Final answer:
Seiches and tsunamis are classified as shallow-water waves due to their long wavelengths, not because of where they travel. The distance between tsunami wave crests at sea can be calculated based on the given speed and frequency, resulting in 150 km. When waves superimpose, they add their disturbances according to the principle of superposition.
Step-by-step explanation:
Despite moving across the deepest ocean basins, seiches and tsunamis are referred to as "shallow-water waves" not because of where they travel but because of their characteristics. Specifically, the classification as shallow-water waves is due to their long wavelengths relative to the water depth. Shallow-water wave theory states that when the depth of the water is much less than the wavelength, the speed of the wave is dependent on the depth of the water.
In the context of a tsunami's wave crest distances at sea, given that a tsunami travels at a speed of 600 km/h and has one wave crest approaching every 15 minutes, we can find the distance between crests. As there are 4 waves per hour, the distance between these crests (wavelength) can be calculated by multiplying the speed (600 km/h) by the time interval for one wave (1/4 hour), leading to a result of 150 km between successive wave crests at sea.
When waves superimpose on one another, they do so by adding their disturbances (options c). This is known as the principle of superposition, whereby individual waves pass through each other without being permanently affected, merely summing their disturbances at each point they meet.