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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.) Autumn 2007 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 Autumn 2007 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 Oilfield Review 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 Autumn 2007 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. Autumn 2007 > 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