “Energetics
of the Tsunami of 26 December 2004 in the Indian Ocean:
A Brief Review,” Marine Geodesy 29, no. 1 (January
2006): 39–47.
3. The term plate tectonics was coined by Bryan Isacks,
Jack Oliver and Lynn Sykes in a 1968 research paper.
Isacks B, Oliver J and Sykes L: “Seismology and the New
Global Tectonics,” Journal of Geophysical Research 73
(September 15, 1968): 5855–5899.
4
On December 26, 2004, the Sumatra-Andaman
earthquake, with an estimated magnitude of 9.3
on the Richter scale, was one of the largest ever
recorded using modern seismographic equipment. As it shook the west coast of Sumatra,
Indonesia, and proceeded along a fault line at
the eastern edge of the Indian Ocean, the
earthquake generated a tsunami that focused the
world’s attention on the devastating power of this
natural phenomenon. With estimates of more
than 232,000 deaths and 2,000,000 people
displaced in 12 countries in South Asia and
East Africa, the impact of the tsunami was
truly global.1
In addition to being one of the worst natural
disasters in human history, the tsunami was
unique in other aspects. It was the first global
tsunami to occur since modern sea-level
monitoring networks were established and the
first to be continuously tracked and recorded by
a satellite. No other seismic event of this
magnitude has occurred with so many datagathering sources available. From a scientific
perspective, the event provided a wealth of
information for analysis. These data will be
used to better understand and prepare for
future incidents.
The earthquake and tsunami exacted an
observable physical toll—on houses, bridges and
businesses—that can be seen by comparing
before and after photographs (next page, bottom).
These images reveal the damage that emanated
from events that began below the surface.
However, a full understanding of the earthquake and the subsequent tsunami requires a
multifaceted approach.
To develop an appreciation for the magnitude
of this event—the energy released temporarily
altered the Earth’s rotation—we present a basic
review of the theory of plate tectonics as it
relates to the earthquake.2 A discussion of the
physics of ocean waves and tsunamis follows. We
also examine some of the tools used—such as
seismic and ocean monitoring networks, landbased global positioning systems (GPS) and
tsunami modeling software—to better comprehend the scope of this event. Details of the
WesternGeco tsunami seismic survey will be
included, along with some preliminary findings.
This article also reviews the status of ongoing
efforts to develop an integrated monitoring and
early-warning system in the Indian Ocean region.
Tectonic Foundations for a Tsunami
On a geological time-scale, the surface of the
Earth is constantly changing—oceans form and
disappear, continents collide with one another,
and mountains rise and fall or erode away. To
explain the processes that shaped and continue
to shape the surface of the Earth, the theory of
plate tectonics was proposed.3 It states that the
Earth’s lithosphere, the outermost layer, is
broken into rigid plates that are moving relative
Oilfield Review
Sri Lanka
Andaman Islands
Sumatra
> Courtesy of US Geological Survey (USGS).
Before
After
> High-resolution imaging satellite photographs of Banda Aceh, Indonesia, before and after the tsunami. Banda Aceh is located at the northern tip of
Sumatra. With a population of 260,000, it was the closest major city to the epicenter of the Sumatra-Andaman earthquake. (Photographs courtesy of
DigitalGlobe.)
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5
Sumatra-Andaman
earthquake, 2004
Pacific
Ring of Fire
Indian
Ocean
Crustal plate boundaries
Earthquake epicenters, MW >5, 1980 to 1990
> Plate boundaries defined by seismic activity. The mapping of medium to large seismic events (red) helps identify crustal
plate boundaries (yellow). The area known as the Pacific Ring of Fire is the most active region on the planet, with 90% of
recorded seismic events. By comparison, the Indian Ocean is most active along the eastern edge—especially in the vicinity
of the December 2004 Sumatra-Andaman earthquake. [Adapted from an image courtesy of the US National Oceanic and
Atmospheric Administration (NOAA).]
E U R A S I A N
P L A T E
Him
alay
as
India
today
Bangladesh
Equator
Location of India
70 million
years ago
Indian
Ocean
Sri Lanka
> India in motion. India was an island off the
east coast of Africa 100 million years ago.
It is part of the Indo-Australian plate and has
been advancing into the Eurasian plate as it
journeys northward. During this movement, the
Himalaya Mountains were formed along India’s
northern border.
6
to one another, “floating” on the asthenosphere,
a hotter, denser, more mobile layer. Below the
asthenosphere are the upper mantle, the mantle,
the outer core and, at the center of the Earth, the
inner core. The major plates have been identified
and, by plotting seismic activity, their boundaries
have been defined (above).4
Tectonic plates are constantly diverging,
converging or transforming. In divergent zones,
the plates move away from each other, allowing
basaltic magma to ooze to the seafloor and create
the dense oceanic crust at midocean rift zones.
The magma cools as it meets seawater and
forms a series of underwater mountain ridges
that are carried away from the rift by the
diverging plates.
Landmasses above sea level form the
continental crust, which is usually thicker and
much less dense than oceanic crust. The dense
oceanic plate slides beneath the overriding plate
in what is termed a subduction zone. Eventually,
the subducting plate melts and returns to the
asthenosphere. As the subducting material
dewaters, the fluid migrates upward, mixing with
the material of the overriding plate, reducing its
melting point. This produces magmatic melts,
rich in dissolved gases, that exert enormous
upward pressure on the overriding plate; these
can erupt if a weakness in the crust develops
(next page, top).5
Along boundaries where crust is neither
created nor destroyed, changes still occur,
transforming the surface of the Earth. Over time,
as landmasses collide, an ocean that separated
the masses may disappear, while the previous
ocean bottom is lifted above sea level. Plates may
deform along their borders into mountain ranges.
Landmasses that make up the continental crust
may slide horizontally, creating earthquakes as
plates stick and slip.
The Indo-Australian plate, which played a key
role in the Sumatra-Andaman earthquake,
comprises both continental and oceanic crust. The
landmasses of India and Australia make up the
majority of the continental portion, while the
oceanic segment lies beneath the Indian Ocean.
According to theory (and data), 100 million years
ago, India was an island off the east coast of Africa,
south of the equator, and it has been making a
relentless journey northward, creating the
Himalaya Mountain system along the way. Today,
India is penetrating the Eurasian plate at a rate
of 45 mm/yr [1.8 in./yr] while slowly rotating
counterclockwise.6 Mount Everest, the tallest of the
Himalayan chain, grows 4 mm [0.1576 in.] per year
because of this movement (left).7 The oceanic
crustal portion of the plate is subducting under the
Burma microplate and the Eurasian plate.
To the west of Sumatra, the Sunda (or Java)
trench marks the edge of the subduction zone.
Oilfield Review
Divergent boundary
(rift zone)
Shield volcano
Oceanic crust
Convergent plate
boundary
Continental
rift zone
Tre
n
ch
Oceanic spreading ridge
Continental crust
Lithosphere
Asthenosphere
Su
bdu
ctin
gp
lat
e
ch
Indo-Australian
plate
Tre
n
Burma
microplate
Convergent
plate boundary
Depth indication, m
4,000
1,000
> The ever-changing face of our planet. According to the theory of plate tectonics, the lithosphere is composed of variously sized rigid plates, which are
diverging, converging or transforming along boundaries. At rift zones, plates move away from each other, leaving spaces that are filled with dense basaltic
magma rising from the asthenosphere. At convergent plate boundaries, subduction takes place as dense oceanic crust dives beneath the more buoyant
continental crust, eventually returning to the asthenosphere. Earthquakes occur along these boundaries as stress created by friction between plates
is released, often catastrophically. The sudden movements of submerged plates play an important role in the generation of tsunamis. Bathymetry data
(inset) from a section of the December 2004 earthquake zone shows the Indo-Australian plate subducting beneath the Burma microplate. A trench forms
at their boundaries.
The trench extends some 3,000 km [1,865 mi],
from the Andaman Islands in the northwest to
the Lesser Sunda Islands in the southeast, and
has a depth in excess of 7,700 m [4.8 mi].8 The
Burma microplate is wedged between the IndoAustralian and the Eurasian plates (right). As
the Indo-Australian plate subducts beneath
Eurasian
plate
December 26, 2004
Indo-Australian
plate
0
0
km
a
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Andaman Islands
tr
ma
Su
4. Oreskes N (ed): Plate Tectonics: An Insider’s History of
the Modern Theory of the Earth. Boulder, Colorado, USA:
Westview Press, 2001.
5. Volcanoes result from these upward flows, creating
conduits through the overriding plate for molten magma
to reach the surface.
6. Bilham R: “Earthquakes in India and the Himalaya:
Tectonics, Geodesy and History,” Annals of Geophysics 47,
no. 2 (2004): 839–858.
7. http://www.nationalgeographic.com/features/99/
everest/roof_content.html (accessed October 14, 2007).
8. The Sunda trench was once thought to be the deepest
point in the Indian Ocean until the 8,000-m [26,250-ft]
Diamantina Deep was discovered in 1961.
Burma
microplate
1,000
miles
1,000
> Tectonics of the Sumatra-Andaman earthquake. The eastern edge of the Indo-Australian plate is
subducting beneath the Eurasian plate and Burma microplate at a rate of 52 mm/yr [2.05 in./yr]. The
Indo-Australian plate is moving northward while slowly rotating counterclockwise. The December
2004 Sumatra-Andaman earthquake began at the epicenter (star) and continued north for 1,200 km
[745 mi] along the fault line (blue), terminating at the Andaman Islands. Boundaries of plates (triangles)
and microplates (gray lines) are indicated.
7
these plates, stresses build when the plates
become stuck. Because the plates continue to
move, the time between major earthquakes and
the extent of the area where their relative
motions are constrained determine the potential
earthquake severity.
Although the Indian Ocean has its share of
earthquakes in seismically active zones, the
boundaries of the Pacific Ocean are actually the
most active in the world, with 90% of all
earthquakes—80% of the major ones—occurring
within the Pacific basin. The primary mechanism
for this seismic activity is the movement of the
subducting plate described above.9
Because the Pacific basin is so seismically
active, an extensive network of sensors has been
established for earthquake and tsunami
detection. Although there were plans to develop
a system modeled after the one used in the
Pacific, at the time of the tsunami, there was no
such network for the Indian Ocean. Large
tsunamigenic events were infrequent, with only
one major tsunami occurring there during the
previous century and only four reported in the
1800s. The tsunami created by the well-known
eruption of Krakatoa in 1883, and by its ensuing
collapse, was one of those four. Historical data
combined with the high level of seismic activity
suggested a likelihood of tsunamis occurring
in the region, but nothing on the scale of the
tsunami of 2004 was anticipated.10
Making Waves
Ocean waves—tsunamis being one category—
are classified as gravity waves. Although the
mechanisms that generate them are different,
the physics that describe gravity waves are
applicable to those in a pond, on the open ocean
or after a significant impact such as the SumatraAndaman earthquake. To understand tsunamis,
it is essential to recognize how they are
generated and how they differ from windgenerated waves.
Most ocean waves are primarily generated by
wind turbulence creating friction along the
surface of the water. Turbulence produces ripples
that are capillary waves—waves that travel
between two fluids. Gravity and surface tension
pull the peaks of the ripples back toward
equilibrium, but the ripples overshoot the
original level of the water, causing the surface to
oscillate. Should the wind stop, the oscillations
will die out due to friction. Once the oscillations
have a wavelength greater than 2 cm [0.8 in.],
wind-induced ripples can become gravity waves.
This occurs at the point where the effects of
gravity are greater than the effects of surface
tension. Dispersion from gravity cancels
dispersion caused by surface tension of the
water, resulting in a radiating wave that has the
potential for traveling great distances. As wind
continues providing energy to the waves, the
period, wavelength and speed increase, and the
Wave height increases
Orbital path of
water molecules
resulting waves can even travel faster than the
wind that generated them.
Waves can travel great distances, often
gaining strength and speed by combining with
other waves or by the addition of more wind
energy. A wave in Hawaii might have begun
during a storm in Alaska, arriving on the beach
with little loss of speed or energy. Although the
wave began many miles away, the molecules of
water were not displaced any great distance until
just before the wave reached the shore.
In deep water, if the wavelength is much
shorter than the water depth, the motion of the
water can be described as circular during the
trough-peak-trough cycle. In shallow water, or
when the wavelength is greater than the water
depth, the motion is more elliptical, with the ratio
of the horizontal to vertical motions proportional
to the ratio of wavelength to depth. For a tsunami,
because of its long wavelength, this occurs even
in the deep ocean, and the horizontal motion can
be much greater than the vertical motion. At the
shore, the elliptical motion transforms into forward motion, and the water molecules advance
with the wave (below).
In the ocean, with all its variability, wave
motion is more complex. Gravity, tides, crosswinds, submarine and shoreline features, water
depth and wave arrivals from various angles will
act upon the wave to affect wave height, speed
and direction. Because of the long distances
Surf zone
Elliptical path
> Wave basics. Wind-generated swells move across the surface of the ocean. The water molecules generally have a
circular motion that becomes more elliptical as the wave approaches the shore. The velocity of a wave slows as it
approaches the shore, forcing the water upward. The tip of the wave continues moving faster than the base until it
reaches the surf zone, where the peak of the wave breaks over due to gravity.
8
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Wavelen
gth
Mean se
a lev
el
Trough
Crest
Wavelen
gth
Run-up
Wave
amplitud
e
> A tsunami approaching the shoreline. When the tsunami arrives at the shore, its velocity decreases
rapidly and its height increases and rises well above the average sea level. The original long-wavelength
wave becomes somewhat shorter at the coastline. The distance the wave travels inland—inundation—
and the height of the wave at the shoreline—run-up—are determined by coastal geometry and the
characteristics of the individual tsunami. Contrary to popular belief, a tsunami rarely has a break-over,
rising much like a fast-moving tide. After the wave inundates the low-lying coastal regions, the outrush of water returning to the ocean carries debris from inland. Since the tsunami is actually a series
of waves, subsequent surges return the debris, acting like battering rams along the coastline.
open to wave travel in the oceans, the simple
wave train can develop into swells, which are
long-wavelength waves. As the swells reach
shallow-water depths, they rise higher than they
were when over deep water and form peaks.
These peaks will eventually break over because
of the steepness of the wavefront, the pull of
gravity and the peak moving faster than the base
of the wave.11
Whether the water movement is created by
the wind, the sudden movement of the seafloor
during an earthquake, the downward force from
a landslide or even the impact from an asteroid,
these forces all generate oscillatory motion that
translates into gravity waves. A tsunami differs
from waves produced by the wind in that it is an
impact-generated wave, deriving its speed and
power from the event that created it. Large
impact-generated waves also have extremely
long wavelengths. Tsunamis can have wavelengths in excess of 100 km [62 mi], whereas
wind-generated swells have wavelengths on the
order of 150 m [500 ft].
Wavelength is a useful characteristic for
classifying wave types. A shallow-water gravity
wave is characterized by the fact that the ratio
between the water depth and the wavelength is
quite small. These waves travel at a speed that is
equal to the square root of the product of the
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acceleration due to gravity (9.8 m/s2) [32 ft/s2]
and the water depth. Because of a tsunami’s long
wavelength, it acts like a shallow-water wave
even in deep water, and its speed can be
approximated if the water depth is known. With a
water depth of 7,700 m, the Sunda trench was a
perfect incubator for a fast-moving tsunami,
which attained speeds of more than 900 km/h
[560 mi/h], rivaling the speed of a modern
commercial jetliner.
Not only do tsunamis travel at high rates of
speed, they maintain their wave height, or
amplitude, for great distances. The amplitudes of
water waves decay as they propagate for three
reasons: the waves spread out over the surface of
the water; the waves disperse because longer
wavelengths travel faster; and energy is
attenuated by viscous damping in the water. For
a large tsunami, all three effects are minimal.
Since the energy for initiation occurs along an
extended fault, the waves spread out linearly
rather than cylindrically, resulting in little
spreading. For extremely long wavelengths, the
waves are not highly dispersive because the
velocity is proportional to the square root of the
water depth, resulting in little dispersion in the
open ocean. Attenuation loss is inversely related
to the wavelength, and thus there is little
attenuation. As a result, a tsunami propagates at
high speeds and travels great distances with
limited energy loss.
As a wave moves into shallow water, the
propagation speed developed in deeper water
cannot be maintained. For a tsunami that
originally traveled at 900 km/h in deep water, the
maximum sustainable velocity would be less than
50 km/h [31 mi/h] in a water depth of 10 meters
[33 ft]. Energy continues pushing the wave
forward, leaving only one direction for the water
to go—upward. Wave height on shore, or run-up,
of 35 m [115 ft] was reported on the island of
Sumatra (above).
Ironically, the tsunami would have been
hardly noticed near the epicenter of the quake. A
rise in ocean levels would have felt like a larger
than average swell. For example, the
WesternGeco survey vessel Geco Topaz was
acquiring seismic data off the coast of India
1,500 km [930 mi] from the epicenter. The
tsunami passed under the vessel 2 to 3 hours
after the initial earthquake and was only a few
tens of centimeters in height—in the open water
of the Indian Ocean.
9. Volcanic activity around the subduction zones has resulted
in the area being known as the Pacific Ring of Fire.
10. For an in-depth review of plate tectonics, see the
Schlumberger SEED Web site: http://www.seed.slb.com/
en/scictr/watch/living_planet/index.htm (accessed
August 18, 2007).
11. Stewart RH: Introduction to Physical Oceanography.
College Station, Texas: Texas A&M University, 2005.
http://oceanworld.tamu.edu/resources/ocng_textbook/
(accessed September 17, 2007).
9
A Wakeup Call
At approximately 8 a.m. local time on
December 26, 2004, the Sumatra-Andaman
megathrust fault earthquake began. The largest
recorded earthquakes have been along thrust
faults, where subducting and overriding plates
suddenly shift to relieve built-up stresses. Over an
eight-minute period, the rupture traveled from
the epicenter off the coast of Sumatra, northward
along the fault plane for about 1,200 km [745 mi]
as the Indo-Australian plate slipped beneath the
Burma microplate. This long section of locked
plates broke apart and the overriding plate, no
longer constrained, heaved upward.
Not all earthquakes produce tsunamis; it
requires the right set of circumstances. In this
case, the fault plane of the earthquake extended
from 30 km [19 mi] below Sumatra to the
seafloor of the Indian Ocean. From a surface
damage standpoint, an earthquake centered in
Area of
plate sticking
Subductin
g plate
the ocean might seem fortuitous. However, this
location facilitated direct transfer of energy
from the plate movement to the water. With a
1,200-km long fault plane, a subduction zone
thickness of 500 m [1,640 ft], and a vertical
displacement of 5 to 15 m [16 to 50 ft], the uplift
of the overriding plate and downdrop of the
subducting plate sent water oscillations traveling
away from the source of the energy, initiating a
tremendous tsunami (below).
Within 15 minutes of the quake, the tsunami
arrived along the Sumatra shoreline. There was
little warning of its approach, although it is likely
that because of its proximity, the earthquake
would have been felt by those living in the region.
The first indication of an approaching tsunami
was probably a forerunner, a swell ahead of the
larger waves.12 Preceding the forerunner would
be a sudden out-rush of water, exposing large
sections of the nearshore seabed. Based on
Overriding
plate
Tsunami begins
Stuck area
ruptures
Tsunami waves
spread
> A tsunami-generating earthquake. The Indo-Australian plate is sliding
beneath the Burma microplate along a subduction zone, developing
stresses between the plates (top). The overriding plate became stuck and
buckled upward. The rupture relieved the stress created by the locked
plates and upward buckling (dashed line) and caused the overriding plate
to move upward and outward (middle). It heaved an estimated 5 to 15 m,
raising the overlying water, and initiating the tsunami (bottom). The rupture
zone was more than 1,200 km in length.
10
eyewitness accounts, this oddity drew people out
along the exposed seafloor, placing them in the
path of the approaching wave.13 Several minutes
passed, and depending on the distance from the
source and speed of the tsunami, the first wave
inundated the exposed beach and rushed inland
to flood the low-lying coastlands. The danger
does not end with the first wave, since the third
to eighth waves generally are even larger. In Sri
Lanka, arrival of the surges came at approximately
40-minute intervals, indicating a wavelength in
the hundreds of kilometers.14
A Measure of Perspective
For the general public, earthquakes are often
classified using a magnitude based on the wellknown Richter scale. Seismologists use more
meaningful measures such as the moment
magnitude scale. Richter and moment magnitude are logarithmic measures of the amplitude
observed on seismograms and are related to the
energy released in an earthquake.
Dr. Charles F. Richter developed his scale to
quantify earthquake magnitude, and it is
designated ML, with the L referring to local. By
comparing the seismic data for numerous
California earthquakes as measured by shear
waves recorded on a Wood-Anderson seismometer, Richter correlated the amplitude of the
measured signal to the size of the earthquake.
The Richter magnitude is the logarithm of the
peak amplitude of the seismic record, with a
distance correction applied. Since it is a logarithmic scale, each whole number on the
Wood-Anderson seismometer represents an
amplitude 10 times greater than the lesser whole
number. Because the energy is proportional to
the square of the amplitude, and larger earthquakes radiate more low-frequency energy not
recorded by the Wood-Anderson seismometer,
each whole number in the magnitude scale
actually represents about a 30-fold increase in
energy for very large earthquakes.
Moment magnitude, MW, more accurately
describes the physical attributes of an earthquake and is used by modern seismologists,
especially when ranking large earthquakes.
Moment is a function of the total energy released
and is a physical quantity proportional to the slip
distance and the average slip area along the fault
surface. Seismic data are used to estimate the
moment and then converted, using a standard
formula, into a number representative of other
earthquake measurements, such as the Richter
magnitude.15 Depending on the source quoted,
the Sumatra-Andaman earthquake received a 9.0
to 9.3 MW rating.
Oilfield Review
Within minutes after the earthquake, reports
were issued from seismic monitoring stations
around the globe. The first magnitude estimate
was 6.2 MW, using the arrivals of early body waves
measured at the reporting station in Hawaii. Bodywave magnitudes are known to underestimate
very large earthquakes. A preliminary-magnitude
report (8.5 ML) was issued by the United States
Geological Survey (USGS) and Pacific Tsunami
Warning Center (PTWC) one hour and 15 minutes
after the event, which was as soon as sufficient
surface wave data were available. Estimates were
later increased to 9.1 MW, which is the estimate
published by the USGS.16 Post-earthquake analysis
has put the figure as high as 9.3 MW, but there is
no figure for which a consensus has been
reached.17 Much of the difficulty is due to relating
the seismic information to the volume of earth
that moved.
The magnitude of an earthquake is crucial
because the strength of the initiating event is a
critical component of the modeling programs
used to predict tsunami generation. A 6.2 MW
earthquake would not have generated a tsunami
bulletin. The PTWC’s report was upgraded as
soon as information became available, but the
discrepancy underscores the difficulty inherent
in an early-warning system.
Data are available from sources other than
seismic monitoring stations, and an earthquake
of this magnitude has never been scrutinized
with such an array of scientific tools. With a
network of approximately 60 GPS monitoring
stations in the vicinity of the earthquake,
accurate ground movement could be quantified.
The GPS network was part of an ongoing
collaborative project, Southeast Asia: Mastering
Environmental Research Using Geodetic Space
Techniques (SEAMERGES), with additional GPS
data coming from monitoring stations with the
International GPS Service. The GPS data
provided the actual earth displacement
information, which was then used to estimate
energy released in the earthquake—but this
could not be accomplished in real time.
Reconciling the data from the seismic
monitoring and the GPS stations resulted in
assigning a magnitude of 9.3 MW to the SumatraAndaman earthquake.18
12. A forerunner is a series of oscillations of the water level
preceding the arrival of the main tsunami waves.
13. Barber B: Tsunami Relief. US Agency for International
Development, Bureau for Legislative and Public Affairs
(April 2005): 4. http://www.reliefweb.int/library/
documents/2005/usaid-tsunami-30apr.pdf (accessed
October 31, 2007).
14. Cyranoski D: “Get Off the Beach—Now!,” Nature 433,
no. 7024 (2005): 354–354.
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> The Geco Searcher in action. The WesternGeco vessel Geco Searcher acquired the data for the
Sumatra Earthquake Deep Seismic Reflection survey. The data will be made available for future
academic research.
Looking Deeper
Within days following the earthquake, humanitarian relief poured into the region surrounding
the Indian Ocean. Individuals and organizations
around the world offered help in the form of
donations and services. Schlumberger made a
threefold promise of funding, volunteers and
technology. The funding and volunteers came
immediately, addressing human aspects of the
tragedy. On the technology front, one project
quickly emerged: a deep seismic survey along the
fault line to improve the understanding of the
complex tectonics in the region of the earthquake.
Previous surveys, using academic research
vessels, could not image structures at 30 km, the
depth inferred from historical seismic activity.
Understanding the distribution and geometry of
the faults that control seafloor displacement is
critical in determining the mechanisms that
generated the tsunami.19
This is not the first time Schlumberger has
been an active participant in earthquake-related
scientific studies. The San Andreas Fault
Observatory at Depth (SAFOD) Project incorporated many oilfield technologies in the
assessment of the seismically active San Andreas
Fault.20 The ability to deploy, acquire and analyze
data using tools developed for oil and gas
exploration has been invaluable in understanding
the mechanisms that generate seismic events
in regions such as the Sumatra-Andaman
earthquake zone.
WesternGeco committed resources to acquire
and process the data for the Sumatra Earthquake
Deep Seismic Reflection survey, or “the tsunami
survey.” The vessel Geco Searcher was used for
the acquisition of seismic data (above). In
conjunction with Schlumberger Cambridge
Research in England and Institut de Physique du
Globe de Paris in France, WesternGeco donated
its services, including logistical and technical
support. The survey was conducted cooperatively
with the Indonesian Agency for the Assessment
and Application of Technology, which retains
the rights to the data. In the future,
WesternGeco plans to make its data available to
15. Hanks T and Kanamori H: “A Moment Magnitude Scale,”
Journal of Geophysical Research 84, no. B5 (1979):
2348–2350.
16. http://earthquake.usgs.gov/eqcenter/eqinthenews/
2004/usslav/#summary (accessed August 22, 2007).
17. Ishii M, Shearer PM, Houston H and Vidale JE: “Extent,
Duration and Speed of the 2004 Sumatra–Andaman
Earthquake Imaged by the Hi-Net Array,” Nature 435,
no. 7044 (2005): 933–936.
18. Vigny C, Simons WJF, Abu S, Bamphenyu R, Satirapod C,
Choosakul N, Subarya C, Socquet A, Omar K, Abidin HZ
and Ambrosius BAC: “Insight into the 2004 Sumatra–
Andaman Earthquake from GPS Measurements in
Southeast Asia,” Nature 436, no. 7048 (2005): 201–206.
19. Singh S: “Seismic Investigation of the Great SumatraAndaman Earthquake,” First Break 24, no. 12
(December 2006): 37–40.
20. Coates R, Haldorsen JBU, Miller D, Malin P, Shalev E,
Taylor ST, Stolte C and Verliac M: “Oilfield Technologies
for Earthquake Science,” Oilfield Review 18, no. 2
(Summer 2006): 24–33.
11
offsets with a single-vessel operation. The source
and streamer depths were maximized for the
acquisition of low-frequency data, and after
modeling and analysis, the decision was made to
tow sources and streamers at a depth of
15 meters. An additional shorter streamer was
towed at 7.5 m [25 ft] to provide high-resolution
images for defining features nearer the surface.
Compared with surveys used in oil and gas
exploration, this survey design was elaborate and
extensive: tripled streamer depth, tripled
streamer length, tripled energy source and
tripled recording time (next page, top).
Concurrent with the seismic survey, the
French research vessel Marion Dufresne
deployed 56 ocean-bottom seismometers along
the route of two of the seismic lines. The widely
spaced OBS sensors recorded naturally occurring
seismic activity but were also able to acquire
seismic data during the WesternGeco acquisition. Using 5- to 20-km [3- to 12-mi] spacing, the
sensors recorded the shots from the survey and
the reflections from the subducting layer. The
seismic reflection data from the WesternGeco
operations and the refraction data from the OBS
sensors are complementary because the reflection data provide high-resolution images of the
crust, and the OBS refraction data provide
deeper images of the crust and upper mantle.24
The volume of data acquired is massive.
Preliminary processing and analysis were carried
out by WesternGeco staff aboard the Geco
Searcher, and later on shore in Indonesia, but
more analysis will be required to identify the
significant features and fully utilize the data
(next page, bottom).
Depth, m
0
Nicobar
442
Burma
microplate
972
1,354
Sumatra
fault
West Andaman fault
1,680
1,985
W
G2
the global academic community for additional
scientific analysis.
The survey was part of a larger initiative, the
Sumatra-Andaman Great Earthquake Research
(SAGER) project, which included high-resolution
sea-bottom bathymetry and an ocean-bottom
seismometer (OBS) refraction survey deployed
by the French research vessel Marion
Dufresne.21 The Institut Polaire Français made
the Marion Dufresne available for the survey and
provided technical support. OBS sensors were
placed on the seabed to record seismic activity
(below left).
In July 2006, the Geco Searcher acquired
three seismic lines, totaling 926 surface km
[575 mi] of deep seismic profiling (below right).
The seismic survey had several objectives:
• image active faults along the subduction zone
• quantify the volume of water that penetrated
along these faults
• provide information to optimize the location
of a future borehole for the Integrated Ocean
Drilling Program.22
Providing an image of faults at a depth of
30 km required long offsets.23 In the oil and gas
industry such depths would not be considered
because they are beyond the reach of any drilling
operation. The Geco Searcher used the Q-Marine
single-sensor marine seismic system to provide
the technology needed to acquire 12-km [7.5-mi]
IndoAustralian
plate
2,290
2,616
Deformation
front
2,998
3,528
Aceh basin
5,216
Sumatra
Simeulue
plateau
December 26, 2004
epicenter
G3
W
G1
W
0
0
> Deploying an OBS. The research vessel Marion Dufresne
deployed 56 ocean-bottom seismometers along the path of
the WesternGeco seismic survey. Intended for monitoring
seismic activity at the seafloor, the OBSs were used to
record reflections from the sources used by WesternGeco.
(Photograph courtesy of First Break, reference 19.)
12
km
Sim
eu
lue
100
miles
100
52 mm/yr
> The survey area. In the vicinity of the Sumatra-Andaman earthquake, three seismic lines
(WG1, WG2 and WG3), totaling 926 surface kilometers, were acquired. Preliminary
processing has provided high-resolution imaging to depths greater than 30 km. The map
also contains bathymetry data for the area that was under study.
Oilfield Review
1.5
Time, s
2.0
2.5
3.0
1.5
Time, s
2.0
2.5
3.0
> Seismic images from two streamer depths. The image from the 7.5-m streamer (top) shows finer details nearer the surface. The image from the
15-m streamer (bottom) uses deeper penetrating seismic energy. Features deeper than 30 km can be studied using these data.
0
Autumn 2007
WG1
Active main thrust fault
4
Active frontal thrust
NE
Backthrust
8
12
Simeulue fore-arc basin
West Andaman fault
Thrust reflectors
2
10
21. Bathymetry is the surveying or mapping of harbors,
inlets or deepwater locations. Echo sounder techniques
are used in the measurement and study of water depths
to create bathymetric maps or charts of seafloor relief
for navigation purposes.
22. For more on the Integrated Ocean Drilling Program:
Brewer T, Endo T, Kamata M, Fox PJ, Goldberg D,
Myers G, Kawamura Y, Kuramoto S, Kittredge S,
Mrozewski S and Rack F: “Scientific Deep-Ocean
Drilling: Revealing the Earth’s Secrets,” Oilfield Review
16, no. 4 (Winter 2004/2005): 24–37.
23. Offsets are the distance between the airgun array and
the sensors.
24. Singh, reference 19.
SW
Simeulue plateau
Accretionary wedge
6
Time, s
The seismic data, along with SAGER
bathymetry and refraction data, are being used
to understand the features that control plate
movement. Preliminary analysis of the data
confirmed that a fault plane, from the
earthquake epicenter at 33 km, extends to the
seabed. The seismic images validated the
Oceanic M
oho
14
0
16
0
km
Continental
Moho
25
miles
25
> Preliminary results. From the WG1 seismic line, preliminary interpretation reveals faulting and
deep boundaries. The main thrust fault can be seen on this image, as well as other reflectors.
The Moho, short for the Mohorovi čić discontinuity, is the boundary between the Earth’s crust and
the mantle, and can be identified here.
13
Accretionary
wedge
Indo-Australian plate
0
Frontal thrust fault
5
Main thrust fault
Sediments
Indicates motion
into page
Upper
seismogenic zone
10
Depth, km
15
20
Simeulue plateau
Crustal-scale
thrust fault
Oceanic
Simeulue fore-arc basin
West Andaman fault
Burma
microplate
Indicates motion
out of page
st
hru
ckt
Ba
December 26, 2004
Eurasian
plate
25
30
Indo-Australian
Plate
0
45
0
km
50
miles
December 26, 2004
50
Mantle wedge
50
> Detailed interpretation of the seismic data. The epicenter of the December 26, 2004 earthquake was
beneath the Simeulue plateau, located west of Sumatra. The earthquake occurred when the continental
plate broke free of the oceanic plate along the subduction zone (red line). The zone extends more than
150 km [93 mi] from the epicenter to the ocean floor. (Adapted from Singh, reference 19.)
premise that a large upheaval of the seabed
contributed to the strength of the tsunami
(above). Early analysis has also identified a very
wide locked zone, greater than 135 km [85 mi],
whose rupture contributed to the magnitude of
the earthquake.25
On September 12, 2007, an 8.4 MW earthquake occurred on the December 2004 fault line,
but produced relatively little tsunami energy
(above right). Scientists can use the seismic
images and information acquired during both
earthquakes to better understand the mechanisms that initiated the earthquakes and
produced (or failed to produce) a large tsunami.
Ultimately, the information can be integrated into
modeling programs to improve tsunami forecasts.
Subduction zones typified by the area that
created the Sumatra-Andaman earthquake exist
in other places around the world. Technology
such as the Q-Marine system can be applied
elsewhere to better understand seismically active
regions. Collaboration between the academic
world and companies like Schlumberger will
equip scientists and researchers with advanced
tools to prepare at-risk locations.
Moving Towards Early Warning
The following is a timeline of the early events
that occurred December 25, 2004, at the National
Oceanic and Atmospheric Administration
(NOAA) Pacific Tsunami Warning Center
(PTWC) in Honolulu, Hawaii:
25. Singh, reference 19.
26. http://www.noaanews.noaa.gov/stories2004/s2358.htm
(accessed August 18, 2007).
27. http://ioc3.unesco.org/itic/ (accessed September 27, 2007).
• 2:59 p.m. local time, the Sumatra-Andaman
earthquake begins
• 3:07 p.m., first seismic arrivals detected at the
PTWC
• 3:10 p.m., PTWC issues an alert that a 8.0 MW
earth quake has occurred near Sumatra,
Indonesia
• 3:14 p.m., PTWC issues bulletin 1—no tsunami
threat to Pacific Ocean basin. There was no
established protocol to contact other regions.
• 3:15 p.m., first tsunami wave strikes Sumatra.
As per standard operating procedure, a text
message was distributed to participants of the
Tsunami Warning System (TWS) in the Pacific,
and e-mail notification was sent to 25,000
interested parties. Alerts were issued by
telephone to various agencies, including the
Hawaii Civil Defense and the International
Tsunami Information Center.26
With 80% of major earthquakes occurring
around the Pacific Ocean, it is critical to have an
effective tsunami early-warning system that
operates as described above. The PTWC is just
one part of a cooperative network coordinated by
the Intergovernmental Oceanographic Commission
(IOC), functioning under the United Nations
Educational, Scientific, and Cultural Organization (UNESCO).27 The Pacific TWS comprises
hundreds of seismic monitoring stations
worldwide, sophisticated tsunameters monitoring wave heights in the open ocean and
strategically placed tidal gauges (next page, top).
0
0
km
tra
ma
Su
40
September 12, 2007
Continental Moho
Oceanic mantle
35
14
Andaman Islands
Subducti
ng ocea
nic cru
Moho
st
1,000
miles
1,000
> Two major earthquakes, with very different
results. The epicenter of a September 12, 2007
earthquake, 8.4 MW, was in the vicinity of the
December 2004 Sumatra-Andaman earthquake,
9.3 MW. Although the 2007 earthquake was
powerful enough to generate a tsunami, the
rupture did not extend from the epicenter as it
did in the 2004 earthquake (red). The small
tsunami produced during the 2007 earthquake
had little effect on the region.
Various organizations representing 26 countries
from that region collaborate to alert the public
whenever the danger of a tsunami is present.
By their very nature, warning networks such
as the Pacific TWS are expensive, having to
contend with vast stretches of open water,
expensive monitoring equipment on land and in
the oceans, and the need for continuous staffing
of monitoring stations with qualified personnel.
The events of December 2004 demonstrate just
how costly the lack of an early-warning system
can be. The Pacific Tsunami Warning Center is
well established and is the model for the Indian
Ocean Tsunami Warning Center (IOTWC). The
PTWC relies on four primary tools: seismic
monitoring, ocean monitoring, fast modeling
software and communication.
Listening to the Earth
Three key earthquake parameters can be
determined from seismic waveform data to
predict an earthquake’s tsunamigenic potential:
• location—whether the earthquake is located
under or near the sea
• depth—whether the earthquake is located
near enough to the Earth’s surface to create
significant displacement
• magnitude—whether the size of the earthquake is sufficient to produce a tsunami.
28. ICG/IOTWS-II, Communications Plan for the Interim
Tsunami Advisory Information Service for the Indian
Ocean Region, ver. 15, January 2006. http://ioc3.
unesco.org/indotsunami/documents/IOTWS_
CommunicationPlan_15Jan06.pdf (accessed
October 25, 2007).
Oilfield Review
KBS
ALE
COR
MIDW
H2O
CMB
PAS
PFO
RSSD
ANMO
TUC
HKT
SLBS
KIP
HRV
WCI SSPA
WVT
BBSR
DWPF
CCM
TEIG
POHA
CMLA
OBN
KIEV
GRFO
ABKT
JTS
GUMO
DBIC
SDV
OTAV
FURI
LCO
PLCA
MSEY
TARA
BTDF
FUNA
HNR
KAPI
ASCN
LSZ
SHEL
TSUM
LVC
RPN
KMBO
DGAR
BDFB
LPAZ
RAP
PTON
RCBR
SAML
KWAJ
DAV
PALK
MSKU
PTGA
NNA
WAKE
QIZ
CHTO
KOWA
BGCA
MBAR
RAO
UAE
SUG
XMAS
AFI
MA2
PET
TLY HIA
MDJ
ULN
YSS
WMQ
ERM
INCN
BJT
MAJO
XAN
NIL
SSE
LSA
ENH
TATO
KMI
BRVK
KURK
MAKZ
AAK
RAYN
SACV
PAYG
ARU
KIV
GNI
ANTO
PAB
MACI
JOHN
KANT
BILL
YAK
DPC
BFO
TIXI
NRIL
KGNO
BORG
ESK
FFC
KDAK
ADK
LVZ
KEV
SEJD
COLA
PMG
COCO
ABPO
CTAO
MBWA
CPUP
BOSA
SUR
MSVF
WRAB
LBTB
NWAO
TRIS
TRQA
TAU
SNZO
EFI
Installed Planned
IRIS/USGS stations
IRIS/IDA stations
USGS/CU stations
Affiliated GSN stations
PMSA
HOPE
CASY
VNDA
SBA
QSPA
> Global Seismographic Network (GSN). With a large number of seismic monitoring stations, the GSN comprises a multinational, multidisciplinary network
of cooperating research seismometer stations, including those affiliated with the Incorporated Research Institutions for Seismology (IRIS). The network, as
of April 2007, includes the following stations: 86 operated by the United States Geological Survey (USGS), 39 operated by International Deployment of
Accelerometers (IDA), a global network of broadband and very long period seismometers, and other affiliated stations. The University of California San
Diego (UCSD), CU in the legend, is a major participant in the network, with funding from the National Science Foundation. For more on GSN, IRIS, UCSD
and IDA: http://www.iris.edu/. (Modified from Global Seismic Network, http://www.iris.edu/about/GSN/map_family.html.)
Seismic monitoring is primarily accomplished
using monitoring stations supported by various
governmental agencies and educational
institutions. The Global Seismographic Network
(GSN) is a primary source of data. It comprises
225 monitoring stations in more than 80 countries. In addition, the PTWC and the IOTWC
receive data from other seismic monitoring
networks such as the International Monitoring
System (part of the Comprehensive Nuclear Test
Ban Treaty Organization) and those coordinated
by the Incorporated Research Institutions for
Seismology (IRIS).
The warning centers receive seismic data
over the Internet. However, because reliable
transmission of data from the Internet is not
guaranteed, especially in the case of
infrastructure damage during and after a major
earthquake, additional sources for data are
available. The Matsushiro Seismic Array System
of Matsushiro Seismological Observatory
(Nagano, Japan) and the Large Aperture Array
comprising Japanese seismological observation
Autumn 2007
Earthquake
Depth
< 100 km
≥ 100 km
Earthquake
Location
Earthquake
Magnitude, Mw
Description of Tsunami Potential
6.5 to 7.0
Very small potential for a
destructive tsunami
Tsunami
information
7.1 to 7.5
Potential for a destructive
local tsunami
Local tsunami
watch
7.6 to 7.8
Potential for a destructive
regional tsunami
Regional
tsunami watch
≥ 7.9
Potential for a destructive
ocean-wide tsunami
Ocean-wide
tsunami watch
Inland
≥ 6.5
No tsunami potential
Tsunami
information
All locations
≥ 6.5
No tsunami potential
Tsunami
information
Under or very
near the sea
Bulletin
Type
> Tsunami bulletin criteria. The Tsunami Warning Centers use magnitude, location (under sea or
under land) and depth of the earthquake to determine the potential for a tsunami and issue bulletins
based on those criteria. (Source of data is reference 28.)
networks—are examples of the warning centers’
contingent data sources.
When a seismic event occurs, data are
processed at the warning centers to evaluate the
potential for a tsunami. The warning centers use
an established criterion, based on the magnitude
of the earthquake, to decide which type of bulletin
to issue (above). A reliable location can be
determined using the least-squares method, with
P-wave arrival times and various reflected phases
used to provide epicenter depth estimations.28
15
The seismic data are the first piece of the
puzzle. If an earthquake is sufficiently large,
takes place in a shallow portion of the Earth’s
crust, and occurs in a location under or close to
the sea, it has the potential for generating a
tsunami. Whether or not a tsunami has actually
been created can be determined only at the
ocean’s surface.
The Ocean’s Pulse
Identifying the formation of a tsunami and
accurately forecasting its arrival times and wave
amplitudes depends on precise ocean-level
monitoring. This is accomplished using two
primary sources—NOAA’s DART Deep-Ocean
Assessment and Reporting of Tsunamis buoys in
deep water and tidal gauges near coastlines.
Although DART buoys have been deployed
globally, the Pacific Ocean has the majority, with
28 DART buoys in place, and four more to be
deployed by the end of 2008 (right). The DART
buoy consists of an anchored seafloor bottompressure recorder (BPR) and a tethered surface
buoy that provides real-time communications
(next page). An acoustic link transmits
DART Locations
34
4
3
NOAA
Planned
Other
> Buoy network for monitoring ocean activity. The Pacific Ocean is encircled by DART Deep-Ocean
Assessment and Reporting of Tsunamis monitoring buoys, with more planned. The network supplies
information to the Pacific Tsunami Warning System. NOAA operates the majority of the buoys,
although a few are maintained by other agencies. As of October 2007, two DART buoys are active in
the Indian Ocean. (Adapted from NOAA, http://www.ndbc.noaa.gov/dart.shtml.)
GLOSS Tidal-Gauge Locations
> Global Sea Level Observing System (GLOSS). With more than 290 sea-level monitoring stations, GLOSS is at work around the world monitoring long-term
climate change and oceanographic sea-level variations. In the event of a tsunami, these data are incorporated into modeling software to refine forecasts
and inundation estimations.
16
Oilfield Review
Iridium satellite
Bidirectional
communication
and control
Tsunami
warning
center
Iridium and
GPS antennas
Electronic systems
and batteries
Surface buoy,
2.5-m diameter,
4,000-kg displacement
Acoustic transducers
(2 each)
Tsunameter
Signal
flag
etr
y
Glass ball
flotation
Autumn 2007
~ 75 m
rec
tio
na
la
co
us
tic
1,000 to 6,000 m
Bid
A Model Forecast
When a seismic or other event of sufficient
magnitude triggers the need for tsunami modeling,
various software programs may be used to estimate
a tsunami’s potential severity. The seismic
information is the initial source, but real-time sealevel data are incorporated into the model as they
become available. These modeling programs
provide estimated wave-arrival time, and waveheight and inundation patterns. It is critical that a
simulation model be able to provide accurate
forecasts as rapidly as possible. The elapsed time
of 15 minutes between the earthquake and the
first wave arrival in Sumatra underscores the need
for speed in model predictions.
The United States National Oceanic and
Atmospheric Administration (NOAA) has developed a cutting-edge modeling program, known as
Method of Splitting Tsunami (MOST).31 The MOST
program uses a suite of numerical simulation
codes to compute predetermined wave behavior
for three stages of a tsunami—generation,
propagation and run-up. The program can provide
coarse grids in deep water, where the wavelength
is long and fewer node points are needed. In
shallow water, the tsunami wavelength shortens
and the amplitude rises. To better model the
wave, the program narrows its focus to highresolution grids.
The early-warning system issues alerts and
notifications to potential at-risk areas based on
the MOST outputs. The MOST program is first run
in a research mode to create scenarios using
tel
em
temperature and pressure data from the BPR to
the surface, which are converted to an estimated
sea-surface height. The accuracy of the
measurement is ± 1 mm in 6,000 m [20,000 ft] of
water depth. These data are transmitted to an
Iridium commercial satellite that relays the
information to monitoring stations. Turnaround
time for data is less than three minutes, from
buoy to warning center.29
Tidal gauges record coastal sea-level
variations using an international network of
monitors. The Global Sea Level Observing System
(GLOSS) is a network of more than 290 sea-level
monitoring stations coordinated under the
auspices of the Joint Technical Commission for
Oceanography and Marine Meteorology
(JCOMM) of the World Meteorological
Organization (WMO) and the Intergovernmental
Oceanographic Commission (IOC). GLOSS
provides high-quality global and regional sealevel data for application to climate,
oceanographic and coastal sea-level research
(previous page, bottom).30
Acoustic
transducer
Anchor, 325 kg
Anchors, 3,100 kg
> NOAA’s DART II system. Anchored to the ocean floor, the tsunameter monitors temperature and
pressure. These data are passed to a separate surface buoy by means of acoustic pulses. The buoy
communicates with the tsunami warning centers using a commercial Iridium satellite link. Firstgeneration DART systems featured an automatic detection and reporting algorithm triggered by a
threshold wave-height value. Today's design permits two-way communications, enabling data
transmission on demand, independent of the automatic triggering. This ensures the measurement and
reporting of tsunamis with amplitudes below predetermined threshold limits. When a seismic event
occurs, the tsunami warning centers use predictive software to model tsunami magnitude and
severity, but until empirical data, such as wave height from DART buoys, become available, the
centers can only forecast the likelihood of a tsunami. DART system information is used to confirm
and refine tsunami characteristics. With these data, more accurate reporting is possible, improving
watches, warnings or evacuation bulletins. (Adapted from NOAA, http://nctr.pmel.noaa.gov/Dart/.)
predetermined inputs—such as earthquake
magnitude, directionality and location. These
simulations can take hours to run, which would
be inappropriate for an early-warning system. To
speed the process, when an earthquake is
detected, the software attempts to match the
real-time data to a preexisting scenario to
predict the likelihood and potential of a tsunami.
29. http://nctr.pmel.noaa.gov/Dart/dart_home.html (accessed
October 1, 2007).
30. http://www.gloss-sealevel.org/ (accessed October 18,
2007).
31. Titov VV and Synolakis CE: “Numerical Modeling of Tidal
Wave Run-Up,” Journal of Waterway, Port, Coastal
and Ocean Engineering 124, no. 4 (July/August 1998):
157–171. For more on tsunami modeling: http://nctr.pmel.
noaa.gov/model.html (accessed August 10, 2007).
17
> Model of the Sumatra-Andaman earthquake tsunami. Using NOAA’s
Method of Splitting Tsunami (MOST) program, the tsunami (arrow) was
modeled as it traveled across the Indian Ocean. Shown here at approximately
1 hour after initiation, the wave will take three more hours to reach the
African coastline. (Adapted from NOAA/PMEL/Center for Tsunami
Research, http://nctr.pmel.noaa.gov/model.html.)
As additional information, such as DART and
tidal-gauge data, becomes available, the model is
adjusted (above).
Another tool used in analyzing the 2004
tsunami, the Jason-1 earth-imaging satellite,
provided modelers with accurate wave-height
data for the duration of the tsunami. Measured
from space, the resolution was in the centimeter
range. Rather than having only point-to-point
measurements, such as from tidal gauges or
DARTs, the waves could be measured continuously with satellite data. Unfortunately, the lag
time is much too great and the coverage area too
sparse to use satellite data in real time. However,
satellite information can provide validation and
improvement for the current modeling programs.
Even with all the data at their disposal,
experts were challenged to explain how the
Sumatra-Andaman earthquake produced a
tsunami whose magnitude exceeded initial waveheight predictions. NOAA’s tsunami forecast,
running the model with seismic data alone,
originally underestimated tsunami heights in the
open ocean by a factor of 10. Integration of
tsunami amplitudes from tidal gauges improved
the results iteratively, but the results were not
considered satisfactory. Analyses of the shock’s
strong seismic waves indicated that the initial
fault break traveled northward from Sumatra at
2.5 km/s [1.6 mi/s]. The analysis also pinpointed
the areas of greatest slip—and thus of the
greatest wave generation. The problem for
tsunami modelers was that none of these seismic
solutions included enough overall fault motion to
reproduce either the satellite observations of
wave heights in the open ocean or the severe
flooding in Banda Aceh.
The critical piece of the puzzle came from
elevation and displacement data provided by
land-based global positioning system (GPS)
monitors, used to track ground movements. The
GPS sensors, recording at a much slower rate
than seismic monitors, revealed that the fault
continued to move long after it stopped
emanating seismic energy. Although there is a
limit to how slowly a fault can slip and still
generate a tsunami, this often overlooked
phenomenon, called after-slip, accounted for the
observed tsunami wave heights. Incorporating
GPS readings into modeling programs will be an
32. Geist EL, Titov VV and Synolakis CE: “Tsunami: WAVE
of CHANGE,” Scientific American 294, no. 1
(January 2006): 56–63.
33. http://nctr.pmel.noaa.gov/sumatra20070912.html
(accessed September 21, 2007).
34. Imamura F, Shuto N, Ide S, Yoshida Y and Abe K:
“Estimate of the Tsunami Source of the 1992 Nicaraguan
Earthquake from Tsunami Data,” Geophysical Research
Letters 20, no. 14 (1993): 1515-1518.
35. “Tsunami 2004: Waves of Death,” The History Channel
Web site, http://www.history.com/shows.do?action=
detail&episodeId=173117 (accessed September 27, 2007).
36. http://www.sciencedaily.com/releases/2006/07/
060710085816.htm (accessed October 1, 2007).
18
important component in improving the accuracy
of tsunami warning systems in the future.32
Another challenge is integrating the data in a
timely manner.
A major drawback in developing and using
modeling software is that there is so little
empirical data to compare with the model. On
September 12, 2007, an 8.4 MW earthquake
occurred in the vicinity of the December 2004
earthquake. This was the first major event since
the deployment of a DART buoy in the Indian
Ocean. The MOST program predicted a 2-cm
[0.75-in.] rise in wave height at the location of
the buoy with an arrival time of approximately
2 hours and 50 minutes. The observed wave heights
and arrival times matched MOST predictions
(next page).33
Inundation models, estimates of how far
inland a tsunami will travel, are another critical
component. Scientists use measurements
recorded near the coast from tidal gauges or postevent estimates from water damage to determine
run-up. Early programs calculated wave heights
at the shore’s edge but had difficulty projecting
the effects onto the shore. A 1992 Nicaraguan
tsunami gave scientists an opportunity to make
comprehensive measurements and compare them
with model predictions.34
Using large-scale laboratory experiments and
field measurements, investigators refined their
models until they could match the empirical
tsunami inundation measurements. Using highresolution land imagery, accurate bathymetry
data, coastal and offshore topographical data,
historical information from previous tsunamis
and software to make rapid calculations, they
demonstrated that an early-warning system
could provide reliable estimations.
Sounding the Alarm
Three months prior to the December 2004
tsunami, a working group for the Southwest
Pacific and Indian Ocean Tsunami Warning
System was established. Under the auspices of
the International Tsunami Information Center
(ITSU), a UNESCO organization, this group’s
charter was to expand the Pacific warning
system to include other regions with the
potential for tsunamis, including the Indian
Ocean. When the earthquake occurred, the
Pacific Tsunami Warning Center (PTWC)
attempted to contact affected countries across
the Indian Ocean; unfortunately, it was Sunday
as well as a holiday for many. Most offices were
closed, and the warnings did not reach the
inhabitants of the affected coastlines. One result
Oilfield Review
Amplitude, cm
Thailand DART buoy
Epicenter of 2007 earthquake
4
DART data
MOST model
2
0
–2
–4
0
1
2
3
4
5
6
7
8
9
10
11
12
Time after earthquake, h
> MOST predictions compared with tsunami data. Shown on the map of the
Indian Ocean (top), the Thailand DART buoy (yellow circle) was installed in
August 2007. On September 12, 2007, an 8.4 MW earthquake (red star)
occurred with an epicenter just south of the Sumatra-Andaman earthquake
of 2004. A minimal tsunami was generated by the event. In a comparison of
wave heights (bottom), the MOST wave-height simulation (red curve) after
eight hours compares favorably with the data recorded by the Thailand
DART buoy (blue curve) both in wave amplitude and arrival time. (Adapted
from data courtesy of NOAA/PMEL/Center for Tsunami Research.)
of the Sumatra-Andaman earthquake was to
accelerate the pace of developing global earlywarning networks.
Post-tsunami analysis confirmed that
communication within the region and links to
other monitoring sites were lacking. A striking
example of the importance of having an
emergency management system in place is
evidenced by comparing the tsunami mortality
rate in Kenya and Somalia. Kenya did not have a
tsunami warning system, but it did have a
chemical and oil spill-alerting system. When
word of the approaching tsunami reached
Kenyan officials (tsunami travel from Sumatra to
Kenya took four hours), they activated the spillalerting system. Approximately 800,000 people
were warned to move inland or seek higher
ground. Four hours after the earthquake, the
tsunami reached the shores of Kenya and
Somalia. The death toll for Kenya was one. In
neighboring Somalia, where there was no
warning system, the death toll was 150.35
Autumn 2007
With modern Internet and satellite connectivity, communication over a wide area is almost
instantaneous, but communications can be
challenging in developing countries. Problems
also arise when the alert must be communicated
to the general population. Planning for events
like this must assume that infrastructures are
likely to be severely damaged. Satellite links
make it possible to communicate in the absence
of land lines, but contingencies must also be in
place to alert the general population if local
systems are destroyed.
Effective warning systems for natural hazards
require public information and preparedness
components. Early warning is largely a social
issue, and technology alone will not solve the
problem. Early-warning systems may fail at times
of crisis if warnings are not received by the
people at risk, or are not understood, or are not
acted upon. An effective early-warning system
needs to be people-centered in addition to
having sound technical methods of communication. Trained and experienced emergency
management personnel are critical to ensure
that warnings are clearly communicated, well
understood and rapidly implemented. In
addition, regional coordination is important, as
earthquakes and tsunamis do not restrict
themselves to territorial borders.
Even with the best data, the accuracy of the
models used to predict tsunamis is limited by
errors in bathymetry and uncertainties in the
triggering mechanism. Each earthquake is
unique, and every tsunami has a unique
combination of wavelengths, wave heights and
directionality. From a warning perspective, this
makes the problem of forecasting tsunamis in
real time difficult. In the case of the December
2004 tsunami, the areas north and south of the
earthquake epicenter had little damage
compared with areas to the east and west. Cocos
Island is 1,500 km to the south, and Sri Lanka is
1,500 km to the west of the epicenter. The
maximum wave height on Cocos was 42 cm
[16 in.], while sections along the Sri Lankan
coast experienced run-up in excess of 8 meters
[26 ft]. Warning center personnel understand the
need for a delicate balance between creating
undue panic and underestimating the severity,
potentially causing an even greater tragedy.
The Way Forward
In 2006, UNESCO Director-General Koïchiro
Matsuura announced that, after much cooperative effort, 26 national tsunami information
centers had been established around the Indian
Ocean.36 As part of the Indian Ocean Tsunami
Warning System (IOTWS), this is the first stage
in the development of an integrated organization modeled after the Pacific Tsunami
Warning System.
As of October 2007, seismographic reporting
stations have been upgraded and two DART buoys
have been deployed in the Indian Ocean. Twentyfive additional monitoring stations will be added
and linked in real time to analysis centers.
Information bulletins are being issued from
Japan and Hawaii, pending a decision on the final
locations of Indian Ocean regional centers. In the
future, additional DART buoys and satellite links
will be deployed. The work certainly is not over; it
has taken 40 years to develop the Pacific Tsunami
Warning System, and developing a comparable
system for the Indian Ocean will also take some
time. However, a basic system is now in place for
when—not if—the next great Indian Ocean
tsunami occurs.
—TS
19