# 11.3: Measuring Earthquakes - Geosciences

There are two main ways to measure earthquakes. The first of these is an estimate of the energy released, and the value is referred to as magnitude. It is often referred to as “Richter magnitude,” but that is a misnomer, and it should be just “magnitude.” There are many ways to measure magnitude—including Charles Richter’s method developed in 1935—but they are all ways to estimate the same number, which is proportional to the amount of energy released.

The other way of assessing the impact of an earthquake is to assess what people felt and how much damage was done. This is known as intensity. Intensity values are assigned to locations, rather than to the earthquake itself, and therefore intensity can vary widely, depending on the proximity to the earthquake and the types of materials underneath and the local conditions.

## Earthquake Magnitude

Before we look more closely at magnitude we need to review what we know about body waves, and look at surface waves. Body waves are of two types, P waves, or primary or compression waves (like the compression of the coils of a spring), and S waves, or secondary or shear waves (like the flick of a rope). An example of P and S seismic wave records is shown in Figure (PageIndex{1}). The critical parameters for the measurement of magnitude are labelled, including the time interval between the arrival of the P- and S-waves—which is used to determine the distance from the earthquake to the seismic station, and the amplitude of the S-waves—which is used to estimate the magnitude of the earthquake.

When body waves (P or S) reach Earth’s surface, some of their energy is transformed into surface waves, of which there are two main types, as illustrated in Figure (PageIndex{2}). Rayleigh waves are characterized by vertical motion of the ground surface, like waves on water, while Love waves are characterized by horizontal motion. Both Rayleigh and Love waves are about 10% slower than S-waves (so they arrive later at a seismic station). Surface waves typically have greater amplitudes than body waves, and they do more damage.

Other important terms for describing earthquakes are hypocentre (or focus) and epicentre. The hypocentre is the actual location of an individual earthquake shock at depth in the ground, and the epicentre is the point on the land surface vertically above the hypocentre (Figure (PageIndex{3})).

A number of methods for estimating magnitude are listed in Table 11.1. Local magnitude (ML) was widely used until late in the 20th century, but moment magnitude (MW) is now more commonly used because it gives more accurate estimates (especially with larger earthquakes) and can be applied to earthquakes at any distance from a seismometer. Surface-wave magnitudes can also be applied to measure distant large earthquakes.

Because of the increasing size of cities in earthquake-prone areas (e.g., China, Japan, California) and the increasing sophistication of infrastructure, it is becoming important to have very rapid warnings and magnitude estimates of earthquakes that have already happened. This can be achieved by using P-wave data to determine magnitude because P-waves arrive first at seismic stations, in many cases several seconds ahead of the more damaging S-waves and surface waves. Operators of electrical grids, pipelines, trains, and other infrastructure can use the information to automatically shut down systems so that damage and casualties can be limited.

Type M Range Dist. Range Comments [Skip Table] Local or Richter (ML) 2 to 6 0 to 400 kilometers The original magnitude relationship defined in 1935 by Richter and Gutenberg. It is based on the maximum amplitude of S-waves recorded on a Wood‑Anderson torsion seismograph. ML values can be calculated using data from modern instruments. L stands for local because it only applies to earthquakes relatively close to the seismic station. Moment (MW) Greater than 3.5 All Based on the seismic moment of the earthquake, which is equal to the average amount of displacement on the fault times the fault area that slipped. It can also be estimated from seismic data if the seismometer is tuned to detect long-period body waves. Surface wave (MS) 5 to 8 20 to 180° A magnitude for distant earthquakes based on the amplitude of surface waves measured at a period near 20 seconds. P-wave 2 to 8 Local Based on the amplitude of P-waves. This technique is being increasingly used to provide very rapid magnitude estimates so that early warnings can be sent to utility and transportation operators to shut down equipment before the larger (but slower) S-waves and surface waves arrive.

Exercise 11.2 Moment magnitude estimates from earthquake parameters

Use this moment magnitude calculation tool to estimate the moment magnitude based on the approximate length, width, and displacement values provided in the following table:

Length (kilometers) Width (kilometers) Displacement (meters) Earthquake [Skip Table] 60 15 4 The 1946 Vancouver Island earthquake 0.4 0.2 .5 The small Vancouver Island earthquake shown in Figure (PageIndex{1}) 20 8 4 The 2001 Nisqually earthquake described in Exercise 11.3 1,100 120 10 The 2004 Indian Ocean earthquake 30 11 4 The 2010 Haiti earthquake

The largest recorded earthquake had a magnitude of 9.5. Could there be a 10? You can answer that question using this tool. See what numbers are needed to make MW = 10. Are they reasonable?

See Appendix 3 for Exercise 11.2 answers.

The magnitude scale is logarithmic; in fact, the amount of energy released by an earthquake of M4 is 32 times higher than that released by one of M3, and this ratio applies to all intervals in the scale. If we assign an arbitrary energy level of 1 unit to a M1 earthquake the energy for quakes up to M8 will be as shown on the following table:

Table 11.3 The energy of an earthquake increases by 32 times at each magnitude level.
MagnitudeEnergy
11
232
31,024
432,768
51,048,576
633,554,432
71,073,741,824
834,359,738,368

In any given year, when there is a large earthquake on Earth (M8 or M9), the amount of energy released by that one event will likely exceed the energy released by all smaller earthquake events combined.

## Earthquake Intensity

The intensity of earthquake shaking at any location is determined not only by the magnitude of the earthquake and its distance, but also by the type of underlying rock or unconsolidated materials. If buildings are present, the size and type of buildings (and their inherent natural vibrations) are also important.

Intensity scales were first used in the late 19th century, and then adapted in the early 20th century by Giuseppe Mercalli and modified later by others to form what we know call the modified Mercalli intensity scale (Table 11.4). Intensity estimates are important because they allow us to characterize parts of any region into areas that are especially prone to strong shaking versus those that are not. The key factor in this regard is the nature of the underlying geological materials, and the weaker those are, the more likely it is that there will be strong shaking. Areas underlain by strong solid bedrock tend to experience much less shaking than those underlain by unconsolidated river or lake sediments.

Level of intensity Description [Skip Table] Not felt (1) Not felt except by a very few under especially favourable conditions Weak (2) Felt only by a few persons at rest, especially on upper floors of buildings Weak (3) Felt quite noticeably by persons indoors, especially on upper floors of buildings; many people do not recognize it as an earthquake; standing motor cars may rock slightly; vibrations similar to the passing of a truck; duration estimated Light (4) Felt indoors by many, outdoors by few during the day; at night, some awakened; dishes, windows, doors disturbed; walls make cracking sound; sensation like heavy truck striking building; standing motor cars rocked noticeably Moderate (5) Felt by nearly everyone; many awakened; some dishes, windows broken; unstable objects overturned; pendulum clocks may stop Strong (6) Felt by all, many frightened; some heavy furniture moved; a few instances of fallen plaster; damage slight Very Strong (7) Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken Severe (8) Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse; damage great in poorly built structures; fall of chimneys, factory stacks, columns, monuments, walls; heavy furniture overturned Violent (9) Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; damage great in substantial buildings, with partial collapse; buildings shifted off foundations Extreme (10) Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; rails bent Extreme (11) Few, if any (masonry), structures remain standing; bridges destroyed; broad fissures in ground; underground pipelines completely out of service; earth slumps and land slips in soft ground; rails bent greatly Extreme (12) Damage total; waves seen on ground surfaces; lines of sight and level distorted; objects thrown upward into the air

An example of this effect is the 1985 M8 earthquake that struck the Michoacán region of western Mexico, southwest of Mexico City. There was relatively little damage in the area around the epicentre, but there was tremendous damage and about 5,000 deaths in heavily populated Mexico City some 350 kilometers from the epicentre. The key reason for this is that Mexico City was built largely on the unconsolidated and water-saturated sediment of former Lake Texcoco. These sediments resonate at a frequency of about two seconds, which was similar to the frequency of the body waves that reached the city. For the same reason that a powerful opera singer can break a wine glass by singing the right note, the amplitude of the seismic waves was amplified by the lake sediments. Survivors of the disaster recounted that the ground in some areas moved up and down by about 20 centimeters every two seconds for over two minutes. Damage was greatest to buildings between 5 and 15 storeys tall, because they also resonated at around two seconds, which amplified the shaking.

Exercise 11.3 Estimating intensity from personal observations

The following observations were made by residents of the Nanaimo area during the M6.8 Nisqually earthquake near Olympia, Washington in 2001. Estimate the Mercalli intensities using Table 11.4.

Building Type Floor Shaking Felt How long it lasted (in seconds) Description of Motion [Skip Table] House 1 no 10 Heard a large rumble lasting not even 10 seconds, mirror swayed House 2 moderate 60 Candles, pictures and CDs on bookshelf moved, towels fell off racks House 1 no Pots hanging over stove moved and crashed together House 1 weak Rolling feeling with a sudden stop, picture fell off mantle, chair moved Apartment 1 weak 10 Sounded like a big truck then everything shook for a short period House 1 moderate 20-30 Teacups rattled but didn’t fall off Institution 2 moderate 15 Creaking sounds, swaying movement of shelving House 1 moderate 15-30 Bed banging against the wall with me in it, dog barking aggressively

See Appendix 3 for Exercise 11.3 answers.

An intensity map for the 1946 M7.3 Vancouver Island earthquake is shown in Figure (PageIndex{4}). The intensity was greatest in the central island region where, in some communities, chimneys were damaged on more than 75% of buildings, some roads were made impassable, and a major rock slide occurred. The earthquake was felt as far north as Prince Rupert, as far south as Portland Oregon, and as far east as the Rockies.

#### Image Descriptions

Figure (PageIndex{1}) image description: P-waves and S-waves from a small (M4) earthquake near Vancouver Island in 1997. The P-wave arrived in 0.7 seconds with an amplitude ranging from negative 0.7 millimeters per second to 1.1 millimeters per second and lasting until the arrival of the S-wave. The S-wave arrived at 8.7 seconds, with a minimum amplitude of negative 2.8 millimeters per second and a maximum amplitude of 2.7 millimeters per second. The S-wave’s net amplitude gradually decreased over the next 5 seconds. [Return to Figure (PageIndex{1})]

Figure (PageIndex{4}) image description: The graduated intensity of the 1945 M7.3 Vancouver Island earthquake based on the modified Mercalli intensity scale. The area surrounding the epicentre of the earthquake which included central Vancouver Island ranged between a very strong (7) and severe (8) intensity. The next ring included the northern and southern parts of Vancouver Island, as well as a part of the main land coast including Vancouver and much of the Sunshine coast a strong (6) intensity. The next ring, which reached experienced a moderate (5) intensity, included Seattle and much of the BC interior. The outermost ring ranged between not felt (1) and light (4) intensity. It was felt as far north as Prince Rupert and the southern tip of Haida Gwaii, south eastern BC, and as far south as north western Oregon. [Return to Figure (PageIndex{4})]

• Figure (PageIndex{1}): © Steven Earle. CC BY.
• Figure (PageIndex{2}) (left): “Rayleigh Wave.” Adapted by Steven Earle. Public domain.
• Figure (PageIndex{2}) (right): “Love Wave” © Nicoguaro. Adapted by Steven Earle. CC BY.
• Figure (PageIndex{3}): © Steven Earle. CC BY.
• Figure (PageIndex{4}): “Vancouver Island, British Columbia June 23, 1946 – Magnitude 7.3,” © National Resources Canada. Used under the terms allowing for non-commercial reproduction. This reproduction is a copy of an official work that is published by the Government of Canada. This reproduction has not been produced in affiliation with, or with the endorsement of the Government of Canada.

• Table 11.4: The modified Mercalli intensity scale © Wikipedia. CC BY-SA.

## Chapter 1: Section 12 - Earthquake Magnitude

In this section you will find materials that support the implementation of EarthComm, Section 12: Earthquake Magnitude.

### Learning Outcomes

• Analyze and interpret data from personal observations of the effects of an earthquake to locate an earthquake’s epicenter.
• Examine an online tool used by scientists to collect and analyze data from people’s experiences of an earthquake.
• Obtain information about the extent of the damage caused by earthquakes of different intensities.

### Using Technology

Measurement of Earthquake Wave Amplitude

1. Visit the Virtual Earthquake website at http://www.sciencecourseware.com/VirtualEarthquake/. The Virtual Earthquake website will help you to simulate a new earthquake.
1. Follow the directions to calculate the magnitude of the earthquake.
2. How does the amplitude (height) of a seismic wave change when the size of an earthquake changes?
3. How would you expect the amplitude as recorded on a seismogram to change as you get farther from the epicenter?

### Inquiring Further

Did You Feel It?, USGS
Take part in citizen science by sharing your intensity observations for an earthquake you experienced.

Earthquakes and Seismicity - Magnitude vs Intensity, USGS
Describes the energy released by an earthquake. Includes a good explanation of the difference between magnitude and intensity.

Global Seismographic Network, USGS
Description of the Global Seismic Network (GSN) and how it is used to study earthquakes around the world.

Monitoring Earthquakes Across the United States, USGS
Brief description of how seismograph networks are used to monitor earthquakes across the country.

Tsunami: the Great Waves, NOAA
Description of how the Tsunami Warning System (TWS) for the Pacific region works.

NOAA Center for Tsunami Research, NOAA
Describes research and development efforts in tsunami monitoring networks. Includes data on recent tsunamis and spectacular animation.

Local Tsunamis in the Pacific Northwest, USGS
History of local tsunamis, including geological evidence and legends surrounding these giant waves.

Tsunami Fact Sheet, FEMA
Reviews what a tsunami is and what kinds of damage are typically associated with a tsunami occurrence.

Tsunami, NOAA
What does it mean to be tsunami ready? Find out this an more tsunami news from NOAA.

Multiple topics concerning tsunamis to chose among. Including video of similuations such as the Alaska earthquake of 1964.

The 2004 Sumatra Quake and Tsunami, IRIS
Visualization of the propogration of the earrthquake waves showing record of waves arriving at different monitoring stations.

### Resources

Seismometers

A Brief History of Seismology to 1910, Institute for Crustal Studies, University of California at Santa Barbara
Read about early explanations for earthquakes and the start of the "modern era" of seismology around 1750. Follow the development of the science of earthquake study.

Latest Earthquakes, USGS
Updated list by date, magntude, location, and depth. Zoom in and out of map or select region from menu.

Seismic Monitor, IRIS Consortium, University of Washington
Map is updated every 30 minutes - shows locations of earthquakes as well as seismic events on or near nuclear test sites. Also include earthquake news and teachable moments.

Seismometers, Seismographs, and Seismograms, etc., USGS
Use the Earthquake Glossary to see how seismographs work, how P and S waves move and learn to read Travel-Time curves.

Interpreting Seismograms

How Do I Read a Seismogram?, USGS

Seismographic Networks Improve Volcano Warnings, USGS
Descriptions of how seismometry is used to monitor volcanic eruptions. Contains good images of technology.

Different Processes: Different Seismic Signals, USGS
Learn how to distinguish the seismic signal from a landslide compared to a glacier sliding or an actual tectonic event.

## 11.3: Measuring Earthquakes - Geosciences

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Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

## 11.3: Measuring Earthquakes - Geosciences

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

## How do we record earthquakes?

Geoscience Australia monitors, analyses and reports on significant earthquakes to alert the Australian Government, State and Territory Governments and the public about earthquakes in Australia and overseas.

Earthquakes are detected by scientific instruments called seismometers. The word seismo originates from the Greek word seismos which means to shake or move violently and was later applied to the science and equipment associated with earthquakes. Seismographs, such as the Teledyne Geotech Helicorder, were used in the past to detect earthquake activity and relied on a mechanical system to record the seismic energy in the Earth onto paper. In contrast, modern seismometers such as the raspberry shake (pictured) detect and convert any small movement in the Earth into an electrical signal for use in computer systems, as shown in the digital seismogram image of seven seismic sensors which detected the magnitude 5.4 earthquake near Moe in Victoria on 19 June 2012.

Digital seismogram image of seven seismic sensors which detected the Mw 7.2 earthquake in the Banda Sea on 24th June 2019. The tremors from this earthquake were felt in Darwin.

### Determining the location of an earthquake

The accurate locations of seismometers are stored in a database accessible by an earthquake monitoring computer system. The system also has access to crustal velocity models which provide approximate information on how fast the various earthquake waves travel through the different layers which make up the Earth in the area between the earthquake and the seismometers. The times at which the differing seismic waves arrive at various seismometers are identified by Seismic Analysts or by a computer system. The arrival times of the seismic waves at the seismometers, together with the locations of the seismometers and the speed at which the seismic waves travel to the seismometers are all used to determine the location of the earthquake. This location is also known as its focus or hypocentre which is represented by the latitude, longitude and depth below the surface.

## Earthquake Intensity

Intensity scales were first used in the late 19th century, and then adapted in the early 20th century by Giuseppe Mercalli and modified later by others to form what we now call the Modified Mercalli Intensity Scale (Table 12.1). To determine the intensity of an earthquake, reports are collected about what people felt and how much damage was done. The reports are then used to assign intensity ratings to regions where the earthquake was felt.

 Table 12.1 Modified Mercalli Intensity Scale I Not felt Not felt except by a very few under especially favourable conditions II Weak Felt only by a few persons at rest, especially on upper floors of buildings III Weak Felt quite noticeably by persons indoors, especially on upper floors of buildings many people do not recognize it as an earthquake standing motor cars may rock slightly vibrations similar to the passing of a truck duration estimated IV Light Felt indoors by many, outdoors by few during the day at night, some awakened dishes, windows, doors disturbed walls make cracking sound sensation like heavy truck striking building standing motor cars rocked noticeably V Moderate Felt by nearly everyone many awakened some dishes, windows broken unstable objects overturned pendulum clocks may stop VI Strong Felt by all, many frightened some heavy furniture moved a few instances of fallen plaster damage slight VII Very Strong Damage negligible in buildings of good design and construction slight to moderate in well-built ordinary structures considerable damage in poorly built or badly designed structures some chimneys broken VIII Severe Damage slight in specially designed structures considerable damage in ordinary substantial buildings with partial collapse damage great in poorly built structures fall of chimneys, factory stacks, columns, monuments, walls heavy furniture overturned IX Violent Damage considerable in specially designed structures well-designed frame structures thrown out of plumb damage great in substantial buildings, with partial collapse buildings shifted off foundations X Extreme Some well-built wooden structures destroyed most masonry and frame structures destroyed with foundations rails bent XI Extreme Few, if any (masonry), structures remain standing bridges destroyed broad fissures in ground underground pipelines completely out of service earth slumps and land slips in soft ground rails bent greatly XII Extreme Damage total waves seen on ground surfaces lines of sight and level distorted objects thrown upward into the air Source: U. S. Geological Survey (1989). The Severity of an Earthquake. USGS General Interest Publication 1989-288-913 view source

Intensity values are assigned to locations, rather than to the earthquake itself. This means that intensity can vary for a given earthquake, depending on the proximity to the epicentre and local conditions. For the 1946 M7.3 Vancouver Island earthquake, intensity was greatest in the central island region (Figure 12.15). In some communities within this region, chimneys were damaged on more than 75% of buildings. Some roads were made impassable, and a major rock slide occurred. The earthquake was felt as far north as Prince Rupert, as far south as Portland Oregon, and as far east as the Rockies, but with less intensity.

Figure 12.15 Intensity map for the M7.3 Vancouver Island earthquake on June 23, 1946. Source: Earthquakes Canada, Natural Resources Canada (2016) view source. Click the image for terms of use.

Intensity estimates are important as a way to identify regions that are especially prone to strong shaking. A key factor is the nature of the underlying geological materials. The weaker the underlying materials, the more likely it is that there will be strong shaking. Areas underlain by strong solid bedrock tend to experience far less shaking than those underlain by unconsolidated river or lake sediments.

An example of this effect is the 1985 M8 earthquake that struck the Michoacán region of western Mexico, southwest of Mexico City. There was relatively little damage near the epicentre, but 350 km away in heavily populated Mexico City there was tremendous damage and approximately 5,000 deaths. The reason is that Mexico City was built largely on the unconsolidated and water-saturated sediment of former Lake Texcoco. These sediments resonate at a frequency of about two seconds, which was similar to the frequency of the body waves that reached the city. Consequently, the shaking was amplified. Survivors of the disaster recounted that the ground in some areas moved up and down by approximately 20 cm every two seconds for over two minutes. Damage was greatest to buildings between 5 and 15 storeys tall, because they also resonated at around two seconds, which amplified the shaking.

## Chapter 8 Measuring Geological Time

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

• Apply basic geological principles to the determination of the relative ages of rocks
• Explain the difference between relative and absolute age-dating techniques
• Summarize the history of the geological time scale and the relationships between eons, eras, periods, and epochs
• Understand the importance and significance of unconformities
• Estimate the age of a rock based on the fossils that it contains
• Describe some applications and limitations of isotopic techniques for geological dating
• Use isotopic data to estimate the age of a rock
• Describe the techniques for dating geological materials using tree rings and magnetic data
• Explain why an understanding of geological time is critical to both geologists and the public in general

Time is the dimension that sets geology apart from most other sciences. Geological time is vast, and Earth has changed enough over that time that some of the rock types that formed in the past could not form today. Furthermore, as we’ve discussed, even though most geological processes are very, very slow, the vast amount of time that has passed has allowed for the formation of extraordinary geological features, as shown in Figure 8.1.

Figure 8.1 Arizona’s Grand Canyon is an icon for geological time 1,450 million years are represented by this photo. The light-coloured layered rocks at the top formed at around 250 Ma, and the dark ones at the bottom (within the steep canyon) at around 1,700 Ma. [SE]

We have numerous ways of measuring geological time. We can tell the relative ages of rocks (for example, whether one rock is older than another) based on their spatial relationships we can use fossils to date sedimentary rocks because we have a detailed record of the evolution of life on Earth and we can use a range of isotopic techniques to determine the actual ages (in millions of years) of igneous and metamorphic rocks.

But just because we can measure geological time doesn’t mean that we understand it. One of the biggest hurdles faced by geology students, and geologists as well, in understanding geology, is to really come to grips with the slow rates at which geological processes happen and the vast amount of time involved.

## An Expert System for measuring shear-wave splitting above small earthquakes

As part of the development of a system for routinely measuring shear-wave splitting, this paper introduces an Expert System (ES) to measure the polarisations and time-delays of seismic shear-wave splitting in three-component seismograms above small earthquakes. Expert Systems are rule-based computer techniques designed to provide expertise in particular topics, where the rules are algorithms developed from previous knowledge and experience. The technique is tested on data recorded by the seismic network in Iceland. The statistics suggests that the ES is reasonably successful and provides appropriate initial input parameters for a more precise analysis, which leads to the success of the comprehensive Shear-Wave Analysis System (SWAS) for measuring shear-wave splitting.

## Japan Earthquake Shifts Earth's Mass and Moves Its Axis

Republished from a March, 2011 press release by Alan Buris of NASA Jet Propulsion Laboratory.

### A Shift in the Distribution of Earth's Mass

The March 11, magnitude 9.0 earthquake in Japan may have shortened the length of each Earth day and shifted its axis. But don't worry-you won't notice the difference.

Using a United States Geological Survey estimate for how the fault responsible for the earthquake slipped, research scientist Richard Gross of NASA's Jet Propulsion Laboratory, Pasadena, Calif., applied a complex model to perform a preliminary theoretical calculation of how the Japan earthquake-the fifth largest since 1900-affected Earth's rotation. His calculations indicate that by changing the distribution of Earth's mass, the Japanese earthquake should have caused Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds (a microsecond is one millionth of a second).

### A Shift in the Position of Earth's Axis

The calculations also show the Japan quake should have shifted the position of Earth's figure axis (the axis about which Earth's mass is balanced) by about 17 centimeters (6.5 inches), towards 133 degrees east longitude. Earth's figure axis should not be confused with its north-south axis they are offset by about 10 meters (about 33 feet). This shift in Earth's figure axis will cause Earth to wobble a bit differently as it rotates, but it will not cause a shift of Earth's axis in space-only external forces such as the gravitational attraction of the sun, moon and planets can do that.

Both calculations will likely change as data on the quake are further refined.

### The Impact of Other Earthquakes

In comparison, following last year's magnitude 8.8 earthquake in Chile, Gross estimated the Chile quake should have shortened the length of day by about 1.26 microseconds and shifted Earth's figure axis by about 8 centimeters (3 inches). A similar calculation performed after the 2004 magnitude 9.1 Sumatran earthquake revealed it should have shortened the length of day by 6.8 microseconds and shifted Earth's figure axis by about 7 centimeters, or 2.76 inches. How an individual earthquake affects Earth's rotation depends on its size (magnitude), location and the details of how the fault slipped.

Gross said that, in theory, anything that redistributes Earth's mass will change Earth's rotation.

### Other Forces Can Change Earth's Rotation

"Earth's rotation changes all the time as a result of not only earthquakes, but also the much larger effects of changes in atmospheric winds and oceanic currents," he said. "Over the course of a year, the length of the day increases and decreases by about a millisecond, or about 550 times larger than the change caused by the Japanese earthquake. The position of Earth's figure axis also changes all the time, by about 1 meter (3.3 feet) over the course of a year, or about six times more than the change that should have been caused by the Japan quake."

### Measurement Limitations

Gross said that while we can measure the effects of the atmosphere and ocean on Earth's rotation, the effects of earthquakes, at least up until now, have been too small to measure. The computed change in the length of day caused by earthquakes is much smaller than the accuracy with which scientists can currently measure changes in the length of the day. However, since the position of the figure axis can be measured to an accuracy of about 5 centimeters (2 inches), the estimated 17-centimeter shift in the figure axis from the Japan quake may actually be large enough to observe if scientists can adequately remove the larger effects of the atmosphere and ocean from the Earth rotation measurements. He and other scientists will be investigating this as more data become available.

### Impact on Our Daily Lives?

Gross said the changes in Earth's rotation and figure axis caused by earthquakes should not have any impacts on our daily lives. "These changes in Earth's rotation are perfectly natural and happen all the time," he said. "People shouldn't worry about them."