Which of the following statements with reference to Richter scale is/are correct ? 1. It is the intensity scale of an earthquake. 2. Richter indicates the amount of energy released during the earthquake. Select the answer using the code given below :

Correct — B, 2 only. The Richter scale is a magnitude scale, and magnitude is precisely a measure of the energy an earthquake released — which makes the second statement right and the first one wrong. NCERT sets the two apart in consecutive sentences: the magnitude scale is the Richter scale, magnitude relates to the energy released during the quake and is expressed in numbers from 0 to 10; the intensity scale is named after Mercalli, takes account of the visible damage caused, and runs from 1 to 12. The physical difference matters more than the names. Magnitude is computed from the amplitude of the ground motion a seismograph records, corrected for how far the instrument sits from the source, so an earthquake has exactly one magnitude no matter where you measure it. Intensity is a judgement about consequences — cracked plaster, toppled chimneys, ground fissures — so a single earthquake has many intensities, high near the epicentre and fading outwards. Statement 1 hands the Richter name to the wrong job.

• (a)1 only — This swaps the two scales outright. Intensity belongs to Mercalli and is read off the damage; Richter reads the seismogram. Choosing this also throws away statement 2, which is the standard definition of magnitude.
• (c)Both 1 and 2 — The tempting pick, because both statements sound like textbook lines and a student who knows Richter measures 'how big' may accept either label. Only one scale can be the intensity scale, and it is Mercalli's.
• (d)Neither 1 nor 2 — Rejects statement 2, which is true by definition. Richter designed his scale in 1935 exactly so that a single number would stand for the size of the shock at its source.

An earthquake is described by two quite different numbers. Magnitude measures the event at its source — how much elastic energy the rupture released — and is worked out from the amplitude recorded on a seismograph after allowing for the distance to the focus. Intensity measures the effect at a place: what shook, what cracked, what fell. The Richter scale gives magnitude; the Modified Mercalli scale gives intensity in Roman numerals from I to XII. One earthquake, one magnitude, many intensities.

Papers like this one test whether the famous name has been filed under the right heading. The reason the confusion survives is that news reports say 'an earthquake of 6.5 on the Richter scale' in the same breath as describing collapsed buildings, so the number gets attached to damage in the reader's mind. Reading the two statements against each other helps: they cannot both describe the Richter scale, since one calls it an intensity scale and the other has it reporting energy. Once you know magnitude is the energy measure, statement 1 must go, and only (b) survives. Worth knowing for the present day: seismological agencies no longer report Richter values for large earthquakes. Charles Richter's original formula was calibrated for shallow, moderate shocks in southern California and saturates above about magnitude 7, so the USGS and others publish moment magnitude, written Mw, which is derived from the physical size of the rupture. Newspapers still print the words 'Richter scale' out of habit.

• NCERT: 'The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in numbers, 0-10.'
• NCERT: 'The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.'
• The Richter scale is logarithmic: one whole unit of magnitude means about ten times the recorded wave amplitude and roughly 31.6 times the energy release.
• Magnitude is a property of the earthquake; intensity is a property of a place, and falls off with distance from the epicentre.
• Large modern earthquakes are reported in moment magnitude (Mw), because Richter's 1935 formula saturates for the biggest events.

• Attaching the Richter name to damage. Damage is Mercalli's business.
• Treating the Richter scale as linear — an increase of two units is a hundredfold change in amplitude, not double.
• Assuming an earthquake has one intensity. It has one magnitude and a whole map of intensities.

Either as a two-statement item pairing the Richter and Mercalli names with the wrong definitions, or as a numerical asking what a one-unit rise on the scale means for amplitude or energy.

• practice — not a real PYQ

  The intensity of an earthquake, as distinct from its magnitude, is measured on which one of the following scales?

  • (a)Richter scale
  • (b)Modified Mercalli scale
  • (c)Beaufort scale
  • (d)Saffir-Simpson scale

  Answer(b) Modified Mercalli scale — it grades the visible damage from I to XII; Beaufort grades wind force and Saffir-Simpson grades hurricanes.

• practice — not a real PYQ

  An increase of one whole unit on the Richter scale corresponds to an increase in the recorded wave amplitude by a factor of

  • (a)2
  • (b)10
  • (c)31.6
  • (d)100

  Answer(b) 10 — the scale is logarithmic, so one unit is a tenfold jump in amplitude; the corresponding jump in energy is about 31.6 times.