Seismology and Earthquake Origin Questions and Answers

Answer: A

Correct answer: remains locked and then slips suddenly

Stick-slip is the alternation of long periods in which friction holds the fault locked while elastic strain accumulates in the surrounding rock, and brief episodes in which friction is overcome and the accumulated strain is released as slip. That sudden release is the earthquake. A fault that slid steadily instead would creep aseismically and produce no earthquakes at all.

Answer: B

Creeping sections release their strain gradually and therefore accumulate little to be released suddenly, so they generate few large earthquakes. Locked sections do the opposite, which is why identifying which is which matters for hazard.

Answer: C

Correct answer: accumulating strain to be released later

Locked means held by friction, not permanently fixed: the plates continue to move, so strain builds in the rock around the locked patch and must eventually be released in an earthquake. This is why locked segments are the ones that concern hazard analysts, while segments that creep steadily release their strain harmlessly as they go.

Answer: D

A larger earthquake means a larger area of fault slipping, so great earthquakes involve ruptures hundreds of kilometres long while a moderate one may break only a few kilometres. This is why the length of an active fault limits the magnitude it can produce.

Answer: A

Correct answer: length and width available to rupture

The seismic moment is the product of rupture area, average slip and rock rigidity, and moment magnitude is derived from it - so the biggest earthquake a fault can generate is set by how large an area can rupture at once. A short fault, or one with a narrow seismogenic width, simply cannot produce a great earthquake, which is why fault dimensions are mapped for hazard work.

Answer: B

Definitions vary but all rest on evidence of movement within a stated recent period, commonly the Holocene or Quaternary, on the reasoning that a fault that has moved recently may move again. Surface expression is useful evidence but not the criterion.

Answer: C

Correct answer: does not reach the ground surface

A blind fault terminates at depth, often beneath a fold, so it leaves no trace to be mapped at the surface. It is nonetheless capable of damaging earthquakes - the 1994 Northridge event is the standard example - which makes blind faults a serious problem for hazard assessment, since the usual method of mapping surface traces will miss them entirely.

Answer: D

Trenching across a fault reveals displaced and buried layers that can be dated, extending the record of large earthquakes back thousands of years. That matters because instrumental records are far shorter than the recurrence intervals of great events.

Answer: A

Correct answer: cover only about a century

Reliable instrumental records began around the start of the twentieth century, but great earthquakes on a given fault may recur only every few hundred or few thousand years. A century of data therefore contains at most one such event on most structures, and often none, which is why recurrence estimates rely on palaeoseismology and on historical accounts to extend the record.

Answer: B

Correct answer: how quickly strain accumulates on it

Slip rate is the long-term average rate at which the two sides of a fault move past each other, established from offset geological features or from geodetic measurement. It sets the pace of the earthquake cycle: a fault slipping at 20 mm a year accumulates a given slip deficit ten times faster than one at 2 mm a year, and so produces its earthquakes ten times as often.