Earthquakes and Volcanoes: Waves, Belts and Types

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 B, magnitude of energy released by an earthquake. The Richter scale, devised by Charles F. Richter in 1935, expresses the energy released at the source of the shock. It is logarithmic and open-ended, so each whole number means about ten times the wave amplitude and roughly thirty-two times the energy of the number below it, and a single earthquake has only one magnitude value.

Option A describes the modified Mercalli scale, which grades the shaking and damage actually felt at a place in twelve degrees from I to XII, and which gives many values for the same earthquake. Option C, the depth of the focus, is worked out from the arrival times of P and S waves at several stations and is not what any named scale reports. Option D, the height of tsunami waves, is measured directly by tide gauges and ocean buoys. Keep the pair fixed: Richter for magnitude, Mercalli for intensity.

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.

The correct answer is C, Stromboli. Stromboli is a small island volcano in the Lipari or Aeolian group north of Sicily, and it has been in almost continuous mild eruption for centuries. The glow of its frequent bursts of incandescent material can be seen at night far out to sea, which earned it the name Lighthouse of the Mediterranean.

Option A, Mount Etna, also in Italy on the island of Sicily, is the tallest active volcano in Europe and erupts often, but the lighthouse epithet belongs to Stromboli. Option B, Krakatoa, lies in the Sunda Strait of Indonesia between Java and Sumatra, far from the Mediterranean, and is remembered for its catastrophic eruption of 1883. Option D, Mount Fuji or Fujiyama, is the composite cone of Japan and the standard textbook example of that type. This nickname is a favourite one-line question, so tie it to Stromboli alone.

The correct answer is B, Primary waves. P waves are longitudinal, moving the particles of the medium back and forth along the direction of travel by compression and rarefaction. They are the fastest of all seismic waves and therefore always reach a station first and appear first on the seismogram, which is exactly why they are named primary.

Option A, secondary or S waves, are transverse and travel more slowly, so they arrive after the P waves; the time gap between the two arrivals is used to calculate the distance to the epicentre. Options C and D, surface waves, which include Love and Rayleigh waves, are generated when body waves reach the surface; they are the slowest of all and arrive last, yet they cause the heaviest damage because of their long rolling motion along the ground. The order of arrival to remember is P, then S, then surface waves.

The correct answer is C, Pacific Ocean. The circum-Pacific belt, popularly called the Ring of Fire, runs from the Andes of South America up through Mexico, the western coast of North America and the Aleutians, then down through Kamchatka, Japan, the Philippines and Indonesia to New Zealand. It follows the subduction zones where the Pacific plate is pushed beneath the surrounding plates, so it carries the greatest share of the world's active volcanoes and of its earthquakes.

Option A, the Atlantic Ocean, has the Mid-Atlantic ridge, where plates move apart and give quiet basaltic eruptions as in Iceland, but far fewer explosive volcanoes. Option B, the Indian Ocean, has volcanic activity along the Sunda arc and at islands such as Barren Island and Reunion, but no comparable ring. Option D, the Arctic Ocean, is the least active of the four. Link the Ring of Fire to subduction along the Pacific margin.

The correct answer is D, Shield volcanoes. Basic lava is poor in silica, very hot and thin, so it travels a long way from the vent before it solidifies. Layer upon layer of such flows builds a wide dome with slopes of only a few degrees, shaped like a warrior's shield laid on the ground, of which Mauna Loa in Hawaii is the textbook example. Eruptions of this type are usually quiet rather than explosive.

Option A, composite or strato volcanoes such as Fujiyama and Vesuvius, are built of alternating layers of ash and viscous acidic lava and therefore rise as steep cones. Option B, cinder cones, are small steep hills built only of loose fragments thrown out around a vent. Option C, caldera volcanoes, are the most explosive kind, named after the wide hollow left when the summit collapses into an emptied magma chamber. The key idea to carry into the exam is that thin lava makes gentle slopes and thick lava makes steep ones.