28.2. Earthquake Magnitude

• Developed in 1935 by Charles F. Richter. • Based on the amplitude of seismic waves recorded on a Wood-Anderson seismograph. • Logarithmic scale: each unit increase corresponds to 10 times more amplitude and ~32 times more energy release. • Formula: M = log(A) − log(A₀) Where: – A = maximum amplitude of seismic waves – A₀ = amplitude for a standard earthquake at 100 km • Suitable for small to medium earthquakes (typically < 6.8).

The Richter Magnitude Scale was introduced in 1935 to provide a systematic way of measuring the energy produced by earthquakes. It uses a device called a seismograph to measure the seismic waves generated by an earthquake. The scale is logarithmic, meaning that for each whole number increase in the Richter scale, the amplitude of the earthquake waves increases by a factor of 10, and the energy released increases by about 32 times. For example, a 5.0 magnitude earthquake will produce waves that are ten times larger than those of a 4.0 magnitude quake. The formula used to calculate the magnitude includes the maximum amplitude of the seismic wave and a standard amplitude reference for earthquakes occurring 100 kilometers away. However, it is most effective for smaller to medium-sized earthquakes, typically those below a magnitude of 6.8, beyond which its readings may not be as reliable.

Think of the Richter Scale like a sound volume scale for music. If you increase the volume from 4 to 5, it’s 10 times louder. If you increase it from 5 to 6, it’s again 10 times louder than last level. Similarly, each step in magnitude is like turning up the music, with significant changes in impact.

• Currently the most widely used magnitude scale. • Based on seismic moment (Mo): M = log(M₀) − 10.7 Where: – M₀ = µ·A·D – µ = shear modulus of rocks (~30 GPa) – A = rupture area – D = average displacement • Provides consistent values across a wide range of earthquake sizes. • Does not saturate like the Richter scale for large earthquakes.

The Moment Magnitude Scale (Mw) has become the go-to method for measuring earthquake magnitudes today because it accounts for the actual physical properties of the earthquake. It is based on the seismic moment, which considers the area of the fault that slipped, the average distance it slipped, and the stiffness of rocks involved. This scale provides a more accurate measure of the earthquake's size, especially for larger quakes, where the Richter scale may reach its limits (saturate). Its logarithmic formula gives a more reliable value across various earthquake sizes.

Imagine measuring the size of a water balloon. The Moment Magnitude Scale measures not just how big the balloon looks from outside (like Richter) but also how much water fills it and how much pressure is inside. This way, you get a complete picture—just like when we measure earthquakes using Mw.

• Body Wave Magnitude (Mb): Based on P-wave amplitude. • Surface Wave Magnitude (Ms): Based on Rayleigh surface waves. • Duration Magnitude: Based on the duration of shaking. Each scale serves different purposes and may be used based on the nature of the seismic event and available data.

There are multiple scales used to measure earthquake magnitude, each suited to different types of seismic waves. The Body Wave Magnitude (Mb) uses the amplitude of P-waves, or primary waves, which are the fastest seismic waves. The Surface Wave Magnitude (Ms) looks at Rayleigh waves, which travel along the earth's surface and are responsible for much of the shaking. Additionally, the Duration Magnitude takes into account how long the shaking lasts. Depending on the location and type of earthquake, certain scales may be more appropriate than others.

Think of each magnitude scale as different tools for measuring a lumber's thickness. Just like you would use a caliper for precision measurements or a ruler for quick estimates, scientists choose different scales based on what they want to measure from an earthquake.

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