20.12.1. Magnitude Scales

Magnitude represents the energy released at the source of the earthquake.

The term 'magnitude' refers to a numerical measure that captures the total energy released during an earthquake. This energy is crucial for understanding how powerful an earthquake is, as it influences the potential damage and effects on structures and the environment.

Think of magnitude like the volume of music. Just as louder music can fill a room and affect how we feel in it, a higher magnitude earthquake indicates a larger release of energy that can cause more destruction.

Richter Scale (ML): Logarithmic scale introduced in 1935 by Charles Richter. Each increase by 1.0 unit represents a tenfold increase in amplitude and ~31.6 times more energy release.

The Richter Scale, developed by Charles Richter in 1935, is a way to quantify the size of earthquakes. It is logarithmic, meaning that each whole number increase on the scale corresponds to a tenfold increase in measured amplitude (the height of the waveform on a seismograph) and approximately 31.6 times more energy released. For instance, an earthquake of magnitude 5 is ten times larger in amplitude and roughly 31.6 times as energetic as a magnitude 4 earthquake.

Consider the Richter Scale like measuring temperature in Celsius – each degree can feel very different. If you increase your drink’s temperature from 20°C to 30°C, it feels much warmer, similar to how every point on the Richter Scale increases perceived size and energy of an earthquake significantly.

Moment Magnitude Scale (Mw): More accurate for large earthquakes, based on seismic moment (fault area × slip × rock rigidity).

The Moment Magnitude Scale (Mw) is more modern and is used for measuring larger earthquakes. It is based on the seismic moment of an earthquake, which is determined by three factors: the area of the fault that slipped, the average amount of slip during the earthquake, and the rigidity of the rocks involved. This scale provides a more accurate measure of an earthquake's size compared to the Richter scale, particularly for very large events.

Imagine you are measuring the area of a puddle, where traditional methods only measure how deep it is. The Moment Magnitude Scale is like using a comprehensive method that accounts for the size of the puddle (area) and how deep it is, giving a fuller picture of its volume.

Body Wave (Mb) and Surface Wave (Ms) Magnitudes: Used for different types of waves but now largely replaced by Mw.

Body Wave (Mb) and Surface Wave (Ms) magnitudes were earlier used to assess the energy of earthquakes by focusing on P-waves (primary waves that travel through the Earth) and S-waves (secondary waves that travel along the surface). However, these measures have been largely replaced by the Moment Magnitude Scale because it gives a better overall picture of the energy released. The distinction is essential because different waves carry different types of information about the earthquake's source.

Consider Body Waves like the sound of a train passing underground, which is felt deeply, while Surface Waves are like the vibrations you feel on the surface when the train passes overhead. Both can show how strong the earthquake can be, but the Moment Magnitude Scale provides a better overall measure of its total energy.

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