Geophysics Questions and Answers

Answer: C

Kepler's Second Law says the line joining a planet to the Sun sweeps out equal areas in equal intervals of time.

Because the orbit is an ellipse, the planet moves faster near perihelion, where the radius is short, and slower near aphelion, where it is long, so that the area swept per unit time stays constant. The law is a consequence of conservation of angular momentum.

'Equal area in equal orbits' and 'larger area in smaller orbits' talk about different orbits, but the law concerns one planet in its own orbit. 'Smaller area in larger time interval' contradicts it outright.

Answer: C

Seismic moment is Slip × Area × Elastic Modulus.

The seismic moment of an earthquake is the product of the rigidity (shear modulus) of the rock, the area of the fault that ruptured and the average slip on it:

\[ M_{0} = \mu A D \]

The moment magnitude is then calculated from the logarithm of \(M_{0}\). Slip times modulus leaves out the rupture area. The other two options use the P-S arrival time or amplitude, which belong to locating an earthquake or to the older Richter magnitude, not to the moment magnitude.

Answer: C

The core-mantle boundary lies at a depth of about 2900 km.

The boundary, also called the Gutenberg discontinuity, is at about 2,890 km. There P-wave velocity drops sharply and S-waves disappear, because the outer core below it is liquid. This is what produces the S-wave shadow zone beyond about 103° from an earthquake.

About 5,150 km is the depth of the inner core boundary, so 5500 km falls within the inner core region. 3900 km lies inside the outer core, and 1200 km is within the lower mantle.

Answer: C

The average densities are 2·7 gm/cm\(^{3}\) and 3 gm/cm\(^{3}\).

Continental crust is broadly granitic, rich in quartz and feldspar, with an average density of about 2.7 g/cm\(^{3}\). Oceanic crust is basaltic and gabbroic, richer in iron and magnesium, at about 3.0 g/cm\(^{3}\). This contrast is why continents stand high and oceanic crust sinks back into the mantle at subduction zones.

Option A reverses the two and inflates the first. 8 and 12 gm/cm\(^{3}\) are values for the core, not the crust. Option D gives gm/m\(^{3}\), a unit a million times too small.

Answer: B

These are Rayleigh waves.

In a Rayleigh wave, a surface wave, each particle moves in a vertical ellipse in the plane containing the direction of travel: it has a component along the direction of propagation and a component perpendicular to it, out of phase, so the ground rolls like an ocean wave.

Love waves (L-waves) are surface waves too, but their particle motion is horizontal and purely perpendicular to the direction of travel. P-waves and S-waves are body waves, with motion parallel and perpendicular to propagation respectively, not surface waves.

Answer: A

The Richter scale is based on the amplitude of seismic waves at a distance of 100 km.

Charles Richter defined local magnitude in 1935 as the logarithm of the maximum trace amplitude, in micrometres, recorded on a standard Wood-Anderson seismograph 100 km from the epicentre. Records at other distances are corrected to that reference, and each whole number step means a tenfold increase in amplitude.

Ground motion, velocity and the epicentre are not the measured quantity, and 200, 300 and 400 km are not the reference distance.

Answer: A

The ratio of P and S wave velocities remains same.

Both waves travel through the same rock, and the ratio of their speeds depends on its elastic properties, through Poisson's ratio. For most crustal and mantle rocks Vp/Vs is close to \(\sqrt{3}\), about 1.73. Because the S-wave is always slower by a nearly fixed proportion, the lag between P and S arrivals grows steadily with distance, which is how the S-P time is used to find the distance to an earthquake.

The ratio does not decrease logarithmically or linearly, nor increase periodically, with distance.

Answer: A

The geothermal gradient is about 17.17°C/km, the first option.

The gradient is the rise in temperature divided by the depth:

\[ \begin{aligned} G &= \frac{80 - 25}{3.2} \\ &= \frac{55}{3.2} \approx 17.2\ ^{\circ}\text{C/km} \end{aligned} \]

The exact quotient is 17.19, and 17.17°C/km is the option that matches it; the others differ by whole tens of degrees per kilometre. 07.17 would be obtained by ignoring most of the temperature rise, and 27.17 or 37.17 by adding to it, so none follows from the data.

Answer: D

The S-P interval increases with distance and the ratio of P and S velocities remains the same.

P waves travel faster than S waves, so the further a station is from the source, the more the S wave lags behind the P wave. This growing S-P interval is what allows epicentral distance to be estimated from a single station.

The ratio of P to S velocity depends on the elastic properties of the rock (about 1.73 for a Poisson solid) rather than on distance, so it stays nearly constant. The options in which the separation decreases or stays the same, or the ratio changes with distance, are therefore wrong.

Answer: B

The key gives thermal convection as the main energy source of the geodynamo.

The Earth's magnetic field is generated in the liquid iron outer core by convection: hot, buoyant fluid rises and cooler fluid sinks, and the Earth's rotation organises this motion into columns that sustain the field. Convection is driven by heat lost through the core-mantle boundary and by the light elements and latent heat released as the inner core freezes.

Mantle convection takes place in solid rock outside the core and controls only how fast the core loses heat. Rotation organises the flow but is not its energy source, and radioactivity in the core is generally thought to be small.