EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earths
EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earths surface caused byrapid movement of the earths rocky outer layer. Earthquakes occur when energy stored withinthe earth, usually in the form of strain in rocks, suddenly releases.
This energy is transmitted tothe surface of the earth by earthquake waves. The study of earthquakes and the waves they createis called seismology. Scientists who study earthquakes are called seismologists. (Webstersp.423) The destruction an earthquake causes, depends on its magnitude or the amount of shakingthat 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 createtsunamis (large sea waves).
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Many Lives can be lost because of this destruction. (The Road toJaramillo p.211) Several hundred earthquakes, or seismic tremors, occur per day around theworld.
A worldwide network of seismographs detect about one million small earthquakes peryear. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8.6 on theRichter 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 7.0), andthe 1995 tremor in Kbe, Japan (magnitude 6.8), occur about 20 times a year.
Moderateearthquakes 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 byearthquakes 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 constructing stronger, moreflexible structures, can limit the loss of life and decrease the damage caused by earthquakes. (TheRoad to Jaramillo p.213-215,263) AN EARTHQUAKES ANATOMY Seismologists examinethe parts of an earthquake, like what happens to the earths surface during an earthquake, how theenergy of an earthquake moves from inside the earth to the surface, and how this energy causesdamage.
By studying the different parts and actions of earthquakes, seismologists learn moreabout their effects and how to predict ground shaking in order to reduce damage. (On ShiftingGround p.109-110) Focus and Epicenter The point within the earth along the rupturinggeological fault where an earthquake originates is called the focus, or hypocenter. The point onthe earths surface directly above the focus is called the epicenter. Earthquake waves begin toradiate out from the focus and follow along the fault rupture. If the focus is near the surfacebetween 0 and 70 km (0 and 40 mi.
) deep shallow focus earthquakes are produced. If it is deepbelow the crust between 70 and 700 km (40 and 400 mi.) deep a deep focus earthquake willoccur. 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 morestrain. (The Ocean of Truth p.
76 & The road to Jaramillo p.94-97) Seismologists know fromobservations that most earthquakes originate as shallow-focus earthquakes and most of themoccur near plate boundaries areas where the earths crustal plates move against each other. Otherearthquakes, including deep-focus earthquakes, can originate in subduction zones, where onetectonic plate subducts, or moves under another plate. (The Ocean of Truth p.54-56) I FaultsStress in the earths crust creates faults places where rocks have moved and can slip, resulting inearthquakes. The properties of an earthquake depend strongly on the type of fault slip, ormovement along the fault, that causes the earthquake.
Geologists categorize faults according tothe direction of the fault slip. The surface between the two sides of a fault lies in a plane, and thedirection of the plane is usually not vertical; rather it dips at an angle into the earth. When therock hanging over the dipping fault plane slips downward into the ground, the fault is called anormal fault. When the hanging wall slips upward in relation to the bottom wall, the fault iscalled a reverse fault or a thrust fault. Both normal and reverse faults produce verticaldisplacements, or the upward movement of one side of the fault above the other side, that appearat the surface as fault scarps. Strike slip faults are another type of fault that produce horizontaldisplacements, or the side by side sliding movement of the fault, such as seen along the SanAndreas fault in California.
Strike-slip faults are usually found along boundaries between twoplates that are sliding past each other. (Plate Tectonics p.49-53) II Waves The sudden movementof rocks along a fault causes vibrations that transmit energy through the earth in the form ofwaves. Waves that travel in the rocks below the surface of the earth are called body waves, andthere are two types of body waves: primary, or P, waves, and secondary, or S, waves. The Swaves, also known as shearing waves, cause the most damage during earthquake shaking, as theymove the ground back and forth.
(Plate tectonics p.133) Earthquakes also contain surface wavesthat travel out from the epicenter along the surface of the earth. Two types of these surface wavesoccur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, namedafter British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as theyshake the ground underneath the foundations of buildings and other structures.
Body waves, or Pand S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P wavesare compression waves because the rocky material in their path moves back and forth in the samedirection as the wave travels alternately compressing and expanding the rock. P waves are thefastest seismic waves; they travel in strong rock at about 6 to 7 km (4 mi.) per second. P wavesare 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 eitherside to side or up and down perpendicular to the direction the waves are traveling, thus shearingthe rocks. Both P and S waves help seismologists to locate the focus and epicenter of anearthquake. As P and S waves move through the interior of the earth, they are reflected andrefracted, or bent, just as light waves are reflected and bent by glass.
Seismologists examine thisbending 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 thatcontains the direction of the wave travel. This circular movement is somewhat like a piece ofseaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type ofsurface wave, the Love wave, causes rock to move horizontally, or side to side at right angles tothe direction of the traveling wave, with no vertical displacements. Rayleigh and Love wavesalways travel slower than P waves and usually travel slower than S waves.
(The Floor of the Seap.76-78, 112-115) III CAUSES Most earthquakes are caused by the sudden slip along geologicfaults. The faults slip because of movement of the earths tectonic plates. This concept is calledthe elastic rebound theory.
The rocky tectonic plates move very slowly, floating on top of aweaker rocky layer. As the plates collide with each other or slide past each other, pressure buildsup within the rocky crust. Earthquakes occur when pressure within the crust increases slowlyover hundreds of years and finally exceeds the strength of the rocks.
Earthquakes also occurwhen human activities, such as the filling of reservoirs, increase stress in the earths crust.(Encarta 98) ELASTIC REBOUND THEORY In 1911 American seismologist Harry FieldingReid studied the effects of the April 1906 California earthquake. He proposed the elastic reboundtheory to explain the generation of earthquakes that occur in tectonic areas, usually near plateboundaries. 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 causesvibrations. The slip changes the local strain out into the surrounding rock. The change in strainleads to aftershocks, which are produced by further slips of the main fault or adjacent faults in thestrained region.
The slip begins at the focus and travels along the plane of the fault, radiatingwaves out along the rupture surface. On each side of the fault, the rock shifts in oppositedirections. The fault rupture travels in irregular steps along the fault; these sudden stops andstarts of the moving rupture give rise to the vibrations that propagate as seismic waves. After theearthquake, strain begins to build again until it is greater than the forces holding the rockstogether, then the fault snaps again and causes another earthquake. (Plate tectonics p.
56-59)DISTRIBUTION Seismologists have been monitoring the frequency and locations of earthquakesfor most of the 20th century. They have found that the majority of earthquakes occur along platetectonic boundaries, while there are relatively few intraplate earthquakes, that occur within atectonic plate. The categorization of earthquakes is related to where they occur, as seismologistsgenerally classify naturally occurring earthquakes into one of two categories: interplate andintraplate. Interplate earthquakes are the most common; they occur primarily along plateboundaries. Intraplate earthquakes occur within the plates at places where the crust is fracturinginternally. Both interplate and intraplate earthquakes may be caused by tectonic or volcanicforces.
(Naked Earth p.134-135) I Tectonic Earthquakes Tectonic earthquakes are caused by thesudden release of energy stored within the rocks along a fault. The released energy is producedby the strain on the rocks due to movement within the earth, called tectonic deformation. Theeffect is like the sudden breaking and snapping back of a stretched elastic band. (The Ocean oftruth p.
122) II Volcanic Earthquakes Volcanic earthquakes occur near active volcanoes but havethe same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by theupward movement of magma under the volcano, which strains the rock locally, and leads to anearthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rockmasses and causes continuous tremors that can last up to several hours or days.
Volcanicearthquakes occur in areas that are associated with volcanic eruptions, such as in the CascadeMountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such asHawaii. (Plate tectonics p.74) LOCATIONS Seismologists use global networks of seismographicstations to accurately map the focuses of earthquakes around the world. After studying theworldwide distribution of earthquakes, the pattern of earthquake types, and the movement of theearths rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as theymove over another weaker rocky layer, was the main underlying cause of earthquakes. The theoryof plate tectonics arose from several previous geologic theories and discoveries.
Scientists nowuse the plate tectonics theory to describe the movement of the earth’s plates and how thismovement causes earthquakes. They also use the knowledge of plate tectonics to explain thelocations of earthquakes, mountain formation, deep ocean trenches, and predict which areas willbe damaged the most by earthquakes. It is clear that major earthquakes occur most frequently inareas with features that are found at plate boundaries: high mountain ranges and deep oceantrenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousandsof 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, atthe edges of the Pacific plate, are responsible for an average of 80 percent of the energy releasedin earthquakes worldwide.
Japan is shaken by more than 1000 tremors greater than 3.5 inmagnitude each year. The western coasts of North and South America are very also activeearthquake zones, with several thousand small to moderate