# Earthquakes Chapter 8 Created by Rachel Joseph. An Earthquake is… the shaking and trembling that results from the movement of rock beneath Earth's surface.

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Earthquakes Chapter 8 Created by Rachel Joseph

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 stress, a force that acts on rock to change its volume or shape (deformation) This stores energy in the rock. When the rock snaps from all the stress, it releases energy as seismic waves.

Where Do Earthquakes Occur? Most earthquakes take place near the edges of tectonic plates. This figure shows the Earth’s tectonic plates and the locations of recent major earthquakes.

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 (  shearing  ) (  compression  ) (  tension  )

Faults A fault is a break or crack 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, releasing energy, creating an earthquake Faults usually occur along plate boundaries, where the forces of plate motion compress, pull, or shear the crust too much so the crust finally breaks.

Section 1 What Are Earthquakes? Chapter 8 Elastic Deformation and Elastic Rebound Click below to watch the Visual Concept. Visual Concept

San Andreas Fault San Andreas fault in California. Notice it is not one long fault, but many, somewhat connected smaller ones. This line will parallel the transform boundary between the North American Plate and the Pacific Plate. Transform boundaries tend to have the most earthquakes.

Hanging Wall? Foot Wall? See the block farthest to the right that is shaped kind of like a foot? That’s the foot wall. Now look at the block on the other side of the fault. See how it’s resting or hanging on top of the footwall block? That’s the hanging wall. Here’s another way to think of it: the hanging wall block is always above the fault plane, while the foot wall block is always below the fault plane.

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 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 Tension forces cause the hanging wall to slip 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 fault This fault has the same set up as a normal fault, but reversed, which explains it’s name So the hanging wall gets pushed up the footwall. This 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 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 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 colliding 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 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 points downward (sinks) These 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 rock Like a lasagna, a plateau consists of many layers, so it is wider than it is tall

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 from the focus They ripple like when you through a stone into a lake or pond Epicenter: the point on the surface directly above the focus.

P, S, and Surface Waves

How Earthquakes Form Everyday, about 8,000 earthquakes hit Earth, but most of them are too little to feel Earthquakes will always begin when the stress builds up the rock in a fault. Friction keeps the rocks from moving, so the stress builds up. Eventually the energy is so great the friction is overcome and the rock breaks or moves. This release of energy and movement of rock is an earthquake Focus: the point beneath Earth's surface where rock that is under stress breaks

Aftershocks An earthquake that occurs after a larger earthquake in the same area May strike hours, days, or even months later

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 (called body waves as they travel through the interior of the Earth). When they reach the top of the epicenter, surface waves form

Primary Waves Also known as P Waves The first waves to come are these waves (fastest) P waves are earthquake waves that compress and expand the ground like an accordion (compressional) P waves cause buildings to expand and contract Travel through solids and liquids.

Secondary Waves Also known as S Waves (shear) After P waves, S waves come next (slower) S waves are earthquake waves that vibrate from one side to the other as well as down and up (transverse) 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 (Love/Rayleigh Waves) When S waves and P waves reach the top, some of them are turned into surface waves (L-waves) S+P 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

Section 1 What Are Earthquakes? Chapter 8 Seismic Waves: Surface Waves Click below to watch the Visual Concept. Visual Concept

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 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 The weight on the seismograph never moves. The machine moves back and forth.

Locating the Epicenter Since 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. Two stations can isolate the epicenter to two possible locations. If three stations combine their data, the quake occurred where the three circles overlap.

Section 2 Earthquake Measurement Chapter 8 S and P Time Method: Finding an Epicenter Click below to watch the Visual Concept. Visual Concept

Measuring Earthquakes There are many ways to measure an earthquake There are at least 20 different types of scales. 3 of them are the Mercalli scale, Richter scale, and the Moment Magnitude scale Magnitude is a measurement of earthquake strength (energy) based on seismic waves and movement along faults

The Mercalli Scale Developed in the twentieth century to rate earthquakes according to their intensity. The intensity of an earthquake is the strength felt by people and the damage caused. Is not a precise measurement, the rating is based on the damage done. 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 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 away ones

Richter Magnitude v. Energy

The Moment Magnitude Scale Geologists use this scale today It’s a rating system that estimates the total energy released by an earthquake (basically the Richter scale modified for distance) 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

We are in a place of moderate danger of earthquakes happening

Section 3 Earthquakes and Society Chapter 8 Earthquake-Hazard Level Click below to watch the Visual Concept. Visual Concept

Earth’s Liquid Core This was one of the proofs that the Earth has a liquid core. You have an S- wave shadow where the S waves do not pass through liquids.

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.

Earthquake Hazard Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity. The greater the seismic activity, the higher the earthquake-hazard level.

Earthquake Forecasting Forecasting when and where earthquakes will occur and their strength is difficult. By studying areas of seismic activity, seismologists have discovered some patterns in earthquakes that allow them to make some general predictions.

Earthquake Forecasting, continued Strength and Frequency Earthquakes vary in strength. The strength of earthquakes is related to how often they occur. This table shows more detail about the relationship.

Earthquake Forecasting, continued Another method of forecasting an earthquake’s strength, location, and frequency is the gap hypothesis. The gap hypothesis is based on the idea that a major earthquake is more likely to occur along the part of an active fault where no earthquakes have occurred for a certain period of time.

Earthquake Forecasting, continued An area along a fault where relatively few earth-quakes have occurred recently but where strong earthquakes have occurred in the past is called a seismic gap.

Earthquake Forecasting, continued Using the Gap Hypothesis Not all seismologists believe the gap hypothesis is an accurate method of forecasting earthquakes. But some seismologists think the gap hypothesis helped forecast the approximate location and strength of the 1989 Loma Prieta earthquake in California.

Section 3 Earthquakes and Society Chapter 8 Gap Hypothesis and Seismic Gaps Click below to watch the Visual Concept. Visual Concept

Earthquakes and Buildings Earthquakes can easily topple buildings and destroy homes. Today, older structures in seismically active places, such as California, are being made more earthquake resistant. Retrofitting is the name given to the process of making older structure more earthquake resistant.

Earthquakes and Buildings, continued A common way of retrofitting an older home is to securely fasten it to its foundation. Steel is often used to strengthen buildings and homes made of brick.

Earthquakes and Buildings, continued Earthquake-Resistant Buildings A lot has been learned from building failure during earthquakes. With this knowledge, architects and engineers use new technology to design and construct buildings and bridges to better withstand earthquakes. The next slide shows some of the technology used to make earthquake- resistant buildings.

Earthquakes and Buildings, continued

Are You Prepared for an Earthquake? Before the Shaking Starts The first thing should do safeguard your home against earthquakes. Place heavier objects on lower shelves so they do not fall during an earthquake. Find safe places within each room of your home and outside of your home. Make a plan with others to meet in a safe place after the earthquake is over. These students are participating in an earthquake drill.

Earthquake Preparations, continued When the Shaking Starts If you are indoors, crouch or lie face down under a table or desk. If you are outside, cover your head with your hands and lie face down away from buildings, power lines, or trees. If you are in a car on an open road, you should stop the car and remain inside.

Earthquake Preparations, continued After the Shaking Stops Try to calm down and get your bearings. Remove yourself from immediate danger, such as downed power lines, broken glass, and fire hazards. Do not enter any damaged buildings unless you are told it is safe by someone in authority. Beware that aftershocks may cause more damage. San Francisco: after 1906 quake

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