Seismology and Earthquake Origin Questions and Answers

Answer: C

Correct answer: stops distinguishing between events of increasing size

The Richter scale is measured from a wave of a particular period, and for a very large earthquake the rupture lasts far longer than that period, so the amplitude at that period stops growing even as the rupture gets bigger. Above about magnitude 7 the scale therefore compresses, and events of quite different size return similar values - which is why great earthquakes are quoted as moment magnitude.

Answer: D

The product of these three quantities describes the physical size of the source, which is why moment magnitude tracks great earthquakes faithfully. A long rupture with large slip gives a very large moment however the waves happen to be recorded.

Answer: A

Correct answer: 10

The magnitude scale is the base ten logarithm of amplitude, so one unit is a tenfold increase in the amplitude recorded on the seismogram. The factor of about 32 belongs to energy rather than amplitude, and the two are constantly confused - the energy grows faster because it depends on the square of the amplitude and on the duration of the rupture.

Answer: B

Energy rises as roughly the three-halves power of the amplitude ratio, giving about thirty-two times per magnitude unit and about a thousand times for two units. That is why a magnitude 8 event dwarfs a magnitude 6 rather than merely exceeding it.

Answer: C

It states that the logarithm of the number of events exceeding a magnitude falls linearly with that magnitude, so small earthquakes vastly outnumber large ones. The slope is the b-value and is close to one in most regions.

Answer: D

Correct answer: 1.0

The Gutenberg-Richter relation states that the logarithm of the number of earthquakes above a given magnitude falls linearly with magnitude, and the slope \( b \) is close to 1 in most regions. A \( b \) of one means that for every earthquake of magnitude 6 there are about ten of magnitude 5 and a hundred of magnitude 4, which is why the smallest events overwhelmingly outnumber the rest.

Answer: A

Correct answer: foreshock

A foreshock is a smaller event preceding a larger one on the same structure, and it can only be identified as such after the larger event has happened - which is exactly why foreshocks have proved so disappointing as a prediction tool. Aftershocks follow the mainshock and are far more numerous, and microseisms are continuous background vibration from ocean waves and wind.

Answer: B

They arise as the surrounding rock adjusts to the redistributed stress, are most frequent immediately after the mainshock and decay with time. They are a serious hazard because they can collapse already damaged structures.

Answer: C

Correct answer: tectonic

The great majority of earthquakes, and virtually all the damaging ones, come from sudden slip on faults as accumulated tectonic strain is released. The other classes exist but are minor: volcanic earthquakes are small and local, collapse earthquakes smaller still, and induced events from reservoirs, mining or fluid injection are limited in size by the volume involved.

Answer: D

Correct answer: collapse earthquakes

When the roof of a mine working or a natural cavern falls, the sudden movement of rock radiates seismic waves and is recorded as an earthquake. Such events are small and shallow and their effects are local, but they are genuine earthquakes by any instrumental definition - they are simply not driven by tectonic strain.