Published on September 20, 2007
Earthquakes: Earthquakes Chapter 5 Created by Rachel Joseph An Earthquake is…: An Earthquake is… the shaking and trembling that results from the movement of rock beneath Earth's surface The movement of Earth's plates produces strong forces that squeeze or pull the rock in the crust This is an example of stress, a force that acts on rock to change its volume or shape Stress: Stress There are three different types of stress that occur on the crust, shearing, tension, and compression These forces cause some rocks to become fragile and they snap Some other rocks tend to bend slowly like road tar softened by the suns heat Faults: Faults A fault is a break in the crust where slabs of crust slip past each other. The rocks on both sides of a fault can move up or down or sideways When enough stress builds on a rock, the rock shatters, creating faults Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust too much so the crust smashes Strike-Slip Faults: Strike-Slip Faults Shearing creates this fault In this fault, rocks on both sides of the fault slide past each other with a little up and down motion When a strike-slip fault forms the boundary between two plates, it becomes a transform boundary Normal Faults: Normal Faults Tension forces in Earth's crust causes these types of faults Normal faults are at an angle, so one piece of rock is above the fault, while the other is below the fault The above rock is called the hanging wall, and the one below is called the footwall When movement affects along a normal fault, the hanging wall slips downward Normal faults occur along the Rio Grande rift valley in New Mexico, where two pieces of Earth's crust are diverging Reverse Faults: Reverse Faults Compression forces produce this fault This fault has the same setup as a normal fault, but reversed, which explains it’s name Just like the normal fault, one side of the reverse fault is at an angle of the other This fault produced part of the Appalachian Mountains in the eastern United States How Do Mountains Form?: How Do Mountains Form? The forces of plate movement can build up Earth's surface, so over millions of years, movement of faults can change a perfectly flat plain into a gigantic mountain range Sometimes, a normal fault uplifts a block of rock, so a fault-block mountain forms When a piece of rock between two normal faults slips down, a valley is created Mountains Formed by Folding : Mountains Formed by Folding Sometimes, under current conditions, plate movement causes the crust to fold Folds are bends in rock that form when compression shortens and thickens part of Earth's crust The crashing of two plates can cause folding and compression of crust These plate collisions can produce earthquakes because rock folding can fracture and lead to faults Anticlines and Synclines : Anticlines and Synclines Geologists use the terms syncline and anticline to describe downward and upward folds in rock An anticline is a fold in a rock that arcs upward A syncline is a fold in a rock that arcs downward These folds in rocks are found on many parts of the earths surface where compression forces have folded the crust Plateaus: Plateaus The forces that elevate mountains can also raise plateaus, a large area of flat land elevated high above sea level Some form when a vertical fault pushes up a large flat piece of rock Like a lasagna, a plateau consists of many layers, so it is wider than it is tall How Earthquakes Form: How Earthquakes Form Everyday, about 8,000 earthquakes hit Earth, but most of them are too little to feel Earthquakes will always begin in a rock beneath the surface A lot of earthquakes begin in the lithosphere within 100 km of Earth's surface The focus triggers an earthquake Focus: the point beneath Earth's surface where rock that is under stress breaks Seismic Waves: Seismic Waves Seismic Waves: vibrations that travel through Earth carrying the energy released during an earthquake an earthquake produces vibrations called waves that carry energy while they travel out through solid material During an earthquake, seismic waves go out in all directions to the focus They ripple like when you through a stone into a lake or pond Seismic Waves Ctd.: Seismic Waves Ctd. There are three different types of seismic waves: P waves, S waves, and surface waves An earthquake sends out two of those waves, P and S waves When they reach the top of the epicenter, surface waves form Primary Waves: Primary Waves Also known as P Waves The first waves to come are these waves P waves are earthquake waves that compress and expand the ground like an accordion P waves cause buildings to expand and contract Secondary Waves: Secondary Waves Also known as S Waves After p waves, S waves come S waves are earthquake waves that vibrate from one side to the other as well as down and up They shake the ground back and forth When S waves reach the surface, they shake buildings violently Unlike P waves, which travel through both liquids and solids, S waves cannot move through any liquids Surface Waves: Surface Waves When S waves and P waves reach the top, some of them are turned into surface waves Surface waves move slower than P waves and S waves, but they can produce violent ground movements Some of them make the ground roll like ocean waves Other surface waves move buildings from side to side Detecting Seismic Waves: Detecting Seismic Waves Geologists use instruments called seismographs to measure the vibrations of seismic waves Seismographs records the ground movements caused by seismic waves as they move through the Earth Mechanical Seismographs: Mechanical Seismographs Until just recently, scientists have used a mechanical seismograph a mechanical seismograph consists of a heavy weight connected to a frame by a wire or spring When the drum is not moving, the pen draws a straight line on paper wrapped around the drum Seismic waves cause the drum to vibrate during an earthquake the pen stays in place and records the drum's vibrations The higher the jagged lines, the more severe earthquake Measuring Earthquakes: Measuring Earthquakes There are many things to know about the measures of an earthquake There are at least 20 different types of measures 3 of them are the Mercalli scale, Richter scale, and the Moment Magnitude scale Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults The Mercalli Scale: The Mercalli Scale Developed in the twentieth century to rate earthquakes according to their intensity The intensity of an earthquake is the strength of ground motion in a given place Is not a precise measurement But, the 12 steps explain the damage given to people, land surface, and buildings The same earthquake could have different Mercalli ratings because of the different amount of damage in different spots The Mercalli scale uses Roman numerals to rank earthquakes by how much damage they cause The Richter Scale: The Richter Scale The Richter scale is a rating of the size of seismic waves as measured by a particular type of mechanical seismograph Developed in the 1930’s All over the world, geologists used this for about 50 years Electric seismographs eventually replaced the mechanical ones used in this scale Provides accurate measurements for small, nearby earthquakes Does not work for big, far ones The Moment Magnitude Scale: The Moment Magnitude Scale Geologists use this scale today It’s a rating system that estimates the total energy released by an earthquake Can be used for any kind of earthquakes, near or far Some news reports may mention the Richter scale, but the magnitude number they quote is almost always the moment magnitude for that earthquake Locating the Epicenter: Locating the Epicenter Sine the P waves travel faster than the S waves, scientists can use the difference in arrival times to see how far away the earthquake occurred. It does not tell the direction however. Determining Direction: Determining Direction One station can only learn how far away the quake occurred. They would draw a circle at that radius. If three stations combine their data, the quake occurred where the three circles overlap. How Earthquakes Cause Damage: How Earthquakes Cause Damage The severe shaking provided by seismic waves can damage or destroy buildings and bridges, topple utility poles, and damage gas and water mains With their side to side, up and down movement, S waves can damage or destroy buildings, bridges, and fracture gas mains.