Earthquake Properties
http://www.worldphoto360.com/wp-content/uploads/2009/12/Earthquake.jpg
Physical Properties
•Focus – point of rockfailure where theearthquake originates
•Epicenter - The pointon the earth's surfacedirectly above theearthquake's focus
Seismic Waves
•Vibrationsgenerated duringan earthquake
Types of Earthquake Waves
Surface waves
•Travel on Earth’s surface outward from theepicenter in a circular motion pattern
•Complex motion
•Slowest velocity of all waves
Types of Earthquake Waves
Body waves - travel through interior of Earth
1. Primary (P) waves
•Push-pull (compressional) motion
•Travel through solids, liquids, and gases
•Greatest velocity of all earthquake waves
Types of Earthquake Waves
Body waves - travel through interior of Earth
2. Secondary (S) waves
•"Shake" motion
•Travel only through solids
•Slower velocity than P waves
A seismogram records waveamplitude vs. time
http://ww2.lafayette.edu/~malincol/Geol120/seismogram.GIF
The machine that producesseismograms is called a seismograph.
http://upload.wikimedia.org/wikipedia/commons/0/0f/Kinemetrics_seismograph.jpg
http://www.cartage.org.lb/en/themes/Sciences/Earthscience/Geology/Earthquakes/MeasuringEarthquakes/Seismographs/seismograph.GIF
Locating an epicenter requires 3 seismograph stations.
Distance from the epicenter to a station isdetermined by the difference in arrival time betweenP and S waves.
Calculate distances from 3 stations and see wherethe circles intersect.
300 km
500 km
100 km
Measuring The Size of An Earthquake
•Intensity – qualitative; determined by effects onpeople, structures, and the environment
•Magnitude – quantitative; measures energyreleased at the source of the earthquake
Magnitude Scales
•Richter scale - measures earthquake magnitude based on size of largest wave
•Moment magnitude scale - accounts for size of faultrupture, amount of movement at fault, and rocks’stiffness more precise
•Each step represents a 10-fold increase in wave size,or a 30-fold increase in energy released
A magnitude 6 earthquake is 10 times bigger than amagnitude 5 earthquake and releases 30 times more energy.
A magnitude 7 earthquake is 100 times bigger than amagnitude 5 earthquake and releases 900 times more energy.
Moment Magnitude
Moment = µ A D
µ = shear modulus
A = LW = areaD = average displacement duringrupture
Source: USGS
The seismic momentis a measure of thesize of an earthquakebased on the area offault rupture, theaverage amount ofslip, and the forcethat was required toovercome the frictionsticking the rockstogether that wereoffset by faulting.
Magnitude From Amplitude
http://www.earthquakes.bgs.ac.uk/earthquakes/education/eq_booklet/dia_richter_scale.jpg
Let’s try it!
An earthquake has been recorded on 3seismograms.
•Can you locate its epicenter?
•Can you determine its magnitude?
http://www.divinationpower.com/UserFiles/2009/12/1/Earthquake%20measuring%203.7%20in%20magnitude%20jolts%20northeastern%20Bulgaria.jpg
Intensity
Witness observations
I
Felt by very few people; barely noticeable
II
Felt by a few people, especially on upper floors.
III
Noticeable indoors, especially on upper floors, but may not be recognized asan earthquake. Hanging objects swing.
IV
Felt by many indoors, by few outdoors. May give the impression of a heavytruck passing by.
V
Felt by almost everyone, some people awakened. Small objects move. Treesand poles may shake.
VI
Felt by everyone. Difficult to stand. Some heavy items of furniture move,plaster falls. Slight damage to chimneys possible.
VII
Slight to moderate damage in well-built, ordinary structures. Considerabledamage to poorly built structures. Some walls may fall.
VIII
Little damage in specially built structures. Considerable damage to ordinarybuildings, severe damage to poorly built structures. Some walls collapse.
IX
Considerable damage to specially built structures, buildings shifted offfoundations. Noticeable cracks in ground. Wholesale destruction. Landslides.
X
Most masonry and frame structures and their foundations destroyed. Groundbadly cracked. Landslides. Wholesale destruction.
XI
Total damage. Few, if any, structures standing. Bridges destroyed. Widecracks in ground. Waves seen on ground.
XII
Total damage. Waves seen on ground. Objects thrown up into air.
Abbreviated Modified Mercalli Intensity Scale
Intensity
•Intensity when focus depth
•Intensity when distance fromepicenter
•Also depends on what ground is madeof – why?
http://earthquake.usgs.gov/eqcenter/dyfi.php
Did You Feel It?