the science of tsunamis

4 Oilfield Review

The Science of Tsunamis

Tim BuntingKuala Lumpur, Malaysia

Chris ChapmanPhil ChristieCambridge, England

Satish C. SinghUniversity of CambridgeCambridge, England

Jim SledzikGatwick, 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, CollegeStation, USA.Q-Marine is a mark of Schlumberger. DART is a registeredtrademark of the US National Oceanic and AtmosphericAdministration (NOAA).

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.

On December 26, 2004, the Sumatra-Andamanearthquake, with an estimated magnitude of 9.3on the Richter scale, was one of the largest everrecorded using modern seismographic equip - ment. As it shook the west coast of Sumatra,Indonesia, and proceeded along a fault line atthe eastern edge of the Indian Ocean, theearthquake generated a tsunami that focused theworld’s attention on the devastating power of thisnatural phenomenon. With estimates of morethan 232,000 deaths and 2,000,000 peopledisplaced 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 naturaldisasters in human history, the tsunami wasunique in other aspects. It was the first globaltsunami to occur since modern sea-levelmonitoring networks were established and thefirst to be continuously tracked and recorded bya satellite. No other seismic event of thismagnitude has occurred with so many data-gathering sources available. From a scientificperspective, the event provided a wealth ofinformation for analysis. These data will be used to better understand and prepare for future incidents.

The earthquake and tsunami exacted anobservable physical toll—on houses, bridges andbusinesses—that can be seen by comparingbefore and after photographs (next page, bottom).

These images reveal the damage that emanatedfrom events that began below the surface.However, a full understanding of the earth-quake and the sub se quent tsunami requires amultifaceted approach.

To develop an appreciation for the magnitudeof this event—the energy released temporarilyaltered the Earth’s rotation—we present a basicreview of the theory of plate tectonics as itrelates to the earthquake.2 A discussion of thephysics of ocean waves and tsunamis follows. Wealso examine some of the tools used—such asseismic and ocean monitoring networks, land-based global positioning systems (GPS) andtsunami modeling software—to better com -prehend the scope of this event. Details of theWesternGeco tsunami seismic survey will beincluded, along with some preliminary findings.This article also reviews the status of ongoingefforts to develop an integrated monitoring andearly-warning system in the Indian Ocean region.

Tectonic Foundations for a TsunamiOn a geological time-scale, the surface of theEarth is constantly changing—oceans form anddisappear, continents collide with one another,and mountains rise and fall or erode away. Toexplain the processes that shaped and continueto shape the surface of the Earth, the theory ofplate tectonics was proposed.3 It states that theEarth’s lithosphere, the outermost layer, isbroken into rigid plates that are moving relative

1. “Indian Ocean Earthquake & Tsunami Emergency Update December 29, 2005,” Center of Excellence inDisaster 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: “Energeticsof the Tsunami of 26 December 2004 in the Indian Ocean:A Brief Review,” Marine Geodesy 29, no. 1 (January2006): 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 NewGlobal Tectonics,” Journal of Geophysical Research 73(September 15, 1968): 5855–5899.

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> Courtesy of US Geological Survey (USGS).

Sri Lanka

Andaman Islands

Sumatra

> High-resolution imaging satellite photographs of Banda Aceh, Indonesia, before and after the tsunami. Banda Aceh is located at the northern tip ofSumatra. With a population of 260,000, it was the closest major city to the epicenter of the Sumatra-Andaman earthquake. (Photographs courtesy ofDigitalGlobe.)

Before After


to one another, “floating” on the asthenosphere,a hotter, denser, more mobile layer. Below theasthenosphere are the upper mantle, the mantle,the outer core and, at the center of the Earth, theinner core. The major plates have been identifiedand, by plotting seismic activity, their boundarieshave been defined (above).4

Tectonic plates are constantly diverging,converging or transforming. In divergent zones,the plates move away from each other, allowingbasaltic magma to ooze to the seafloor and createthe dense oceanic crust at midocean rift zones.The magma cools as it meets seawater and forms a series of underwater mountain ridgesthat are carried away from the rift by thediverging plates.

Landmasses above sea level form thecontinental crust, which is usually thicker andmuch less dense than oceanic crust. The denseoceanic plate slides beneath the overriding platein what is termed a subduction zone. Eventually,the subducting plate melts and returns to theasthenosphere. As the subducting materialdewaters, the fluid migrates upward, mixing withthe material of the overriding plate, reducing itsmelting point. This produces magmatic melts,rich in dissolved gases, that exert enormousupward pressure on the overriding plate; thesecan erupt if a weakness in the crust develops(next page, top).5

Along boundaries where crust is neithercreated nor destroyed, changes still occur,transforming the surface of the Earth. Over time,as landmasses collide, an ocean that separatedthe masses may disappear, while the previousocean bottom is lifted above sea level. Plates maydeform along their borders into mountain ranges.Landmasses that make up the continental crustmay slide horizontally, creating earthquakes asplates stick and slip.

The Indo-Australian plate, which played a keyrole in the Sumatra-Andaman earthquake,comprises both continental and oceanic crust. Thelandmasses of India and Australia make up themajority of the continental portion, while theoceanic segment lies beneath the Indian Ocean.According to theory (and data), 100 million yearsago, India was an island off the east coast of Africa,south of the equator, and it has been making arelentless journey northward, creating theHimalaya 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 rotatingcounterclockwise.6 Mount Everest, the tallest of theHimalayan chain, grows 4 mm [0.1576 in.] per yearbecause of this movement (left).7 The oceaniccrustal portion of the plate is subducting under theBurma microplate and the Eurasian plate.

To the west of Sumatra, the Sunda (or Java)trench marks the edge of the subduction zone.

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> India in motion. India was an island off theeast coast of Africa 100 million years ago. It is part of the Indo-Australian plate and hasbeen advancing into the Eurasian plate as itjourneys northward. During this movement, theHimalaya Mountains were formed along India’snorthern border.

O c e a nI n d i a nLocation of India

70 millionyears ago

Bangladesh

Indiatoday

E U R A S I A N P L A T E

Sri Lanka

Equator

Himalayas

> 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% ofrecorded 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 andAtmospheric Administration (NOAA).]

Crustal plate boundaries Earthquake epicenters, MW >5, 1980 to 1990

IndianOcean

PacificRing of Fire

Sumatra-Andamanearthquake, 2004


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The trench extends some 3,000 km [1,865 mi],from the Andaman Islands in the northwest tothe Lesser Sunda Islands in the southeast, andhas a depth in excess of 7,700 m [4.8 mi].8 TheBurma microplate is wedged between the Indo-Australian and the Eurasian plates (right). Asthe Indo-Australian plate subducts beneath

4. Oreskes N (ed): Plate Tectonics: An Insider’s History ofthe Modern Theory of the Earth. Boulder, Colorado, USA:Westview Press, 2001.

5. Volcanoes result from these upward flows, creatingconduits through the overriding plate for molten magmato 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 deepestpoint in the Indian Ocean until the 8,000-m [26,250-ft]Diamantina Deep was discovered in 1961.

> The ever-changing face of our planet. According to the theory of plate tectonics, the lithosphere is composed of variously sized rigid plates, which arediverging, converging or transforming along boundaries. At rift zones, plates move away from each other, leaving spaces that are filled with dense basalticmagma rising from the asthenosphere. At convergent plate boundaries, subduction takes place as dense oceanic crust dives beneath the more buoyantcontinental 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.

Shield volcanoOceanic spreading ridge

Divergent boundary(rift zone)

Continentalrift zone

Convergent plateboundary

Trench

LithosphereOceanic crust

AsthenosphereSubducting plate

Continental crust

Burma

microplate

Convergent

plate boundary

Indo-Australianplate

Tren

ch

Depth indication, m

1,0004,000

> Tectonics of the Sumatra-Andaman earthquake. The eastern edge of the Indo-Australian plate issubducting beneath the Eurasian plate and Burma microplate at a rate of 52 mm/yr [2.05 in. /yr]. TheIndo-Australian plate is moving northward while slowly rotating counterclockwise. The December2004 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.

Eurasianplate

Burmamicroplate

Indo-Australianplate

December 26, 2004

Andaman Islands

Sumatra

km

miles 1,000

0

0

1,000


these plates, stresses build when the platesbecome stuck. Because the plates continue tomove, the time between major earthquakes andthe extent of the area where their relativemotions are constrained determine the potentialearthquake severity.

Although the Indian Ocean has its share ofearthquakes in seismically active zones, theboundaries of the Pacific Ocean are actually themost active in the world, with 90% of allearthquakes—80% of the major ones—occurringwithin the Pacific basin. The primary mechanismfor this seismic activity is the movement of thesubducting plate described above.9

Because the Pacific basin is so seismicallyactive, an extensive network of sensors has beenestablished for earthquake and tsunamidetection. Although there were plans to developa system modeled after the one used in thePacific, at the time of the tsunami, there was nosuch network for the Indian Ocean. Largetsunamigenic events were infrequent, with onlyone major tsunami occurring there during theprevious century and only four reported in the1800s. The tsunami created by the well-knowneruption of Krakatoa in 1883, and by its ensuingcollapse, was one of those four. Historical datacombined with the high level of seismic activitysuggested a likelihood of tsunamis occurring in the region, but nothing on the scale of thetsunami of 2004 was anticipated.10

Making Waves Ocean waves—tsunamis being one category—are classified as gravity waves. Although themechanisms that generate them are different,the physics that describe gravity waves areapplicable to those in a pond, on the open oceanor after a significant impact such as the Sumatra-Andaman earthquake. To understand tsunamis,it is essential to recognize how they aregenerated and how they differ from wind-generated waves.

Most ocean waves are primarily generated bywind turbulence creating friction along thesurface of the water. Turbulence produces ripplesthat are capillary waves—waves that travelbetween two fluids. Gravity and surface tensionpull the peaks of the ripples back towardequilibrium, but the ripples overshoot theoriginal level of the water, causing the surface tooscillate. Should the wind stop, the oscillationswill die out due to friction. Once the oscillationshave a wavelength greater than 2 cm [0.8 in.],wind-induced ripples can become gravity waves.This occurs at the point where the effects ofgravity are greater than the effects of surfacetension. Dispersion from gravity cancelsdispersion caused by surface tension of thewater, resulting in a radiating wave that has thepotential for traveling great distances. As windcontinues providing energy to the waves, theperiod, wavelength and speed increase, and the

resulting waves can even travel faster than thewind that generated them.

Waves can travel great distances, oftengaining strength and speed by combining withother waves or by the addition of more windenergy. A wave in Hawaii might have begunduring a storm in Alaska, arriving on the beachwith little loss of speed or energy. Although thewave began many miles away, the molecules ofwater were not displaced any great distance untiljust before the wave reached the shore.

In deep water, if the wavelength is muchshorter than the water depth, the motion of thewater can be described as circular during thetrough-peak-trough cycle. In shallow water, orwhen the wavelength is greater than the waterdepth, the motion is more elliptical, with the ratioof the horizontal to vertical motions proportionalto the ratio of wavelength to depth. For a tsunami,because of its long wavelength, this occurs evenin the deep ocean, and the horizontal motion canbe much greater than the vertical motion. At theshore, the elliptical motion transforms into for -ward motion, and the water molecules advancewith the wave (below).

In the ocean, with all its variability, wavemotion is more complex. Gravity, tides, cross -winds, submarine and shoreline features, waterdepth and wave arrivals from various angles willact upon the wave to affect wave height, speedand direction. Because of the long distances

8 Oilfield Review

> Wave basics. Wind-generated swells move across the surface of the ocean. The water molecules generally have acircular motion that becomes more elliptical as the wave approaches the shore. The velocity of a wave slows as itapproaches the shore, forcing the water upward. The tip of the wave continues moving faster than the base until itreaches the surf zone, where the peak of the wave breaks over due to gravity.

Wave height increases Surf zone

Orbital path ofwater molecules

Elliptical path


Autumn 2007 9

open to wave travel in the oceans, the simplewave train can develop into swells, which arelong-wavelength waves. As the swells reachshallow-water depths, they rise higher than theywere when over deep water and form peaks.These peaks will eventually break over becauseof the steepness of the wavefront, the pull ofgravity and the peak moving faster than the baseof the wave.11

Whether the water movement is created bythe wind, the sudden movement of the seafloorduring an earthquake, the downward force froma landslide or even the impact from an asteroid,these forces all generate oscillatory motion thattranslates into gravity waves. A tsunami differsfrom waves produced by the wind in that it is animpact-generated wave, deriving its speed andpower from the event that created it. Largeimpact-generated waves also have extremelylong wavelengths. Tsunamis can have wave -lengths in excess of 100 km [62 mi], whereaswind-generated swells have wavelengths on theorder of 150 m [500 ft].

Wavelength is a useful characteristic forclassifying wave types. A shallow-water gravitywave is characterized by the fact that the ratiobetween the water depth and the wavelength isquite small. These waves travel at a speed that isequal to the square root of the product of the

acceleration due to gravity (9.8 m/s2) [32 ft/s2]and the water depth. Because of a tsunami’s longwavelength, it acts like a shallow-water waveeven in deep water, and its speed can beapproximated if the water depth is known. With awater depth of 7,700 m, the Sunda trench was aperfect incubator for a fast-moving tsunami,which attained speeds of more than 900 km/h[560 mi/h], rivaling the speed of a moderncommercial jetliner.

Not only do tsunamis travel at high rates ofspeed, they maintain their wave height, oramplitude, for great distances. The amplitudes ofwater waves decay as they propagate for threereasons: the waves spread out over the surface ofthe water; the waves disperse because longerwavelengths travel faster; and energy isattenuated by viscous damping in the water. Fora large tsunami, all three effects are minimal.Since the energy for initiation occurs along anextended fault, the waves spread out linearlyrather than cylindrically, resulting in littlespreading. For extremely long wavelengths, thewaves are not highly dispersive because thevelocity is proportional to the square root of thewater depth, resulting in little dispersion in theopen ocean. Attenuation loss is inversely relatedto the wavelength, and thus there is littleattenuation. As a result, a tsunami propagates athigh speeds and travels great distances withlimited energy loss.

As a wave moves into shallow water, thepropagation speed developed in deeper watercannot be maintained. For a tsunami thatoriginally traveled at 900 km/h in deep water, themaximum sustainable velocity would be less than50 km/h [31 mi/h] in a water depth of 10 meters[33 ft]. Energy continues pushing the waveforward, leaving only one direction for the waterto go—upward. Wave height on shore, or run-up,of 35 m [115 ft] was reported on the island ofSumatra (above).

Ironically, the tsunami would have beenhardly noticed near the epicenter of the quake. Arise in ocean levels would have felt like a largerthan average swell. For example, theWesternGeco survey vessel Geco Topaz wasacquiring seismic data off the coast of India1,500 km [930 mi] from the epicenter. Thetsunami passed under the vessel 2 to 3 hoursafter the initial earthquake and was only a fewtens of centimeters in height—in the open waterof the Indian Ocean.

9. Volcanic activity around the subduction zones has resultedin the area being known as the Pacific Ring of Fire.

10. For an in-depth review of plate tectonics, see theSchlumberger SEED Web site: http://www.seed.slb.com/en/scictr/watch/living_planet/index.htm (accessedAugust 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).

> A tsunami approaching the shoreline. When the tsunami arrives at the shore, its velocity decreasesrapidly and its height increases and rises well above the average sea level. The original long-wavelengthwave 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 thecharacteristics 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 out-rush of water returning to the ocean carries debris from inland. Since the tsunami is actually a seriesof waves, subsequent surges return the debris, acting like battering rams along the coastline.

WavelengthMean sea levelCrest

Trough

Waveamplitude

Run-up

Wavelength


A Wakeup CallAt approximately 8 a.m. local time onDecember 26, 2004, the Sumatra-Andamanmegathrust fault earthquake began. The largestrecorded earthquakes have been along thrustfaults, where subducting and overriding platessuddenly shift to relieve built-up stresses. Over aneight-minute period, the rupture traveled fromthe epicenter off the coast of Sumatra, northwardalong the fault plane for about 1,200 km [745 mi]as the Indo-Australian plate slipped beneath theBurma microplate. This long section of lockedplates broke apart and the overriding plate, nolonger constrained, heaved upward.

Not all earthquakes produce tsunamis; itrequires the right set of circumstances. In thiscase, the fault plane of the earthquake extendedfrom 30 km [19 mi] below Sumatra to theseafloor of the Indian Ocean. From a surfacedamage standpoint, an earthquake centered in

the ocean might seem fortuitous. However, thislocation facilitated direct transfer of energy from the plate movement to the water. With a1,200-km long fault plane, a subduction zonethickness of 500 m [1,640 ft], and a verticaldisplacement of 5 to 15 m [16 to 50 ft], the upliftof the overriding plate and downdrop of thesubducting plate sent water oscillations travelingaway from the source of the energy, initiating atremendous tsunami (below).

Within 15 minutes of the quake, the tsunamiarrived along the Sumatra shoreline. There waslittle warning of its approach, although it is likelythat because of its proximity, the earthquakewould have been felt by those living in the region.The first indication of an approaching tsunamiwas probably a forerunner, a swell ahead of thelarger waves.12 Preceding the forerunner wouldbe a sudden out-rush of water, exposing largesections of the nearshore seabed. Based on

eyewitness accounts, this oddity drew people outalong the exposed seafloor, placing them in thepath of the approaching wave.13 Several minutespassed, and depending on the distance from thesource and speed of the tsunami, the first waveinundated the exposed beach and rushed inlandto flood the low-lying coastlands. The dangerdoes not end with the first wave, since the thirdto eighth waves generally are even larger. In SriLanka, arrival of the surges came at approxi mately40-minute intervals, indicating a wave length inthe hundreds of kilometers.14

A Measure of Perspective For the general public, earthquakes are oftenclassified using a magnitude based on the well-known Richter scale. Seismologists use moremeaningful measures such as the momentmagnitude scale. Richter and moment magni -tude are logarithmic measures of the amplitudeobserved on seismograms and are related to theenergy released in an earthquake.

Dr. Charles F. Richter developed his scale toquantify earthquake magnitude, and it isdesignated ML, with the L referring to local. Bycomparing the seismic data for numerousCalifornia earthquakes as measured by shearwaves recorded on a Wood-Anderson seismom e -ter, Richter correlated the amplitude of themeasured signal to the size of the earthquake.The Richter magnitude is the logarithm of thepeak amplitude of the seismic record, with adistance correction applied. Since it is a loga -rithmic scale, each whole number on theWood-Anderson seismometer represents anamplitude 10 times greater than the lesser wholenumber. Because the energy is proportional tothe square of the amplitude, and larger earth -quakes radiate more low-frequency energy notrecorded by the Wood-Anderson seismom e ter,each whole number in the magnitude scaleactually represents about a 30-fold increase inenergy for very large earthquakes.

Moment magnitude, MW, more accuratelydescribes the physical attributes of an earth -quake and is used by modern seismologists,especially when ranking large earthquakes.Moment is a function of the total energy releasedand is a physical quantity proportional to the slipdistance and the average slip area along the faultsurface. Seismic data are used to estimate themoment and then converted, using a standardformula, into a number representative of otherearthquake measurements, such as the Richtermagnitude.15 Depending on the source quoted,the Sumatra-Andaman earthquake received a 9.0to 9.3 MW rating.

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> A tsunami-generating earthquake. The Indo-Australian plate is slidingbeneath the Burma microplate along a subduction zone, developingstresses between the plates (top). The overriding plate became stuck andbuckled upward. The rupture relieved the stress created by the lockedplates and upward buckling (dashed line) and caused the overriding plateto move upward and outward (middle). It heaved an estimated 5 to 15 m,raising the overlying water, and initiating the tsunami (bottom). The rupturezone was more than 1,200 km in length.

Subducting plate

Overridingplate

Area ofplate sticking

Stuck arearuptures

Tsunami begins

Tsunami wavesspread


Autumn 2007 11

Within minutes after the earthquake, reportswere issued from seismic monitoring stationsaround the globe. The first magnitude estimatewas 6.2 MW, using the arrivals of early body wavesmeasured at the reporting station in Hawaii. Body-wave magnitudes are known to under estimatevery large earthquakes. A preliminary-magnitudereport (8.5 ML) was issued by the United StatesGeological Survey (USGS) and Pacific TsunamiWarning Center (PTWC) one hour and 15 minutesafter the event, which was as soon as sufficientsurface wave data were available. Estimates werelater increased to 9.1 MW, which is the estimatepublished by the USGS.16 Post-earthquake analysishas put the figure as high as 9.3 MW, but there isno figure for which a consensus has beenreached.17 Much of the difficulty is due to relatingthe seismic information to the volume of earththat moved.

The magnitude of an earthquake is crucialbecause the strength of the initiating event is acritical component of the modeling programsused to predict tsunami generation. A 6.2 MW

earthquake would not have generated a tsunamibulletin. The PTWC’s report was upgraded assoon as information became available, but thediscrepancy underscores the difficulty inherentin an early-warning system.

Data are available from sources other thanseismic monitoring stations, and an earthquakeof this magnitude has never been scrutinizedwith such an array of scientific tools. With anetwork of approximately 60 GPS monitoringstations in the vicinity of the earthquake,accurate ground movement could be quantified.The GPS network was part of an ongoingcollaborative project, Southeast Asia: MasteringEnvironmental Research Using Geodetic SpaceTechniques (SEAMERGES), with additional GPSdata coming from monitoring stations with theInternational GPS Service. The GPS dataprovided the actual earth displacementinformation, which was then used to estimateenergy released in the earthquake—but thiscould not be accomplished in real time.Reconciling the data from the seismicmonitoring and the GPS stations resulted inassigning a magnitude of 9.3 MW to the Sumatra-Andaman earthquake.18

Looking DeeperWithin days following the earthquake, humani -tarian relief poured into the region surroundingthe Indian Ocean. Individuals and organizationsaround the world offered help in the form ofdonations and services. Schlumberger made athreefold promise of funding, volunteers andtechnology. The funding and volunteers cameimmediately, addressing human aspects of thetragedy. On the technology front, one projectquickly emerged: a deep seismic survey along thefault line to improve the understanding of thecomplex tectonics in the region of the earthquake.Previous surveys, using academic researchvessels, could not image structures at 30 km, thedepth inferred from historical seismic activity.Understanding the distribution and geometry ofthe faults that control seafloor displacement iscritical in determining the mechanisms thatgenerated the tsunami.19

This is not the first time Schlumberger hasbeen an active participant in earthquake-relatedscientific studies. The San Andreas Fault

Observatory at Depth (SAFOD) Project incor -porated many oilfield technologies in theassessment of the seismically active San AndreasFault.20 The ability to deploy, acquire and analyzedata using tools developed for oil and gasexploration has been invaluable in understandingthe mechanisms that generate seismic events in regions such as the Sumatra-Andamanearthquake zone.

WesternGeco committed resources to acquireand process the data for the Sumatra EarthquakeDeep Seismic Reflection survey, or “the tsunamisurvey.” The vessel Geco Searcher was used forthe acquisition of seismic data (above). Inconjunction with Schlumberger CambridgeResearch in England and Institut de Physique duGlobe de Paris in France, WesternGeco donatedits services, including logistical and technicalsupport. The survey was conducted cooperativelywith the Indonesian Agency for the Assessmentand Application of Technology, which retains the rights to the data. In the future, WesternGeco plans to make its data available to

12. A forerunner is a series of oscillations of the water levelpreceding the arrival of the main tsunami waves.

13. Barber B: Tsunami Relief. US Agency for InternationalDevelopment, Bureau for Legislative and Public Affairs(April 2005): 4. http://www.reliefweb.int/library/documents/2005/usaid-tsunami-30apr.pdf (accessedOctober 31, 2007).

14. Cyranoski D: “Get Off the Beach—Now!,” Nature 433,no. 7024 (2005): 354–354.

and Ambrosius BAC: “Insight into the 2004 Sumatra–Andaman Earthquake from GPS Measurements inSoutheast Asia,” Nature 436, no. 7048 (2005): 201–206.

19. Singh S: “Seismic Investigation of the Great Sumatra-Andaman 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 Technologiesfor Earthquake Science,” Oilfield Review 18, no. 2(Summer 2006): 24–33.

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–AndamanEarthquake 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

> The Geco Searcher in action. The WesternGeco vessel Geco Searcher acquired the data for theSumatra Earthquake Deep Seismic Reflection survey. The data will be made available for futureacademic research.


the global academic community for additionalscientific analysis.

The survey was part of a larger initiative, theSumatra-Andaman Great Earthquake Research(SAGER) project, which included high-resolutionsea-bottom bathymetry and an ocean-bottomseismometer (OBS) refraction survey deployedby the French research vessel MarionDufresne.21 The Institut Polaire Français madethe Marion Dufresne available for the survey andprovided technical support. OBS sensors wereplaced on the seabed to record seismic activity(below left).

In July 2006, the Geco Searcher acquiredthree 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 OceanDrilling Program.22

Providing an image of faults at a depth of30 km required long offsets.23 In the oil and gasindustry such depths would not be consideredbecause they are beyond the reach of any drillingoperation. The Geco Searcher used the Q-Marinesingle-sensor marine seismic system to providethe technology needed to acquire 12-km [7.5-mi]

offsets with a single-vessel operation. The sourceand streamer depths were maximized for theacquisition of low-frequency data, and aftermodeling and analysis, the decision was made totow sources and streamers at a depth of15 meters. An additional shorter streamer wastowed at 7.5 m [25 ft] to provide high-resolutionimages for defining features nearer the surface.Compared with surveys used in oil and gasexploration, this survey design was elaborate andextensive: tripled streamer depth, tripledstreamer length, tripled energy source andtripled recording time (next page, top).

Concurrent with the seismic survey, theFrench research vessel Marion Dufresnedeployed 56 ocean-bottom seismometers alongthe route of two of the seismic lines. The widelyspaced OBS sensors recorded naturally occurring

seismic activity but were also able to acquireseismic data during the WesternGeco acquisi -tion. Using 5- to 20-km [3- to 12-mi] spacing, thesensors recorded the shots from the survey andthe reflections from the subducting layer. Theseismic reflection data from the WesternGecooperations and the refraction data from the OBSsensors are complementary because the reflec -tion data provide high-resolution images of thecrust, and the OBS refraction data providedeeper images of the crust and upper mantle.24

The volume of data acquired is massive.Preliminary processing and analysis were carriedout by WesternGeco staff aboard the GecoSearcher, and later on shore in Indonesia, butmore analysis will be required to identify thesignificant features and fully utilize the data(next page, bottom).

12 Oilfield Review

> Deploying an OBS. The research vessel Marion Dufresnedeployed 56 ocean-bottom seismometers along the path ofthe WesternGeco seismic survey. Intended for monitoringseismic activity at the seafloor, the OBSs were used torecord reflections from the sources used by WesternGeco.(Photograph courtesy of First Break, reference 19.)

> The survey area. In the vicinity of the Sumatra-Andaman earthquake, three seismic lines(WG1, WG2 and WG3), totaling 926 surface kilometers, were acquired. Preliminaryprocessing has provided high-resolution imaging to depths greater than 30 km. The mapalso contains bathymetry data for the area that was under study.

Depth, m 0

442

972

1,354

1,680

1,985

2,616

2,290

2,998

3,528

5,216

Sumatra fault

Sumatra

Simeulueplateau

December 26, 2004epicenter

Simeulue

52 mm/yr

WG1

Deformationfront

Indo-Australian

plate

WG2

West Andaman fault

Burmamicroplate

Aceh basin

Nicobar

1000 miles

0 100km

WG3


Autumn 2007 13

The seismic data, along with SAGERbathymetry and refraction data, are being usedto understand the features that control platemovement. Preliminary analysis of the dataconfirmed that a fault plane, from theearthquake epicenter at 33 km, extends to theseabed. The seismic images validated the

21. Bathymetry is the surveying or mapping of harbors,inlets or deepwater locations. Echo sounder techniquesare used in the measurement and study of water depthsto create bathymetric maps or charts of seafloor relieffor 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-OceanDrilling: Revealing the Earth’s Secrets,” Oilfield Review16, no. 4 (Winter 2004/2005): 24–37.

23. Offsets are the distance between the airgun array andthe sensors.

24. Singh, reference 19.

> Seismic images from two streamer depths. The image from the 7.5-m streamer (top) shows finer details nearer the surface. The image from the15-m streamer (bottom) uses deeper penetrating seismic energy. Features deeper than 30 km can be studied using these data.

Tim

e, s

1.5

2.0

2.5

3.0

Tim

e, s

1.5

2.0

2.5

3.0

> Preliminary results. From the WG1 seismic line, preliminary interpretation reveals faulting anddeep boundaries. The main thrust fault can be seen on this image, as well as other reflectors.The Moho, short for the Mohorovicic discontinuity, is the boundary between the Earth’s crust andthe mantle, and can be identified here.

Simeulue fore-arc basinSimeulue plateau

Accretionary wedgeSW

Tim

e, s

16

14

12

10

8

6

4

2

0

Active main thrust fault

Active frontal thrust

Oceanic Moho

West Andaman fault

Backthrust

ContinentalMoho

NEThrust reflectors

WG1

0 miles

0 km 25

25


premise that a large upheaval of the seabedcontributed to the strength of the tsunami(above). Early analysis has also identified a verywide locked zone, greater than 135 km [85 mi],whose rupture contributed to the magnitude ofthe earthquake.25

On September 12, 2007, an 8.4 MW earth -quake occurred on the December 2004 fault line,but produced relatively little tsunami energy(above right). Scientists can use the seismicimages and information acquired during bothearthquakes to better understand the mecha -nisms that initiated the earthquakes andproduced (or failed to produce) a large tsunami.Ultimately, the information can be integrated intomodeling programs to improve tsunami forecasts.

Subduction zones typified by the area thatcreated the Sumatra-Andaman earthquake existin other places around the world. Technologysuch as the Q-Marine system can be appliedelsewhere to better understand seismically activeregions. Collaboration between the academicworld and companies like Schlumberger willequip scientists and researchers with advancedtools to prepare at-risk locations.

Moving Towards Early WarningThe following is a timeline of the early eventsthat occurred December 25, 2004, at the NationalOceanic and Atmospheric Administration(NOAA) Pacific Tsunami Warning Center(PTWC) in Honolulu, Hawaii:

• 2:59 p.m. local time, the Sumatra-Andamanearthquake begins

• 3:07 p.m., first seismic arrivals detected at thePTWC

• 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 tsunamithreat to Pacific Ocean basin. There was noestablished 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 theTsunami Warning System (TWS) in the Pacific,and e-mail notification was sent to 25,000interested parties. Alerts were issued bytelephone to various agencies, including theHawaii Civil Defense and the InternationalTsunami Information Center.26

With 80% of major earthquakes occurringaround the Pacific Ocean, it is critical to have aneffective tsunami early-warning system thatoperates as described above. The PTWC is justone part of a cooperative network coordinated bythe Intergovernmental Oceanographic Commission(IOC), functioning under the United NationsEducational, Scientific, and Cultural Organi -zation (UNESCO).27 The Pacific TWS compriseshundreds of seismic monitoring stationsworldwide, sophisticated tsunameters monitor -ing wave heights in the open ocean andstrategically placed tidal gauges (next page, top).

Various organizations representing 26 countriesfrom that region collaborate to alert the publicwhenever the danger of a tsunami is present.

By their very nature, warning networks suchas the Pacific TWS are expensive, having tocontend with vast stretches of open water,expensive monitoring equipment on land and inthe oceans, and the need for continuous staffingof monitoring stations with qualified personnel.The events of December 2004 demonstrate justhow costly the lack of an early-warning systemcan be. The Pacific Tsunami Warning Center iswell established and is the model for the IndianOcean Tsunami Warning Center (IOTWC). ThePTWC relies on four primary tools: seismicmonitoring, ocean monitoring, fast modelingsoftware and communication.

Listening to the EarthThree key earthquake parameters can bedetermined from seismic waveform data topredict 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 createsignificant displacement

• magnitude—whether the size of the earth-quake is sufficient to produce a tsunami.

14 Oilfield Review

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).

28. ICG/IOTWS-II, Communications Plan for the InterimTsunami Advisory Information Service for the IndianOcean Region, ver. 15, January 2006. http://ioc3.unesco.org/indotsunami/documents/IOTWS_CommunicationPlan_15Jan06.pdf (accessed October 25, 2007).

> Detailed interpretation of the seismic data. The epicenter of the December 26, 2004 earthquake wasbeneath the Simeulue plateau, located west of Sumatra. The earthquake occurred when the continentalplate broke free of the oceanic plate along the subduction zone (red line). The zone extends more than150 km [93 mi] from the epicenter to the ocean floor. (Adapted from Singh, reference 19.)

Dept

h, k

m

50

0

5

10

15

20

25

30

35

40

45 500 miles

0 50km

Oceanic mantle

Oceanic Moho

Sediments

Upperseismogenic zone

Mantle wedge

Continental Moho

December 26, 2004

Backthrust

Subducting oceanic crust

Accretionarywedge Simeulue plateau

West Andaman faultSimeulue fore-arc basinIndo-Australian plate

Frontal thrust fault

Crustal-scalethrust fault

Main thrust faultIndicates motion

into pageIndicates motionout of page

> Two major earthquakes, with very differentresults. The epicenter of a September 12, 2007earthquake, 8.4 MW, was in the vicinity of theDecember 2004 Sumatra-Andaman earthquake,9.3 MW. Although the 2007 earthquake waspowerful enough to generate a tsunami, therupture did not extend from the epicenter as itdid in the 2004 earthquake (red). The smalltsunami produced during the 2007 earthquakehad little effect on the region.

Eurasianplate

Burmamicroplate

Indo-AustralianPlate

December 26, 2004

Andaman Islands

Sumatra

km

miles 1,000

0

0

1,000

September 12, 2007


Autumn 2007 15

Seismic monitoring is primarily accomplishedusing monitoring stations supported by variousgovernmental agencies and educationalinstitutions. The Global Seismographic Network(GSN) is a primary source of data. It comprises225 monitoring stations in more than 80 coun -tries. In addition, the PTWC and the IOTWCreceive data from other seismic monitoringnetworks such as the International MonitoringSystem (part of the Comprehensive Nuclear TestBan Treaty Organization) and those coordinatedby the Incorporated Research Institutions forSeismology (IRIS).

The warning centers receive seismic dataover the Internet. However, because reliabletransmission of data from the Internet is notguaranteed, especially in the case ofinfrastructure damage during and after a majorearthquake, additional sources for data areavailable. The Matsushiro Seismic Array Systemof Matsushiro Seismological Observatory(Nagano, Japan) and the Large Aperture Arraycomprising Japanese seismological observation

networks—are examples of the warning centers’contingent data sources.

When a seismic event occurs, data areprocessed at the warning centers to evaluate thepotential for a tsunami. The warning centers use

an established criterion, based on the magnitudeof the earthquake, to decide which type of bulletinto issue (above). A reliable location can bedetermined using the least-squares method, withP-wave arrival times and various reflected phasesused to provide epicenter depth estimations.28

> Global Seismographic Network (GSN). With a large number of seismic monitoring stations, the GSN comprises a multinational, multidisciplinary networkof cooperating research seismometer stations, including those affiliated with the Incorporated Research Institutions for Seismology (IRIS). The network, asof April 2007, includes the following stations: 86 operated by the United States Geological Survey (USGS), 39 operated by International Deployment ofAccelerometers (IDA), a global network of broadband and very long period seismometers, and other affiliated stations. The University of California SanDiego (UCSD), CU in the legend, is a major participant in the network, with funding from the National Science Foundation. For more on GSN, IRIS, UCSDand IDA: http://www.iris.edu/. (Modified from Global Seismic Network, http://www.iris.edu/about/GSN/map_family.html.)

BBSRDWPF

WVT

CCMWCI SSPA

HRV

FFC

ANMO

TEIGSUG

SDV

RSSDCOR

TUCHKT

JTS

PFOPASCMB

COLA

KDAKADK

H2O

KIPPOHA

MIDWSLBS

JOHN

XMASKANT

AFI

RAP

RAOPTON

RPN

PAYG OTAV

NNALPAZ

LCO

LVC

PTGA

SAML

CPUP

TRQA

BDFB

RCBR

SACV

CMLA

MACI

KOWADBIC

ASCN

MSKU

BGCAMBAR

SHELTSUM

LSZ

PLCA

EFI

PMSA HOPE

TRIS

LBTB

SURBOSA

QSPAVNDA

SBA

CASY

ABPO

KMBO

FURI

DGAR

MSEYPALK

COCO

NWAO

MBWA

BTDF

TAU SNZO

WRAB

KAPI

PMG

CTAOMSVF

HNRFUNA

TARA

KWAJDAV

QIZ

GUMO

WAKE

TATO

ERM

MAJO

SSEENH

XAN

INCNBJT

MDJ YSS

PET

BILL

YAKMA2

HIATLYULNWMQ

NILLSA

KMI

CHTO

UAERAYN

ABKT

AAKMAKZ

KURK

GNIKIV

BRVKARU

NRILTIXI

OBNKIEV

ANTO

GRFO

PAB

BFODPC

LVZKEV

KGNO

ESKBORG

KBS

SEJD

ALE

Installed Planned

IRIS/USGS stationsIRIS/IDA stationsUSGS/CU stationsAffiliated GSN stations

> Tsunami bulletin criteria. The Tsunami Warning Centers use magnitude, location (under sea orunder land) and depth of the earthquake to determine the potential for a tsunami and issue bulletinsbased on those criteria. (Source of data is reference 28.)

EarthquakeDepth

EarthquakeLocation

EarthquakeMagnitude, Mw

Description of Tsunami PotentialBulletin

Type

Very small potential for adestructive tsunami

Potential for a destructivelocal tsunami

Potential for a destructiveregional tsunami

Potential for a destructiveocean-wide tsunami

No tsunami potential

No tsunami potential

Tsunamiinformation

Local tsunamiwatch

Regionaltsunami watch

Ocean-widetsunami watch

Tsunamiinformation

Tsunamiinformation

Under or verynear the sea

< 100 km

≥ 100 km

Inland

All locations

≥ 7.9

≥ 6.5

≥ 6.5

7.6 to 7.8

7.1 to 7.5

6.5 to 7.0


The seismic data are the first piece of thepuzzle. If an earthquake is sufficiently large,takes place in a shallow portion of the Earth’scrust, and occurs in a location under or close tothe sea, it has the potential for generating atsunami. Whether or not a tsunami has actuallybeen created can be determined only at theocean’s surface.

The Ocean’s PulseIdentifying the formation of a tsunami andaccurately forecasting its arrival times and waveamplitudes depends on precise ocean-levelmonitoring. This is accomplished using twoprimary sources—NOAA’s DART Deep-OceanAssess ment and Reporting of Tsunamis buoys indeep water and tidal gauges near coastlines.Although DART buoys have been deployedglobally, the Pacific Ocean has the majority, with28 DART buoys in place, and four more to bedeployed by the end of 2008 (right). The DARTbuoy consists of an anchored seafloor bottom-pressure recorder (BPR) and a tethered surfacebuoy that provides real-time communications(next page). An acoustic link transmits

16 Oilfield Review

> Buoy network for monitoring ocean activity. The Pacific Ocean is encircled by DART Deep-OceanAssessment and Reporting of Tsunamis monitoring buoys, with more planned. The network suppliesinformation 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 inthe Indian Ocean. (Adapted from NOAA, http://www.ndbc.noaa.gov/dart.shtml.)

DART Locations

4 PlannedNOAA34

Other3

> Global Sea Level Observing System (GLOSS). With more than 290 sea-level monitoring stations, GLOSS is at work around the world monitoring long-termclimate change and oceanographic sea-level variations. In the event of a tsunami, these data are incorporated into modeling software to refine forecastsand inundation estimations.

GLOSS Tidal-Gauge Locations


Autumn 2007 17

temperature and pressure data from the BPR tothe surface, which are converted to an estimatedsea-surface height. The accuracy of themeasurement is ± 1 mm in 6,000 m [20,000 ft] ofwater depth. These data are transmitted to anIridium commercial satellite that relays theinformation to moni toring stations. Turnaroundtime for data is less than three minutes, frombuoy to warning center.29

Tidal gauges record coastal sea-levelvariations using an international network ofmonitors. The Global Sea Level Observing System(GLOSS) is a network of more than 290 sea-levelmonitoring stations coordinated under theauspices of the Joint Technical Commission forOceanography and Marine Meteorology(JCOMM) of the World MeteorologicalOrganization (WMO) and the IntergovernmentalOceanographic Commission (IOC). GLOSSprovides high-quality global and regional sea-level data for application to climate,oceanographic and coastal sea-level research(previous page, bottom).30

A Model ForecastWhen a seismic or other event of sufficientmagnitude triggers the need for tsunami modeling,various software programs may be used to estimatea tsunami’s potential severity. The seismicinformation is the initial source, but real-time sea-level data are incorporated into the model as theybecome available. These modeling programsprovide estimated wave-arrival time, and wave-height and inundation patterns. It is critical that asimulation model be able to provide accurateforecasts as rapidly as possible. The elapsed timeof 15 minutes between the earthquake and thefirst wave arrival in Sumatra underscores the needfor speed in model predictions.

The United States National Oceanic andAtmospheric Administration (NOAA) has devel -oped a cutting-edge modeling program, known asMethod of Splitting Tsunami (MOST).31 The MOSTprogram uses a suite of numerical simula tioncodes to compute predetermined wave behaviorfor three stages of a tsunami—generation,propagation and run-up. The program can providecoarse grids in deep water, where the wavelengthis long and fewer node points are needed. Inshallow water, the tsunami wavelength shortensand the amplitude rises. To better model thewave, the program narrows its focus to high-resolution grids.

The early-warning system issues alerts andnotifications to potential at-risk areas based onthe MOST outputs. The MOST program is first runin a research mode to create scenarios using

predetermined inputs—such as earthquakemagnitude, directionality and location. Thesesimulations can take hours to run, which wouldbe inappropriate for an early-warning system. To

speed the process, when an earthquake isdetected, the software attempts to match thereal-time data to a preexisting scenario topredict the likelihood and potential of a tsunami.

29. http://nctr.pmel.noaa.gov/Dart/dart_home.html (accessedOctober 1, 2007).

30. http://www.gloss-sealevel.org/ (accessed October 18,2007).

> NOAA’s DART II system. Anchored to the ocean floor, the tsunameter monitors temperature andpressure. These data are passed to a separate surface buoy by means of acoustic pulses. The buoycommunicates with the tsunami warning centers using a commercial Iridium satellite link. First-generation DART systems featured an automatic detection and reporting algorithm triggered by athreshold wave-height value. Today's design permits two-way communications, enabling datatransmission on demand, independent of the automatic triggering. This ensures the measurement andreporting of tsunamis with amplitudes below predetermined threshold limits. When a seismic eventoccurs, the tsunami warning centers use predictive software to model tsunami magnitude andseverity, but until empirical data, such as wave height from DART buoys, become available, thecenters 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, improvingwatches, warnings or evacuation bulletins. (Adapted from NOAA, http://nctr.pmel.noaa.gov/Dart/.)

Iridium andGPS antennas

Electronic systemsand batteries

Acoustic transducers(2 each)

Surface buoy,2.5-m diameter,

4,000-kg displacement

Bidirectionalcommunication

and control

Iridium satellite

Tsunamiwarningcenter

Tsunameter

Signalflag

Glass ballflotation

Acoustictransducer

Anchor, 325 kgAnchors, 3,100 kg

1,000 to 6,000 m

~ 75 m Bidrec

tiona

l aco

ustic

telem

etry

31. Titov VV and Synolakis CE: “Numerical Modeling of TidalWave 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).


As additional information, such as DART andtidal-gauge data, becomes available, the model isadjusted (above).

Another tool used in analyzing the 2004tsunami, the Jason-1 earth-imaging satellite,provided modelers with accurate wave-heightdata for the duration of the tsunami. Measuredfrom space, the resolution was in the centimeterrange. Rather than having only point-to-pointmeasurements, such as from tidal gauges orDARTs, the waves could be measured continu -ously with satellite data. Unfortunately, the lagtime is much too great and the coverage area toosparse to use satellite data in real time. However,satellite information can provide validation andimprovement for the current modeling programs.

Even with all the data at their disposal,experts were challenged to explain how theSumatra-Andaman earthquake produced atsunami whose magnitude exceeded initial wave-height predictions. NOAA’s tsunami forecast,running the model with seismic data alone,originally underestimated tsunami heights in theopen ocean by a factor of 10. Integration oftsunami amplitudes from tidal gauges improved

the results iteratively, but the results were notconsidered satisfactory. Analyses of the shock’sstrong seismic waves indicated that the initialfault break traveled northward from Sumatra at2.5 km/s [1.6 mi/s]. The analysis also pinpointedthe areas of greatest slip—and thus of thegreatest wave generation. The problem fortsunami modelers was that none of these seismicsolutions included enough overall fault motion toreproduce either the satellite observations ofwave heights in the open ocean or the severeflooding in Banda Aceh.

The critical piece of the puzzle came fromelevation and displacement data provided byland-based global positioning system (GPS)monitors, used to track ground movements. TheGPS sensors, recording at a much slower ratethan seismic monitors, revealed that the faultcontinued to move long after it stoppedemanating seismic energy. Although there is alimit to how slowly a fault can slip and stillgenerate a tsunami, this often overlookedphenomenon, called after-slip, accounted for theobserved tsunami wave heights. IncorporatingGPS readings into modeling programs will be an

important component in improving the accuracyof tsunami warning systems in the future.32

Another challenge is integrating the data in atimely manner.

A major drawback in developing and usingmodeling software is that there is so littleempirical data to compare with the model. OnSeptember 12, 2007, an 8.4 MW earthquakeoccurred in the vicinity of the December 2004earthquake. This was the first major event sincethe deployment of a DART buoy in the IndianOcean. The MOST program predicted a 2-cm[0.75-in.] rise in wave height at the location ofthe buoy with an arrival time of approximately 2 hours and 50 minutes. The observed wave heightsand arrival times matched MOST predictions(next page).33

Inundation models, estimates of how farinland a tsunami will travel, are another criticalcomponent. Scientists use measurementsrecorded near the coast from tidal gauges or post-event estimates from water damage to determinerun-up. Early programs calculated wave heightsat the shore’s edge but had difficulty projectingthe effects onto the shore. A 1992 Nicaraguantsunami gave scientists an opportunity to makecomprehensive measurements and compare themwith model predictions.34

Using large-scale laboratory experiments andfield measurements, investigators refined theirmodels until they could match the empiricaltsunami inundation measurements. Using high-resolution land imagery, accurate bathymetrydata, coastal and offshore topographical data,historical information from previous tsunamisand software to make rapid calculations, theydemonstrated that an early-warning systemcould provide reliable estimations.

Sounding the AlarmThree months prior to the December 2004tsunami, a working group for the SouthwestPacific and Indian Ocean Tsunami WarningSystem was established. Under the auspices ofthe International Tsunami Information Center(ITSU), a UNESCO organization, this group’scharter was to expand the Pacific warningsystem to include other regions with thepotential for tsunamis, including the IndianOcean. When the earthquake occurred, thePacific Tsunami Warning Center (PTWC)attempted to contact affected countries acrossthe Indian Ocean; unfortunately, it was Sundayas well as a holiday for many. Most offices wereclosed, and the warnings did not reach theinhabitants of the affected coastlines. One result

18 Oilfield Review

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 ResearchLetters 20, no. 14 (1993): 1515-1518.

35. “Tsunami 2004: Waves of Death,” The History ChannelWeb 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).

> Model of the Sumatra-Andaman earthquake tsunami. Using NOAA’sMethod of Splitting Tsunami (MOST) program, the tsunami (arrow) wasmodeled as it traveled across the Indian Ocean. Shown here at approximately1 hour after initiation, the wave will take three more hours to reach theAfrican coastline. (Adapted from NOAA/PMEL/Center for TsunamiResearch, http://nctr.pmel.noaa.gov/model.html.)


Autumn 2007 19

of the Sumatra-Andaman earthquake was toaccelerate the pace of developing global early-warning networks.

Post-tsunami analysis confirmed thatcommunication within the region and links toother monitoring sites were lacking. A strikingexample of the importance of having anemergency management system in place isevidenced by comparing the tsunami mortalityrate in Kenya and Somalia. Kenya did not have atsunami warning system, but it did have achemical and oil spill-alerting system. Whenword of the approaching tsunami reachedKenyan officials (tsunami travel from Sumatra toKenya took four hours), they activated the spill-alerting system. Approximately 800,000 peoplewere warned to move inland or seek higherground. Four hours after the earthquake, thetsunami reached the shores of Kenya andSomalia. The death toll for Kenya was one. Inneighboring Somalia, where there was nowarning system, the death toll was 150.35

With modern Internet and satellite connec -tivity, communication over a wide area is almostinstantaneous, but communications can bechallenging in developing countries. Problemsalso arise when the alert must be communicatedto the general population. Planning for eventslike this must assume that infrastructures arelikely to be severely damaged. Satellite linksmake it possible to communicate in the absenceof land lines, but contingencies must also be inplace to alert the general population if localsystems are destroyed.

Effective warning systems for natural hazardsrequire public information and preparednesscomponents. Early warning is largely a socialissue, and technology alone will not solve theproblem. Early-warning systems may fail at timesof crisis if warnings are not received by thepeople at risk, or are not understood, or are notacted upon. An effective early-warning systemneeds to be people-centered in addition tohaving sound technical methods of communi -cation. Trained and experienced emergency

management personnel are critical to ensurethat warnings are clearly communicated, wellunderstood and rapidly implemented. Inaddition, regional coordination is important, asearthquakes and tsunamis do not restrictthemselves to territorial borders.

Even with the best data, the accuracy of themodels used to predict tsunamis is limited byerrors in bathymetry and uncertainties in thetriggering mechanism. Each earthquake isunique, and every tsunami has a uniquecombination of wavelengths, wave heights anddirectionality. From a warning perspective, thismakes the problem of forecasting tsunamis inreal time difficult. In the case of the December2004 tsunami, the areas north and south of theearthquake epicenter had little damagecompared with areas to the east and west. CocosIsland is 1,500 km to the south, and Sri Lanka is1,500 km to the west of the epicenter. Themaximum wave height on Cocos was 42 cm[16 in.], while sections along the Sri Lankancoast experienced run-up in excess of 8 meters[26 ft]. Warning center personnel understand theneed for a delicate balance between creatingundue panic and underestimating the severity,potentially causing an even greater tragedy.

The Way ForwardIn 2006, UNESCO Director-General KoïchiroMatsuura announced that, after much coopera -tive effort, 26 national tsunami informationcenters had been established around the IndianOcean.36 As part of the Indian Ocean TsunamiWarning System (IOTWS), this is the first stage in the development of an integrated organi -zation modeled after the Pacific TsunamiWarning System.

As of October 2007, seismographic reportingstations have been upgraded and two DART buoyshave been deployed in the Indian Ocean. Twenty-five additional monitoring stations will be addedand linked in real time to analysis centers.Information bulletins are being issued fromJapan and Hawaii, pending a decision on the finallocations of Indian Ocean regional centers. In thefuture, additional DART buoys and satellite linkswill be deployed. The work certainly is not over; ithas taken 40 years to develop the Pacific TsunamiWarning System, and developing a comparablesystem for the Indian Ocean will also take sometime. However, a basic system is now in place forwhen—not if—the next great Indian Oceantsunami occurs. —TS

> MOST predictions compared with tsunami data. Shown on the map of theIndian Ocean (top), the Thailand DART buoy (yellow circle) was installed inAugust 2007. On September 12, 2007, an 8.4 MW earthquake (red star)occurred with an epicenter just south of the Sumatra-Andaman earthquakeof 2004. A minimal tsunami was generated by the event. In a comparison ofwave heights (bottom), the MOST wave-height simulation (red curve) aftereight hours compares favorably with the data recorded by the ThailandDART buoy (blue curve) both in wave amplitude and arrival time. (Adaptedfrom data courtesy of NOAA/PMEL/Center for Tsunami Research.)

0 1 2 3 4 5 6 7 8 9 10 11 12

MOST modelDART data

Time after earthquake, h

Ampl

itude

, cm

0

2

4

–2

–4

Thailand DART buoy Epicenter of 2007 earthquake


the science of tsunamis

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