Earthquake

An Earthquake is in fact the shaking of the ground caused by sudden movements in the earth's The biggest earthquakes are set off by the movement of the geological plates that make up the earth's crust. Some plates slide past each other gently, but others can cause a heavy pressure on the rocks, so they finally crack and slide past each other. By this, vibrations or shock waves are caused, which go through the ground. It is these vibrations or seismic waves which cause an earthquake. The closer to the source of the earthquake (the focus or hypocenter), the more damage occurs. Earthquakes are classified according to the depth of the focus.

0-43 miles (0-70 km) below ground → Shallow Earthquakes
43-186 miles (70-300 km) below ground → Intermediate Earthquakes
Deeper than 186 miles (300 km) below ground → Deep Earthquakes

The closer the focus to the surface, the heavier the earthquake. The earthquake is always the most intense on the surface directly above the focus (Epicenter). In general big earthquakes begin with light vibrations (foreshocks). These are the initial fractures in the rocks. After the main shock, there may be minor aftershocks, most of the time for months. This occurs as the rocks settle down.

Plate Tectonic

Geologists came to the conclusion in the 1960's that the Earth's rigid outer layer (crust and outer, rigid layer of the mantle) was not a single piece, but was broken up into about 12 large pieces called plates. There are three types of plate boundaries divided on the basis of their movement during an earthquake.


Convergent boundaries - two plates collide to form mountains or a subduction zone.

Divergent boundary - two plates are moving in opposite directions as in a mid-ocean ridge.

Transform boundary - two plates are sliding past each other as in the San Andreas fault of California. A transform boundary is like a tear in the Earth's crust. These plates move very slowly across the surface of the Earth as though they were on a conveyor belt. The convection currents in the much hotter mantle continually move the plates about 1/2 to 4 inches per year.

When the plates move they collide or spread apart allowing the very hot molten material called lava to escape from the mantle. When collisions occur they produce mountains, deep underwater valleys called trenches, and volcanoes. As mountains and valleys are being formed natural disasters such as earthquakes and volcanic activity can occur which has affected humans for thousands of years.

Seismology

Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves.

Seismic Waves

Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or by an explosion. It is the energy that travels through the earth and is recorded on seismographs.

Types of Seismic Waves

There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.

P Waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P waves push and pull on rock. Sometimes animals can hear the P waves of an earthquake. Usually we only feel the bump and rattle of these waves.

S Waves

The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock. This wave moves rock up and down, or side-to-side.

Richter Magnitude Scale

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph.

The Richter Scale has no upper limit. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes.

The Richter Scale is not used to express damage. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans.