Earth Science 8.1 Earthquakes Earthquakes
Earthquakes • Each year more than 30,000 earthquakes happen worldwide. Most are minor and do very little damage. • Only about 100 major earthquakes happen each year on the average. • If one of these major earthquakes happens near a city or heavily populated area, the results can be very destructive
Earthquakes • An earthquakeis the vibration of Earth produced by the rapid release of energy within the lithosphere. • Earthquakes are produced by slippage along a break in the lithosphere called a fault.
Earthquakes • The point within the earth where the earthquake starts is called the focus. The focus of an earthquake is located along a fault underneath the surface. • The energy released by the earthquake travels in all directions from the focus in the form of seismic waves. These waves are similar to the waves produced when a stone is dropped in a pond.
Earthquakes • When you see a news report about an earthquake, the reporter always mentions the place on the Earth’s surface where the earthquake was centered. • This place is referred to as the epicenter. • The epicenter is the location on the surface directly above the focus.
Earthquakes • The movement that occurs along faults during earthquakes is a major factor in changing the surface above. • The land above a fault can shift tens of meters in a single earthquake. • Over time, this type of movement can push up coastlines, mountains, and plateaus.
Earthquakes • The crust can move vertically or horizontally as a result of fault movements. • If the crust moves vertically, scientists say it has been uplifted. Vertical movements create a sharp edge called a fault scarp. • If the crust moves horizontally we say it has been offset or displaced.
Earthquakes • The San Andreas fault: • The San Andreas fault in California is one of the most studied fault systems in the world. • This fault extends about 1300 kilometers through the state and into the Pacific ocean. • Studies have shown that displacement has occurred along the fault in areas as much as 200 kilometers long.
Earthquakes • The San Andreas fault: • The San Andreas fault is 1300 kilometers long but segments of it 100 to 200 kilometers long tend to act individually. • Some segments show a slow, gradual slip known as fault creep. Other sections regularly slip and produce small earthquakes. • Some segments remain unmoved for hundreds of years and then move suddenly with explosive force producing large earthquakes,
Earthquakes • The San Andreas fault: • One great earthquake along the fault was the San Francisco Earthquake of 1906. • During this earthquake, the land on the western side of the fault moved 4.7 meters in relation to the land on the east.
Earthquakes • Before the San Francisco Earthquake, scientists did not understand what caused them. Studies afterward led to the development of a hypothesisthat explained earthquakes. • According to the elastic rebound theory, 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 it’s energy as seismic waves.
Earthquakes • Deformation of Rock: • Forces inside the earth slowly deform the rock that makes up the Earth’s crust, causing the rock to change it’s shape. • As the rocks bend, they store energy, just as a wooden ruler does when you bend it. Elastic energy is the same kind of energy that is stored when you stretch a rubber band. • As the rock is stretched farther, as each side of the fault moves in an opposite direction, the elastic energy builds as tension in the rock increases.
Earthquakes • Deformation of Rock: • Suddenly the tension becomes too much; the rock slips at it’s weakest point,(the focus of an earthquake) releasing the energy. • Like a rubber band snapping, the energy is released in a moment along the fault line. • The tendency for the rock under pressure to spring back after an earthquake is calledelastic rebound. • The energy released from the elastic rebound moves along the fault line releasing the pent up energy as seismic waves.
Earthquakes • Aftershocks and Foreshocks: • Even a major earthquake like the 1906 earthquake does not release all the energy in one moment. • Aftershocks and foreshocks also release some of the fault’s stored elastic energy. • An aftershockis an earthquake that occurs sometime after a major earthquake. • Aftershocks may occur anywhere from hours to weeks after an earthquake. • Small earthquakes called foreshockssometimes precede major earthquakes days or even a year before a large event.
Earth Science 8.2 Measuring Earthquakes Measuring Earthquakes
Earth Science 8.2 Measuring Earthquakes • In 2003, a powerful earthquake shook Alaska wilderness south of Fairbanks along the Denali front. This earthquake was so strong it shook ponds in Louisiana and Texas. • Seismic waves carry the energy released from an earthquake hundreds of kilometers away. • Seismic waves transmit the energy of these vibrations from particle to particle. Like a bell hit with a hammer, the vibrations ring through the rocks the bedrock that makes up the lithosphere.
Earth Science 8.2 Measuring Earthquakes • Earthquakes produce two main types of seismic waves • Body waves • Surface waves • These seismic waves differ in • their type of wave motion, • their behavior as they travel through the Earth, • and their speed. • The waves that travel through the Earth’s interior are called body waves. • There are two types of body waves • P waves • S waves
Earth Science 8.2 Measuring Earthquakes • P waves and S waves: • P wavesare push-pull waves that push and pull particles as they move in a direction. They could also be said to compress-expand. • P waves are also called compressional waves. P waves travel faster than S waves. P waves can travel through both liquids and solids. • S wavesshake particles at right angles to the waves direction. Like taking one end of a rope and shaking it, the waves move in a curve. S waves are also called transverse waves.
Earth Science 8.2 Measuring Earthquakes • Surface waves: When body waves reach the surface, they produce surface waves. Surface waves travel more slowly than body waves do. • Surface waves move up and down as well as side-to-side. • Surface waves are usually much larger than body waves, so surface waves are usually the most destructive seismic waves.
Earth Science 8.2 Measuring Earthquakes • Recording Seismic Waves: • Scientists have developed an instrument to record seismic waves: the seismograph. • A seismograph produces a time record of the ground motion during an earthquake. • How it works: A seismograph has a weight suspended from a support that is attached to the bedrock. • When seismic waves reach the bedrock, the inertia of the suspended weight keeps the weight arm almost stationary while the bedrock moves. • In older seismographs, a pen suspended drew on a rotating cylinder and the traces showed the movement of the waves.
Earth Science 8.2 Measuring Earthquakes • Seismogram: • The time record that a seismograph produces is called a seismogram. • A seismogram shows all three types of seismic waves. • The stronger the earthquake, the larger the waves on the seismogram. • The first wave to arrive is the P wave, followed soon after by the S wave. • After the S waves, the Surface waves arrive showing much stronger movement. seismogram seismograph
Measuring Earthquakes: Richter Scale • Intensityis a measure of the amount of earth shaking that happens at a given location in an earthquake. • Magnitude(M) is a measure of the size of the seismic waves or the amount of energy released at the source of the earthquake. • The Richter Scale and the Moment Magnitude Scale both measure earthquake’s magnitude. • The Modifed Mercalli Scale is based on earthquake intensity. seismogram seismograph
Measuring Earthquakes: Richter Scale • Richter Scale: • A familiar but outdated scale for measuring the magnitude of earthquakes is the Richter Scale. • The Richter Scale is based on the height of the largest seismic wave (P,S, or surface wave) recorded on a seismograph. • A 10X increase in wave height equals an increase of 1 on the Richter Scale. • An increase for a M5.0 earthquake is 10 times greater than the shaking produced by a magnitude 4.0 on the Richter Scale. seismogram
Measuring Earthquakes: Momentary Magnitude Scale • Moment Magnitude Scale: • Today scientists use the moment magnitude scale to measure earthquakes. • The moment magnitude scaleis derived from the amount of displacement that occurs along a fault. • The moment magnitude scale is also the only scale that estimates the energyreleased by earthquakes.
Measuring Earthquakes: Momentary Magnitude Scale • Moment Magnitude Scale: • The moment magnitude is generated using several factors in addition to seismographic data. • These factors include • Average amount movement • Area of surface break • Strength of the broken rock • Together, these factors give a measure of how much energy is released when the rock slips and breaks and releases it’s energy as an earthquake.
Measuring Earthquakes: Modified Mercalli Scale • Modified Mercalli Scale: • Another scale used to rate earthquakes is the modified Mercalli scale. • This scale rates an earthquake’s intensity in terms of an earthquake’s effects at different locations (how much shaking it creates or damage it does). • The scale has 12 steps expressed as Roman numerals An earthquake that is barely felt rates a I. An earthquake which creates almost total destruction rates a XII • The same earthquake can rate different Mercalli scale ratings at different locations.
Measuring Earthquakes: Locating an Epicenter P waves S Waves • Locating an Earthquake Epicenter: • The difference in speeds between the P and the S waves provides a way to locate the epicenter. • The P wave always travels faster, arriving before the S wave. • The longer the distance away the epicenter, the greater the amount of time between the waves arriving.
Measuring Earthquakes: Triangulation • Locating an Earthquake Epicenter: • With this information, a receiver can judge the distance to the epicenter of the earthquake. • If multiple receiving stations ( 3 or more) at different spots on the globe can all identify and coordinate their information, they can triangulatea location. Each station knows it’s distance to the epicenter. • Triangulatemeans to use three or positions to determine an exact location.
Measuring Earthquakes: Time Travel Graphs • Locating an earthquake epicenter: • A time travel graph , data from seismographs from at least three different locations, and a globe can be used to determine an earthquakes epicenter. • First, using the data from the each of the seismograph readings, you estimate the distance from each receiver to the epicenter. • Second, you determine a circle from each receiver that shows the distance away the epicenter could be in any direction. • Third, the point where all three circles intersect marks the epicenter
Vocabulary: Earthquakes • Focus: the point beneath Earth's surface where rock that is under stress breaksEpicenter: point directly above the focusSeismic waves: vibrations that travel through Earth's surface carrying the energy created by an earthquakePrimary waves: the earthquake waves that compress and expand the ground like an accordionSecondary waves: the waves that vibrate from side to side as well as up and downSurface waves: when Primary waves and Secondary waves reach the surface some of them are transformed into Surface wavesSeismograph: the instrument that geologist use to record and measure the vibration of seismic waveMagnitude: the measurement of earthquake strength based on seismic waves and movement a long faultsMercalli Scale: rate earthquakes by their intensityRichter Scale: rate earthquakes by mechanical seismographMoment Magnitude Scale: rates earthquakes by estimating the total energy released