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Tsunamis: Generation and Propagation
Dr. Samir G. Khoury, Ph.D, P.G.
Overview
This course on “Tsunamis:
Generation and Propagation” is presented as a complement to course C-3008 on “Earthquakes
and Tsunamis: Fundamental Concepts” for the benefit of those students who
want to learn more about this particularly devastating natural phenomenon.
Whereas Course C-3008 addressed primarily the topic of earthquakes, this course
addresses primarily the topic of tsunamis. This course is a stand-alone
presentation and Course C-3008 is not a pre-requisite. For continuity, this
course contains a few descriptive sections about tsunami effects that are
included in course C-3008. However, the emphasis here is on explaining the
mechanical principles of tsunami generation and propagation.
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.
The
student must take a multiple-choice quiz consisting of twenty (20) questions at the end of this course
to earn PDH credits.
Specific Knowledge or Skill Attained
In this course you will learn
that large earthquakes that affect the ocean floor are capable of generating
immense sea waves called “tsunamis”, a Japanese word that means “harbor wave”.
You will also understand why this is an appropriate descriptive term for this
phenomenon. In the open ocean these waves have relatively low heights, but are
of enormous longitudinal dimension (at right angle to the direction of
propagation). As they approach the coast, these waves undergo significant
transformations that determine the ensuing wave run-up above mean sea level
along the coastline. Tsunamis can travel at great speed for very large distances
such as across the entire widths of oceans, inflicting significant damage to far
away coastal towns. You will specifically learn:
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How the wavelength, wave
height and period are used to describe the geometry and behavior of wind
generated waves and tsunamis,
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That wind generated waves
and tsunamis are easily distinguishable,
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That the earth’s crust is
formed of interlocking rigid plates that interact with each other,
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That the cycles of strain
accumulation and slippage along these plate boundaries is the most common
mechanism for the triggering of large earthquakes and tsunamis,
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How the near shore
transformations of tsunamis determine the ultimate run-up of the waves above
the coastal mean sea level,
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That the devastating
effects of tsunamis led NOAA to develop an early tsunami warning system that
was first deployed in the Pacific Ocean in the mid-1990s,
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That the initiation of a
tsunami can be readily detected by the monitoring stations of the warning
system,
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How to compute the
propagation velocity of a tsunami in open water,
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That tsunamis can also be
generated by massive submarine landslides, the collapse of marine volcanoes,
and even by the impact of large meteorites or asteroids, and
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That tsunamis occurred
periodically throughout the geologic record indicating that these events
represent one of the most potent recurring natural hazards of our planet.
Course
Click on the following PDF attachment to review
the course prior to taking a
quiz for credit.
Tsunamis: Generation and Propagation (693 KB)
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