the focus and epicenter of an earthquake the point within earth where rock under stress breaks is...
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The Focus and Epicenter of an Earthquake
• The point within Earth where rock under stress breaks is called the focus
• The point directly above the focus on the surface is the epicenter
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WHAT ARE SEISMIC WAVES?
Vibrations that travel through Earth carrying the energy released during an earthquake
Energy starts at the focus and moves outward
Two types: Body waves
P and S Surface waves
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BODY WAVES: P AND S WAVES Body waves
P or primary/pressure waves
fastest waves travel through
solids, liquids, or gases
Waves compress and expand like an accordian
S or secondary/shear waves
slower than P waves
travel through solids only
Move up and down as well as side to side
Undisturbed Material
P Waves
S Waves
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SURFACE WAVES:
Surface Waves Travel just below or along the ground’s surface Slower than body waves Surface waves are the most damaging and are
especially damaging to buildings.
R waves
L waves
R waves L waves
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Seismographs record earthquake events
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HOW IS AN EARTHQUAKE’S EPICENTER LOCATED? Seismic wave behavior
P waves arrive first, then S waves, then surface waves Average speeds for all these waves is known After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance from the seismograph to the epicenter.
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WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN?
~80% of all earthquakes occur in the circum-Pacific belt
~15% occur in the Mediterranean-Asiatic belt
~remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150,000 quakes strong enough to be felt are
recorded each year
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HOW DO YOU LOCATE AN EPICENTER?
Surface
focus
epicenter
A BWhich seismicStation will Receive the Wave first? A or B
The Focus and Epicenter of an Earthquake•The point within Earth where rock under stress breaks is called the Focus •The point directly above the focus on the surface is the Epicenter.
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HOW DO YOU LOCATE AN EPICENTER?
You must have three different seismographs at three differentLocations - then you can pinpoint the different times each pickedup the earthquake wave. A scientist will do this in a lab.
Surface
focus
epicenter
A B
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HOW IS AN EARTHQUAKE’S EPICENTER LOCATED?
Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter
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HOW IS AN EARTHQUAKE’S EPICENTER LOCATED?
Time-distance graph shows the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves
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HOW ARE THE SIZE AND STRENGTH OF AN EARTHQUAKE MEASURED?
Magnitude: the measurement of earthquake strength based on seismic waves and movement along faults
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THREE WAYS OF MEASURING EARTHQUAKES:
Mercalli Scale measures the intensity of an earthquake
Richter Scale is a rating of the size of seismic waves
Moment Magnitude Scale estimates the total energy released by an earthquake
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MERCALLI SCALE
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RICHTER SCALE
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THE ECONOMICS AND SOCIETAL IMPACTS OF EQS
Damage in Oakland, CA, 1989• Building collapse• Fire• Tsunami• Ground failure
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WHAT ARE THE DESTRUCTIVE EFFECTS OF EARTHQUAKES?
Ground Shaking The earthquake’s amplitude, how long it
lasts, and damage increases in poorly consolidated rocks
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EARTHQUAKE HAZARDS
Local soil conditions: loose soil shakes more violently than the surrounding rock (house built on solid rock will shake less than a house build on sandy soil)
Liquefaction: earthquakes violent shaking suddenly turns loose soil into liquid mud
Aftershocks: buildings weakened by an earthquake may collapse during aftershocks
Tsunamis: earthquakes on the ocean floor cause large waves that can grow to the height of a six story building
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CAN EARTHQUAKES BE PREDICTED?
Forecasting when and where earthquakes will occur and their strength is difficult.
One way seismologist forecast earthquakes in a place is to observe their past strength and frequency.
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EARTHQUAKE HAZARD
Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity.
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FORECASTING EARTHQUAKES A seismic gap is an area along a fault where
relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past. Ex: California
The gap hypothesis states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future.