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Earthquakes : 

Earthquakes Alaa M. Omar Geophysics Department 2nd Year

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Contents: Overview on Earth structure and plate tectonics. Definitions around earthquakes. Causes of earthquakes. Recording of earthquakes. Effects of earthquakes. Safety precautions.

Introduction: : 

Introduction: Each year, more than 150,000 earthquakes are recorded by the WWNSS and are analyzed, with the aid of computers, at the earthquake data center in Boulder, Colorado. With this system the exact location, depth, and magnitude of the detectable earthquakes are plotted on regional maps, and information about the direction of fault movement associated with the shock is determined. As a result we able to monitor the details of present plate motion. Indeed, seismology provides some evidence in support of the theory of plate tectonics. But that’s not all. In addition, seismic waves provide the most effective probe to the earth’s interior.

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Basic Definitions: Earthquake: Vibrations of the earth caused by the rapture and sudden movement of rocks that have been strained beyond their elastic limit. Focus: The point within the earth where the initial slippage occurs to generate earthquake energy. Epicenter: The point on the earth’s surface directly above the focus.

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Three Types of Waves Generated by an Earthquake: P (primary) waves: In which particles move back and forth in the direction the wave travels. S (secondary) waves: In which particles move back and forth at right angles to the direction the wave travels. Surface waves: Which travel only in the outer layers of the earth and are similar to waves on water, and it is divided to Rayleigh, and Love waves .

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Difference between magnitude and intensity of earthquake: Magnitude: A measure of the amount of energy released, and is much more precise measure of earthquake than intensity, based on direct measurement of the amplitude of seismic wave. Intensity: It is measured on relation to the effects the earthquake had on human life and generally described in terms of destruction ــــ buildings, dams, bridges, based on reports by witnesses, and it depend on: Total amount of energy released. The distance from the epicenter. Type of rock and degree of consolidation.

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Richter scale…An expression of magnitude with an arbitrary zero for the lowest limits of detection. Each step on the scale represents an increase in amplitude by a factor of 10. Thus, an earthquake with a mag. of 2 produces vibrations with 10 times the amp. of one with mag. of 1, and an earthquake with a mag. of 8 has an amp. 1 million times greater than one with a mag. of 2.

Causes of Earthquake: : 

Causes of Earthquake: Faults ( Kalabsh fault in Aswan). Volcanic activity. Other causes include meteorite impacts, undersea landslides and explosion of nuclear bombs.

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shallow-focus earthquake (70 km) Intermediate-focus earthquake (70 to 300 km) Deep-focus earthquake (300 to 700 km) at convergent plate boundary Depth of focus α 1/ Max. energy released

Recording of Earthquake: : 

Recording of Earthquake:

Calculating the magnitude: : 

Calculating the magnitude: Measure the distance between the first P wave and the first S wave. In this case, the first P and S waves are 24 seconds apart. Find the point for 24 seconds on the left side of the chart below and mark that point. According to the chart, this earthquake's epicenter was 215 kilometers away. Measure the amplitude of the strongest wave. The amplitude is the height (on paper) of the strongest wave. On this seismogram, the amplitude is 23 millimeters. Find 23 millimeters on the right side of the chart and mark that point. Place a ruler on the chart between the points you marked for the distance to the epicenter and the amplitude. The point where your ruler crosses the middle line on the chart marks the magnitude (strength) of the earthquake. This earthquake had a magnitude of 5.

Finding the epicenter: : 

Finding the epicenter: Check the scale on your map. It should look something like a piece of a ruler. All maps are different. On your map, one centimeter could be equal to 100 kilometers or something like that. Figure out how long the distance to the epicenter (in centimeters) is on your map. For example, say your map has a scale where one centimeter is equal to 100 kilometers. If the epicenter of the earthquake is 215 kilometers away, that equals 2.15 centimeters on the map. Using your compass, draw a circle with a radius equal to the number you came up with in Step #2. The center of the circle will be the location of your seismograph. The epicenter of the earthquake is somewhere on the edge of that circle. Do the same thing for the distance to the epicenter that the other seismograms recorded (with the location of those seismographs at the center of their circles). All of the circles should overlap. The point where all of the circles overlap is the approximate epicenter of the earthquake.

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The focus of an earthquake is determined from the time elapsing between the arrivals of the three types of seismic waves. The time interval between the arrival of the P wave and S wave is a function of the distance from the epicenter. Seismologists have constructed time-distance graphs that can be used to determine the distance to the epicenter of a new quake. The seismic records indicate the distance, but not the direction, to the epicenter; therefore, records from at least three stations are necessary to locate the epicenter.

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Time-distance curve:

Effects of Earthquake: : 

Effects of Earthquake: Violent ground motion accompanying movement along a fracture. This motion can shear and collapse buildings, dams, tunnels. Landslides, Tsunamis, and regional or local submergence of the land. Fires. Flooding (seiches).

Most Destructive Known Earthquakes on Record in the World: : 

Most Destructive Known Earthquakes on Record in the World:

Safety Precautions: : 

Safety Precautions: What to Do Before an Earthquake? Make sure you have a fire extinguisher, first aid kit, a battery-powered radio, a flashlight, and extra batteries at home. Learn first aid. Make up a plan of where to meet your family after an earthquake. Don't leave heavy objects on shelves (they'll fall during a quake). Anchor heavy furniture, cupboards, and appliances to the walls or floor. What to Do During an Earthquake? If you're indoors, stand against a wall near the center of the building, stand in a doorway, or crawl under heavy furniture (a desk or table). Stay away from windows and outside doors. If you're outdoors, stay in the open away from power lines or anything that might fall. Stay away from buildings (stuff might fall off the building or the building could fall on you). Don't use matches, candles, or any flame. Broken gas lines and fire don't mix. If you're in a car, stop the car and stay inside the car until the earthquake stops. Don't use elevators (they'll probably get stuck anyway). What to Do After an Earthquake? Check yourself and others for injuries. Provide first aid for anyone who needs it. Check water, gas, and electric lines for damage. If any are damaged, shut off the valves. Check for the smell of gas. If you smell it, open all the windows and doors, leave immediately. Be careful around broken glass and debris. Wear boots or sturdy shoes to keep from cutting your feet. Stay away from beaches. Tsunamis sometimes hit after the ground has stopped shaking. Expect aftershocks.

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