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« في: يناير 17, 2003, 05:44:45 مساءاً »
Earthquakes
Shaking of earth due to movement of rocks along a fault.
Rocks under stress accumulate strain energy over time.
When stress exceeds strength of rocks, rock breaks.
Strain energy is released as seismic waves. The longer that energy is stored up and is maintained without release, the more likely that a strong earthquake will occur.
*-Types of seismic waves
1. Body waves -- travel through interior
2. Surface waves -- travel on surface of earth
*-Specific Body Waves
Primary or "P" Waves: Primary waves
Highest velocity
Causes compression and expansion in direction of wave travel.
Secondary or "S" Waves:Secondary or shear waves
Slower than P waves but faster than surface waves.
Causes shearing of rock perpendicular to direction of wave
propagation
Cannot travel through liquids
*-Surface Waves or "Love" Waves
Cause vertical & horizontal shaking
Travel exclusively along surface of earth
*-Determining the location of an earthquake
First, distance to earthquake is determined.
1.Seismographs record seismic waves
2. From seismograph record, measure time delay between P & S wave arrival
3. Use travel time curve to determine distance to earthquake as function of P-S time delay
Now we know distance waves traveled, but we don't know the direction from which they came.
Plot a circle around seismograph location; radius of circle is the distance to the quake.
Quake occurred somewhere along that circle.
Do the same thing for at least 3 seismograph stations; circles intersect at epicenter. Thus, point is triangulated and epicenter is located.
*-Determining the magnitude of an earthquake
Magnitude -- measure of energy released during earthquake.
There are several different ways to measure magnitude.
Most common magnitude measure is Richter Magnitude, named for the renowned seismologist, Charles Richter.
*-Richter Magnitude
Measure amplitude of largest S wave on seismograph record.
Take into account distance between seismograph & epicenter.
*-Richter Scale
Remember Richter Scale is logarithmic.
Increasing one whole unit on Richter Scale represents 10 times greater magnitude.
Going up one whole unit on Richter Scale represents about a 30 times greater release of energy.
*-Hazards associated with Quakes
1. Shaking:
Frequency of shaking differs for different seismic waves.
High frequency body waves shake low buildings more.
Low frequency surface waves shake high buildings more.
Intensity of shaking also depends on type of subsurface material.
Unconsolidated materials amplify shaking more than rocks do.
Fine-grained, sensitive materials can lose strength when shaken. They lose strength by liquefaction.
Buildings respond differently to shaking depending on construction styles, materials
Wood -- more flexible, holds up well
Earthen materials -- very vulnerable to shaking.
2. Ground displacement:
Ground surface may shift during an earthquake (esp. if focus is shallow).
Vertical displacements of surface produce fault scarps.
3. Tsunamis
Tsunamis are huge waves generated by earthquakes undersea or below coastal areas.
If earthquake displaces sea surface, wave is generated that can grow as it moves over sea surface.
4. Fires
Usually occurs from shifting of subsurface utilities (gas lines)
*-Earthquake Prediction (?)
How can scientists predict an earthquake?
Currently, that is not possible.
Future technology will monitor subsurface seismic waves and periodic shifting indicative of future slippage.
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