Understanding the Richter Scale for Earthquake Magnitude

The question asks us to identify the statement that is FALSE about the Richter scale. Let's analyze each statement based on our knowledge of how earthquakes are measured using the Richter scale.

Analyzing Each Statement about the Richter Scale

• Statement A: It was developed by Charles Richter and Gutengberg in 1935.

  This statement is largely correct. The magnitude scale, often referred to as the Richter magnitude scale, was indeed developed by Charles F. Richter in collaboration with Beno Gutenberg at the California Institute of Technology in 1935. So, this statement is TRUE.

• Statement B: It is a logarithmic scale.

  This is a fundamental characteristic of the Richter scale. It is a base-10 logarithmic scale. This means that an earthquake of magnitude 5 is ten times larger in wave amplitude recorded by a seismometer than an earthquake of magnitude 4, and releases approximately 32 times more energy.

  Mathematically, the magnitude $M$ is related to the amplitude $A$ of the largest wave recorded by a seismometer at a standard distance by the formula:

  $$M = \log_{10} A + \text{correction factor}$$

  The correction factor accounts for the distance from the epicenter. Because it uses a logarithm, this statement is TRUE.

• Statement C: It can be measured using a seismometer.

  Yes, the data needed to calculate the Richter magnitude comes directly from seismographs (or seismometers). These instruments record the ground motion produced by seismic waves. The amplitude of the waves recorded by the seismometer is used in the calculation. Therefore, this statement is TRUE.

• Statement D: Magnitude of 8-9 on a Richter scale means it is a micro earthquake.

  This statement is incorrect. Earthquake magnitudes are categorized based on their effects and the energy released. A 'micro earthquake' is typically defined as an earthquake with a very low magnitude, often less than 2.0, which is usually not felt by people and is only detected by sensitive seismometers. Earthquakes with magnitudes ranging from 8.0 to 9.9 are classified as 'Great' earthquakes. These are capable of causing tremendous destruction over wide areas. Therefore, a magnitude of 8-9 signifies a major or great earthquake, not a micro earthquake. This statement is FALSE.

Conclusion on the False Statement

Based on the analysis, the statement that is FALSE about the Richter scale is that a magnitude of 8-9 means it is a micro earthquake. Magnitudes in this range represent very large, powerful earthquakes.

Revision Table: Key Facts about Richter Scale

Additional Information: Earthquake Magnitude Scales and Effects

While the Richter scale was historically significant, it has limitations, particularly for very large earthquakes. Today, seismologists commonly use the Moment Magnitude Scale ($M_w$) as the standard measure for larger earthquakes because it better reflects the total energy released. However, the concept of magnitude and the use of seismometers remain central to earthquake measurement.

Here's a general classification of earthquakes based on magnitude:

• Micro: Less than 2.0 (Not usually felt)
• Minor: 2.0 - 2.9 (Not usually felt, but recorded)
• Minor: 3.0 - 3.9 (Often felt, but rarely causes damage)
• Light: 4.0 - 4.9 (Noticeable shaking, minor damage possible)
• Moderate: 5.0 - 5.9 (Can cause significant damage to poorly constructed buildings)
• Strong: 6.0 - 6.9 (Can be destructive in populated areas)
• Major: 7.0 - 7.9 (Causes serious damage over large areas)
• Great: 8.0 and higher (Can cause widespread destruction and loss of life)

As you can see from this classification, a magnitude of 8-9 falls into the 'Great' category, not 'Micro'.