Earthquakes
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earth·quake
a shaking or trembling of the earth that is volcanic or tectonic in origin
What causes an Earthquake to happen?
Earthquakes occur on faults. A fault is a fracture or area of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. The Earth's tectonic plates are always slowly moving but occasionally they get stuck at their edges due to friction. If the movement occurs suddenly or rapidly, it is referred to as an earthquake. Movement can also occur slowly in the form of a creep. Faults can be a few miles to thousands of miles long.
Where do earthquakes occur?
While earthquakes are sometimes believed to be a West Coast phenomenon, there are actually 45 states and territories throughout the United States that are at moderate to high risk for earthquakes including the New Madrid fault line in Central U.S.
Earthquakes can strike any location at any time but they normally occur along the circum-Pacific seismic belt. About 81 percent of the world's largest
earthquakes occur along this belt thus earning it's nickname of the "Ring of Fire". This belt extends from Chile, northward along the South American coast, through Central America, Mexico, along the West Coast of the United States and the Southern part of Alaska, through the Aleutian Islands to Japan, the Philippine Islands, New Guinea, the island groups of the Southwest Pacific and to New Zeland. This
earthquake belt has created havoc and caused extensive damage.
The second most important belt is the Alpide that extends from Java to Sumatra through the Himalays, the Mediterranean and out in to the Atlantic. This belt is responsible for 17 percent of the world's earthquakes like the August 1968 quake that killed 11,000 people in Iran. A third prominent belt follows the submerged mid-Atlantic Ridge.
What does an earthquake feel like?
The way an earthquake feels depends on several factors. It depends on where you are in relationship to the center of the earthquake and how powerful the earthquake is.
Large Earthquake nearby:
This will feel like a sudden large jolt followed by more strong shaking that can last for only a few seconds to a few minutes. The shaking will be violent and you will find it hard to stand or move around without falling.
Large Earthquake faraway:
This will feel like a gentle bump followed a few seconds later by stronger rolling shaking that may feel like sharp shaking for a little while
Small Earthquake nearby:
You will feel a small sharp jolt followed by a few stronger sharp shakes that pass quickly.
Small Earthquake Far away:
You will most likely not even feel this but if you do it will only be a gentle shake or two. Usually easier to feel if you are still and sitting down.
How are Earthquakes recorded and measured?
Earthquakes are monitored and measured by a seismographic network. Seismic stations have been installed around the world. Each station in the network is responsible for measuring movement of a specific area. Magnitude is the most common measure of an earthquake's size. It is measured by the Richter Scale that was first developed by Charles Richter in the 1930's. It measures the largest wiggle but as more and more stations were installed, it became apparent that an extension of Richter's original scale (Ml-Magnitude Local) was needed. The new scale include body-wave magnitude (Mb) and surface wave magnitude (MS). Intensity is also measured. It is a measure of the shaking and damage caused by the earthquake.
What are the earthquake magnitude classes?
- Great - Magnitude greater than or equal to 8
- Major - Magnitude between 7 and 7.9
- Strong - Magnitude between 6 and 6.9
- Moderate - Magnitude between 5 and 5.9
- Moderate - Magnitude between 5 and 5.9
- Light - Magnitude between 4 and 4.9
- Minor - Magnitude between 3 and 3.9
- Micro - Magnitude less than 3
What are the effects of earthquakes?
Ground Shaking
Ground shaking is used to describe the vibrations of the ground that happen during an earthquake. The severity of ground shaking increases as the magnitude of the earthquake increases. The shaking is caused by the different waves that are created when an earthquake occurs. The primary waves travel through the earth with a speed of about 15,000 miles per hour and are the first waves to create the vibration of a building. The secondary waves arrive next and cause a structure to vibrate from side to side. These can be damaging to buildings because of the horizontal movement they create. The primary and secondary waves are high=frequency vibrations. Finally, the surface waves arrive. These waves are more complex and are low-frequency vibrations. The surface waves have two classifications.
The Rayleigh Waves move up and down like ocean swells along the Earth's surface and the Love Waves move side to side. These are the most damaging to man-made structures because they are complex in their movement.
Surface Faulting or Rupture
This occurs when movement on a fault deep within the earth breaks through to the surface. Not all earthquakes result in a surface rupture.
Damages to the structures located in the fault zone can be very high. The size of the displacement varies have ranged from less than 1 mile to over 200 miles. Most are confined to a narrow zone of 6 to 1000 feet in width.
Ground Failure
The strong motions from an earthquake also cause damages to the ground and soil area. These damages can take many different forms: cracking, fissuring, weakening, sinking, liquefaction and more. Liquefaction occurs when loosely packaged, water-logged sediments lose their strength. If this occurs beneath buildings or other structures, major damage can occur if the ground can no longer support the weight of the building. When the soil supporting a building or some other structure liquefies and loses strength, large deformations can occur within the soil, allowing the structure to settle and tip. Lateral spreads can happen in response to the ground shaking and usually develop on gentle slopes. The horizontal movements on lateral spreads are as much as 10-15 feet but can be as much as 100-150 feet and usually break up internally forming many fissures and scraps.
Landslides
Large earthquake induced rock avalanches, soil avalanches and underwater landslides can be very destructive. The area affected by earthquake-induced landslides depend on the magnitude of the earthquake along with the topography and geologic conditions near the fault. The length of time the ground shakes is a major factor on landslides. In 1970, a Peruvian earthquake caused a rock avalanche that killed over 18,000 people. For more information about landslides, click here.
Tsunamis
Tsunamis are caused by a sudden vertical movement of the sea floor during an undersea earthquake. The movement creates a wave of water and pushes it outward from the area of disturbance. The tsunami is only about a 1 foot high in the deep ocean but the wave crests are very long, sometimes over 60 miles apart. Tsunamis can travel very quickly but as the water depth decreases, the wave will slow. In the mid-Pacific, where the depths reach over 3 miles, tsunami can travel more than 430 miles per hour. As they reach shallow water around islands, the waves increase in height quickly sometimes reaching as high as 80 feet. The great distance between the crest prevents the tsunamis from dissipating energy like other waves as they break surf instead they cause water levels to rise rapidly along the coast lines. For more information about tsunamis, click here.
Earthquake Frequently Asked Questions
Q: During an earthquake you should head for the doorway?
A: Only if you live in an old, unenforced adobe house. In modern homes doorways are no stronger than any other parts of the house and usually have doors that will swing and can injure you. YOU ARE SAFER PRACTICING THE DUCK, COVER, AND HOLD under a sturdy piece of furniture.
Q:What is the biggest earthquake ever?
A:Since 1900, the earthquake in Chile on May 22, 1960, is the biggest in the World with magnitude 9.5 Mw.
Q: What is the biggest earthquake in the United States?
A: Alaska and California
Q: Can I get on a list to receive an email message when there is an earthquake? How do I sign up for earthquake notifications?
A: Yes, please go to Earthquake Notification Services (ENS) to sign up for free.