Question

Explain the difference between magnitude and intensity as expressions of the size of an earthquake.

Short answer

The difference between magnitude and intensity of an earthquake lies in their definitions, measurements, and variations. Magnitude is a measure of the seismic energy released at the earthquake's source and is measured on a logarithmic scale such as the Richter scale or moment magnitude scale. It provides an objective measure of the size of an earthquake. Intensity describes the effects or impacts of an earthquake at specific locations, considering factors like local geology and infrastructure. It varies from place to place and is measured on scales such as the Modified Mercalli Intensity scale. While magnitude represents the overall size of the seismic event, intensity gives a more subjective assessment of the impact and damage experienced at specific locations.

Step-by-step solution

Define Magnitude #

Magnitude is a measure of the energy released at the source of the earthquake. It is a logarithmic scale based on the amplitude of the seismic waves, which are the vibrations caused by an earthquake. The most commonly used scale for measuring magnitude is the Richter scale, developed in 1935 by Charles F. Richter. The scale ranges from 0 to 10, with each whole number representing a tenfold increase in the amplitude of the seismic waves. This means that a magnitude 7 earthquake is 10 times more powerful than a magnitude 6 earthquake and 100 times more powerful than a magnitude 5 earthquake. Another widely used scale for measuring magnitude is the moment magnitude scale (Mw), which is more accurate for larger earthquakes.

Define Intensity #

Intensity, on the other hand, describes the effects or impacts of an earthquake at a specific location, taking into account the local geology, distance from the earthquake's epicenter, and human infrastructure. Intensity is a subjective measure and varies from place to place, depending on factors such as the type of buildings and their construction. The Modified Mercalli Intensity (MMI) scale is a widely used scale for measuring intensity. It ranges from I (not felt) to XII (total destruction), with values assigned based on observations and reports from people who experienced the earthquake and the extent of the damage.

Comparing Magnitude and Intensity #

Now that we have defined magnitude and intensity, let's compare them: 1. Measurement: Magnitude is a measure of the seismic energy released at the source, whereas intensity measures the effects of the earthquake at a specific location. 2. Scale: Magnitude is measured on a logarithmic scale, such as the Richter scale or the moment magnitude scale. Intensity is measured on a scale that evaluates the effects of the earthquake, such as the Modified Mercalli Intensity scale. 3. Variation: Magnitude is a single value for any given earthquake, representing the overall size of the seismic event. Intensity varies from place to place within the affected area, depending on distance from the epicenter, local geology, and human infrastructure. In summary, while magnitude provides an objective measure of the size of an earthquake, intensity gives us a more subjective assessment of the impact and damage experienced at specific locations. Understanding both magnitude and intensity is essential for effective earthquake preparedness and response.

Key concepts

Richter scale

The Richter scale, named after Charles F. Richter, is a traditional method for quantifying the energy released by earthquakes. Established in the 1930s, this logarithmic scale measures the amplitude of the seismic waves produced during a seismic event. The scale starts at 0 and theoretically has no upper limit, although the most destructive earthquakes recorded have not exceeded magnitude 10.

When we refer to an earthquake's magnitude on the Richter scale, we're talking about a measurable quantity that indicates the size of the earthquake at its source. For example, a difference of one unit in magnitude, say from 5.0 to 6.0, signifies a tenfold increase in wave amplitude and approximately 31.6 times more energy release. This scale has been pivotal in understanding the potential energy an earthquake can unleash.

Seismic waves

Seismic waves are the energy waves that travel through the Earth's layers, radiating outward from the earthquake's focus, the point where the quake originates. These waves are what seismographs detect and record, and they come in different types, including primary (P) waves, which are compressional and travel fastest, and secondary (S) waves, which are shear and follow P waves.

Surface waves are another category that travel on the Earth's exterior and cause most of the shaking felt by humans. Their movement is mainly responsible for the damage to structures during an earthquake. Understanding seismic waves is crucial for seismologists to determine the magnitude of an earthquake, its epicenter, and potential impacts on populated areas.

Modified Mercalli Intensity scale

While magnitude gives us a quantitative measure of an earthquake, the Modified Mercalli Intensity (MMI) scale offers a qualitative assessment of its effects. Created in 1931 by Harry O. Wood and Frank Neumann, it is an adaptation of an earlier version designed by Italian volcanologist Giuseppe Mercalli.

The MMI scale ranges from I, indicating an earthquake that is not felt by people, to XII, which signifies total destruction. Its values are determined by factors such as reported human experiences, observed damage to buildings, and changes in natural features. Unlike magnitude, which remains constant for an earthquake, intensity can vary greatly depending on location and local geology, making it a more personalized measure of an earthquake's impact.

Moment magnitude scale

The moment magnitude scale (Mw) is a more modern and comprehensive scale used for measuring the size of larger earthquakes. Developed in the 1970s as an extension of the work by seismologist Hiroo Kanamori, it is based on the total energy released by an earthquake.

The Mw scale calculates magnitude using several factors, including the area of the fault that slipped, the average slip on the fault, and the rigidity of the surrounding rock. This scale does not saturate at higher values, unlike the Richter scale, making it more precise for large-scale seismic events. It is considered the standard scale by the seismological community for comparing the sizes of earthquakes.

Earthquake preparedness

Earthquake preparedness refers to the steps and measures that individuals and communities take to minimize risks related to earthquakes. Preparedness involves creating emergency plans, conducting drills, securing heavy furniture, creating disaster supply kits, and ensuring structures are built or retrofitted to resist seismic forces.

Knowledge of magnitude and intensity scales aids in preparedness by helping to evaluate potential earthquake scenarios and their likely impacts. Public education on how to react during and after earthquakes also forms an essential component of preparedness, aiming to protect lives and reduce economic losses when an earthquake occurs.

Seismic event measurement

Measuring a seismic event involves determining both its magnitude and intensity, which provide different but complementary information. Magnitude measures the absolute size of the event at its source, allowing comparisons between different earthquakes, while intensity measures its effects on the Earth's surface, highlighting the variability of impacts in different locations.

Reliable seismic event measurement is essential for earthquake response and preparedness. Seismologists use a network of seismographs to measure ground motions, which are then analyzed to understand an earthquake's characteristics. This information is crucial for developing building codes, emergency response plans, and insurance assessments, ultimately serving to mitigate the risks that earthquakes pose to society.