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Physics Of Tsunamis

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Running heading: Tsunamis

Physics of Tsunamis

Wendy M. Blevins

PS102 Explorations in Physics

Embry-Riddle Aeronautical University

Abstract

This paper will discuss the physics and warning systems of tsunamis, a destructive wave force that researchers have been studying for many years. Tsunamis are different than tides or surface waves because undersea earthquakes, instead of winds or the gravitational pull of the moon or sun, generate them. They can reach speeds of up to 700 kilometers per hour but can be undetected until they reach shallow water, then unexpectedly arise as deadly waves.

Tsunamis evolve from three physical processes, which are generation, propagation, and inundation of dry land. The propagation phase is the most understood, whereas generation and inundation are more difficult to model with computer simulations. Researchers apply a linear wave theory to the propagation phase, which assumes that the small height of the wave compared with the wavelength does not affect the wave's behavior. Their theory predicts that the deeper the water and longer the wave, the faster the tsunami. Upon inundation, the wave height is so high that the linear wave theory fails to describe the interaction between the water and shoreline.

Emergency planners have struggled with getting reliable confirmation of the existence of tsunamis. This has snowballed into a seventy-five percent false alarm rate since the 1950's. There are plans being put into place to upgrade the warning systems, but the success of improved safety will also depend on the people's response. The education of coastal communities on evacuation routes and procedures is crucial to improvement of the current tsunami emergency evacuation plans.

Physics of Tsunamis

To fully understand tsunamis, it will be helpful to first distinguish them from wind generated waves or tides. Ocean breezes can crinkle the surface into relatively short waves that create currents that are restricted to a shallow layer. Strong winds are able to whip up waves that are 30 meters or higher but even these do not move deep water as the tsunamis do. Tides, which sweep around the globe twice a day, also do not produce currents that reach the ocean bottom. Unlike true tidal waves, however, tsunamis are not generated by the gravitational pull of the moon or sun. A tsunami is produced by an undersea earthquake, or much less frequently, volcanic eruptions, meteorite impacts, or underwater landslides. Even though tsunamis can reach speeds of up to 700 kilometers per hour and its length can exceed 750 kilometers, a tsunami is not dangerous in deep water. The Japanese word tsunami translates literally as "harbor wave", because a tsunami can speed silently and undetected across the ocean, then unexpectedly arise as destructively high waves in shallow coastal waters.

Tsunamis have a very powerful, long reach. They can transport destructive energy from their source to coastlines thousands of kilometers away. Hawaii, because of its mid-ocean location, is especially vulnerable to tsunamis. Regardless of their origin, tsunamis evolve through three overlapping but distinct physical processes; generation by any force that disturbs the water column, propagation from deeper water near the source to shallow coastal areas, and inundation of dry land. Of these, the propagation phase is most understood, whereas generation and inundation are more difficult to model with computer simulations.

Generation is the process by which a seafloor disturbance, such as movement along a fault, reshapes the sea surface into a tsunami. Direct measurements of the seafloor motion have never been available but modelers assume that the displacement is identical to that of the ocean floor. Researchers use an idealized model of the quake and assume the crustal plates slip past one another along a simple, rectangular plane inside the earth. Even then, predicting the tsunami's initial height requires at least ten parameters that researchers must estimate, which is the reason that this first simulation frequency underestimates inundation, sometimes by factors of five or ten.

In the propagation phase, the tsunami transports seismic energy away from the earthquake site through undulations of the water, just as shaking moves the energy through the earth. At this point, researchers apply the linear wave theory, which assumes that the small height compared with the wavelength, does not affect the wave's behavior. The theory predicts that the deeper the water and longer the wave, the faster the tsunami. This dependence of wave speed on water depth means that any refraction by bumps and grooves on the seafloor can shift the wave's direction, especially as it travels into shallow water. The wave fronts tend to align parallel to the shoreline so they wrap around a protruding headland before smashing into it with greatly focused energy. At the same time, each individual wave must also slow down because of the decreasing water depth, so they begin to overtake on another, decreasing the distance between them in a process called shoaling. This squeezes the same amount of energy into a smaller volume of water, creating higher waves and faster currents.

The final stage of inundation, in which tsunamis run ashore as a breaking wave or wall of water, is perhaps the most difficult to model.

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