EarthquakesChapter 5

An Earthquake is…

• The movement of Earth's plates produces strong forces that squeeze or pull the rock in the crust. We feel this as shaking and trembling.

• Stress is a force that acts on rock to change its volume or shape

3 Types of Stress

There are three different types of stress that occur on the crust, shearing, tension, and compressionThese forces cause some rocks to become fragile and they snapSome other rocks tend to bend slowly like road tar softened by the suns heat

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 faultsFaults 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

Shearing creates this faultIn this fault, rocks on both sides of the fault slide past each other with a little up and down motionWhen a strike-slip fault forms the boundary between two plates, it becomes a transform boundary

Normal Faults

Tension forces in Earth's crust causes these types of faultsNormal faults are at an angle, so one piece of rock is above the fault, while the other is below the faultThe above rock is called the hanging wall, and the one below is called the footwallWhen 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

Compression forces produce this faultThis fault has the same setup as a normal fault, but reversed, which explains it’s nameJust like the normal fault, one side of the reverse fault is at an angle of the otherThis fault produced part of the Appalachian Mountains in the eastern United States

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 rangeSometimes, a normal fault uplifts a block of rock, so a fault-block mountain formsWhen a piece of rock between two normal faults slips down, a valley is created

Mountains Formed by Folding

Sometimes, under current conditions, plate movement causes the crust to foldFolds 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 crustThese plate collisions can produce earthquakes because rock folding can fracture and lead to faults

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 downwardThese folds in rocks are found on many parts of the earths surface where compression forces have folded the crust

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 rockLike a lasagna, a plateau consists of many layers, so it is wider than it is tall

How Earthquakes FormEveryday, about 8,000 earthquakes hit Earth, but most of them are too little to feelEarthquakes will always begin in a rock beneath the surfaceA lot of earthquakes begin in the lithosphere within 100 km of Earth's surface Focus: the point beneath Earth's surface where rock that is under stress breaks The focus triggers an earthquake

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 materialDuring an earthquake, seismic waves go out in all directions to the focusThey ripple like when you through a stone into a lake or pond

Seismic Waves Ctd.

There are three different types of seismic waves: Primary waves, Secondary waves, and surface waves

Primary Waves

Also known as P WavesThe first waves to come are these wavesP waves are earthquake waves that compress and expand the ground like an accordion P waves cause buildings to expand and contract

Secondary Waves

Also known as S WavesAfter p waves, S waves comeS waves are earthquake waves that vibrate from one side to the other as well as down and upThey shake the ground back and forthWhen S waves reach the surface, they shake buildings violentlyUnlike P waves, which travel through both liquids and solids, S waves cannot move through any liquids

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 movementsSome of them make the ground roll like ocean wavesOther surface waves move buildings from side to side

Detecting Seismic Waves

Geologists use instruments called seismographs to measure the vibrations of seismic wavesSeismographs records the ground movements caused by seismic waves as they move through the Earth

Mechanical SeismographsUntil just recently, scientists have used a mechanical seismographa 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 earthquakethe pen stays in place and records the drum's vibrations The higher the jagged lines, the more severe earthquake

Measuring EarthquakesThere are many things to know about the measures of an earthquakeThere are at least 20 different types of measures3 of them are the Mercalli scale, Richter scale, and the Moment Magnitude scaleMagnitude is a measurement of earthquake strength based on seismic waves and movement along faults

The Mercalli ScaleDeveloped in the twentieth century to rate earthquakes according to their intensityThe intensity of an earthquake is the strength of ground motion in a given placeIs not a precise measurementBut, the 12 steps explain the damage given to people, land surface, and buildingsThe 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 ScaleThe Richter scale is a rating of the size of seismic waves as measured by a particular type of mechanical seismograph Developed in the 1930’sAll over the world, geologists used this for about 50 yearsElectric seismographs eventually replaced the mechanical ones used in this scaleProvides accurate measurements for small, nearby earthquakesDoes not work for big, far ones

The Moment Magnitude Scale

Geologists use this scale todayIt’s a rating system that estimates the total energy released by an earthquakeCan be used for any kind of earthquakes, near or farSome 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

• 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

• 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

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.