EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earth's surface caused by rapid movement of the earth's 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. (Webster " sp. 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 miles 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 toJaramillo 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 Prieto, California (magnitude 7.
0), and the 1995 tremor in K^one, 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 toJaramillo 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 T'ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage.
Good precautions, such as education, emergency planning, and constructing 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) AN EARTHQUAKES ANATOMY Seismologists examine the parts of an earthquake, like what happens to the earth's 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 earth's 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 earthquakes 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 earth's crustal plates move against each other. Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subduct's, or moves under another plate. (The Ocean of Truth p. 54-56) I Faults Stress in the earth's 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 the 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 sudden 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 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 physicist 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 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 followed 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. (The Floor of the Sep. 76-78, 112-115) III CAUSES Most earthquakes are caused by the sudden slip along geologic faults.
The faults slip because of movement of the earth's tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the earth's crust.
(Encarta 98) ELASTIC REBOUND THEORY In 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906 California earthquake. He proposed the elastic rebound theory to explain the generation of earthquakes that occur in tectonic areas, usually near plate boundaries. This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault slips, movement in the crustal rock causes vibrations. The slip changes the local strain out into the surrounding rock. The change in strain leads to aftershocks, which are produced by further slips of the main fault or adjacent faults in the strained region.
The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. On each side of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves. After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake. (Plate tectonics p. 56-59) DISTRIBUTION Seismologists have been monitoring the frequency and locations of earthquakes for most of the 20 th century.
They have found that the majority of earthquakes occur along plate tectonic boundaries, while there are relatively few intra plate earthquakes, that occur within a tectonic plate. The categorization of earthquakes is related to where they occur, as seismologists generally classify naturally occurring earthquakes into one of two categories: inter plate. Inter plate earthquakes are the most common; they occur primarily along plate boundaries. Intra plate earthquakes occur within the plates at places where the crust is fracturing internally.
Both inter plate and intra plate earthquakes may be caused by tectonic or volcanic forces. (Naked Earth p. 134-135) I Tectonic Earthquakes Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault. The released energy is produced by the strain on the rocks due to movement within the earth, called tectonic deformation.
The effect is like the sudden breaking and snapping back of a stretched elastic band. (The Ocean of truth p. 122) II Volcanic Earthquakes Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally, and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.
(Plate tectonics p. 74) LOCATIONS Seismologists use global networks of seismographic stations to accurately map the focuses of earthquakes around the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake types, and the movement of the earth's rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as they move over another weaker rocky layer, was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the earth's plates and how this movement causes earthquakes. They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, deep ocean trenches, and predict which areas will be damaged the most by earthquakes.
It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intra plate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging. (On shifting ground p. 17-19) Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide.
Japan is shaken by more than 1000 tremors greater than 3. 5 in magnitude each year. The western coasts of North and South America are very also active earthquake zones, with several thousand small to moderate.