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Tsunamis: Basic Principles

Samir G. Khoury, Ph.D., P.G.

This course on “Tsunamis” is presented as a complement to course G175 on “Earthquakes: Basic Principles” for the benefit of those students who want to learn more about this particularly devastating natural phenomenon. Course G175, however, is not a pre-requisite for taking this course. This course is a stand-alone presentation that can be taken independently of course G175.

In this course you will learn that any number of large-scale, short duration disturbances of the ocean floor can generate tsunamis. Most frequently, however, it is the strong, shallow submarine earthquakes that are the most likely triggering mechanism of these monstrous sea waves.

Following a brief explanation of the origin of the word, you will learn how to describe the geometry and general behavior of water waves and what distinguishes wind-generated waves from tsunamis. The geologic concept of plate tectonics is then presented, which explains that the earth’s crust is divided into a number of rigid plates that interact with one another causing seismic activity along their boundaries. In fact, it is the interaction between the oceanic and continental plates that most frequently trigger the large seismic events that deform the ocean floor and spawn the most devastating tsunamis. A step by step illustration of this process is presented along with an explanation of how the initial tsunami wave splits into two waves that start traveling in opposite directions. The wave that travels out to the deep ocean is known as the distant tsunami, while the other wave that travels towards the nearby coast is referred to as the local tsunami. The wave transformations that occur at shallow oceanic depths are explained as they control the ensuing wave run-up that occurs all along the coastline. The equations used to compute the velocity of tsunamis are presented in an appendix at the end of the course.

The characteristics of the most notable tsunamis that occurred since 1900 are presented and discussed. Experiences gained from the study of these events led the US National Oceanic and Atmospheric Administration (NOAA) to develop and deploy several tsunami-warning stations in the Pacific Ocean in the mid-1990s. When data collected from these stations confirm the detection of a tsunami, scientists begin immediately to predict the propagation course and velocity of the waves. Warnings are then issued to the most susceptible areas likely to be affected. Following the devastating tsunami of December 26, 2004, several additional tsunami detection stations have been added to the existing network.

Finally, other mechanisms that could also trigger tsunamis are presented and explained. Examples from the historical and geological records are presented as evidence that tsunamis have occurred in the past and are therefore one of the recurring and potent geologic hazards of our planet.

A glossary of terms and acronyms used is provided at the end of this course as a reference to assist the student in following the concepts that are discussed throughout the text.

The information presented in this course is based on the professional experience gained by the author in dealing with various aspects of seismic issues he dealt with in association with the major engineering projects he managed around the world.

This course includes a multiple choice quiz at the end, which is designed to enhance the understanding of the course materials.

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