Seismology and Earthquake Origin Questions and Answers

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: seismicity concentrates along plate boundaries, and deep focus events are essentially confined to the cold descending slabs of subduction zones. Statement 3 is wrong - mid-ocean ridge earthquakes are shallow and mostly small, because the crust there is thin, hot and weak, so only a small area can store strain and rupture at once.

Answer: D

Correct answer: 1 and 3 only

Statements 1 and 3 are correct: an earthquake has one magnitude, since magnitude describes the source, and the scale is logarithmic. Statement 2 confuses magnitude with intensity - the distance correction is applied precisely so that stations at different distances arrive at the same magnitude, and a systematic dependence on distance would mean the correction was wrong.

Answer: A

Correct answer: Erosion of a river valley

Erosion removes load slowly over geological time and does not produce the sudden slip that radiates seismic waves. The other three all do: tectonic fault movement accounts for almost all significant earthquakes, magma movement produces volcanic events, and impounding a deep reservoir can induce seismicity by raising pore pressure, as at Koyna.

Answer: B

Mercalli intensity comes from what was felt and what was damaged, not from a seismogram, which is why it is the more subjective of the two measures and can be applied to earthquakes that predate instruments. The other three statements are correct.

Answer: C

Correct answer: A greater focal depth

A deeper focus means the waves travel further through rock to reach the surface and so arrive attenuated and spread over a wider area, which reduces the damage at any one place. The other three all worsen it: proximity to the epicentre, poor construction, and soft alluvium that amplifies the ground motion and may liquefy.

Answer: D

The Mid-Atlantic ridge is where plates move apart, the San Andreas is where they slide past one another, and the Sumatra trench is where one descends beneath the other. Each boundary type produces a characteristic depth and size of earthquake.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

Three terms describing three different things. The focus is the point at depth where rupture begins. The epicentre is its vertical projection on the surface. An isoseismal is a contour drawn on a map joining places that experienced the same intensity, so it belongs to the description of effects rather than of the source.

Answer: B

Correct answer: false, because the ratio is about 32

Energy scales as \( 10^{1.5} \) per magnitude unit, giving a factor of about 32 rather than 2. The error usually comes from treating the logarithmic scale as if one unit meant a doubling. Even the factor of ten, which is the amplitude ratio, would understate the energy difference by a factor of three.

Answer: C

Correct answer: false, since the epicentre lies on the surface

The epicentre is by definition a point on the ground surface, vertically above the focus. It is the focus that lies inside the earth and where rupture begins. The two terms are constantly interchanged in casual reporting, and the distinction matters because focal depth is one of the main controls on how damaging a given magnitude turns out to be.

Answer: D

Correct answer: 2, 1, 3, 4

The elastic rebound cycle begins with the gradual accumulation of strain as the plates move while the fault stays locked. When friction is overcome the fault ruptures suddenly and the strain is released. Aftershocks then adjust the stresses around the rupture over weeks to years, and the cycle begins again as strain reaccumulates for the next event.