Storm Surge and Tsunami Effects Questions and Answers

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: thrust earthquakes are the commonest generator, and submarine landslides - often triggered by an earthquake - can produce very large local tsunamis. Statement 3 is the reverse of the truth: purely horizontal movement displaces little water vertically, which is why a strike-slip earthquake of a given magnitude generates a much smaller tsunami than a thrust of the same size.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A tsunami is generated by the vertical displacement of the water column, and strike-slip motion slides the two blocks past each other horizontally with little vertical offset, so little water is displaced. A thrust earthquake lifts one block over the other and displaces the whole column above it, which is why the great tsunamigenic earthquakes are all subduction thrusts.

Answer: C

Correct answer: A-2, B-4, C-1, D-3

Four sources with four characters. A subduction thrust ruptures over hundreds of kilometres, so its wave is long and travels ocean-wide with little dispersion. A submarine landslide has a short source, giving extreme local runup that disperses quickly with distance. A volcanic collapse displaces a large water volume suddenly - Krakatoa and Anak Krakatau are the examples. A meteorite impact is vanishingly rare and potentially enormous.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Wave dispersion depends on the ratio of wavelength to depth, and a short source produces relatively short waves that spread and lose height rapidly as they travel. So a landslide tsunami may produce runup of tens of metres within a few kilometres - Lituya Bay is the extreme case - and be barely detectable across the ocean, unlike a thrust earthquake tsunami.

Answer: A

Wavelengths of hundreds of kilometres dwarf even the deepest ocean basins. Celerity therefore depends on the depth throughout its whole journey. Nothing about it resembles a wind generated wave. The whole water column takes part in the motion.

Answer: B

The square root of gravity times depth gives about two hundred metres per second. That is comparable to the cruising speed of a commercial aircraft. Ocean basins are crossed in a matter of hours. Warning across an ocean is correspondingly tight.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason supplies the figure. In deep water the celerity is the square root of g times the depth, which at 4000 metres gives about 200 metres per second - the speed of a jet airliner. A tsunami therefore crosses the Pacific in about a day and reaches a coast a thousand kilometres away in under two hours, which is what makes the warning problem so difficult.

Answer: D

Green's law gives the growth in height as the depth falls. The period is conserved throughout, which is why the wave shortens as it slows. Period is the one property conserved throughout. Height rises as wavelength and speed both fall.

Answer: A

It is the shallow water limit of the shoaling relation. A hundredfold reduction in depth therefore roughly triples the wave height. The exponent is small but the depth change is large. Shoaling alone can triple the wave height.

Answer: B

The number of crests passing a point per unit time cannot change. Wavelength and celerity both fall while the height grows. Conservation of period follows from simple continuity. Crest count per unit time cannot alter.