ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

What Happens Underground During an Earthquake?

Updated on August 11, 2012
unvrso profile image

Unvrso has been writing varied topics of literature since 2005 and started writing for hubpages in 2009.

What's an Earthquake?

An earthquake is the sudden shaking of the Earth, which is created by the movement or vibrations of rocks and soil along fractures in the earth's crust. During an earthquake, the earth is suddenly shaken by the readjusting of tectonic plates which compose the lithosphere. This shift produces vibrations on the surface rocks and soil known as seismic waves. When a big earthquake occurs, the aftershocks create several earthquakes of all magnitudes which can last for months. The shifts and shockwaves represent the way in which the Earth's crust is settling into new ground arrangements.

Most earthquakes occur along continental and oceanic plate boundaries where these plates converge, diverge or slip past one another. The Earth’s lithosphere comprises seven distinct huge tectonic plates, along with many other small plates. The great majority of earthquakes occur around the Ring of Fire in the Pacific Ocean at depths of no more than a few tens of kilometers. On continental land, earthquakes may occur at depths of less than 70 km (43 miles), and they're termed shallow earthquakes. Those occurring at depths ranging from 70-300 km (43-186 miles) are known as intermediate. If the focus of an earthquake is above 300 km (186 miles), they’re called deep earthquakes.

Oceanic and Continental Plates Boundaries

Tectonic Plates
Tectonic Plates | Source

The sudden movement of tectonic plates is caused by the motion of a layer lying underneath the crust called the mantle. It is believed that convection currents occurring in the upper mantle are the principal driving force producing the movement of the tectonic plates. Earthquakes also occur along cracks in the Earth’s crust, known as faults, where the release of built up stress produces a sudden slip among locked up plates. When these plates pull apart, collide or slip sideways, they produce ground vibrations. These ground vibrations are known as seismic waves. Seismic waves travel through the interior of the Earth’s crust.

Seismic Waves Radiating From the focus

Epicenter of an Earthquake
Epicenter of an Earthquake | Source

Faults in the Earth's Crust

Faults are fractures in the Earth's crust where rock mass has moved against another rock mass in the oceanic or continental crust. The blocks on either side of a fault may just slide and pass one another without producing an earthquake; however, if the blocks get interwined, over time, they will reach a certain point where they will have stored enough energy, causing a sudden break between them, and releasing tremendous energy in the form of an earthquake.

Faults may cause breaks through the ground surface or they may go deep into the Earth's interior. The location at which a rock slip first occurs is called the focus, and the position directly above it on the surface of the Earth is known as the epicenter. Normally, earthquakes take place at some depth within the Earth. Based on the depths of the focus, earthquakes are classified as shallow if the focus is at 60 km; intermediate if the focus is between 60-300 km; and deep if the focus is above 300 km.

How Does an Earthquake Occur?

Seismic Experiment
Seismic Experiment | Source


1. Break one block of foam board into two pieces

2. Place both broken pieces on a table

3. Place both of your hands on top of each piece of foam

4. Push half of one block with your left hand. At the same time pull the other half of the foam with your right hand.

5. Maintain pushing and pulling until the two blocks slide freely

This is what occurs during an earthquake:

The crack between the two blocks represent the fault, and the point at the very center of the two blocks is where the friction unlocked the tension accumulated throughout a very long expanse of time.

The blocks represent the Earth's crust, and the force exerted upon the blocks with the hands represents the movement during an earthquake.

Seismic Waves

Soon after an earthquake strikes, the energy is released in the form of seismic waves, moving in all directions. These waves can either radiate underground from the point of the focus, and are known as body waves, or from above the ground, and are known as surface waves. Body waves radiate through the Earth's interior, while surface waves radiate only within the outer surface layer of the crust. There are two types of body waves, P waves and S waves. P waves move back and forth, and they can go through liquids, solids and gases. P waves move at about 6 km/ second (4 miles/second), and cause lots of damage.

P waves can temporarily change the density and volume of the ground where buildings are built on. When the ground contracts or expands during an earthquake, many constructions tend to fall down because of the ground surface modifications. Many public services in big cities become affected. For example, many subterranean ducts and pipelines may rupture or explode due to sudden ground movements. This is the reason why expolsions and fires are often seen during an earthquake.

S waves usually follow P waves. S waves can only be transmitted by solids not liquids. S waves shake ground material at right angles, or sideways motions, to the direction the wave is traveling. This type of motion produces changes in the shape of the material they encounter, although not its volume. S waves move at the speed of 3.5 km/second (2.2 miles/second). They're responsible for knocking down buildings from their foundations. Although, S waves travel slower than P waves, they cause more damage than P waves due that they shake the ground in a perpendicular direction to the direction the wave is traveling.

Surface waves, including Rayleigh and love waves, can be dramatically destructive due to their complicated circular and twisting motions. A Rayleigh wave produces elliptical motions on the ground with no perpendicular or transverse motion. A love wave has a horizontal motion to the direction in which the wave is traveling. Surface waves move slower than body waves; however, because of their large amplitud and duration, they can be the most destructive types of seismic waves.

Diffracted P waves at the Mantle

Seismic Waves
Seismic Waves | Source

Mohorovičić discontinuity

Seismic wave velocities change significantly when they reach the mantle. The change in velocity is determined by a denser mantle. P and S waves vary in speed at about 35-40 km (22-25 miles) under the continents, and at approximately 10 km (6.2 miles) below the oceans. Below high mountain ranges, the speed of seismic waves may change at depths of approximately 70 km (43 miles).

This boundary where the seismic waves tend to change is known as the Mohorocicic discontinuity. Seismic waves usually travel an approximate 20% slower speed below this discontinuity than above it, and the Mohorocicic discontinuity is considered to be the deepest limit of the Earth's crust. Seismic wave velocity tends to increase with depth; however, the heat present in the mantle slows down seismic waves. Partially molten areas within the mantle tend to slow down P waves, and since S waves are not able to move through liquids, they also prevent the displacement of S waves.

The following video describes with detail the locations at which earthquakes most commonly occur, as well as the geological processes involved in their creation.

Interesting Information About Earthquakes

Alaska is the most seismically active regions in the world, and experience a magnitude 7 earthquake every year, as well as a magnitude 8 earthquake every fourteen years.

Since millions of earthquakes are produced on the planet, there is no way one can be predicted; however, scientists are studying the problem.

The worst earthquake in terms of loss of human lives was one that occurred in Shensi, China, on January 23, 1556. This earthquake claimed the lives of 830,000 people.

The Largest earthquake of the twentieth century was one registered in Chile on May 22, 1960. This earthquake reached a magnitude of 9.5 on the Richter scale.

Percentage of visitors who have experienced an earthquake?

Have you ever experienced an earthquake?

See results

© 2012 Jose Juan Gutierrez


Submit a Comment
  • unvrso profile imageAUTHOR

    Jose Juan Gutierrez 

    7 years ago from Mexico City

    Thanks for stopping by and leaving your comment!

  • profile image

    honey rose 

    7 years ago

    very nice.....


This website uses cookies

As a user in the EEA, your approval is needed on a few things. To provide a better website experience, uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at:

Show Details
HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
LoginThis is necessary to sign in to the HubPages Service.
Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
AkismetThis is used to detect comment spam. (Privacy Policy)
HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the or domains, for performance and efficiency reasons. (Privacy Policy)
Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
MavenThis supports the Maven widget and search functionality. (Privacy Policy)
Google AdSenseThis is an ad network. (Privacy Policy)
Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
Index ExchangeThis is an ad network. (Privacy Policy)
SovrnThis is an ad network. (Privacy Policy)
Facebook AdsThis is an ad network. (Privacy Policy)
Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
AppNexusThis is an ad network. (Privacy Policy)
OpenxThis is an ad network. (Privacy Policy)
Rubicon ProjectThis is an ad network. (Privacy Policy)
TripleLiftThis is an ad network. (Privacy Policy)
Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
ClickscoThis is a data management platform studying reader behavior (Privacy Policy)