Earthquakes and Volcanoes » Magnitude and the Richter Scale

What you'll learn this session

Study time: 30 minutes

How earthquake magnitude is measured using the Richter scale
The logarithmic nature of the Richter scale
How to interpret Richter scale readings
The difference between magnitude and intensity
Alternative measurement scales for earthquakes
The relationship between magnitude and earthquake impacts
Case studies of significant earthquakes and their magnitudes

Understanding Earthquake Magnitude

When an earthquake strikes, one of the first things reported is its magnitude. But what exactly does this number mean and how is it calculated? In this session, we'll explore how scientists measure the power of earthquakes and what these measurements tell us about their potential impacts.

Key Definitions:

Magnitude: A measure of the energy released during an earthquake.
Richter Scale: A logarithmic scale developed in 1935 by Charles Richter to measure earthquake magnitude.
Seismograph: An instrument that detects and records ground movements during earthquakes.
Seismic Waves: Vibrations that travel through the Earth following an earthquake.

The Richter Scale Explained

The Richter scale was developed by American seismologist Charles Richter in 1935. It was initially designed to compare the sizes of earthquakes in Southern California, but it soon became used worldwide. The scale measures the amplitude (height) of the largest seismic wave recorded on a seismograph.

📈 Logarithmic Scale

The Richter scale is logarithmic, not linear. This means that each whole number increase on the scale represents a tenfold increase in the measured amplitude of the earthquake and approximately a 31.6-fold increase in energy released.

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 earthquake releases about 31.6 times more energy than a magnitude 6.0 (or about 1,000 times more energy than a magnitude 5.0).

📊 Calculating Magnitude

The Richter scale magnitude (M) is calculated using this formula:

M = log₁₀A - log₁₀A₀

Where A is the maximum amplitude measured by the seismograph and A₀ is a standard reference value that depends on the distance between the seismograph and the earthquake.

Don't worry - you won't need to calculate this for your exam! Just understand that it's based on measurements from seismographs.

Interpreting the Richter Scale

The Richter scale has no upper limit, but in practice, the largest recorded earthquakes have been just over magnitude 9.0. Here's how to interpret different magnitudes:

🔻 Minor (Less than 4.0)

Often felt by people, but rarely cause damage. Millions of these occur worldwide each year.

You might notice objects shaking slightly, but structural damage is unlikely.

⚠ Moderate (4.0-5.9)

Can cause damage to poorly constructed buildings but limited damage to well-designed structures.

At the higher end of this range, even well-built structures may experience some damage.

⚡ Major (6.0+)

Can be destructive in populated areas. Magnitude 6.0-6.9 earthquakes can be devastating locally.

Magnitude 7.0+ earthquakes can cause serious damage over larger areas and are considered major events.

Beyond the Richter Scale

While the Richter scale is the most well-known measure of earthquake magnitude, scientists now use more sophisticated scales for different types of earthquakes and distances:

📝 Moment Magnitude Scale (Mw)

The Moment Magnitude Scale has largely replaced the Richter scale for measuring large earthquakes. It measures the total energy released by an earthquake and provides more accurate readings for larger earthquakes.

This scale is based on the physical properties of the earthquake, including the area of the fault that ruptured and how much the rocks moved.

When you hear about earthquake magnitudes in the news today, they're usually moment magnitudes, even if they're still referred to as "Richter scale" readings.

💭 Magnitude vs. Intensity

It's important to distinguish between magnitude and intensity:

Magnitude is the measure of energy released at the source of the earthquake. It's a single value for each earthquake.
Intensity measures the effects of an earthquake at a specific location. The same earthquake will have different intensities in different places, usually decreasing with distance from the epicentre.

Intensity is often measured using the Modified Mercalli Intensity Scale, which uses Roman numerals from I to XII to describe observed effects.

Factors Affecting Earthquake Impact

While magnitude is important, the impact of an earthquake depends on several factors beyond just its magnitude:

Depth: Shallow earthquakes (less than 70km deep) typically cause more damage than deeper ones of the same magnitude.
Distance from epicentre: The closer to the epicentre, the stronger the shaking.
Ground conditions: Soft soils can amplify shaking, while solid bedrock minimises it.
Building design: Earthquake-resistant structures can withstand stronger shaking.
Population density: The same earthquake will cause more casualties in densely populated areas.
Time of day: Earthquakes during the day when people are active may lead to different outcomes than those at night.

Case Study Focus: 2011 Tōhoku Earthquake and Tsunami, Japan

On 11 March 2011, a magnitude 9.0-9.1 earthquake struck off the coast of Japan. It was one of the most powerful earthquakes ever recorded worldwide and the most powerful to ever hit Japan.

Key facts:

The earthquake triggered powerful tsunami waves that reached heights of up to 40.5 metres.
The tsunami caused nuclear accidents at the Fukushima Daiichi Nuclear Power Plant.
Nearly 20,000 people died, mainly from the tsunami rather than the earthquake itself.
The earthquake was so powerful it moved Japan's main island, Honshu, 2.4 metres east and shifted the Earth on its axis by an estimated 10-25 cm.
Economic damage was estimated at $235 billion, making it the costliest natural disaster in history.

This case study demonstrates how a high-magnitude earthquake can have devastating consequences, especially when combined with secondary hazards like tsunamis.

Comparing Significant Earthquakes by Magnitude

To put earthquake magnitudes in perspective, here are some significant earthquakes from history:

Year	Location	Magnitude	Deaths	Notes
1960	Valdivia, Chile	9.5	1,655-6,000	Largest earthquake ever recorded
2004	Indian Ocean	9.1-9.3	230,000+	Caused devastating tsunami affecting multiple countries
2011	Tōhoku, Japan	9.0-9.1	19,747	Triggered tsunami and nuclear disaster
2010	Haiti	7.0	100,000-316,000	High death toll despite lower magnitude due to poor building standards and high population density
1989	Loma Prieta, USA	6.9	63	Relatively low death toll despite occurring in San Francisco Bay Area due to better building standards

Notice how the relationship between magnitude and death toll isn't always straightforward. The Haiti earthquake had a much lower magnitude than the Chile earthquake but resulted in far more deaths due to factors like building quality, population density and emergency response capabilities.

Key Points to Remember

The Richter scale is logarithmic - each whole number increase represents a tenfold increase in amplitude and roughly 31.6 times more energy.
Modern seismologists often use the Moment Magnitude Scale rather than the Richter scale, especially for larger earthquakes.
Magnitude measures the energy released at the source, while intensity measures the effects at a specific location.
The impact of an earthquake depends on many factors beyond just magnitude, including depth, local geology, building standards and population density.
Earthquakes below magnitude 4.0 are usually felt but rarely cause significant damage.
Earthquakes above magnitude 6.0 can cause serious damage and those above 7.0 are considered major events.
The largest earthquakes ever recorded have been just over magnitude 9.0.

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