Seismology and Earthquake Origin Questions and Answers

Answer: C

Correct answer: along plate boundaries

If plates are rigid and deform mainly at their edges, then the strain that earthquakes release must accumulate there, and about ninety per cent of global seismicity does lie along plate boundaries. Intraplate earthquakes do occur, and can be damaging because they strike regions with no tradition of seismic design, but they are the exception.

Answer: D

Correct answer: divergent boundary

At a divergent boundary the plates move apart, mantle material rises to fill the gap, and new oceanic crust is created - the process that builds the mid-ocean ridges. Convergent boundaries destroy crust, and transform boundaries neither create nor destroy it, so the three boundary types together keep the earth’s surface area constant.

Answer: A

Correct answer: the oceanic plate descends beneath the continental plate

Oceanic lithosphere is denser than continental, so where the two converge it is the oceanic plate that is driven down into the mantle. This produces the whole subduction assemblage: an oceanic trench, a dipping zone of earthquakes reaching several hundred kilometres, and a volcanic arc on the overriding continent.

Answer: B

Correct answer: transform boundary

The San Andreas is the boundary along which the Pacific plate slides north-westward past the North American plate, with motion horizontal and parallel to the fault. No crust is created or destroyed, which is what defines a transform. It is the type example of a continental transform and of the strike-slip earthquakes such boundaries produce.

Answer: C

The Indian plate continues to move northward and underthrust Eurasia, and the strain released along the resulting thrust systems has produced several great earthquakes. This makes the Himalayan belt the most seismically active part of India.

Answer: D

The belt of subduction zones around the Pacific accounts for the great majority of the world's earthquakes and a large share of its active volcanoes. The Alpide belt running through the Mediterranean and the Himalaya is the second most active.

Answer: A

Such events are far less frequent than boundary earthquakes but occur in stable continental regions where crust is strong and attenuation is low, so the shaking can be felt over enormous areas. The 1993 Latur earthquake in peninsular India is an Indian example.

Answer: B

Correct answer: normal fault

On a normal fault the hanging wall drops relative to the footwall, which lengthens the crust horizontally - so normal faulting is the signature of extension. The names come from mining: the hanging wall is the block above the inclined fault plane, the footwall the one below it, and a miner standing in the working would hang a lamp on the one and stand on the other.

Answer: C

Correct answer: compression

On a reverse fault the hanging wall rides up over the footwall, which shortens the crust horizontally. Shortening requires compression, so reverse faulting is characteristic of convergent settings - subduction zones and continental collisions such as the Himalaya. Normal faulting is the tensional counterpart, and strike-slip faulting the shear one.

Answer: D

Correct answer: horizontal and parallel to the fault strike

Strike is the horizontal direction of the fault trace, and on a strike-slip fault the two blocks slide past each other along it with little or no vertical component. The crust is neither shortened nor lengthened, only sheared. The San Andreas is the classic example, and such faults often show a long, remarkably straight surface trace.