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Earthquakes occur on faults. Active Fault. Earthquakes Create Seismic Waves. (also hypocenter). 3 Types of Seismic Waves. P wave: Primary S wave: Secondary Surface waves. Fastest Slowest. Depiction of Seismic Waves. Types of Earthquake Waves. Wave terminology - PowerPoint PPT Presentation
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Earthquakes occur on faults
Active Fault
Earthquakes Create Seismic Waves
(also hypocenter)
3 Types of Seismic Waves
• P wave: Primary• S wave: Secondary• Surface waves
Fastest
Slowest
Depiction of Seismic Waves
Types of Earthquake Waves• Wave terminology
– Period: time for one complete cycle between successive wave peaks to pass
– Wavelength: distance between wave crests– Amplitude: amount of positive, negative wave motion– Frequency: number of peaks per second
Seismograph or seismometer is the machine, seismogram is the record
Northridge earthquake seismograms
Keller, 2002
How do we measure Earthquakes?
• How big is it?• Perceived effects: intensity• Amount of energy released: magnitude
#1: Intensity Scale
• Mercalli Intensity Scale developed in 1902
• Based on effects• Local small quake
is similar to distant large quake
http://www.scec.org/instanet/01news/images/NorthridgeSMap.gif
An example of felt effects using Mercalli Scale
Hypothetical M7.8 earthquake
Felt effects using Mercalli Scale
#2: Richter Magnitude Scale – ML
– Amplitude– Logarithmic scale– Less accurate >M6.5
#3: Moment Magnitude Scale• Moment magnitude is measure of total energy
expended during earthquake– Moment = (shear strength of rocks) x (surface area
of rupture) x (slip distance on fault)– Mw– This is the most common scale for quakes >3.5
Comparison of two >M7.0 quakes
Haiti• 1/10/10• Mw=7.0• Mercalli=IX• Strike-slip fault• Depth=13 km• Deaths: 92,000-230,000
Chile• 2/27/10• Mw=8.8• Mercalli=VIII• Subduction zone• Depth=35 km• Deaths: 521
Logarithmic Scales
Each number is 32X energyEach 2 numbers =1000X
Difference between 7.0 and 8.8 is >500X the energy released
Another example of M~1/f
Ground Shaking causes most damage
• Ground shaking depends on– How much fault moved– Where fault moved
• Local conditions amplify shaking and increase damage
Earthquake Hazards:
Material amplification
Collapsed Floors Punctured by Load-Bearing ColumnSevere resonance oscillations of the buildings caused strain at the juncture between columns and ceiling slabs. The vertical columns were punched through the heavy floors that collapsed around them.
Generalized geologic map of Mexico city showing ancient lake deposits where greatest damage occurred.
Keller, 2002
Ground Acceleration and Shaking Time
• Acceleration – percentage of gravity (g)• Duration of shaking depends on size of earthquake• Quake >magnitude 6 increases area and total time
of shaking• Amount of shaking decreases with distance from
earthquake• Severity of shaking also depends on type of
material waves travel through– Softer material more intense shaking
Shaking in 1995Kobe earthquake,JapanCauses collapseof freeway
Shaking in 1989Loma Prieta earthquake,SF Bay Area
Causes collapse
Secondary Ground Effects• Surface rupture-scarp• Earthquakes often trigger landslides• Can also cause liquefaction
– Soils become almost liquid when shaken, solidify when shaking stops
– Significant damage to structures atop liquefied sediments
• Fires
Surface rupture-scarp
1906 earthquake surface rupture.8’ fence offset above
http://mnw.eas.slu.edu/Earthquake_Center/1906EQ/1906thumb.htmlAnd http://quake.wr.usgs.gov/info/1906/images/fenceoffset_big.html
Australia, 1968, M6.8
Armenia, 1988, M6.9
Borah Peak, ID: 1983 M7.3
California, 1979, M6.9
Landslides caused by 2002 Denali Fault earthquake
1965 Seattle quake M6.5
Liquifaction
Water in sediment causes solid rock to behave like a liquid.
This residential and commercial building sank more than three feet into the partially liquefied soil.
:
Liquifaction: Niigata, Japan, 1964
San Francisco 1906 M8.3
Secondary effects: Fire
Summary• Earthquake wave types• Locating earthquakes• Intensity and scales• Primary Effects: Shaking• Secondary Effects: Landslides, scarps, liquefaction