Earthquakes occur on faults

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