Word Count: 1917EARTH QUAKE REFERENCE FILES EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earths surface caused byrapid movement of the earths rocky outer layer. 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). 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 Kobe, 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