How Earthquake Magnitudes Are Measured

The magnitude of an earthquake is a measure of the size and energy released during an event. And it plays a significant role in understanding the potential impact, damage, and intensity of the tremors. Seismometers are instruments used to record vibrations, and the data obtained from these devices helps seismologists calculate an earthquake’s magnitude. This is how earthquake magnitudes are measured.

The measurement of earthquake magnitudes has evolved over the years as our understanding of the processes underlying seismic events has improved. The Richter Scale, developed in 1935, was one of the first tools used to measure earthquake magnitudes. It determined values based on the logarithm of the amplitude of waves recorded by seismographs.

Moment Magnitude Scale (Mw) is how earthquake magnitudes are measured best by scientists. The Mw scale accounts better for the size, depth, and characteristics of the fault. From the total energy released during an earthquake, seismologists can estimate the rupture area and the average fault movement.

Contents

Key Takeaways

Earthquake magnitudes provide insight into the size and energy released during seismic events.
Seismometers record ground vibrations, which are used to calculate an earthquake’s magnitude.
Modern measurements predominantly use the Moment Magnitude Scale (Mw), which is an improvement on the Richter Scale.

Earthquakes: A Brief Overview

The sudden release of energy in the Earth’s crust precedes an earthquake. This results in the generation of seismic waves, which travel through the Earth’s layers and cause the ground to shake. The intensity of the shaking depends on the amount of energy released and the distance from the source of the earthquake.

Seismic Waves

Seismic waves are of two main types: body waves and surface waves. Body waves travel through the Earth’s interior, while surface waves move along the Earth’s surface. These waves produce ground motion that can be recorded by instruments called seismographs. The data collected by seismographs help scientists in understanding the earthquake’s characteristics and its impact on the Earth’s crust.

Magnitude

An earthquake’s magnitude is the measure of the energy it releases. This energy is caused by the movement of the Earth’s tectonic plates, which constantly interact with each other. The magnitude of an earthquake is determined by analysing the amplitude of the seismic waves recorded by seismographs. The Richter scale was conceptualised by Charles Richter. It is an example of a method used to determine earthquake magnitudes using the logarithm of the largest seismic wave’s amplitude to base 10.

Intensity

In contrast to magnitude, the intensity of an earthquake refers to the effects of ground motion on people, structures, and the natural environment. Intensity is usually measured using the Modified Mercalli Intensity Scale. This scale considers the observations of people, the amount of damage caused, and other factors.

Takeaway

Earthquakes can have consequences ranging from minor ground motion to catastrophic events. Many cause significant damage to structures and loss of life. Understanding and measuring earthquake magnitudes and intensities is essential for assessing the potential hazards and formulating effective strategies for disaster management and mitigation.

Understanding Earthquake Magnitude

Earthquake magnitude is an essential aspect of understanding the power and impact of earthquakes. Magnitude is a measure of the amplitude of the seismic waves produced by an earthquake’s source, recorded by seismographs. Seismologists use various magnitude scales to quantify the size and energy release of earthquakes.

The Richter Scale

The Richter scale, developed by seismologist Charles F. Richter in 1935, used to be the widely accepted method for measuring earthquake magnitudes. It uses the logarithm of the largest seismic wave’s amplitude to base 10. However, the Richter scale has limitations, especially for larger earthquakes and those occurring at greater distances from the recording instruments.

Moment Magnitude Scale

Today, the moment magnitude scale (Mw) is the standard for measuring earthquake magnitudes. It is based on a more accurate representation of earthquake size. It considers the fault geometry, the amount of slip along the fault, and properties of rocks at the fault. This scale provides a more reliable estimate of the earthquake’s actual energy release, making it the preferred method for seismologists.

Logarithms

Earthquake magnitudes are logarithmic. This means a one-unit increase in magnitude corresponds to a tenfold increase in amplitude and a roughly 32-fold increase in energy release. For example, a magnitude 3 earthquake would release 32 times more energy than a magnitude 2 earthquake. This logarithmic scale helps to differentiate earthquakes of varying sizes without using large, unwieldy numbers.

To grasp the energy release during earthquakes, one can compare the energy released with the energy produced by detonating explosives. For instance, a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. While a magnitude 8 earthquake releases energy equivalent to detonating 6 million tons of TNT.

Takeaway

Understanding earthquake magnitudes is crucial for seismologists and disaster management teams to evaluate the potential impact of earthquakes accurately. By using scales like the Moment Magnitude scale, they can efficiently classify earthquakes, monitor energy release, and predict the potential damage caused by these natural disasters.

How Earthquake Magnitudes are Measured – The Methods used

Earthquake magnitude is a crucial parameter in understanding the size and impact of an earthquake.

Seismographs

Apart from the Richter scale and the Moment Magnitude scale (Mw), seismographs, also known as seismometers, are essential instruments in the measurement of earthquake magnitude. These devices detect and record ground motions caused by seismic waves. A network of global seismometers helps scientists to measure the strength, duration, and overall characteristics of an earthquake. The information provided by seismometers is used to create a digital graphic plotting of the ground motion, known as a seismogram.

Other Methods of Measurement

There are other methods used to measure the magnitude of an earthquake. Some of these methods consider factors such as wave amplitude, fault size, and the amount of slip that occurs during the earthquake. These alternative measurement scales have been developed to address specific limitations and shortcomings of the Richter scale. And they provide more accurate and comprehensive assessments of earthquake magnitude (UPSeis).

The United States Geological Survey (USGS)

The USGS plays an essential role in collecting and analysing data related to earthquakes. As a leading scientific agency, it provides crucial information about earthquakes. This includes their magnitude, location, and potential impact on human populations and environments. With the advancement of technology and the increasing availability of earthquake data, the methods for measuring earthquake magnitude have improved. Thus, enabling scientists to understand and predict earthquake behaviour better.

Takeaway

Measuring earthquake magnitude involves methods and instruments, such as the Richter scale, Moment Magnitude scale, seismographs, and other scales that consider different earthquake characteristics. Agencies like USGS play a significant role in gathering and disseminating this critical information. This helps communities effectively prepare for and respond to earthquake events.

The Seismograph in Detail

A seismograph is a sensitive instrument designed to detect and record the slightest vibrations generated by an earthquake. It measures the movement of the ground through the seismic waves that travel through the Earth’s layers. The data recorded by seismographs is essential to understanding earthquake magnitudes and helping the development of effective earthquake mitigation strategies.

Seismographs consist of a base and a suspended mass, which remains stationary even when the base is subjected to ground movement. The relative motion between the base and the mass is recorded in seismograms. These help determine the wave amplitude of seismic waves. These recordings enable scientists to analyse the intensity of an earthquake and its characteristics.

Seismograhic Network

A collection of seismographs strategically positioned around the world forms a seismographic network. This network allows accurate and timely detection of earthquakes as well as the identification of their epicentres and magnitudes. It works based on the principle that each seismograph in the network registers the ground movement at its specific location. The combined data from multiple stations allows for a broader and more precise understanding of an earthquake’s impact.

Seismic Waves

Seismic waves, the energy released by an earthquake as it travels through the Earth’s layers, can be categorised into two main types:

Body waves – Body waves include primary waves (P-waves) and secondary waves (S-waves), which travel through the Earth’s interior, and
Surface waves – Surface waves only travel along the Earth’s surface and are responsible for most earthquake-related damages.

Amplitude

The seismograms record the various waves, providing valuable information on the depth, distance, and direction of the earthquake.

Wave amplitude, a crucial parameter in studying earthquakes, is affected by multiple factors, including;

the amount of slip along a fault,
fault size, and
the distance from the earthquake to the recording station.

By analysing the wave amplitude information from seismograms, scientists can determine the earthquake’s magnitude and estimate its potential impact on surrounding areas.

Takeaway

Seismographs play an essential role in understanding earthquakes. Thereby enabling the measurement of their magnitudes through the detection and analysis of seismic waves. The data recorded by seismographs, in terms of wave amplitude and other parameters, helps geologists and seismic experts monitor and study earthquakes with a superior degree of accuracy. This not only benefits our knowledge of the Earth’s geophysical processes but also contributes to the development of effective earthquake preparedness and response strategies.

Concept of Moment Magnitude

The Moment Magnitude is an important measurement technique for estimating the size or strength of an earthquake based on its seismic moment. It is a more accurate reflection of the total energy released by an earthquake. Therefore it provides a better estimation of its magnitude compared to other scales such as:

the Richter scale,
body-wave scale, and
surface-wave scale.

The Moment Magnitude scale (MMS) was introduced by Thomas C. Hanks and Hiroo Kanamori in 1979, and is denoted explicitly with Mw. It is calculated by considering the seismic moment. This is a physical quantity proportional to the slip on the fault multiplied by the area of the fault surface that slips. This calculation is related to the total energy released during an earthquake.

Seismograms and geodetic measurements can be used to estimate the moment. Seismograms are digital recordings from seismographs that detect and measure the strength and duration of the earthquake’s waves. The Moment Magnitude scale overcomes the limitations of other magnitude scales that may not provide accurate measurements for very large earthquakes. It is capable of measuring moment release and energy release consistently across a wide range of magnitudes.

Takeaway

It is essential to understand magnitude measurements refer to the size of the earthquake. While shaking intensity can vary from place to place based on factors like distance and types of surface materials. Overall, the Moment Magnitude scale has become a crucial tool for researchers and experts in the field of seismology. And it contributes to the accurate depiction of earthquake events and assists in vital earthquake preparedness and response efforts.

The Richter Scale

The Richter Scale, developed by Charles F. Richter in the 1930s, is a numerical scale used to measure the magnitude of earthquakes. This scale is based on the logarithm of the amplitude of seismic waves recorded by seismographs. This allows scientists to compare the size of one earthquake with another.

Charles Richter’s idea for creating a logarithmic earthquake magnitude scale emerged due to the need for a standardised method to measure the size of earthquakes occurring in southern California. The scale, originally referred to as ML, was specifically designed to measure moderate-sized earthquakes (magnitude 3 to magnitude 7). In this logarithmic scale, an increase of one integer in magnitude equates to a tenfold increase in the amplitude of the seismic waves.

To measure an earthquake’s magnitude, the Richter Scale uses the logarithm of the amplitude of waves recorded by seismographs. They consider adjustments for variations in the distance between seismographs and the earthquake’s epicentre. This allows for a more accurate and consistent measure across different geological conditions.

The Richter Scale uses the Roman numerals I to XII, with I being the lowest and XII being the highest. Lower magnitude earthquakes generally cause less damage and are less noticeable. Whereas higher magnitude earthquakes are often associated with extensive devastation and loss of life.

The Richter Scale remains widely used due to its simplicity and ease of interpretation. In addition to the Richter Scale, scientists also employ other measurement scales such as the Moment Magnitude Scale (MMS) and the Modified Mercalli Intensity Scale. Charles Richter’s contribution to seismology has left a lasting impact on the understanding and assessment of earthquakes. And the Richter Scale is still an important tool for earthquake measurement.

Hierarchy of Earthquake Magnitude

Earthquake magnitude is a crucial factor in understanding the severity and impact of seismic events. It represents the amount of energy released during an earthquake. And it allows scientists and researchers to classify earthquakes based on their size and potential consequences.

Richter and Moment Magnitude Scales

The original Richter scale tool, calculates the magnitude of an earthquake through the logarithm of the amplitude of waves recorded by seismographs. Although effective for some earthquakes, the Richter scale is limited in scope. It works mainly for seismic events in Southern California and within a range of seismometer distances.

Because of these limitations, the Richter scale was superceded by more comprehensive and accurate systems for measuring earthquake magnitudes. The Moment Magnitude Scale (Mw) and the Surface Wave Magnitude Scale (Ms) take in the fault area and rock rigidity. They also calculate the distance the fault sides move past each other. This makes them more suitable for accurately determining earthquake sizes on a global scale.

Categorising Hierarchy

The hierarchy of earthquake magnitudes is categorised based on the level of shaking and potential damage they can cause. People don’t usually feel small earthquakes ranging below magnitude 3.0. And they have minimal, if any, impact on structures. Moderate earthquakes, around magnitudes 4.0 to 5.0, can cause light to moderate damage to poorly constructed buildings.

However, as the magnitude increases, the potential for destruction rises significantly. Major earthquakes, usually categorised as magnitudes 6.0 to 6.9, can cause severe damage to structures and infrastructure. At the top of the hierarchy are great earthquakes, with magnitudes above 7.0. These devastating events can cause widespread destruction over large areas, leading to significant loss of life and property.

A single step increase in magnitude from 5.0 to 6.0, corresponds to a 10-fold increase in the size of an earthquake. Plus a significant increase in the total energy released. This vast difference in energy can mean the difference between a minor shaking and catastrophic devastation.

Takeaway

Understanding the hierarchy of earthquake magnitudes is critical for evaluating seismic hazards and planning mitigation efforts. If we know the logarithmic nature of the scale and the various scales used to classify earthquakes, we can prepare better for, and reduce, the impacts of these powerful natural phenomena.

Intensity and Damage Caused by Earthquakes

When discussing earthquakes, it is important to differentiate between magnitude and intensity. While magnitude measures the energy released at the source of the earthquake, intensity measures the strength of shaking produced by the earthquake at specific locations. Intensity plays a crucial role in determining the damage caused by seismic events, as it directly relates to the effect of earthquakes on buildings, infrastructure, and people.

Assessing Intensity

One widely used method for assessing the intensity of an earthquake is the Modified Mercalli Intensity Scale, or MMI for short. This scale traditionally ranges from I (not felt) to XII (total destruction), with increasing levels of intensity corresponding to more severe shaking and damage. A key feature of the Modified Mercalli Intensity Scale is that it considers the observations of people and the effects of shaking on structures, making it a useful tool for understanding the real-world impacts of earthquakes.

The degree of damage caused by an earthquake depends on several factors, including the intensity of shaking, the type of ground and soil conditions, and the building materials and construction quality. As a rule of thumb, higher intensity shaking, represented by higher values on the Modified Mercalli Scale, tends to result in more serious damage to buildings and infrastructure.

By way of example, an intensity level of V on the Mercalli scale indicates that an earthquake is strong enough to be felt by nearly everyone, but may only cause minor damage to poorly constructed structures. On the other hand, an intensity level of IX is considered violent, with well-built structures likely to be badly damaged, with poorly built structures potentially collapsing. When the shaking reaches intensity levels of XI and XII, destruction will be extreme. Even well-designed buildings may suffer severe damage or complete failure.

Takeaway

The intensity of an earthquake, as measured by the Modified Mercalli Intensity Scale, is a significant factor in determining the damage caused by seismic events. By understanding the relationship between intensity levels and the potential damage to buildings and infrastructure, communities can prepare for and mitigate the impacts of earthquakes much better.

Understanding the Location of Earthquakes

As we now know, earthquakes occur when there is a sudden release of energy in the Earth’s crust, causing the ground to shake. The location of an earthquake is often the key to understanding its cause and potential impact on nearby areas. To determine the location and magnitude of earthquakes, seismologists use a combination of factors, such as;

the type of rock,
proximity to fault lines, and
data from seismometers.

Hypocentre and Epicentre

The point at which an earthquake originates beneath the Earth’s surface is called the hypocentre, while the point on the surface above it is known as the epicentre. The epicentre is typically the location where the shaking is most severe and where potential damage may be highest.

Seismometer Data

To determine the location of an earthquake, seismologists rely on seismometer data to measure vibrations caused by earthquake waves. Seismometers are sensitive instruments installed below ground to accurately record ground movements. By analysing the data recorded, scientists can identify the type of waves and their amplitude, which helps pinpoint the earthquake’s epicentre.

Fault and Earthquake Location

Fault lines play a crucial role in the occurrence of earthquakes. A fault is a fracture or zone of fractures in the Earth’s crust, where rocks on either side have moved past one another. These movements generate stress in the surrounding rock formations, which is released as earthquakes. The properties of the rock surrounding faults, such as their density and elasticity, impact the intensity and duration of an earthquake, as well as the seismic waves generated.

Understanding the location of earthquakes and their connection to the underlying fault systems enables seismologists to better assess the potential hazards associated with such events. By studying patterns of past earthquakes along specific faults, they can provide vital information for designing infrastructure and creating disaster preparedness plans to minimise damage and casualties during future seismic events.

Data Collection and Mapping

The process of measuring earthquake magnitudes involves the collection of data from seismic sensors located across the globe. The U.S. Geological Survey (USGS) plays a critical role in monitoring and reporting earthquake activity. By using an extensive network of seismometers, the USGS is able to gather crucial information on the size, location, and magnitude of earthquakes.

When an earthquake occurs, the seismometers record vibrations that travel through the Earth’s surface. These seismic waves are then analysed to determine the time, location, and magnitude of the earthquake. The data recorded by seismometers is essential in understanding the nature and extent of the event.

One of the tools developed by the USGS for earthquake data collection is the Did You Feel It? system which asks people who experience the feel of an earthquake to talk about it. They then build maps to show the experiences and damage. Suffered. The crowdsourced initiative enables individuals who experience an earthquake to provide information about the intensity of the shaking at their location. By combining this subjective data with objective seismometer readings, the USGS can create more accurate and comprehensive maps of earthquake intensity.

These maps are crucial in assessing the potential impact of earthquakes on infrastructure and human populations. They help governments, emergency services, and other organisations to deploy resources efficiently and plan for future events. The earthquake data gathered by the USGS and other geological agencies around the world is also used by researchers to study the Earth’s internal processes and improve earthquake forecasting models.

Takeaway

Accurate data collection and mapping are essential components to understand and respond to earthquakes. The USGS plays a pivotal role in this process, using a combination of seismometers, crowdsourced information, and innovative mapping techniques to provide invaluable insights into the nature and impact of these natural phenomena.

Special Cases: Subduction Zones and California Earthquakes

Subduction zones are a unique type of geological setting where one tectonic plate slides beneath another, resulting in the formation of deep-sea trenches and volcanic arcs. These zones are responsible for some of the largest and most destructive earthquakes. A prime example of a subduction zone is the Cascadia Subduction Zone, stretching from British Columbia in Canada to northern California in the United States. Earthquakes in this region can reach magnitudes greater than 9.0, posing significant hazards to the surrounding areas.

California earthquakes, on the other hand, are chiefly influenced by the San Andreas Fault, which is not part of a subduction zone. Instead, it is a strike-slip fault, where two tectonic plates slide past each other horizontally. Earthquakes in California primarily occur in the upper 15 miles of the Earth’s crust and can reach magnitudes of around 8.0.

Subduction zone earthquakes and California earthquakes differ in their origin, magnitude, and depth. While both types of earthquakes can be destructive, subduction zones are generally associated with higher magnitudes and tsunami risks due to the greater displacement of the ocean floor.

Takeaway

Understanding the geological differences between subduction zones and strike-slip fault systems is essential for accurately measuring earthquake magnitudes and predicting hazards. By recognising these distinctions, scientists can study and mitigate the risks posed by earthquakes, much better.

Frequently Asked Questions

What differentiates the Moment Magnitude scale from the Richter scale?

The Moment Magnitude scale is a more comprehensive and accurate measure of the size of an earthquake compared to the Richter scale. Moment Magnitude takes into account the total energy the earthquake radiates, the area that moves, the distance that one side of the fault moves past the other, and the rigidity of the rock. The Richter scale, meanwhile, is a local measurement with limitations in accurately estimating larger earthquakes.

What units are used to measure earthquake magnitudes?

Earthquake magnitudes are measured on a logarithmic scale, where each whole number increase represents a tenfold increase in ground motion and a ~31.6-fold increase in the energy released. Magnitude scales, like the Moment Magnitude scale, use these numerical values to represent the size of an earthquake at its source.

How is the intensity of an earthquake determined?

The intensity of an earthquake is determined by the amount of ground shaking experienced at various locations. It is commonly measured using the Modified Mercalli Intensity (MMI) Scale, which uses Roman numerals ranging from I (not felt) to XII (total destruction) to describe the local effects of the earthquake⁵. The intensity varies based on factors such as distance from the epicentre, surface material, and the depth of the earthquake.

What is the maximum possible magnitude on current earthquake scales?

Although there is no theoretical maximum magnitude, the practical limit of earthquake magnitudes is around 9.5, which is the largest earthquake ever recorded (the 1960 Valdivia earthquake in Chile). Earthquakes of such magnitude are extremely rare and occur only in specific geological settings, such as subduction zones.

Why did scientists move away from using the Richter scale?

The Richter scale has limitations in accurately estimating the size of large earthquakes and is less reliable for global comparisons. In contrast, the Moment Magnitude scale provides a more accurate representation of the size and energy of an earthquake, including those of larger magnitudes. As a result, scientists transitioned away from the Richter scale in favour of more precise measures like the moment magnitude scale.

How do various magnitudes of earthquakes feel to people?

The experience of an earthquake depends on its magnitude and the local geological conditions. Earthquakes with magnitudes below 2.0 are rarely felt by people, while those with magnitudes between 2.0 and 3.9 can be felt but typically cause little to no damage. As the magnitude increases, so does the potential for damage and the impact on people. Magnitudes of 7.0 or higher are considered major earthquakes and can cause widespread destruction and casualties.

Sources and Resources

If you need to go deeper, here are some useful authority resources and sources drawn from in this article.

Why not Join Us

If you enjoyed this article, we would like to offer you two gifts – our Starter Pack of 4 James King’s books and our Weekly Digest, which you can receive by email.