• Essential Knowledge:• What are the causes of earthquakes?
UNIT 7 (1) PLATE TECTONICS, EARTHQUAKES AND VALCANOES
Earthquake is series of seismic waves or tremors in the earth’s crust. Caused when pressure builds up at edge of plates as they collide or slide past one another.Pressure released a huge amount of energy is released causing the plates to shake.
Seismology-study of earthquakes.
• Earthquake vocabulary:• Tremors-waver of seismic energy which spread
from the focus.• Aftershocks- smaller tremors which may occur
hours or days after original earthquakes. They can be extremely dangerous because they
may cause already damaged building to collapse.
• Focus- (Hypocenter)place deep in the earth’s crust where earthquake begins.
• Epicenter- place on surface of earth, directly above focus. The tremors are strongest at this point.
• Elastic Rebound-rocks bend until the strength of the rock is exceeded.
• Rupture occurs& rocks quicklyrebound to an Undeformed shape.Energy is releasedin waves radiateoutward from fault.
• Richter scale-used to describe strength of earthquake.
• Seismograph-device that measures strength of earthquake.
• Richter scale is logarithmic (6 is 10X stronger than 5).
Magnitude and Energy
5. Magnitude and Intensity
Magnitude Energy Explosive Power Example
3 One ton of Explosives World Trade Center Collapse
2
1 Topple 50-meter tree
One kilogram of explosives
Head-on colision at 60 mph
0 Drop a car 10 meters
Half stick of dynamite Very bad day skydiving
-1 Impact of bullet One gram of explosives
-2 Hammer blow
-3 Dribbling a basketball
Magnitude and Energy
5. Magnitude and Intensity
Magnitude Energy Explosive Power Example
9 U.S. Energy Use for a month
Alaska 1964Indonesia 2004
8 U.S. Energy Use for a day
San Francisco, 1906
7 One Megaton World Series Earthquake, 1989
6 U.S. Energy Use for a minute
Large Thunderstorm
5 One Kiloton
4
3 One ton of explosives World Trade Center Collapse
Seismic waves-response of material to arrival of energy fronts released by rupture. 2 types body waves P and S Surface waves R and L
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 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 seismograph to the epicenter.
How is an Earthquake’s Epicenter Located?Time-distance graph
showing average travel times for P- and S-waves. Farther away a seismograph is from e focus of an earthquake, longer interval between arrivals of the P- and S- waves
How is an Earthquake’s Epicenter Located?
• Three seismograph stations are needed to locate epicenter of an earthquake
• A circle where radius equals distance to epicenter is drawn
• Intersection of circles locates epicenter
• Modified Mercalli Intensity Map– 1994 Northridge, CA earthquake, magnitude
6.7
• Intensity– subjective
measure of kind of damage done & people’s reactions to it
– isoseismal lines identify areas of equal intensity
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
ESSENTIAL KNOWLEDGE:What are the factors contributing to earthquake damage, and other dangers associated with earthquakes.
Can Earthquakes be Controlled?
• Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per month
• Some have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones
Faults- fracture along which blocks of crust on either side have moved relative to one another parallel to the fracture. Strike-slip-vertical fractures where blocks have mostly moved horizontally. Faults are classified bykinds of movement thatoccurs along them.Strike-slip left lateralfault.
• Strike-slip right lateral fault.
• Dip-slip faults-inclined fractures where blocks have shifted vertically
• Dip-slip normal fault. Rock mass above an inclined fault
moves down.
• Dip-slip reverse fault. Rock above fault moves up.
• Normal faults are caused by extension.• Reverse faults are caused by compression. • Reverse faults are often called thrust faults.
• Earthquake Hazards.• Building collapse
• Landslides
• Fire
• Tsunamis
• Most dangerous buildings adobe house• Unreinforced masonry.• Reinforces concrete.• Steel-frame.• Small wood-frame house.
• Tsunamis usually caused by submarine landslides-travel 400mph
• They pass unnoticed at sea, but cause damage on shore.
• There is a warning network around Pacific ocean, then can forecast arrival.
• Damage from a Tsunamis depends on direction of travel of wave, harbor shape, bottom, and tide & weather.
5. Magnitude and Intensity
Earthquake Fatalities Since 1800
7. Predicting earthquakes is not yet possible
Are Earthquakes Getting More Frequent?
7. Predicting earthquakes is not yet possible
Some Important Earthquakes
1755 - Lisbon, Portugal • Killed 70,000, Raised Waves in Lakes all over
Europe • First Scientifically Studied Earthquake 1811-1812 - New Madrid, Missouri • Felt over 2/3 of the U.S. • Few Casualties 1886 - Charleston, South Carolina • Felt All over East Coast, Killed Several Hundred. • First Widely-known U.S. Earthquake
Some Important Earthquakes 1906 - San Francisco • Killed 500 (later studies, possibly 2,500) • First Revealed Importance of Faults 1923 – Tokyo - Killed 140,000 in firestorm1964 - Alaska • Killed about 200 • Wrecked Anchorage. • Tsunamis on West Coast. 1976 - Tangshan, China • Hit an Urban Area of Ten Million People • Killed 650,000
• History and earthquakes• In last 500 years more then 7 million people
have died.• Destroyed economies and food sources.
• Lisbon Earthquake 1755 -60,000 killed –ocean waves 30-40 feet above high tide level swamped city –fires burned 3 days.