Earthquake, shaking of the earths surface caused by rapid movement of the earths rocky outer layer. Earthquakes occur when energy stored within the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the earth by earthquake waves. The study of earthquakes and the waves they create is called seismology. Scientists who study earthquakes are called seismologists. (Websters p. 423) The destruction an earthquake causes, depends on its magnitude or the amount of shaking that occurs. The size varies from small imperceptible shaking, to large shocks felt iles around.
Earthquakes can tear up the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Many Lives can be lost because of this destruction. (The Road to Jaramillo p. 211) Several hundred earthquakes, or seismic tremors, occur per day around the world. A worldwide network of seismographs detect about one million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8. 6 on the Richter scale and caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California (magnitude 7. ), and the 1995 tremor in Kbe, Japan (magnitude 6. 8), occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people. (The Road to Jaramillo p. 213-215)
In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 Tang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Good precautions, such as education, emergency planning, and onstructing stronger, more flexible structures, can limit the loss of life and decrease the damage caused by earthquakes. The Road to Jaramillo p. 213-215,263)
Seismologists examine the parts of an earthquake, like what happens to the earths surface during an earthquake, how the energy of an earthquake moves from inside the earth to the surface, and how this energy causes damage. By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict ground shaking in order to reduce damage. On Shifting Ground p. 109-110) Focus and Epicenter The point within the earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter.
The point on the earths surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and follow along the fault rupture. If the focus is near the surface between 0 and 70 km (0 and 40 mi. ) deep shallow focus earthquakes are produced. If it is deep below the crust between 70 and 700 km (40 and 400 mi. ) deep a deep focus earthquake will occur. Shallow-focus arthquakes tend to be larger, and therefore more damaging, earthquakes.
This is because they are closer to the surface where the rocks are stronger and build up more strain. The Ocean of Truth p. 76 & The road to Jaramillo p. 94-97) Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries areas where the earths crustal plates move against each other. Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate. (The Ocean of Truth p. 4-56) I Faults Stress in the earths crust creates faults places where rocks have moved and can slip, resulting in earthquakes.
The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When he hanging wall slips upward in relation to the bottom wall, the fault is called a reverse fault or a thrust fault.
Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other. (Plate Tectonics p. 49-53) II Waves The udden movement of rocks along a fault causes vibrations that transmit energy through the earth in the form of waves.
Waves that travel in the rocks below the surface of the earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, cause the most damage during earthquake shaking, as they move the ground back and forth. (Plate tectonics p. 133) Earthquakes also contain surface waves that travel out from the epicenter along the surface of the earth.
Two types of these surface waves occur: Rayleigh waves, named after British hysicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures. Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (4 mi. ) per second.
P waves are ollowed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3. 5 km (2 mi. ) per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake. As P and S waves move through the interior of the earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by glass. Seismologists examine this bending to determine where the earthquake originated.
Encarta 98) On the surface of the earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type of surface wave, the Love wave, causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements. Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves.