Earthquakes Chapter 11

Earthquake Causes- Sec 11-1 When force is applied to rocks, they bend and stretch, this is elastic deformation. When the force becomes too much the rock will break, this is its elastic limit. When rocks break they move along faults, or cracks. They can move forward, backward or to the sides depending on the fault line. The force that causes the faults to form come from inside the earth, when plates move.

Plates are large sections of the surface of the Earth. When these plates move they break rock and cause intense vibrations which cause earthquakes. As rocks move past each other along faults, their surfaces sometimes catch and cause them to stop. However, force continues to push the rocks against each other until they break. Then an earthquake is the result. Earthquakes have a large range of intensity and occur when rocks move along, over, or under each other along faults.

Fault Types There are 3 types of faults and they are categorized by the type of force that are used to form them. Tension is the force that pulls rocks apart and it causes normal faults. Here the rock above the fault surface moves downward.

Normal Fault

Reverse Fault The force that causes these faults is compression, which pushes rocks together. Here rock is pushed up and over the rock below the fault surface.

Reverse Fault

Strike-slip Fault Here rocks on either side of a fault line move past each other. This movement is caused by force that is called shear. H.W. pg 303 ques 1-4 &pg 324 ques 6&7

Strike-slip Fault

Earthquake Features Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth. They are the energy that travels through the earth and is recorded on seismographs. The point where this energy is first released is called the focus of the earthquake. The focus can be anywhere from 65km to 700km beneath the surface.

Primary & Secondary and Surface Waves P-waves cause rock particles to move back and forth in the same direction that the wave is traveling. This is similar to a spring or coil. S-waves cause rock particles to move in right angles to the direction of the wave travel. Surface waves cause rock to be moved in a backward rolling motion, and side to side motion. Cause the most destruction and occur at the epicenter of the earthquake, which is directly above the focus. http://hercules.geology.uiuc.edu/~hsui/classes/geo351/images/QTmovies/pwave.mov

Waves

Measuring Waves P-waves are the fastest followed by s-waves and then surface waves. We use a seismograph to measure these waves. They are made with a pendulum with an attached pen and a drum of paper. When the waves reach the seismograph the pen moves but the paper stays still. They measure both horizontal and vertical movement.

Seismograph

Finding the epicenter Three seismograph stations in the area of the quake are needed to find the epicenter. Since p-waves are faster then s-waves, the p waves reach the seismograph first and then the s-waves. We use the time between the p and s waves to determine how far the epicenter is from each station. If the distance from one station is 8km than we draw a circle with a 8km radius around that station. We do this for each station. Where the circles intersect is where the epicenter is. http://vcourseware4.calstatela.edu/VirtualEarthquake/VQuakeExecute.html

Earth Stucture There are 4 layers of Earth. The inner core at the very center of the earth is a solid, and dense and made of mostly iron with traces of silicon, sulfur and oxygen. Its solid b/c of the pressure from the layers above.. The outer core is liquid and also mostly iron. Next is Earths mantle. It’s the largest layer and is made of an upper and lower parts.

Earths crust The upper mantle contains the crust. The crust and the mantle just below it is called the lithosphere, which is where the plates are located and move. It ranges from 5-60km deep. The crust has more silicon and magnesium and less iron then the mantle. H.W. pg 311 ques. 1-4 & pg 324 ques. 1-5, 8-14

People and Earthquakes Seismologists study earthquakes. The energy released from a quake is called the magnitude, and is measured on a Ricther scale. We use the lines on a seismograph, and apply them to the Ricther scale. 9.5 is the highest value on the scale. Every 1.0 increase on the scale is 32x the amount of energy released. A quake of 2.0 is has 32x more energy than a 1.0 quake.

People and Earthquakes Sometimes the vibrations from an earthquake can cause thick, wet soil to become very loose and it will almost act like a liquid. This is liquefaction and causes houses to collapse. Tsunamis are giant , destructive waves of water that are caused mainly, by earthquakes that occur under water. A sure sign that a tsunami is coming is the sudden rush of the shore water toward the sea. H.W. pg 326 Ques. 1-15

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