My thoughts about the potentially vulnerability of the US to a man-made tsunami were spawned by a History Channel special I saw shortly after the East Asian disaster, which chronicled the historical record of these random freaks of nature, caused by tectonic tension amassed over thousands of years and released in one instantaneous underwater explosion of energy, displacing water on a massive scale with the reverberations felt throughout the body of water in which it originated.
This type of tsunami is unpredictable, and as far as modern technology goes, entirely unpreventable. Though scary, when the earth moves, humans will always be at the whim of the violent forces of nature. However, the most frightening segment of the hour-long feature came toward the end and concerned tsunami waves created by massive landslides down the sides of mountains which provide sufficient force in an adjacent body of water to displace huge amounts and send it careening violently at heights of 160+ feet destroying everything in its path.
The expose did not go into depth about the possibility of humans harnessing these destructive forces of nature, but such a possibility must cause any reflective individual to take pause at the realization that a nuclear explosion is unnecessary to unleash the basic principals of gravity which act upon and provide the energy behind landslides. It is conceivable to me that the surgical placement of charges within the underlying foundation of just such a vulnerable land formation could trigger a landslide on the eastern shores of Asia, or those of western Europe or Africa, which would send a tidal wave of apocalyptic proportions barreling toward the Eastern Seaboard, wreaking havoc virtually without warning on the heart of the American economy.
Another possibility is that such a mode of attack could go weeks, months, even years to fully unravel the cause of, if sufficient evidence is even recoverable at all. Whether or not there is a reasonable policy option for implementing defensive measures in anticipation of such a threat I do not know, but I doubt that is much that could be accomplished in the near term based on limitations in technological capacity. However, no time is better to start drawing up contingency plans than the present.