Earth Structure and Physical Properties Questions and Answers

Answer: C

The reservoir-triggered earthquake is the 1967 Koyna earthquake.

The Koyna Dam in Satara district, Maharashtra, was impounded in 1962. Seismic activity increased after filling, and on 11 December 1967 a magnitude 6.3 earthquake struck, killing about 180 people. It is the largest known reservoir-triggered earthquake, and seismicity in the Koyna-Warna region still follows the annual loading and unloading of the reservoirs.

The 1934 Bihar-Nepal and 1950 Assam (Assam-Tibet) earthquakes were great tectonic earthquakes of the Himalayan collision zone. The 1995 Kobe earthquake was caused by slip on an active strike-slip fault in Japan. None was triggered by a reservoir.

Answer: C

The difference is about 27,000 times.

Energy released rises by a factor of about \(10^{1.5} \approx 31.6\) for each whole unit of magnitude, from \(\log E = 1.5M + \text{constant}\). A difference of 3 units gives:

\[ \begin{aligned} \frac{E_{7}}{E_{4}} &= 10^{1.5 \times 3} = 10^{4.5} \\ &\approx 31{,}600 \end{aligned} \]

Using the rounded factor of 30 per unit, as textbooks often do, gives \(30^{3} = 27{,}000\), the option offered. 30 times is the factor for one unit only, and 900 times for two. 3,000 is not a power of the factor at all. Ground-motion amplitude, not energy, rises by 10 per unit, which would give 1,000.

Answer: B

The point where energy is released is the hypocentre (focus).

An earthquake begins at the point within the Earth where the rupture starts and seismic energy is first released. This is the hypocentre, or focus, and it may be a few kilometres to about 700 km deep.

The epicentre is the point on the Earth's surface directly above the hypocentre, which is where shaking is usually reported, but it is not where the energy is released. Circumcentre is a term from geometry, and 'isocentre' is not a seismological term; lines of equal intensity are isoseismals.

Answer: C

The S wave reflected from the outer core is ScS.

In seismic phase notation, S is a shear wave in the mantle and a lower-case c marks a reflection from the core-mantle boundary, the top of the outer core. So ScS is an S wave that travels down through the mantle, reflects off the outer core and returns as an S wave. PcP is the equivalent compressional phase.

SKP is an S wave that converts to a P wave (K) in the liquid outer core and emerges as a P wave; it passes through the core rather than reflecting from it. 'SOcS' and 'ScOS' are not standard phase names.