Earthquake
The earth's crust is always moving, but usually very slowly. Small movements of the crust that you may or may not feel are called tremors. There are more than 6 million tremors each year. Sudden, strong movements of the earth's crust are called earthquakes. They are capable of causing a lot of damage.
Focus and Epicenter
Earthquakes begin deep inside the earth. The place inside the earth where an earthquake starts is the focus. The place on the surface of the earth directly above the focus is called the epicenter. The surface of the earth shakes the hardest is at the epicenter.
Causes of Earthquakes
Earthquakes are caused mostly by faulting. Usually, the rocks on both sides of a fault are pushed together very tight. The rocks do not move. Geologists say that the fault is "locked". Pressure in the rocks increases. When the pressure becomes too great, the rocks break at a weak point. Rocks first slip and move at the focus. As the rocks move, they release energy in the form of vibrations. These vibrations are called seismic waves, or earthquake waves. Seismic waves travel out from the focus in all directons. Imagine throwing a pebble into a pond. At the point where the pebble hits the water, you see waves move outward in all directions. Earthquake waves move out from the focus the same way.
Measuring Earthquakes
A seismograph is an instrument that detects and measures earthquakes. A seismograph can even measure very small tremors that people cannot feel. It makes a record of the movements in the earth's crust on a piece of paper. The record is called a seismogram. It looks like wavy lines. The higher the wavy lines are on the seismograph, the stronger is the earthquake.
The Richter scale
In 1935, Charles Richter developed a scale to measure the energy released by earthquakes. The scale is called the Richter scale. On the Richter scale, an earthquake is given a number between 1 and 10. If a stronger earthquake occurs, a higher number will be used. The higher the number is, the stronger is the earthquake. An earthquake measuring 7 or more on the scale can cause a great deal of damage. Earthquakes that measure 2.5 or less are usually not felt by people. The largest earthquake recorded so far measured 8.9 on the scale.
The Richter scale
In 1935, Charles Richter developed a scale to measure the energy released by earthquakes. The scale is called the Richter scale. On the Richter scale, an earthquake is given a number between 1 and 10. If a stronger earthquake occurs, a higher number will be used. The higher the number is, the stronger is the earthquake. An earthquake measuring 7 or more on the scale can cause a great deal of damage. Earthquakes that measure 2.5 or less are usually not felt by people. The largest earthquake recorded so far measured 8.9 on the scale.
Earthquake Damage
Many new buildings are built to be "Earthquake-proof". They do not fall during an earthquake. Tall buildings my sway. Sometimes tall buildings my move back and forth so much that they tip over. Earthquakes also damage electrical lines, telephone lines, and water pipes. Explosions are caused by broken electric and gas lines. Fires caused by broken gas lines often destroy many buildings.
Tsunami
A great wave that is sometimes caused by and earthquake is called a tsunami. A tsunami forms when the epicenter of an earthquake is on the ocean floor. Out in the open ocean, a tsunami is not very high. Near the shore, the height of the tsunami increases. A tsunami nay be 30m to 40m high. When a tsunami hits land, it can cause a lot of damage. In 1964, a tsunami was caused by an earthquake in Alaska. An entire fishing fleet was almost destroyed. Some of the fishing boats were swept into downtown Kodiak, Alaska.
Predicting Earthquakes
Scientists use past earthquakes to predict future earthquakes. Their predictions may be off by 25 years or more. Scientist use small movements in the earth's crust as a signal to a future earthquake. They look at the ground in the area to see if it has moved up or down. Laser field sattions record the smalllest movements along faults. Lasers are thin, strong light beams. The laser beam is shot into a reflector. By measuring the time it takes the beam to hit the reflection and come back, scientists can find out if any movement has taken place along a fault. Using earthquake infomation, scientists have developed a Seismic Risk Map. This map shows where earthquakes my occur and the kind of damage they may cause.
What are Fold and Faults?
Folding
Over millions of years, pressure in the earth's crust caused sedimentary rocks layers to bend, or fold. flat rock layers were squeezed from the sides. The pressure caused the flat rock layers to move into new positions without breaking. The rocks may have craked, but the rock layers stayed together. Folds look like waves in rock layers. The layers curve up in some places.
An upward fold is an anticline. A downward fold is syncline. Some folds are small enough to be seen in small rocks. Other folds are very large. You often can see anticlines and synclines in road cuts.
Faulting
Pressure deep inside the earth can break rocks. A break in a rock is called a fracture. If the rocks on either side of a fracture move, the break is called a fault. There are four kinds of faults. Faulting causes rocks to move up and down or side to side. The rock layers move only a short distance. Each time movement along a fault occurs, the rock layers move further apart. The pressure eases. When the pressure builds up again and is released, movement occurs again. The four types of faults are: Normal fault, Thrust fault, Strike-slip fault, and Reverse fault.
San Andreas Fault
Over millions of years, pressure in the earth's crust caused sedimentary rocks layers to bend, or fold. flat rock layers were squeezed from the sides. The pressure caused the flat rock layers to move into new positions without breaking. The rocks may have craked, but the rock layers stayed together. Folds look like waves in rock layers. The layers curve up in some places.
An upward fold is an anticline. A downward fold is syncline. Some folds are small enough to be seen in small rocks. Other folds are very large. You often can see anticlines and synclines in road cuts.
Faulting
Pressure deep inside the earth can break rocks. A break in a rock is called a fracture. If the rocks on either side of a fracture move, the break is called a fault. There are four kinds of faults. Faulting causes rocks to move up and down or side to side. The rock layers move only a short distance. Each time movement along a fault occurs, the rock layers move further apart. The pressure eases. When the pressure builds up again and is released, movement occurs again. The four types of faults are: Normal fault, Thrust fault, Strike-slip fault, and Reverse fault.
San Andreas Fault