Seismology and Earthquake Origin Questions and Answers

Answer: A

Correct answer: a shallower dip angle

Both are compressional faults with the hanging wall moving up, and the distinction is one of geometry: a thrust dips at a shallow angle, conventionally less than about thirty degrees, while a reverse fault is steeper. The shallow dip matters because a thrust can carry rock a long horizontal distance and can produce a very large rupture area, and hence a very large earthquake.

Answer: B

Correct answer: a segment of an active fault that has not slipped recently

The reasoning is that strain accumulates steadily along an active fault, so a segment that has stayed quiet while its neighbours have ruptured has been storing strain for longer and is closer to failure. It is a statement about relative likelihood over decades, not a prediction of date, and a segment that has just ruptured is the least likely to fail again soon.

Answer: C

Magnitude describes the earthquake itself, so a given event has one magnitude and the value does not depend on where the measurement was made. Intensity is the separate idea of how strongly the ground shook at a chosen location.

Answer: D

Correct answer: One

Magnitude is a property of the source: it describes the size of the rupture, so there is one value for the earthquake however many stations record it. In practice different stations give slightly different estimates and the reported figure is an average, but that is measurement scatter. Intensity is the quantity that legitimately differs from place to place.

Answer: A

Correct answer: the local shaking and its observed effects

Intensity is assessed from what happened at a particular place - what people felt, how objects moved, what damage the buildings suffered - so it varies across the affected region. It depends on magnitude and on distance, but also on focal depth, on the local soil, and on how well the buildings were built, which is why two towns at equal distance can record different intensities.

Answer: B

Because it rests on observation rather than instruments it is inevitably more subjective than a magnitude scale, which is computed from records. Its value is that it can be applied to historical earthquakes for which no instrumental data exist.

Answer: C

Roman numerals are used deliberately to signal that the scale is descriptive and ordinal rather than measured, running from imperceptible at I to total destruction at XII. The convention avoids implying a precision the observations cannot support.

Answer: D

Isoseismal maps show how intensity fell away from the source and reveal the influence of geology, since soft ground produces a bulge of higher intensity. The others are lines of equal pressure, temperature and rainfall.

Answer: A

Correct answer: the amplitude of the largest seismic wave recorded

Richter defined his local magnitude from the logarithm of the maximum trace amplitude on a standard instrument, corrected for distance. It is therefore an amplitude measure, not an energy or a rupture-area measure. Moment magnitude, which is derived from the fault area and the slip, was introduced later precisely because the amplitude definition breaks down for the largest events.

Answer: B

Moment magnitude is derived from the seismic moment, which combines rock rigidity, rupture area and average slip, so it keeps increasing as ruptures get bigger. The Richter scale saturates because the single wave amplitude it uses stops growing.