Seismology and Earthquake Origin Questions and Answers

Answer: A

Impounding a deep reservoir both loads the crust and raises pore water pressure at depth, which reduces the effective stress holding a fault and can trigger slip that was already close to occurring. The Koyna region in India is the most studied example.

Answer: B

Correct answer: small and confined to the volcanic area

Volcanic earthquakes are produced by magma forcing its way through rock and by the fracturing that accompanies it, so they are limited in size by the volume of the magmatic system and are concentrated beneath and around the volcano. That local concentration is what makes them useful: swarms of them are one of the best short-term indicators that an eruption is approaching.

Answer: C

Named for the seismologist who identified it from a change in wave velocity, the Moho lies at roughly 35 km under continents and much shallower under oceans. The Gutenberg discontinuity is the deeper mantle-core boundary.

Answer: D

Correct answer: asthenosphere

The asthenosphere is the weak, partially molten upper mantle layer on which the rigid lithospheric plates move. It is the mechanical contrast between the two - brittle above, ductile below - rather than any change in chemistry that makes plate tectonics possible, and it is why the lithosphere can be treated as a set of rigid plates at all.

Answer: A

Refraction and reflection at boundaries, and the failure of shear waves to pass through liquid, revealed the layered structure and the liquid outer core. Seismology remains the principal means of probing depths no drill can reach.

Answer: B

Correct answer: the orientation of the fault and the sense of slip

A fault plane solution uses the pattern of first motions recorded around the earthquake - which stations moved up first and which moved down - to find the two possible orientations of the fault and the direction of slip on it. It reveals the mechanism of the event, and by extension the stress regime, but it says nothing about future earthquakes.

Answer: C

Correct answer: average interval between events of a given size

Return period is a long-term average, obtained from the historical and geological record: if events of magnitude 7 have occurred on average every 250 years, that is the return period. It is emphatically not a countdown - an earthquake is no less likely the year after one occurs than the year before the average interval is up.

Answer: D

No precursor has proved dependable, so practice relies on hazard assessment, which states the probability of shaking over decades, and on engineering the structures to survive it. Forecasting a probability is a different matter from predicting an event.

Answer: A

Correct answer: the probability of shaking over a long period

Hazard assessment answers a probabilistic question: what level of ground motion has a given chance of being exceeded at this site in fifty years, or in the design life of the structure. It is what codes are written from. Prediction, which would name the place, time and size of a specific future event, remains beyond reach, and the two should never be confused.

Answer: B

Correct answer: north-north-east

The Indian plate is converging with Eurasia at roughly 45 to 50 mm per year in a north to north-north-easterly direction. That convergence is what raised the Himalaya and what continues to load the thrust faults along its front, making the Himalayan belt one of the most seismically hazardous regions on earth.