earthquakes waves & seismograms lecture prepared by mr. b
TRANSCRIPT
Topics
•Elastic rebound theory
•Seismic waves
•Seismograph and seismogram
•Finding distance to an earthquake
•Locating an earthquake
•Distribution of earthquakes
•Depth of earthquakes
•Earthquake intensity
•Earthquake magnitude
Can Earthquakes be Predicted?
Earthquake Precursors
1. changes in elevation or tilting of land surface2. fluctuations in groundwater levels3. magnetic field4. electrical resistance of the ground5. Release of gases
Can Earthquakes be Predicted?Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during, and after earthquakes
– monitor activity along major faults
– produce risk assessments
Elastic Rebound TheoryRocks bend under
stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking willoccur along the fault.Stored elastic energy is released as the earthquake. Rocks“snap back”, or rebound to their original condition.
Types of Earthquake (Seismic) Waves
• Body Waves– P-Waves (primary waves)– S-Waves (secondary waves)
• Surface Waves- L – Love Waves- R – Raleigh Waves (surface, vertical)
Body Waves: P and S waves
• Body waves– P or primary waves
• fastest waves• travel through solids,
liquids, or gases• compressional wave,
material movement is in the same direction as wave movement
– S or secondary waves• slower than P waves• travel through solids
only• shear waves - move
material perpendicular to wave movement
Surface Waves: R and L waves
• Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side movement
– Especially damaging to buildings
How is an Earthquake’s Epicenter Located? Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– 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.
How is an Earthquake’s Epicenter Located?
Time-distance graph showing 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
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
How are the Size and Strength of an Earthquake Measured?
• Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake, magnitude 6.7
• Intensity
– subjective measure of the kind of damage done and people’s reactions to it
– isoseismal lines identify areas of equal intensity
Mercalli Scale of Earthquake Intensity
• Advantages:– No high-tech instruments are required.
• Disadvantages:– Damage depends on geologic materials
and type of structures in area– Damage varies with distance from
epicenter– Subjective - different people may view
damage and effects very differently
The Goofy Mercalli Scale
I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale measures total amount of energy released by an earthquake; independent of intensity
– Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale
Richter Scale Math• Increase by 1 whole number means a 10X
increase in the Magnitude of the quake
• For every increase on the Richter Scale, the amount of energy released increases 30X
Compare a 5.0 to a 7.0 quake
7.0 has 10 X 10 = 100 times greater magnitude
7.0 has 30 X 30 = 900 times more energy!
Compared to a 1.0 quake, a 7.0 has:
10 X 10 X 10 X 10 X 10 X 10 = 1 000 000 greater strength
And 30 X 30 X 30 X 30 X 30 X 30 = 729 000 000 more energy
Earthquake Magnitude aand Worldwide
OccurrenceMagnitude Number/YearLess than 2.0600,0002.0-2.9 300,0003.0-3.9 49,0004.0-4.9 6,2005.0-5.9 8006.0-6.9 2667.0-7.9 18Greater than 8,0 1