Seismology and Earthquake Origin Questions and Answers

Answer: A

Tectonic movement loads the rock elastically until its strength is exceeded, at which point it ruptures and the stored strain energy is released as seismic waves and heat. That is why earthquakes recur on the same faults as strain accumulates again.

Answer: B

Reid explained the 1906 San Francisco earthquake by rock on either side of a fault deforming elastically until it snapped back to an unstrained position. Richter and Mercalli gave their names to scales, and Gutenberg worked on magnitude-frequency statistics.

Answer: C

Correct answer: relative displacement of the two sides

A fracture alone is a joint; what makes it a fault is that the rock on one side has moved relative to the other. That displacement is the whole point, because it is the sudden slip on a fault that generates an earthquake. Melting, chemical change and density change may accompany deformation but none of them defines a fault.

Answer: D

Correct answer: focus

The focus, or hypocentre, is the point at depth where rupture nucleates. Slip then spreads outward from it over the fault plane, so the focus marks where the earthquake began rather than where most of the energy came from. The epicentre is its projection on the surface, and an isoseismal is a contour of equal intensity drawn on a map.

Answer: A

Correct answer: on the surface vertically above the focus

The epicentre is a purely geometric construction: drop a vertical line from the focus and the point where it meets the surface is the epicentre. It is convenient for mapping but it is not necessarily where the worst damage occurs, since damage depends on soil, on construction and on the direction the rupture propagated as well as on distance.

Answer: B

It is measured vertically from the point of rupture up to the surface, and it governs how concentrated the shaking is. A shallow focus delivers its energy to a small area and does far more damage than a deep one of the same magnitude.

Answer: C

Correct answer: shallow focus

The conventional divisions are shallow to about 70 km, intermediate from 70 to 300 km, and deep below 300 km. Shallow earthquakes matter disproportionately: the energy has less rock to travel through before it reaches the surface, so for a given magnitude a shallow event shakes the ground far more strongly than a deep one.

Answer: D

A shallow rupture releases its energy close to the surface, so the waves have little distance in which to spread and attenuate before they reach structures. A deep earthquake of equal magnitude produces weaker but more widely felt shaking.

Answer: A

Correct answer: subduction zones

Below about 70 km rock is normally too hot and too confined to fail brittly, so deep earthquakes should not happen at all. They occur only where a cold slab of oceanic lithosphere is being carried down into the mantle, keeping brittle conditions to depths of several hundred kilometres. The distribution of deep foci therefore maps the descending slab, and was among the first strong evidence for subduction.

Answer: B

Correct answer: a number of rigid lithospheric plates

The essential idea of plate tectonics is that the lithosphere behaves as a set of rigid plates moving over the weaker asthenosphere, with deformation concentrated at their edges rather than spread through their interiors. That is exactly why earthquakes cluster along narrow belts instead of occurring uniformly - the belts are the plate boundaries.