19.5. Earthquake Magnitude and Intensity

Quantifies the energy released at the source.

• Richter Scale (ML): Logarithmic; not effective for very large earthquakes.
• Moment Magnitude Scale (Mw): Based on seismic moment; more accurate and widely used.

Magnitude is a way of measuring the energy released by an earthquake at its source. There are two main scales used for measuring magnitude:

1. Richter Scale (ML): This scale measures magnitude on a logarithmic scale, which means that each whole number increase on the scale represents a tenfold increase in measured amplitude. However, it is less effective for very large earthquakes, where other scales are preferred.

2. Moment Magnitude Scale (Mw): This is the more modern and widely used scale, as it is based on the seismic moment of a quake, making it more accurate for all sizes of earthquakes. It takes into account the area of the fault that slipped, the amount of slip, and the rigidity of the rocks involved.

Think of measuring the volume of water using two different containers. The Richter Scale is like using a small cup that can only handle small amounts of water accurately. If you pour in too much, it overflows, similar to how the Richter scale can underestimate large earthquakes. On the other hand, the Moment Magnitude Scale is like using a bucket that can accurately measure any amount of water—small or large—without overflowing.

Qualitative measure of effects on people, structures, and the Earth's surface.

• Modified Mercalli Intensity (MMI) Scale: Ranges from I (not felt) to XII (total destruction).
• Varies with distance from epicenter and local site conditions.

Intensity assesses the effects of an earthquake rather than the energy released. It looks at how people feel the shaking, the damage done to buildings, and the changes to the landscape. The Modified Mercalli Intensity (MMI) Scale is commonly used for this purpose, ranging from I, where the earthquake is not felt, to XII, which indicates total destruction. Intensity can vary widely depending on the earthquake's distance from the epicenter (the point on the Earth's surface directly above where the earthquake originates) and local conditions, such as soil type and structure behavior.

This means that an earthquake could be measured the same in terms of magnitude but have very different effects in different locations based on these factors.

Imagine a rock thrown into a calm pond. The splash created near the rock is much bigger than the ripples felt at the edge of the pond. Similarly, people close to the epicenter of an earthquake may experience intense shaking and damage, while those farther away feel less or even none at all, depending on where they are located and the ground conditions.

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