In describing the way a seafloor disturbance such as movement along a fault reshapes the s

In describing the way a seafloor disturbance such as movement along a fault

reshapes the sea surface into a tsunami, modelers assume the sea-surface

displacement is identical to that of the ocean bottom, but direct measurements

Line of seafloor motion have never been available. Researchers presently use an

(5) idealized model of the quake: they assume that the crustal plates slip past one

another along a simple, rectangular plane. As modelers scramble to guide

tsunami survey teams immediately after an earthquake, only the orientation of

the assumed fault plane and the quake's location, magnitude and depth can be

interpreted from the seismic data alone.

(10) As all other parameters must be estimated, this first simulation frequently

underestimates inundation, which can signify that the initial tsunami height was

also understated when the single-plane fault model distributes seismic energy

over too large an area. Analyses of seismic data cannot resolve energy

distribution patterns any shorter than the seismic waves themselves, which

(15) extend for several hundred kilometers, but long after the tsunami strikes land,

modelers can work backward from records of run-up and additional earthquake

data to refine the tsunami's initial height. For example, months of aftershocks

eventually reveal patterns of seismic energy that are concentrated in regions

much smaller than the original, single-plane fault model assumed. When seismic

(20) energy is focused in a smaller area, the vertical motion of the seafloor-and

therefore the initial tsunami height-is greater. Satisfactory simulations are

difficult, but improve immeasurably scientists' ability to make better

predictions.

Propagation of the tsunami transports seismic energy away from the

(25) earthquake site through undulations of the water, just as shaking moves the

energy through the earth. At this point, the wave height is so small compared

with both the wavelength and the water depth that researchers can apply linear

wave theory, which predicts that the velocity of tsunami increases with the

depth of the water and the length of the wave. This dependence of wave speed

(30) on water depth means that refraction by bumps and grooves on the seafloor can

shift the wave's direction, especially as it travels into shallow water. In

particular, wave fronts tend to align parallel to the shoreline so that they wrap

around a protruding headland before smashing into it with greatly focused

incident energy. At the same time, each individual wave must also slow down

(35) because of the decreasing water depth, so they begin to overtake one another,

decreasing the distance between them in a process called shoaling. Refraction

and shoaling squeeze the same amount of energy into a smaller volume of water,

creating higher waves and faster currents. In the last stage of evolution,

inundation and run-up, in which a tsunami may run ashore as a breaking wave or

(40) a wall of water or a tide-like flood, the wave height is now so large that it is

difficult to assess the complicated interaction between the water and the shoreline.

The primary function of the

A．introduce a new explanation of a physical phenomenon

B．explain how a physical phenomenon is measured and described

C．illustrate the limitations of applying mathematics to complicated physical phenomena

D．indicate the direction that research into a particular physical phenomenon should take

E．clarify the differences between an old explanation of a physical phenomenon and a new model of it