单项选择题

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 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. 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
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
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 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 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 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 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.

According to the model presented in the passage, which of the following normally determines the initial height of a tsunami（）

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