Tsunami Dynamics: Causes, Impact, and Mitigation Strategies

Abstract

A tsunami is a set of ocean waves caused by any large, abrupt disturbance of the sea surface (NOAA, 2007). A very large disturbance can cause local devastation and export tsunami destruction even to thousands of miles away.

Predicting when and where the next tsunami will strike are impossible, but once the tsunami is generated, forecasting tsunami arrival and impact is possible through modeling and measurement technologies.

The recent development of real-time deep ocean tsunami detectors and tsunami inundation models has given coastal communities the means to reduce the impact of future tsunamis.

If these tools are used with a continuing educational program in the communities that may be affected, at least 25% of the tsunami related deaths might be averted. Coastal communities must be educated so that when the next earthquake takes place, evacuation plans can be available and warning systems can be made (Whitmore, 2003; Telford & Cosgrave, 2004; NOAA, 2007).

1. Introduction

The word tsunami is a Japanese word, represented by two characters: tsu, meaning, "harbor", and nami meaning, "wave".

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In the past, tsunamis were often incorrectly referred to as "tidal waves" by many people. Tsunamis, however, are not caused by the tides nor are related to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact. Tides are the result of gravitational influences of the moon, sun, and planets (NOAA, 2007).

A tsunami is a set of ocean waves caused by any large, abrupt disturbance of the sea surface (NOAA, 2007). A very large disturbance can cause local devastation and export tsunami destruction even to thousands of miles away. If the disturbance is close to the coastline, however, local tsunamis can demolish coastal communities within minutes. Predicting when and where the next tsunami will strike are impossible, but once the tsunami is generated, forecasting tsunami arrival and impact is possible through modeling and measurement technologies.

1.1 Objectives

This study primarily aims to identify the root of tsunamis and how they are formed. Due to the fact that tsunamis cannot be predicted nor prevented, it is important that precautionary measures are taken to enable swift evacuation of coastal areas.

This study also examines the possible methods for detecting the arrival of tsunamis using modern technology and determines, whether or not, these equipment are effective and useful. Studies on the possibility of predicting the onset of tsunamis are also scrutinized.

1.2 Scope and Limitation

The information obtained in this experiment is obtained from evaluations of past tsunami disasters; these evaluations were made weeks and even months since the disaster, and so the information obtained may have discrepancies compared to what really happened in when the disaster struck.

2. Review of Related Literature
2.1. Generation of Tsunamis

A tsunami is a series of waves with very long wave lengths and long periods that are generated in a body of water by a disturbance that displaces the water. A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. Tsunamis are primarily associated with earthquakes in oceanic and coastal regions, landslides, volcanic eruptions, nuclear explosions, and even impacts of objects from outer space like meteorites, asteroids, and comets (Ward & Asphaug, 1999; Ward, 2000; Watts, 2000; NOAA, 2007).

Earthquakes generate tsunamis when the sea floor abruptly deforms and displaces the water above it from its equilibrium position. Waves are formed as the displaced water, which acts under the influence of gravity, attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation, which is controlled by the earthquake's magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting. Other factors that influence the size of a tsunami along the coast are the shoreline, the velocity of the sea floor deformation, the water depth near the earthquake source, and the efficiency which energy is transferred from the earth's crust to the water column (NOAA, 2007).

When a tsunami finally reaches the coastline, it may appear as a rapidly rising or falling tide or a series of breaking waves. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the height of the tsunami as it approaches the shore. Tsunamis rarely become great, towering breaking waves, as they sometimes break far offshore, or they may form into a bore: a step-like wave with a steep breaking front.

A bore can happen if the tsunami moves from deep water into a shallow bay or river. The water level on shore can rise many feet, and in extreme cases, water level can rise to more than 50 feet (15 m) for tsunamis of distant origin and over 100 feet (30 m) for tsunami generated near the earthquake's epicenter. Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters (98 ft) (Borrero, 2004; NOAA, 2007).

2.1.1. Earthquake-generated tsunamis

Earthquakes are the most common cause for tsunamis. Earthquakes occur whenever one of the many tectonic plates that make up the Earth’s crust subducts under an adjacent plate; this newly formed area is then called the “subduction zone”. The overriding plate then gets squeezed as its leading edge is dragged down while the area behind it swells upward, building stress for over long periods of time.

After decades, or even centuries of built up stress, an earthquake finally occurs along the subduction zone, because the leading edge of the overriding plate eventually breaks free from the subducting plate. This movement then raises the sea floor and displaces a great mass of seawater upwards, while also relieving the tension as the rest of the overriding plate collapses, thereby the lowering the coastal areas (Atwater et al 2005).

2.1.2. Landslide-generated tsunamis

Submarine landslides, which often occur alongside a large earthquake, can sometimes also create a tsunami. The tsunami created are often termed “surprise tsunami” and can be initiated far outside the epicenter of an associated earthquake or be greater than predicted as according to the magnitude of the earthquake.

During a submarine landslide, the equilibrium sea-level is distorted by sediment moving along the sea-floor. Gravitational forces then propagate the tsunami given the initial disturbance of the sea-level. What makes this cause for a tsunami dangerous is that it arrives without any precursory seismic warning at all (Ward & Asphaug, 1999; Ward, 2000; Watts, 2000; NOAA, 2007).

2.1.3. Tsunamis generated from volcanic eruptions

Likewise, albeit infrequent, a violent marine volcanic eruption can create an impulsive force that displaces the water column and generate a tsunami. Basaltic volcanoes (volcanoes that emit basalt upon eruption) have certain factors that determine the magnitude of the tsunami that can be created; geochemical factors, growth and collapse of lava domes, volcanic explosivity factors and blast geometry factors have an effect on the tsunami that will be generated (Ward & Asphaug, 1999; Ward, 2000; Watts, 2000; Pararas-Carayannis, 2004; NOAA, 2007).

Variations in the chemical composition of volcanic effluents determine whether a volcano will have effusive eruptive or explosive type of eruption will occur. Rapid lava dome growth, on the other hand, indicates a build-up of pressure within a volcano, while its collapse often triggers an outbreak of volcanic eruptions that varies in intensities. Explosivity factors such as the sudden release of gases can create sudden atmospheric pressure disturbances, which can also generate destructive waves.

Furthermore, the geometry of eruption blasts has the potential to create subareal or submarine landslides, which can also cause tsunamis. These blasts can be vertical, lateral or channelized. Vertical blasts can lead to cone collapse, which may result in landslides. Strong blasts can generate disturbances in atmospheric pressure, which can set off destructive waves of varying periods. Lateral and channelized blasts, on the other hand, are far-reaching and can therefore generate more destructive local tsunamis (Ward & Asphaug, 1999; Ward, 2000; Watts, 2000; Pararas-Carayannis, 2004; NOAA, 2007).

2.1.4. Tsunamis generated from objects from outer space

Space born objects can disturb the water from above the surface; the falling debris displaces the water from its equilibrium position and also produces a tsunami. Ward and Aspaug (1999) have investigated on the generation, propagation and probabilistic hazard of tsunami that can be created by oceanic asteroid impacts.

Their method had linked the depth and diameter of parabolic impact cavities to asteroid density, radius, and impact velocity by means of elemental energy arguments and crater scaling rules. They had concluded that the probabilistic hazard of tsunami created by asteroid impacts is comparable to those created by earthquakes and volcanic eruptions, if one is to integrate contributions over all admissible impactor sizes and impact locations (Ward & Asphaug, 1999; Ward, 2000; Watts, 2000; NOAA, 2007).

2.2 The Physics Behind the Waves

Figure 1. Figures of wavelengths when the tsunami generated is in the deep ocean (R) and when the tsunami reaches shallow waters (L).
(Source: http://media.allrefer.com/s1/l/w0061300-wavelength.jpg and http://library.thinkquest.org/03oct/02144/glossary/pics/wavelength.png)

As the tsunami crosses the deep ocean, its length from crest to crest (see Figure 1) may be a hundred miles or more, and its height from crest to trough will only be a few feet or less. Thus, they can not be felt aboard ships nor can they be seen from the air in the open ocean, even as the waves reach speeds exceeding 600 miles per hour (970 km/hr). When the tsunami enters the shallow water of coastlines, however, the velocity of its waves decreases while the wave height increases. It is in these shallow waters that a large tsunami can crest to heights exceeding 100 feet (30 m) and strike with devastating force (NOAA, 2007).

Tsunamis are characterized as shallow-water waves; shallow-water waves are different from wind-generated waves, the waves many of us have observed on the beach. A wave is characterized as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Wind-generated waves usually have periods (time between two succeeding waves) of five to twenty seconds and a wavelength (distance between two succeeding waves) of about 100 to 200 meters (300 to 600 ft). A tsunami can have a period in the range of ten minutes to two hours and a wavelength in excess of 300 miles (500 km).

It is because of their long wavelengths that tsunamis behave as shallow-water waves. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (9.80m/sec2) and the depth of the water, and the rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Therefore, in very deep water, a tsunami will travel at high speeds and travel great distances with limited energy loss (NOAA, 2007).

As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Since the speed of the tsunami is related to the water depth, as the depth of the water decreases, the speed of the tsunami diminishes, but the change of total energy of the tsunami remains constant. Therefore, the speed of the tsunami decreases as it enters shallower water, and the height of the wave grows. Because of this shoaling effect, a tsunami that was imperceptible in deep water may grow to be several feet or more in height (Kowalik et al, 2004; NOAA, 2007).

3. Discussion
3.1 Reduction of impact.

The recent development of real-time deep ocean tsunami detectors and tsunami inundation models has given coastal communities the means to reduce the impact of future tsunamis. If these tools are used with a continuing educational program in the communities that may be affected, at least 25% of the tsunami related deaths might be averted. Coastal communities must be educated so that when the next earthquake takes place, evacuation plans can be available and warning systems can be made (Whitmore, 2003; Telford & Cosgrave, 2004; NOAA, 2007).

3.2 Warning Systems

Since 1946, the tsunami warning system has provided warnings of potential tsunami danger in the pacific seabed by monitoring earthquake activity and the passage of tsunami waves at tide gauges. However, neither seismometers nor coastal tide gauges can provide data that allow accurate prediction of the impact of a tsunami at a particular coastal location. Monitoring earthquakes gives a good estimate of the potential for tsunami generation, based on earthquake size and location, but gives no direct information about the tsunami itself. Partly because of these data limitations, 15 of 20 tsunami warnings issued since 1946 were considered false alarms because the tsunami that arrived was too weak to cause damage (Whitmore, 2003; NOAA, 2007).

However, recent developments by the US Government have produced Deep-ocean Assessment and Reporting of Tsunamis (DART™) Technology (see Figure 2). The information collected by a network of DART™ systems positioned at strategic locations throughout the globe (see Figure 3) plays a critical role in tsunami forecasting. When a tsunami event occurs, the first information available about the source of the tsunami is based only on the available seismic information for the earthquake event.

As the tsunami wave propagates across the ocean and successively reaches the DART™ systems, these systems report sea level information measurements back to the Tsunami Warning Centers, where the information is processed to produce a new and more refined estimate of the tsunami source. The result is an increasingly accurate forecast of the tsunami that can be used to issue watches, warnings or evacuations (NOAA, 2007).

Figure 2. The technology behind DART™ buoys. (Source: http://nctr.pmel.noaa.gov/Dart/)

Figure 3. Locations of DART™ buoys, which relay information to three Tsunami Warning Centers: West Coast/Alaska, Pacific Tsunami Warning Centers and International Tsunami Information Center.
3.3. Public Awareness

A survey was conducted in 2006 by Kurita’s research group in Sri Lanka to assess and evaluate the disaster management system in Sri Lanka and the capacity of a local community to respond to natural disasters. By using different methods to gather information, the group’s findings were devastating. The results of the survey of residents indicate that more than 90 percent of residents lacked tsunami knowledge prior to the 2004 tsunami. They had also discovered that the main source of information during the disaster was direct information from family and neighbors.

The school surveys had revealed that about 30 percent of school children do not yet understand what causes a tsunami, despite the fact that 90 percent of school children have a keen interest in studying natural disasters. These findings imply that comprehensive disaster education has not been provided, mainly because the audio-visual means are thought to be the most effective tool for disaster education, cannot be provided.

In addition, the survey of government officials shows that seminars and drills on natural disaster have not been conducted among general officials other than the military and police. Safety measures need to be developed to safeguard the interests of tourists, as sirens, TV, and radio broadcasts are effective tools for disseminating disaster warnings in the even of another tsunami disaster (Kurita et al 2006).

3.4. Funding

The tsunami response for the 2004 disaster in South Asia has been the most generous and immediately funded international response in history. More than eighteen and a half billion US dollars (US$ 18.5 B) had been pledged or donated internationally fore emergency relief and reconstruction. However, the international system for tracking those funds did not register the very substantial contributions made by the donors and governments in the affected countries.

The generous flow of funding had led to the need for additional people to allocate the funds. Agencies have only relatively small numbers of appropriately experienced personnel who can operate in an emergency at an international level. The pressure for quick results and assessment leads to the recruitment of inexperienced staff members. Thus, new people with insufficient experience and competence, as well as people forced to venture in activities outside their field of expertise, were forced to help in allocating the donations received. As a result, imbalances, misuse and poor traceability and monitoring became evident.

Telford and Cosgrave’s synthesis (2006) had concluded that the allocation and programming of these funds were driven by politics, as opposed to be driven, ideally, by assessment and need. Some donors saw to it that their donations were used favoring recovery and construction, while others funded mainly emergency needs; funding was not based on systematic measurement of the relative effectiveness and efficiency of different agencies and their respective programs (Telford & Cosgrave, 2006).

4. Conclusions and Recommendations

Natural disasters are in no way predictable or escapable. However, this does not give us an excuse to leave everything to chance when disaster strikes. The development of the DART™ technology had proved to be the best way for coast dwellers to be informed of an incoming tsunami, how far it is and how high it might be.

Despite having this kind of technology, it is nothing without the cooperation of the public. The people most vulnerable to the wrath of this disaster must also be educated, so that evacuation plans and safety measure could have been made beforehand. By preparing themselves for unforeseen disasters, the amount of casualties can be lowered at a large percent, if not none at all. Geologists, volcanologists and seismologists have tediously studied on how tsunamis are often generated, and therefore have informed the public on safety tips as to keep steady minds in the event of the disaster.

The tsunamis generated from the earthquake in the Indian Ocean back in 2004 had proved the importance of having evacuation centers that are placed in higher ground for those who dwell along shorelines, as well as to tourists who have sought the seclusion of private beaches in South Asia. Advices from Atwater and his (2005) colleagues on how to survive tsunamis had been compiled from statements of those who had survived the Pacific Ocean tsunami in 1960, and how these had reached from Chile to Hawaii and Japan.

Important advices such as heading to higher ground, abandoning belongings, hanging on to floating objects, climbing trees, expecting a series of waves and expecting waves to leave debris had been explained in detail in their handout. Local governments in coastal areas could ask permission to reproduce this document as it contains information that can be vital to a person’s survival when disaster strikes.

In addition to modern technology, education and preparation for the occurrence of tsunamis, local governments should also have an access to an emergency fund, whether it is provided by local or international governments. The synthesis conducted by Telford and Cosgrave on the 2004 tsunami from the Indian Ocean had revealed how chaotic and disorganized the transfer of funds can be in the occurrence of a disaster. They had also exposed how some donors manage to have a choice on where to spend their donations.

Having a local agency with well trained staff members is another excellent method for preparing for another tsunami disaster. Local agencies involved in social work can create alliances with governments from other countries, creating an organization that benefits all. Countries that are vulnerable to the devastation that can be caused by tsunamis should assemble themselves and organize standard operating procedures that would be shared with each other. In the end, all of the countries will benefit from advices from one another, while ensure strong alliances that can be counted on whenever one of them would be affected.

Upon organizing amongst themselves, these nations should also seek help from richer countries, such as the United States and the United Kingdom, just in case all of them become affected by tsunamis that reverberate through the oceans that they are connected to. The alliance could also deposit their funds in an international bank, to ensure the safety of their accounts, as well as for its fast retrieval during emergencies. In this way, the allocation of funds for disaster relief, reconstruction and recovery will be fast and accounted for.

5. References
Atwater BF, M Cisternas, J Bourgeois, WC Dudley, JW Hendley II and PH Stauffer. (2005). Surviving Tsunami—Lessons from Chile, Hawaii and Japan. USA: U.S. Geological Survey Information Services.

Bernard, E.N. (2007). National Oceanic and Atmospheric Administration. The
Tsunami Story. Retrieved October 24, 2007 from http://www.tsunami.noaa.gov/tsunami_story.html

Borrero, J.C. (2004). Field Survey Sumatra and Banda Aceh, Indonesia and after the Tsunami and Earthquake of 26 Dec 2004 University of Southern California, CA, USA.: Earthquake Engineering Institute

Kowalik, Z., Knight, W., Logan, T. & Whitmore, P. (2005). Numberical Modeling of the Global Tsunami: Indonesian Tsunami of 26 December 2004. Science of Tsunami Hazards, 23(1), 40 – 57.

Kurita, T., Nakamura, A., Kodama, M., Columbage, S.R.N. (2006). Tsunami public awareness and the disaster management system of Sri Lanka. Disaster Prevention and Management 15(1), 92-110.

National Oceanic and Atmospheric Administration. (2007). Physics of Tsunamis. Retrieved October 23, 2007 from http://wcatwc.arh.noaa.gov/physics.htm

Parras-Carayannis, G. (2004). Volcanic Tsunami Generating Source Mechanisms in the Eastern Carribean Region. Science of Tsunami Hazards 22(2), 74-115.

Telford, J. & Cosgrave, J. (2006). Joint Evaluation of the International Response to the Indian Ocean Tsunami: Synthesis Report. London: Tsunami Evaluation Coalition

Ward, S.N. & Asphaug, E. (1999). Asteroid Impact Tsunami: A Probabilistic Hazard Assessment. University of California, USA: Institute of Geophysics and Planetary Physics.

Ward, S.N. (2000). Landslide Tsunami. University of California, USA: Institute of Geophysics and Planetary Physics.

Watts, P. (2000). Tsunami Features of Solid Block Underwater Landslides. Journal of Waterway, Port, Coastal, and Ocean Engineering May/June 2000

Whitmore, P.M. (2003). Tsunami Amplitude Prediction During Events: A Test Based on Previous Tsunamis. Science of Tsunami Hazards, 21, 135-143.