Earthquake and Tsunami Facts
What Is an Earthquake?
An earthquake is the sudden, and often violent, shaking of the earth caused by the movement of the earth’s crust. The outermost layer of the earth’s crust, the lithosphere, is composed of many large sections, or tectonic plates, that are slowly but continually moving on top of the mantle, the layer just below the crust. Along the plate boundaries and sometimes within plates there are faults, which are rock fractures where the two sides have been displaced relative to each other. Earthquakes most often occur along faults, when the two blocks of earth slip past each other or collide.
The edges of the tectonic plates are rough and sometimes two plates get stuck along a fault. Over time as the plates are prevented from slipping by friction along the fault, energy is stored up until the force is strong enough to overcome the friction. When that occurs, the edges become unstuck and the plates suddenly slip, releasing all the stored energy. This energy is what causes earthquakes. The energy radiates outward from the fault in all directions, creating seismic waves that shake the earth as they travel. Earthquakes can last for only seconds or may continue for several minutes. The location under the earth’s surface where the earthquake occurs is called the hypocenter, and the location on the earth’s surface directly above the hypocenter is called the epicenter.
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What Are Aftershocks?
Aftershocks are smaller earthquakes that occur after the mainshock. They can occur hours, days, weeks, months, and even years after a mainshock. Generally, stronger mainshocks will produce stronger and more numerous aftershocks.
How Are Earthquakes Measured?
The seismic waves produced by an earthquake are recorded on instruments called seismographs, which record a zigzag trace from the ground vibrations beneath the seismograph. The readings from multiple seismographs can be used to determine the time, location, and magnitude of an earthquake.
Historically, the Richter scale, developed by Charles F. Richter in 1935, was used as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake was determined from the logarithm of the amplitude of waves recorded by the seismograph.
The Richter scale can underestimate the energy released by large earthquakes, and consequently it is no longer the common method for measuring earthquake magnitude. Many scales can be used to measure the magnitude of an earthquake, but today the most common is the moment magnitude scale. The moment magnitude scale uses seismographs plus what occurs physically during an earthquake, termed the “seismic moment.” The seismic moment describes how much force is needed to create the generated seismic waves. That information is plugged into the scale and the magnitude of the earthquake is determined. Because earthquake magnitude scales are logarithmic, each whole number increase in the magnitude of the earthquake represents a 10-fold increase in the size of seismic waves measured on the seismograph, and a 30-fold increase in the amount of energy released compared to the previous whole number magnitude.
Can Earthquakes Be Predicted?
No. Scientists have not been able to come up with a way to predict earthquakes. They know that earthquakes typically occur along plate boundaries, but they have no means of telling exactly when and where an earthquake will occur.
Where Do Most Earthquakes Occur in Alaska?
Where earthquakes occur has a lot to do with plate tectonics and where plate boundaries are located. In Alaska the Pacific plate is sliding northwest past southeast Alaska and is being pushed under (subducted) the North American plate in southern Alaska, the Alaska Peninsula, and the Aleutian Islands at about 2 inches per year. The majority of earthquakes in Alaska are the result of those two plates sliding past each other. Earthquakes also occur in the Alaska interior along faults in the crust.
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What Damage Can Earthquakes Cause?
Thousands of earthquakes occur each year in the United States, but most are too small for humans to feel. Larger magnitude earthquakes, however, can cause serious damage. The amount of damage is influenced by the duration of shaking, strength of shaking, type of soil (strength of shaking is greater on soft, thick, and wet soils), and type of building construction. Intense shaking can cause multiple hazards, including:
- Landslides or avalanches.
- Surface faulting. The ground along the two sides of the fault is displaced vertically or horizontally.
- Liquefaction. Sand or silt behaves like liquid during the intense shaking of an earthquake.
- Flash floods. Caused by liquefaction near bodies of water, or by damage to dams.
- Building collapse and damage.
- Destruction of transportation systems such as roads, bridges, airports, railways, and ship ports.
- Destruction of utility services such as water and fuel lines, telecommunications, and electrical power.
What Is a Tsunami?
A tsunami is a series of waves generated by a sudden displacement of a large volume of water, usually caused by earthquakes, underwater landslides, or a volcanic eruption. The series of waves created by these disturbances can travel great distances across the ocean, retaining their energy. In the deep ocean these waves travel at speeds of up to 500 miles per hour, but are not high in elevation (a tsunami is not a tidal wave), and usually pass by unnoticed in the open ocean.
As the waves reach shallower water near the coast, they slow down and may rise up several feet, or even tens of feet in severe cases, and push inland with extreme force. Tsunamis can come ashore as a breaking wave, a wall of water, or a tide-like flood, and can bring rushing water miles inland. Be aware that because tsunamis travel over great distances in the ocean, they can occur thousands of miles away from where an earthquake occurred, in places where the earthquake itself was not even felt.
Earthquakes are capable of creating multiple types of tsunamis. Probably the most familiar are tectonic tsunamis, which are produced from the energy released from the fault during an earthquake and come as a series of waves. Another type, seiche waves, are standing waves that oscillate back and forth in semi-enclosed or fully enclosed bodies of water, and can occur in areas that are not directly connected to bodies of water near the source of the earthquake.
Landslide-generated tsunamis can be the most dangerous type, because they can arrive before the shaking of an earthquake stops, or very shortly after, leaving little or no time for a tsunami warning or evacuation. Landslide tsunamis are created when an earthquake (or other force) creates an avalanche of soil and materials into the water. Water is pushed away from the shore forcibly and then comes crashing back onto land. This type of tsunami can occur from landslides above or below the water. Because these tsunamis can occur very quickly, they are hard to warn against. Consequently it is important to evacuate immediately if a strong earthquake lasts for more than 20 seconds near a coast, because if you wait for a tsunami warning it may already be too late.
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What Are the Natural Warning Signs of a Tsunami?
- A strong earthquake where shaking lasts for 20 seconds or more near the coast. Not all large earthquakes cause tsunamis, but many do.
- The rapid, unusual rise or fall of coastal water.
- Abnormal roar of the coastal water.
Is There a Tsunami Warning System?
Yes. The National Oceanic and Atmospheric Administration (NOAA) National Weather Service operates two tsunami warning centers. The National Tsunami Warning Center in Palmer, Alaska, serves Alaska, Washington, Oregon, California, the US Atlantic and Gulf of Mexico coasts, Puerto Rico, the US Virgin Islands, and Canada. The Pacific Tsunami Warning Center in Hawaii serves Hawaii and the US Pacific territories, and is an international warning center for the Pacific and Indian Oceans and the Caribbean Sea.
If a strong earthquake occurs or tsunami activity is detected by one of these warning centers, NOAA issues tsunami warnings, advisories, watches, or information bulletins to the appropriate emergency officials and the public. The National Tsunami Warning Center has definitions of each of these alert levels. However, be aware that if a strong earthquake near the coast occurs for more than 20 seconds, you should evacuate immediately. There may not be time for you to wait for a tsunami warning.