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examBoard: Cambridge
examType: IGCSE
lessonTitle: Magnitude Measurement Scales
When an earthquake strikes, scientists need reliable ways to measure how powerful it was. This isn't just for scientific records โ these measurements help emergency services understand how bad the damage might be and where to send help first. They also help engineers design buildings that can withstand future earthquakes in high-risk areas.
Key Definitions:
There are two main ways we measure earthquakes:
Magnitude scales (like Richter and Moment Magnitude) measure the energy released at the source of the earthquake. These give a single value for the whole earthquake.
Intensity scales (like the Mercalli Scale) measure the effects of the earthquake at different locations. The same earthquake can have different intensity readings in different places.
Different scales serve different purposes:
Magnitude scales help scientists compare earthquakes worldwide and understand the physics behind them.
Intensity scales help emergency services understand the actual impact on people and buildings in specific areas.
Some scales work better for certain types or sizes of earthquakes than others.
Developed by Charles Richter in 1935, the Richter Scale was the first widely used method to measure earthquake magnitude. It works by measuring the amplitude (height) of the largest seismic wave recorded on a seismograph.
The Richter Scale is logarithmic, which means each whole number increase represents a tenfold increase in the amplitude of the wave and approximately 31.6 times more energy release. For example, a magnitude 6.0 earthquake releases about 31.6 times more energy than a magnitude 5.0 earthquake and a magnitude 7.0 releases about 31.6 times more energy than a magnitude 6.0.
Felt by most people. Minor damage possible. Objects might fall off shelves.
Can cause serious damage in populated areas. Buildings may suffer structural damage.
Major earthquake. Can cause serious damage over large areas. Potential for complete building collapse.
While revolutionary for its time, the Richter Scale has important limitations:
Because of these limitations, scientists now prefer to use the Moment Magnitude Scale for most earthquakes, especially larger ones.
Developed in the 1970s, the Moment Magnitude Scale has largely replaced the Richter Scale for measuring earthquake magnitude. It's the scale you'll most often hear reported in the news today.
Unlike the Richter Scale, which measures the largest seismic wave, the Moment Magnitude Scale calculates the total energy released by an earthquake. It takes into account:
Like the Richter Scale, the Moment Magnitude Scale is logarithmic, with each whole number representing about 31.6 times more energy than the previous number.
The Moment Magnitude Scale offers several improvements:
When interpreting MMS readings:
While magnitude scales measure the energy released at the earthquake source, the Mercalli Intensity Scale measures the effects of an earthquake at different locations. It was developed by Giuseppe Mercalli in 1902 and later modified to the Modified Mercalli Intensity (MMI) Scale we use today.
Unlike the numerical Richter and Moment Magnitude scales, the Mercalli Scale uses Roman numerals from I to XII to describe the effects of an earthquake on people, structures and the environment. The same earthquake will have different Mercalli ratings in different locations, usually decreasing as you move further from the epicentre.
From barely perceptible (I) to noticeable shaking of indoor items (IV). Generally no damage.
From everyone feeling it (V) to difficulty standing (VII). Slight to moderate damage to well-built structures.
From structural damage (VIII) to total destruction (XII). Few if any structures remain standing at level XII.
This devastating earthquake struck off the east coast of Japan on March 11, 2011:
Tsunamis, often triggered by underwater earthquakes, have their own measurement scales. Unlike earthquakes, tsunamis are measured by their wave height and the damage they cause.
There are several scales used to measure tsunamis:
This scale measures the energy of a tsunami based on the maximum wave height. Like earthquake scales, it's logarithmic. The formula takes into account the maximum wave height and calculates a magnitude value.
For example, the 2004 Indian Ocean tsunami had an Mt of 9.1, while the 2011 Japan tsunami had an Mt of 9.1-9.3.
Similar to the Mercalli Scale for earthquakes, the Sieberg-Ambraseys Tsunami Intensity Scale measures the impact of tsunamis on land, structures and people. It ranges from I (Very light) to VI (Disastrous).
The 2004 Indian Ocean tsunami reached level VI in many coastal areas, with complete destruction of buildings and massive loss of life.
On December 26, 2004, a massive undersea earthquake triggered one of the deadliest natural disasters in recorded history:
Understanding the different measurement scales helps us interpret earthquake and tsunami reports more accurately:
By using both magnitude and intensity measurements together, scientists and emergency services can better understand both the power of tectonic events and their actual impact on communities. This helps with both immediate disaster response and long-term planning to reduce future risks.
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