Question

Explain the differences among the scales used to describe the size of an earthquake. (E)

Short answer

The main scales used to describe earthquake size are the Richter scale and the moment magnitude scale. The Richter scale, developed in 1935, measures the amplitude of seismic waves, with each whole number increase representing a tenfold increase in amplitude. However, it does not capture the overall energy released by an earthquake. In contrast, the moment magnitude scale, introduced in the 1970s, measures the total energy released by an earthquake and provides a more accurate and consistent measurement for earthquakes of all sizes. Consequently, the moment magnitude scale is widely preferred by seismologists today.

Step-by-step solution

Introducing the Richter Scale

The Richter scale, developed by Charles F. Richter in 1935, is a logarithmic scale used to measure the size of an earthquake. The scale takes into account the amplitude of the seismic waves, and since it's logarithmic, each whole number increase on the scale represents a tenfold increase in the amplitude of the waves. It is important to note that the Richter scale does not measure the overall energy released by the earthquake.

Introducing the Moment Magnitude Scale

The moment magnitude scale (Mw) is another logarithmic scale that was introduced in the 1970s to overcome the limitations of the Richter scale, especially for large-scale earthquakes. This scale takes into account the seismic moment, which is a measure of the total energy released by an earthquake. The moment magnitude scale is widely used by seismologists because it provides a more accurate and consistent measurement of the size of earthquakes.

Comparing the Richter Scale and the Moment Magnitude Scale

While both the Richter scale and the moment magnitude scale are logarithmic and use similar numerical values, they measure different aspects of an earthquake. The Richter scale measures the amplitude of seismic waves, whereas the moment magnitude scale measures the total energy released by the earthquake. The Richter scale is generally limited to measuring local or regional earthquakes and may not give accurate measurements for large-scale earthquakes. The moment magnitude scale, on the other hand, provides more precise and reliable measurements for earthquakes of all sizes.

Conclusion

In conclusion, the scales used to describe the size of an earthquake are primarily the Richter scale and the moment magnitude scale. While both scales use a logarithmic progression to measure the size, they differ in their methodology and what they measure. The Richter scale measures the amplitude of seismic waves, whereas the moment magnitude scale focuses on the total energy released by the earthquake. The moment magnitude scale is currently the preferred method for measuring the size of earthquakes due to its accuracy and consistency.

Key concepts

Richter Scale

When discussing earthquake measurements, the Richter Scale is historically significant. Introduced by Charles F. Richter in 1935, it quantifies the size of an earthquake by observing the amplitude of seismic waves recorded on a seismograph. What sets it apart is its logarithmic nature; each whole number increase on the scale corresponds to a tenfold increase in amplitude.

This means a magnitude 5 earthquake results in shaking with an amplitude ten times greater than a magnitude 4. While monumental at its inception, the Richter Scale is now mostly obsolete, superseded by more comprehensive scales. It's primarily useful for smaller, more localized seismic events and isn't accurate for the largest tremors.

Moment Magnitude Scale

The Moment Magnitude Scale (M_w) was developed in the 1970s as an evolution to the Richter Scale and is currently favored by seismologists. Its calculation involves the seismic moment of an earthquake, which is a combination of the distance a fault moves and the force required to move it. Like the Richter, it's also a logarithmic scale, ensuring each step up represents a significantly larger earthquake.

The true strength of the Moment Magnitude Scale lies in its ability to measure the total energy released by an earthquake, providing a more accurate depiction of an earthquake's size, especially for the most massive seismic events. It's now the standard for quantifying earthquake sizes and is regularly reported by geological institutes around the world.

Seismic Waves

Seismic waves are the energy waves that travel through the Earth's layers following an earthquake. They originate at the earthquake's focus, the point inside the Earth where the seismic event begins, and travel outward in all directions. There are different types of seismic waves: Primary (P) waves, which are compressional and can travel through both solid and liquid layers; Secondary (S) waves, which are shear and can only move through solids; and surface waves that cause the most damage are the ones we feel during an earthquake. These waves are what seismographs record, allowing for the measurement of an earthquake's magnitude.

Seismic Moment

Seismic moment is a fundamental concept in understanding earthquakes. It reflects the total amount of energy released during an earthquake. The calculation of seismic moment takes into account the rigidity of the Earth's crust, the amount of slip on the fault, and the size of the area that slipped. Seismologists use this calculation to assign a single number, the moment magnitude, which provides a consistent and scale-independent estimate of an earthquake's size, making the Moment Magnitude Scale a more universally applicable tool compared to the Richter Scale.

Logarithmic Scale

Both the Richter Scale and the Moment Magnitude Scale are logarithmic scales, a concept that can initially be tricky to grasp. In a logarithmic scale, each step or number represents a value that is a specific multiple of the previous one, rather than a simple addition. For example, on a base-10 logarithmic scale, an increase by 1 equates to a multiplication of the previous number by 10. This is particularly helpful when dealing with a wide range of quantities, such as the energy released by earthquakes, which can vary significantly. The logarithmic nature allows us to succinctly represent and compare very large differences in earthquake energies.