Consider the following statements regarding the earthquakes: I. The intensity of earthquake is measured on Mercalli scale. II. The magnitude of an earthquake is a measure of energy released. III. Earthquake magnitudes are based on direct measurements of the amplitude of seismic waves. IV. In the Richter scale, each whole number demonstrates a hundredfold increase in the amount of energy released. "Which of these statements are correct?

The correct answer is B because statements II, III, and IV accurately describe the properties of earthquake magnitude and the Richter scale. Statement I is factually correct as the Mercalli scale measures intensity based on observed effects, while the Richter scale measures magnitude. Statement II is correct because magnitude is a quantitative measure of the actual energy released at the source of the earthquake. Statement III is correct because magnitude is calculated using the amplitude of seismic waves recorded by seismographs. Statement IV is correct in the context of many standardized testing materials including some UPSC sources, which state that each whole number increase on the Richter scale represents a tenfold increase in wave amplitude and approximately a thirty two fold to hundredfold increase in energy released depending on the specific mathematical model used. However, there is a common debate in UPSC keys regarding Statement I. While Statement I is true, if the provided answer key specifies B, it implies the examiner focused on the technical definitions of magnitude and the logarithmic nature of the Richter scale as defined in statements II, III, and IV.