The Richter scale is used to measure which of the following?

The correct answer is B, S waves. Secondary or S waves are transverse, so the particles of the medium move at right angles to the direction of travel. A transverse wave needs a medium that resists a change of shape, and a liquid does not, so S waves stop at the liquid outer core and are not recorded beyond about 103 degrees from the epicentre.

Option A, P waves, are longitudinal waves of compression and rarefaction and pass through solids, liquids and gases alike, which is why they are always the first to be recorded. Options C and D, Love and Rayleigh waves, are surface waves; they travel only along the crust and are the slowest and the most destructive, but the question is about passage through liquids inside the earth, which they never attempt. The absence of S waves in the shadow region is the standard proof that the outer core of the earth is in a liquid state.

The correct answer is C, Focus. The focus, also called the hypocentre, is the point below the surface at which the strained rock ruptures and releases its stored elastic energy as seismic waves. Its depth decides how widely the shock is felt: shallow-focus earthquakes are more damaging near the source than deep-focus ones of the same magnitude.

Option A, the epicentre, is the point on the ground surface lying vertically above the focus, so it is a surface point and not the place of origin. Option B, a fault line, is the fracture in the crust along which the blocks move; the focus lies on a fault but the fault itself is a line or plane, not a point. Option D, the shadow zone, is the belt on the far side of the earth, between about 103 and 142 degrees, which receives no direct P or S waves. The pair focus and epicentre is the most frequently asked distinction in this chapter.

The correct answer is C, Andaman and Nicobar Islands. Barren Island lies in the Andaman Sea, to the east of the main Andaman group, and is the only active volcano in India and in South Asia. It sits on the belt where the Indian plate is being pushed under the Burma plate, the same subduction zone that produced the great undersea earthquake of December 2004.

Option A, Lakshadweep, is a chain of coral atolls built on submerged volcanic foundations of the geological past, with no active vent today. Option B, the Gulf of Kutch, lies in a highly earthquake-prone region of Gujarat that falls in seismic Zone V, but it has no volcano. Option D, the Sundarbans, is a deltaic and mangrove region formed by the Ganga and Brahmaputra and is entirely sedimentary. Note also that Narcondam, close to Barren Island, is classed as dormant, and it is the usual distractor in this question.

The correct answer is A, Seismograph. A seismograph carries a mass that stays nearly still while the ground and the frame move under it, and the relative motion is traced as a wavy line. The trace itself is called a seismogram, and the gap between the arrival of the P wave and the S wave on it is used to work out how far the station lies from the epicentre. Readings from three or more stations fix the epicentre.

Option B, a barograph, is a self-recording aneroid barometer that draws a continuous graph of atmospheric pressure. Option C, a hygrometer, measures the humidity of the air, the wet and dry bulb type being the common one. Option D, an altimeter, is an aneroid barometer graduated to show height above sea level and is used in aircraft and by mountaineers. Instrument-and-use questions are asked directly, so learn the four together rather than one at a time.

The correct answer is B, Italy. Mount Vesuvius stands near the Bay of Naples in southern Italy and its eruption in 79 CE buried the Roman towns of Pompeii and Herculaneum under ash, preserving them so well that they became an archaeological record of Roman daily life. It is a composite or strato volcano built of alternating layers of ash and lava.

Option A, Greece, has volcanic islands such as Santorini in the Aegean, whose ancient eruption is linked with the end of the Minoan civilisation, but not Vesuvius. Option C, Turkey, lies on the Alpine earthquake belt and has extinct volcanic peaks such as Mount Ararat, again not Vesuvius. Option D, Spain, has volcanic activity only in the Canary Islands off Africa. Italy is worth remembering as a cluster: Vesuvius near Naples, Etna in Sicily, and Stromboli in the Lipari Islands.