Universe Today

Early earthquake picture posted on Twitter by @tapeks.

Although there are earthquakes caused by human activities, such as those that result from the injection of fluids into deep wells for waste disposals, these tremors are not as frequent or as destructive as the large earthquakes due to natural causes.

Earthquakes are mainly caused by plate tectonic activities. These refer to the movements of pieces of the Earth's lithosphere. The same activities are responsible for the formation of volcanoes as well as other imposing geological features on the planet like Mount Everest and the Mariana Trench.

The movement of plate boundaries can be convergent, divergent, or transform (side-to-side). Despite their great size (imagine the size of continents), the movement of plates can carry on smoothly if no locking occurs. However, locking does happen and a lot of stess can build up between the two boundaries as they strive to continue moving in opposite directions.

Once the stress is able to overcome whatever it is that is keeping the boundaries from moving relative to one another, the two surfaces then slip. The whole locking and slipping thing is also known as a stick-slip behavior. The sudden slip can likewise cause an abrupt release of stored energy. This energy is then propagated through seismic waves.

Seismic waves can be detected by a seismograph and is one of main devices used to determine whether a quake happened or not.

This stick-slip behavior can happen in faults, i.e., fractures (large cracks) between two blocks of rocks. Faults can be classified depending on the behavior of the slip. There's the dip slip, wherein the relative motion is nearly vertical. Another one is the transcurrent or strike slip, wherein the relative motion is nearly horizontal. Finally, there's a combination of the two, known as oblique-slip.

In convergent boundaries, one plate can get subducted below another. In the last big Chilean earthquake, which happened last February 27, 2010 and registered a magnitude of 8.8, the event was caused by the subduction of the Nacza plate under the South American plate.

Chile is located in the Pacific Ring of Fire, where most earthquakes and volcanic eruptions occur. This horseshoe-shaped region is also vulnerable to earthquake-generated tsunamis.

If you want to learn more in order to have a more detailed answer to the question, "What causes earthquakes?", I recommend you read the following articles below:

Satellites Show How Earth Moved During Earthquake
Earthquake Should Show a Gravity Scar

There's more about it at USGS. Here are a couple of sources there:

Indian Ocean Earthquake Triggers Deadly Tsunami
The Great 1906 San Francisco Earthquake

Here are two episodes at Astronomy Cast that you might want to check out as well:

Plate Tectonics
Volcanoes, Hot and Cold

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The plate tectonic theory is an amalgamation of the concepts under the continental drift and sea-floor spreading theories.

To grasp the basic concept, let us first familiarize ourselves with the characteristics of the Earth's upper layers: the lithosphere and the mantle. Imagine the lithosphere as the shell of the planet, divided into pieces. This lithosphere floats above the denser but fluid-like asthenosphere, the upper portion of the mantle.

As the name suggests, continental drift refers to the movement of continents that have allowed them to split up from a single land mass to the arrangement they have now. This theory was first suggested by Alfred Wegener in 1912. By just looking at a map, it is easy to see how Wegener might have come up with such an idea.

There is widespread evidence supporting the theory that the continents may have been part of one land mass. Among them: plants and animals (whether alive or fossilized) found in places separated by oceans, opposite coastlines that seem to match like pieces of a jigsaw puzzle, as well as the presence of Permo-Carboniferous glacial sediments on various continents.

Seafloor spreading is the result of diverging motion of mid-ocean ridges, which are basically part of the lithosphere. When this part of the lithosphere diverges, volcanic material from the mantle rises to fill the gap. Eventually, the volcanic material cools, solidifies and forms domes. As the diverging motion continues, more volcanic material pushes the solidified material upward, and this now becomes a new part of the sea floor.

One proof that the seafloor is constantly (although very slowly) spreading is the presence of parallel magnetic stripes on the seafloor. When the volcanic material is still melted, the magnetic minerals that comprise it align with the polarity of the Earth's magnetic field. When they cool and solidify, they form one "stripe".

The magnetic field changes polarity every few hundred thousand years. This change in polarity is observed in the material of the parallel stripes. The material on a ridge is noticeably young. Then as one goes farther from it, the materials on the "stripes" are observed to be older and their polarities alternate.

Plate boundaries are the edges of the tectonic plates. At these regions, the plates converge, diverge, or slide with adjacent plates. Since practically all of the action in plate tectonics happen here, it is at these boundaries where earthquakes, volcanic eruptions, and geological formations happen.

The plate tectonic theory is therefore the most comprehensive theory that explains how towering mountain ranges and deep ocean trenches are formed.

You can read more about plate boundaries here in Universe Today. Here are the links:

Tectonic Plates
Lithosphere

Here are the links of two more articles from USGS:

Main Types of Plate Boundaries
Understanding Plate Motions

Here are two episodes at Astronomy Cast that you might want to check out as well:

Plate Tectonics
Volcanoes, Hot and Cold

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The ESO Very Large Array atop Cerro Paranal, northern Chile (ESO).

The European Southern Observatory, which has several telescopes housed in the mountains of Chile, issued a press release that none of the observatories suffered any damage, and they have no reports of any staff that were injured or killed in the magnitutde 8.8 earthquake that struck central Chile on February 27, 2010:

Despite being the 7th strongest earthquake ever recorded worldwide, the ESO observatory sites did not suffer any damage, partly as they are engineered to withstand seismic activity and partly due to their distances from the epicentre. At La Silla, a power cut caused observations to stop during the night. Paranal Observatory, the APEX telescope and the ALMA Operations Support Facility and Array Operations Site were unaffected.

Additionally, the Gemini South Observatory posted on their website that they experienced no significant damage:
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Astronaut Soichi Noguchi, (@Astro_Soichi) who has taken full advantage of being able to use Twitter live from the International Space Station, has been sending down a stream of images he has taken of Chile following the magnitude 8.8 earthquake that hit the country early Saturday. Just recently, he posted the above image, taken directly over Santiago. "Santiago, the capital city of Chile. One day after the Mega earthquake(M8.8) hit the country. We wish the earliest recovery," Noguchi wrote on Twitter. He also took a video of the ISS astronaut's view as they flew over Chile earlier today, below.

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Map of earthquake and aftershocks in Haiti. Credit: USGS

An earthquake is the result of seismic waves. The most common way earthquakes form is when two tectonic plates push against each other and one finally slips under the other one. The energy from this radiates out as seismic waves and an earthquake is formed. Although earthquakes usually form along fault lines, they can also occur in the middle of a tectonic plate. 

Earthquakes are not a rare occurrence; approximately 8,000 of them happen every day, which adds up to over 3 million every year. Southern California alone has about 10,000 earthquakes every year although most of these quakes cannot be felt. Some powerful earthquakes also go unnoticed because they do not occur in populated areas.

Earthquakes have been traditionally measured using the Richter magnitude scale, although scientists typically use the moment magnitude scale nowadays. There is actually no upper limit to the Richter magnitude scale; people tend to think that it only goes up to ten though because no earthquake that was ten or greater on the Richter scale has ever been recorded. However, scientists believe that a 13 on the Richter scale would release energy equivalent to the impact that caused the Chicxulub Crater on the Yucatan Peninsula in Mexico, which is over 110 kilometers in diameter.

Most of the time, the majority of the destruction is not caused by the earthquake itself. Instead it is caused by other disasters that result from the earthquake including landslides, avalanches, and tsunamis.  Some believe that more people died from the fire after the 1906 San Francisco earthquake than form the quake itself. During some large earthquakes, hundreds of thousands of lives have been lost.

The largest recorded earthquake in the United States struck Prince William Sound, Alaska, on March 28, 1964. The earthquake measured 9.2 on the Richter scale. The largest earthquake recorded in the world was 9.5 on the Richter scale and occurred in Chile in 1960. During the 1960 Chilean earthquake, scientists recorded seismic waves that reached around the Earth.

Earthquakes do not only occur on the West Coast of America. They only occur there more frequently. Between 1975 and 1995, only four states – North Dakota, Iowa, Wisconsin, and Florida – did not experience an earthquake. The state in which the most earthquakes occur every year is not California; it is actually Alaska. Nearly every year, Alaska experiences an earthquake that is 7 on the Richter scale.

Universe Today has articles on earthquakes and biggest earthquakes.

For more information, take a look at how earthquakes work and USGS.

Astronomy Cast has an episode on Earth.

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Caption: NASA Earth Observatory image by Jesse Allen

At 21:53 UTC on January 12, 2010 an earthquake with a magnitude 7.0 struck the Caribbean nation of Haiti. The US Geological Survey (USGS) says that it was the most violent earthquake to strike the impoverished country in a century, and death tolls are reported to be rising into the hundreds of thousands. "Porte-au-Prince is probably one of the worst constructed cities in the world, and even the presidential palace collapsed," said Roger Bilham, from the University of Colorado-Boulder. "This is an earthquake many of us were expecting to occur sooner or later."
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Sri Lanka Tsunami pictures

Like earthquakes, tsunamis can be caused by tectonic plates diverging and converging. When the plates move, it can displace the water above them, causing a tsunami. Most tsunamis are caused by earthquakes. It was actually the Greek historian Thucydides (c. 400 B.C.) who first though that tsunamis were caused by submarine earthquakes particularly subduction earthquakes. Subduction earthquakes are where one tectonic plate slips underneath the other one; they tend to displace more water.

It is often difficult for people to recognize a tsunami in the open ocean because the waves are much smaller further out than they are close to shore. Close to shore, waves can reach at least 30 meters tall. In the open ocean, the tsunami can reach speeds of 800 km per hour. It slows to only about 80 km per hour near shore. The first part of the tsunami that hits is known as the drawback. Instead of a wave coming in, the water actually recedes as if there was an outgoing tide. The drawback can serve as a warning sign that a tsunami is coming. When people do not realize it is a warning sign though, they may check out the exposed areas on shore in order to explore and get in danger when it strikes.

Earthquakes are measured on a scale and so are tsunamis. One of the worst tsunamis in the past decade was the 2004 tsunami in the Indian Ocean. Approximately 300,000 people, most likely more, died in the natural disaster, which caused severe damage to a number of countries.

Universe Today has articles on about tsunamis and causes of tsunamis.

For more information, try tsunami and causes of tsunamis.

Astronomy Cast has an episode on Earth.

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Earthquake near Chengdu, China

Many earthquakes occur every year; however, most of them are not powerful enough to cause any serious damage. In fact, many earthquakes are too slight to even be felt. Major earthquakes do occur however and cause terrible damage. Because most earthquakes are not major, many people do not realize exactly how frequently earthquakes occur. More than 3 million earthquakes occur each year, which means approximately 8,000 earthquakes each day.

Most earthquakes occur in specific regions. This is because they usually happen along the borders of tectonic plates. That is why so many earthquakes happen in California – there is a major fault line near the coast of California.

Even though major earthquakes do not happen that often, they can cause substantial damage. One reason why earthquakes cause so much damage is that they can trigger other natural disasters, such as tsunamis and volcanoes, which are also linked to tectonic plates. Additionally, earthquakes can also trigger fires when gas or electrical lines are damaged and floods when dams break. Some famous earthquakes that resulted in terrible damage to property and life include the 1906 San Francisco Earthquake and the earthquake that destroyed Pompeii in 62 AD.

Scientists measure earthquakes using a seismometer, which is an instrument that has been around for centuries. There is also a method of measuring the intensity of an earthquake. It is known as the Richter scale. The scale starts at one and goes up in intensity. Although there is no upper limit to the scale, most people set ten as the upper limit because no earthquakes greater than ten have been recorded.

Universe Today has articles on famous earthquakes and the Richter scale.

For more information, you should check out earthquakes and how earthquakes work.

Astronomy Cast has an episode on Earth  you will definitely enjoy.

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Beichuan Region of China, before and after Earthquake. Image credit: Formosat 2

Richter magnitude scale is used to measure the size of earthquakes.  It is also known as the local magnitude scale and was created in 1935 by Charles Richter and Beno Gutenberg both from the California Institute of Technology. Originally, the scale was created in order to differentiate between the smaller and larger earthquakes observed in California.  At first, the scale only worked on one type of seismograph. That changed however, and now any calibrated seismograph can use the Richter scale. When Richter first designed the system, the size of the earthquakes was reported using quarter units. However, the system later changed to using decimals.

The Richter scale measures the amplitude of waves created by the earthquake then uses a logarithm to determine the numeric value of the earthquake. For every whole number increase in the scale, the amplitude of the quake is ten times greater. The Richter scale goes from zero with stronger earthquakes in higher numbers. The Richter scale does not have a higher limit. Generally, earthquakes below 2.0 on the Richter scale cannot even be felt; these are known as microearthquakes. Earthquakes between 2.0 and 3.9 almost never cause damage. Earthquakes between 4.0 and 5.9 can begin to cause damage while those 6.0 and above can start causing severe damage to the surroundings. An earthquake 10.0 or above on the Richter has never been recorded. The intensity of an earthquake and the damage it causes also depend on other factors, such as geological conditions and the distance to the origin of the earthquake.

As the quakes increase in magnitude on the Richter scale, they decrease in frequency. For example, around 8,000 microearthquakes are believed to occur each day. However, earthquakes that are between 8.0 and 8.9 on the Richter scale only happen about once a year. Some of these great earthquakes include the Sumatra earthquake in Indonesia in 2007, the 1960 Valvida earthquake in Chile, and the Mexico City earthquake in 1985.

A newer scale to measure earthquakes is the moment magnitude scale (MMS). This method, developed in the 1970’s, measures the earthquakes by how much energy they release. It uses the same values for the quakes that the Richter scale does; the new method is used by the United States Geological Survey to measure all new large earthquakes. For small earthquakes, however, the Richter scale is still generally used. Neither of these scales actually measure the intensity of the earthquake, meaning movement and damage caused by the earthquakes.

Universe Today has articles on the Torino scale and the biggest earthquake.

For more information, check out the Richter magnitude scale and the Richter scale.

Astronomy Cast has an episode on Earth.

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Here are some earthquakes, pictures from space!

Bam Iran

This is a picture of Bam Iran, one day after a powerful earthquake struck the city in 2003. Much of the city just crumbled like sand when the earthquake hit, with 70% of the building flattened by the 6.7 magnitude earthquake.

Earthquake near Chengdu, China

This is a pair pictures of a region in China that was struck by a powerful earthquake. You can see how devastating landslides destroyed regions around the riverbanks, damming up the river.

Beichuan County

This is a picture of Beichuan County before and after the huge earthquake devastated the region. The earthquake changed the direction of rivers and created lakes over night.

We have written many articles about earthquakes for Universe Today. Here's an article about how the ground moves after powerful earthquakes, and powerful new tools to forecast earthquakes.

You can get lots of pictures of volcanoes and earthquakes from NASA's Earth Observatory.

We have recorded an entire episode of Astronomy Cast about our planet. Listen to it here, Episode 51: Earth.

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Ring of Fire Volcanoes

The Ring of Fire is home to the most naturally volatile places in our planet. 90% of the world's earthquakes occur here and 80% of the biggest ones that were ever recorded in the history books happened in this same region. In addition to that, it is where you can find 75% of the Earth's active and dormant volcanoes.

Because tsunamis are caused by disturbances in great bodies of water, the frequent volcanic eruptions and earthquakes in the Ring of Fire have produced tsunamis every now and then. The 9.5-rated Great Chilean Earthquake of 1960 produced a series of tsunamis that pounded the eastern shores of countries like the Philippines and Japan, both located on the opposite side of the Pacific Ocean from Chile.

The 2004 Indian Ocean Tsunami obviously did not take place in the Pacific. However, the devastation it caused prompted governments to speed up the deployment of early warning devices especially in the one place in the world where tsunamis had the highest probability to strike – The Ring of Fire in the Pacific Ocean.

Almost all of the DART buoys are positioned along the Ring of Fire. DART stands for Deep-ocean Assessment and Reporting of Tsunamis and is an advanced tsunami warning system. Tsunami indicators are detected by pressure-sensitive devices below the buoys. The buoys then send these data to a satellite, which in turn transmits the information to land-based stations.

Once the proper authorities are notified, people in places that may be affected can be warned and evacuation procedures can commence.

Among the biggest eruptions that have taken place in the Ring of Fire were Mount St. Helen's in 1980, Mount Krakatoa's in 1883, Mount Pinatubo's in 1991, and Mount Redoubt's in 1989.

All these eruptions and earthquakes are caused by the plate movements in the region. In the east, ring activity is caused by the subduction of the Nazca and Cocos plates under the South American Plate. In the north, it is caused by the subduction of the Pacific Plate under the North American plate. In the west, activity is caused by the subduction of the Pacific Plate along the Kamchatka Peninsula.

There are other plate interactions not mentioned above and you can be sure they contribute to the volatility of the region.

We have some articles in Universe Today that are related to the Ring of Fire. Here are two of them:

• Causes of Tsunamis
• Famous Earthquakes

Ring of Fire articles brought to you by NASA, here are the links:

• FLYING LABORATORY BEGINS PACIFIC RIM EARTH SCIENCE STUDIES
• 7.3-Magnitude Quake off the Honduras Coast

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Ultraviolet Astronomy
• Plate Tectonics

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Tsunamis, translated from Japanese to English as 'harbor waves', are giant waves caused by any major disturbance in a big body of water. Perhaps, for majority of the younger generations around the world, the most deadly natural disaster they've ever witnessed (firsthand, on the Web, or on TV) is the 2004 Indian Ocean Tsunami. That earthquake/tsunami combo claimed over 220,000 lives. The 2004 Indian Ocean Tsunami was caused by a 9.3-magnitude earthquake that lasted for about 10 minutes. Other causes are volcanic eruptions, underwater explosions, and landslides.

Tsunamis are hard to detect offshore because they may only initially rise up to about 30 cm. It is only when they approach land that they rapidly swell. The 2004 tsunami reached 30 meters high (that's about 100 feet). This altitude is easily dwarfed by the biggest tsunami ever recorded: The Lituya Bay Tsunami.

Lituya Bay is found in Alaska. The tsunami that hit that area in 1958 climbed up to a height 524 meters. That's nearly a hundred meters taller than the Empire State Building. Fortunately, its death toll was not proportionate to its size. This particular tsunami was directly caused by a landslide.

A number of the greatest earthquakes in recorded history resulted in tsunamis. The Great Chilean Earthquake of 1960, which also holds the record for the highest magnitude at 9.5, produced a series of gigantic waves that raced across the Pacific from Chile and finally slammed into the Philippines on the other side of the ocean.

The Great Alaska Earthquake of 1964 (magnitude = 9.2) also produced tsunamis that hit the coasts of Japan and Hawaii.

Extensive research, detection, and monitoring of tsunamis is being conducted by the USGS (US Geological Survey) and the NOAA's Center for Tsunami Research. NOAA stands for National Oceanic and Atmospheric Administration.

Mainly because of the 2004 tsunami, deployment of detection devices have been accelerated. DART (Deep-Ocean Assessment and Reporting of Tsunamis) buoys have since been placed in strategic locations on the world's oceans. Most of these buoys have been deployed along the Pacific Ring of Fire, where most earthquakes, and hence, tsunamis, occur.

Each buoy is attached to a bottom pressure recording (BPR) device, which monitors pressure changes below. Data from the BPR is sent to the buoy, then to a satellite. The satellite then sends the information to various NOAA facilities.

The presence of these monitoring systems can help reduce the damage (particularly to life) future tsunamis can make. Many of the deadly tsunamis had to travel great distances and take hours before slamming into shores and wreaking havoc. Once people manning the monitoring systems issue warnings, the public can be given ample time to evacuate to safer areas.

We have some articles in Universe Today that are related to tsunamis. Here are two of them:

• Tsunami
• Famous Earthquakes

Tsunami articles brought to you by USGS. here are the links:

• Life of a Tsunami
• Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Ultraviolet Astronomy
• Large Scale Structure of the Universe

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The flooding caused by the 1960 Earthquake

Here is where it gets sort of tricky. In modern times there have been no 10.0 or greater earthquakes. The best guess would be the one caused by impact of the asteroid that likely killed the dinosaurs. The crater is still visible in the Yucatan peninsula. Other than that, the largest earthquake ever recorded was the 1960 quake in Chile. Known as the 1960 Valdivia earthquake, it was measured at a magnitude of 9.5 on the Richter scale. Ironically it took very few lives as the foreshocks or first tremors caused a lot of people to evacuate buildings before the main event occured. However the earthquake was so powerful that it literally changed the face of Earth in Chile. The effects were dramatic. There were massive landslide and entire rivers were either dammed up or changed their course. The most dramatic effect is that portions of coastal city were flooded because of plate subduction. This is where one tectonic plate is pushed down by another. This normally happens too slowly for anyone to really notice but the earthquake literally sped the process up.

Even for its raw power, how do we know that the 1960 Chile Earthquake is truly the biggest? We know that Earthquakes are quite powerful. As a matter of fact some earthquakes on record have released so much energy that it would take several atomic explosions to be able to artificially reproduce it. The issue is that in the Earth's long history there have been several candidates for the title of biggest earthquake. In the way we now understand it, the biggest Earthquake is the largest one that has occurred since mankind started to have the knowledge and technology to record and measure the strength of earthquakes.

To understand what it takes to be the biggest earthquake we also need to know exactly how they happen. When you get down to the bare facts, an earthquake is merely the side effect of plate tectonics. Beneath the earth crust is a veritable ocean of melted rocks called magma. Like the conventional ocean it has currents. The tectonic plates that make up the Earth's crust are like big barges being moved by the currents of magma. Like boats without guidance these plates collide with each other with one plate going over another and forcing it underground. The stress created by these collisions create cracks or lines of stess in the Earth's crust called fault lines. Normally it takes time for pressure to build between plates. Earthquakes occur when the critical point for that seismic pressure releases.

If you enjoyed this there are other articles on Universe Today that you will enjoy. There is an interesting article on the effects of tsunamis. There is also an interesting article on what causes tsunamis.

There are also great resources on the net. There is a great article on extremescience.com. about the 1960 Chile Earthquake. The USGS website also has great info on the world's strongest earthquakes.

You can also check out Astronomy Cast. Episode 142, Plate Tectonics is quite interesting.

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Tsunamis are some of the most devastating natural disasters known to man. Think of a flood with its source being an ocean and you can grasp a little of how much devastation tsunamis can create. For most people who live in land the greatest threat is from overflowing rivers and creeks. Normally extraordinarily heavy rainfall causes rivers and and other waterways to overflow. The excess water creates deadly currents and sweep away people, causing them to drown. It also does a lot of damage in the initial surge and then with standing water. A tsunami has all of these detrimental effects plus the added destructive power crashing waves.

As you many know a tsunami is caused by a strong earthquake on the ocean bed. The vibrations travel through the water traveling sometimes thousands of kilometers. If you were on the water or deep sea diving in SCUBA gear you would not notice much probably just rough waves or a momentarily strong downward pull if you were underwater. However, a tsunami gains its true destructive power as it approaches land. The water level becomes shallower causing the waves caused by the earthquake to compress and combine. This is what creates the massive and destructive waves that cause so much destruction.

Imagine over several tons worth of water either falling on you or surging towards you. You would have a better chance at the Running of the Bulls. The waves not only sweep people away, but can also destroy even well built structures. The costs to human life can also be devastating. The deadliest tsunamis in recorded history was the Christmas tsunamis of 2004 in the Indian Ocean. On December 24, 2004, a massive 9.2 earthquake occurred of the island of Sumatra. It created a deadly series of tsunamis that swept Indonesia, India, Madagascar, and Ethiopia. The death toll was estimated to be in the neighborhood of 300,000 to 350,000. This was one of the greatest losses of life due to a major natural catastrophe in modern history.

The immediate destruction is only the beginning of the damage. After the waters retreated there was the elevated risk of disease created by stagnant and contaminated water. Since most tsunamis occur south of the Equator and In the Pacific this only raises the risk of disease further.

There can also be more interesting effects that deal solely with scientific curiosity. The Christmas tsunami was so powerful it actually sped up the rotation of the Earth reducing the length of its sidereal day. The earthquake that spawned it also caused the Earth to vibrate all over by as much as 1 cm.

If you enjoyed this article there are several others that you will enjoy on Universe Today. There is a great article on how GPS can be used to predict tsunamis. There is also an article about famous earthquakes.

There are also great resources online. There is PBS Nova has web page that is a companion site to the documentary it did on the Christmas Indian Ocean Tsunamis. The USGS also has a great article that analyzes the tsunamis.

You can also check out Astronomy Cast. Check out the Questions episode for Nov. 10 2008. It has an interesting take on killer asteroids.

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The crust is the top layer of the Earth's Surface. Did you know that there are 2 types, though? One is called the Oceanic Crust, and the other, the Continental Crust. As its name suggests, the Oceanic Crust is the top layer of Earth that forms the ocean floor. The Continental Crust, however, will be our focus.

We walk on top of and dig down through the Continental Crust when we plant or drill. Even if there is an unstable surface at the very top, like sand, the deeper parts of the Crust are made of harder rocks. The large land masses, continents, have bases made from sedimentary, igneous, or metamorphic rocks, as well as any combination thereof. This shield rock is the oldest known; it's been tested, dated, and found to have been here for 3,960,000,000 years!

Geologists, scientists who study the Earth, believe that shield rock was created when hot molten iron, known as magma cooled. If their math's correct, it happened around the time these rocks formed, almost 4 billion years ago, right? Some of those rocks were so big it took a long time for them to cool. So, even if the rocks were formed 3.9 billion years ago, they might not have cooled for quite some time. Many estimate that the Continental Crust wasn't completely hard for another 60,000,000 to 160,000,000 years.

The top portion of this rock has another name, platform rock. The oldest-known platform rocks are approximately 600,000,000 years old, and can be found in central North America. The sedimentary rock ranges from 1,000 to 2,000 meters thick; that is equivalent to more than a half mile to 1.25 miles. When we put the top and bottom portions of the Continental Crust together, we get what scientists call, a craton. Most cratons are stable and haven't been damaged by earthquakes or volcanoes for hundreds of millions of years.

Around the edges are the continental margins, mostly created by sedimentary rock originally found in the oceans. How is that possible, you ask? Well, it's due to earthquake and volcanic activity. In this case, it's mainly due to a phenomenon called, subduction. You see, the Earth fits together like a puzzle; and, if you try to place the wrong piece into a spot where it fits, but isn't quite right, what happens? Another piece might pop out of place. Sometimes, a continental margin works its way under the oceanic crust. When that occurs, the oceanic layer ends up on top of the continental margin. This is subduction. The most well-known place for this is along The Ring of Fire, an area that covers the edges along the Pacific Ocean. This is why so many and such violent earthquakes, volcanic eruptions, and tsunamis occur in that part of the world.

Universe Today has a wealth of information on this and other related topics. Here are just 2 of those available. The first is entitled,
Earth, Barely Habitable?.

The second is called, Interesting Facts About Planet Earth.

Universe Today also hosts Astronomy Cast, a science program that covers a variety of subjects. Episode 51: Earth, explains this subject in greater detail.

The Encyclopedia of Earth , by Michael Pidwirny has some excellent information, too.

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