Richter Scale and Moment Magnitude Scale

Earthquake measurement scales are tools used to quantify the energy released during an earthquake. These scales provide a standardized method to compare earthquake size, making it easier for scientists and the public to understand the magnitude of these natural events.

Measuring earthquakes is crucial for several reasons. Firstly, it allows seismologists and other scientists to develop their understanding of earthquakes and their causes. Secondly, it enables us to make meaningful comparisons between earthquakes, aiding in planning and risk assessment. Lastly, it helps evaluate the effectiveness of earthquake management.

This blog will focus on two primary scales used to measure earthquakes: the Richter scale and the Moment Magnitude scale. The Richter scale, developed by Charles F. Richter in the 1930s, measures the amplitude of seismic waves recorded by seismographs. On the other hand, the Moment Magnitude scale, introduced in the 1970s, measures the total energy released by an earthquake. Both scales have unique characteristics and applications, which we will explore in detail.

The Richter Scale

The Richter scale, also known as the Richter magnitude scale, was developed in 1935 by Charles F. Richter in collaboration with Beno Gutenberg. This scale was a significant advancement in seismology, providing an objective measure of the energy an earthquake releases by quantifying the seismic waves produced.

The Richter scale was originally devised to measure the magnitude of earthquakes of moderate size (that is, magnitude 3 to magnitude 7) by assigning a number that would allow the earthquake size to be compared with another. The scale was developed for temblors occurring in southern California that were recorded using the Wood-Anderson seismograph and whose epicenters were less than 600 km (373 miles) from the location of the seismograph.

On the original Richter scale, the smallest earthquakes measurable at that time were assigned values close to zero on the seismograph of the period. Since modern seismographs can detect seismic waves even smaller than those originally chosen for zero magnitude, it is possible to measure earthquakes having negative magnitudes on the Richter scale.

How Richter Scale Works

The magnitude of an earthquake is determined using the logarithm of the amplitude of the largest seismic wave recorded by seismographs. Adjustments are included to compensate for the variation in the distance between the various seismographs and the earthquake epicenter. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude.

Richter Scale Limitations

Despite its usefulness, the Richter scale has its limitations. It is most accurate for small, local earthquakes but less so for large, distant earthquakes. The Richter scale underestimates the energy released in earthquakes with magnitudes greater than 6.5, as the values calculated after measuring large seismic waves tend to cluster, or “saturate,” near one another2. Furthermore, the Richter scale does not reflect the impact of vertical movement, which can be the wave movement causing the greatest amount of damage8. Modern scientific practice has replaced the original Richter scale with other, more accurate scales, such as the Moment Magnitude scale. However, the Richter scale is still often mentioned in news reports of earthquake severity as the catch-all name for the logarithmic scale upon which earthquakes are measured.

The Moment Magnitude Scale

The Moment Magnitude Scale (Mw) is an earthquake’s magnitude measure based on its seismic moment. This scale was introduced in the 1970s by Thomas C. Hanks and Hiroo Kanamori to overcome the limitations of the Richter scale2. The Moment Magnitude Scale is considered the authoritative magnitude scale for ranking earthquakes by size.

How Moment Magnitude Scale Works

The Moment Magnitude Scale is based on the total moment release of the earthquake. The seismic moment is a physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that slips. It is related to the total energy released in the earthquake. The moment can be estimated from seismograms and also from geodetic measurements. The moment is then converted into a number similar to other earthquake magnitudes by a standard formula. The result is called the moment magnitude.

Why is Moment Magnitude Scale More Accurate and Reliable

The Moment Magnitude Scale is more accurate and reliable than the Richter scale, especially for large, distant, or deep earthquakes. This is because the Moment Magnitude Scale considers the fault’s geometry and the earthquake’s seismic moment. It provides a more accurate accounting of the total energy released by an earthquake. Unlike the Richter scale, the Moment Magnitude Scale does not underestimate magnitudes as other scales do in certain conditions. It provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales.

Comparing the Richter Scale vs Moment Magnitude Scale

Richter scale and Moment Magnitude scale are used to measure earthquake magnitude, but they do so in different ways and are most accurate under different circumstances.

The Richter scale measures the amplitude of seismic waves recorded by seismographs. It is a logarithmic scale, meaning that each whole number increase in magnitude represents a tenfold increase in the measured amplitude of seismic waves. However, the Richter scale is most effective for regional earthquakes no greater than magnitude 5. For larger earthquakes, the Richter scale tends to underestimate the magnitude.

On the other hand, the Moment Magnitude scale is based on the total moment release of the earthquake. The seismic moment is a physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that slips; it is related to the total energy released in the earthquake. The Moment Magnitude scale is more effective for large earthquakes as it uses more variables to calculate the energy released using seismic moment. It provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales.

An example of the difference between the two scales can be seen in the measurement of the Chile earthquake of 1960. This earthquake was registered as 8.6 on the Richter scale, but the Moment Magnitude for this event was measured at 9.54. This discrepancy illustrates the limitations of the Richter scale for very large earthquakes and the more accurate representation of earthquake size provided by the Moment Magnitude scale.

Conclusion

In this blog post, we have explored the two primary scales used to measure earthquakes: the Richter scale and the Moment Magnitude scale. We learned that the Richter scale, developed in 1935, measures the amplitude of seismic waves recorded by seismographs. However, it is most accurate for small, local earthquakes and less so for large, distant earthquakes.

The accurate measurement of earthquakes is of paramount importance. It not only helps us understand these natural phenomena better but also aids in forecasting and preparing for future earthquakes. As our understanding of earthquakes evolves, so too will our methods of measuring them, ensuring we are better prepared to deal with these powerful natural events.

References:
1) USGS. Moment magnitude, Richter scale - what are the different magnitude scales, and why are there so many? Accessed on 01-Aug-2024. Available at: https://www.usgs.gov/faqs/moment-magnitude-richter-scale-what-are-different-magnitude-scales-and-why-are-there-so-many.
2) Quick-Advices. 2019. What are the disadvantages of Richter like magnitude scales? Accessed on 31-Jul-2024. Available at: https://quick-advices.com/what-are-the-disadvantages-of-richter-like-magnitude-scales/.
3) Britannica. Richter scale. Accessed on 01-Aug-2024. Available at: https://www.britannica.com/science/Richter-scale