Earth Science Chapter 8
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- is the vibration of Earth produced by the rapid release of energy within the lithosphere.
- Earthquakes are caused by slippage along a break in the lithosphere.
- are fractures in Earth where movement has occured.
- the point within the Earth where an earthquake starts.
the location on the surface directly above the focus.
the energy released by the earthquake travels in all directions from the focus in the form of seismic waves.
San Andreas Fault
one of the most studied faults in the world.
Cause of Earthquakes
According to the elastic rebound hypothesis, most earthquakes are produced by the rapid release of energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded, it suddenly breaks, releasing some of its stored energy as seismic waves.
Deformation of Rocks
Forces inside Earth slowly deform the rock that makes up Earths crust, causing the rock to change its shape, or bend. As rocks bend, they store elasic energy, just as a wooden stick does when it is bent. Just like elastic energy on a rubber band.
- The tendancy for the deforomed rock along a fault to spring back after an earthquake.
- This is like what happens when you release a streched rubberband.
- Is an earthquake that occurs hours or even weeks after a major earthquake.
- Aftershocks are weaker than the major earthquake.
- Is an earthquake that occurs days or even years before a major earthquake.
- Foreshocks are weaker than major earthquakes.
Earthquakes produce two main types of seismic waves-body waves and surface waves.
- Are push-pull waves that push (compress) and pull (expand) particles in the diection the waves travel.
- These waves are know as compressional waves.
- Can travel trough solids liquids and gases.
- Faster than S and Surface waves.
- Shake particles at right angles to the waves direction of travel.
- Their motion can be modeled by fastening one end of a robe and shaking the other end.
- Travel slower than P Waves.
- Can only travel through solids.
- When body waves reach the surface.
- Surface waves travel slower than P and S waves.
- Move up and down as well as side to side.
- Much bigger than body waves (P and S) meaning they cause more destruction.
Recording Seismic Waves
Scientists have developed an instrument to record seismic waves, the seismograph.
Can consist of a weight suspended from a support attached to bedrock. When seismic waves reach the seismograph, the weight keeps it almost stationary while Earth and the support vibrate.
- a seismograph produces a time record of ground motion during an earthquke.
- Shows all three types of seismic waves. The stronger the earthquake, the larger the waves on the seismogram.
The Richter Scale and the moment magnitue scale measure earthquake magnitude. The Modified Mercalli scale is based on earthquake intensity.
Is based on the height of the largest seismic wave recorded on a seismogram. The Richter Scale is only useful for small, shallow earthquakes within aobut 500 km of the epicenter.
Moment Magnitiude Scale
- Is derived from the amount of displacement that occurs along a fault. Is calculated by several factors in addition to the seismographic data.
- These factors include the average amount of movement along the fault, the area of the surface break, and the strength of the broken rock.
Modified Mercalli Scale
This scale rates an earthquakes intensity in terms of the earthquakes effects at different locations.
Locating an Earthquake
A travel-time graph, data from seismograms made at three or more locations, and a globe can be used to determine an earthquakes epicenter.
Causes of Earthquake Damage
Earthquake-related hazards include seismic shaking, liquefaction, landslides and mudflows, and tsunamis.
- The ground vibrations caused by seismic waves.
- This is the most obvious earthquake hazard.
Where soil and rock are saturated with water, earthquakes can cuase a process called Liquefaction.
Landslides and Mudflows
Earthquakes often cause loose rock and soil on slopes to move. The result is a landslide. Most landslides occur on steep slopes where sediment is loose or where the rocks are highly fractured.
In areas where the water content of soil is high, an earthquake can start a mudflow. During a mudflow, a mixture of soil and water slides rapidly downhill.
- Is a wave formed when the ocean floor shifts suddenly during an earthquake.
- These occur when a earthquake pushes up a slab of ocean floor along a fault. An underwater landslide or volcanic eruption can also trigger an Tsunami.
Reducing Earthquake Damage
Earthquake damage and loss of life can be reduced by determining the earthquake risk for an area, building earthquake resistant structures, and following earthquake saftey precautions.
Layers of the Earth
- Seismic waves travel faster the further into the earth they are.
- change of presure.
- Earths interior consists of three major zones; crust, mantle, and core.
- Crust:thin, rocky, outer layer.varies in thicknessRoughly 7km in oceanic
- regionsContinetal crust averages 8-40km Exceeds 70 km in mountainous
- Continental Crust:
- above sea level
- upper crust composed of granitic rocks
- average density is about 2.7g/cm3
- up to 4 billion years old
- Oceanic Crust:
- basaltic composition
- density about 3.0 g/cm3
- younger (180 million years or less) than the continental crust
- Early seismic data and drilling technology indicate that the continental crust is mostly made of lighter, granitic rocks.
- 82% of earths volume
- below crust to a depth of 2900 km
- composition of the uppermost mantle is the igneous rock peridetite (changes at greater depths)
- Density of 3.4g/cm3
- thought to be mainly dense iorn and nickel, similar to metallic metorites.
- average density of 13g/cm3
crust and uppermost mantle (about 100 km thick) cool, rigid, solid.
- Beneath the Lithosphere
- upper mantle
- to a depth of about 660 km
- soft, weak layer that is easily deformed
- 660-2900 km
- more rigid layer
- rocks are very hot and capable of gradual flow
- liquid layer below mantle
- 2260 km thick
- the flow of metallic iorn generates the earth magnetic field
- radius of 1220 km
- solid state due to the tremendous pressure
- seismic waes passing through the Earth increase in velocity at a level just below 50 km of depth.
- signals a boundary seperating the crust from underlying mantle.
- known for the name of its discoverer.
- P waves coming from an earthquake do not gegister at seismographic stes from about 105 degrees to 140 degrees around the globe.
- Can be explained if Earth contains a core compossed of materials unlike the overlying mantle.
- Refracive bending around the denser outer core.
Discovering Earths Layers
- Meteorites are assumed to have formed a lot of Earths compostion.
- formed of nickle and iorn.
- in liquid Earth, denser material sank into the core as it formed.
- Lighter material stayed on top and formed magma whch comes to the surface.
- Deep-Sea Drilling and seismic data gives details.
The epicenter is located using the difference in the arival times between P and S waves recordings which are related to distance.
Travel time graphs from 3 or more seismographs can be used to find the exact location of an earthquake epicenter.
About 95% of major earthquakes occur in a few narrow zones. Circum Pacific Belt (Ring of Fire) which is Allecutian Islands, Japan, Phillippines, Chile, Mediterranean-Asian Belt, and Ocean Redge Belt
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