Transcript Earthquakes
Faults & Earthquakes Deforming the Earth’s Crust Deformation • The process by which the shape of a rock changes because of stress is called deformation. • There are two basic types of deformation: – Plastic deformation – Elastic deformation • When rock deforms in a plastic manner, it folds like a piece of molded clay. • With elastic deformation, the rock stretches like a rubber band until it breaks. • Elastic deformation can lead to earthquakes. Stress • Stress is a force that acts on rock to change its shape or volume. • Because stress if a force, it adds energy to the rock. • This energy is stored in the rock until the rock either breaks, or changes shape. There are three types of stress that occur in the Earth’s crust: • Compression • Tension • Shearing Compression • The stress force called compression squeezes rock until it folds, or breaks. • Compression makes a mass of rock occupy a smaller space. • When compression occurs at a convergent boundary, large mountain ranges can form. Tension • The stress force called tension pulls on the crust, stretching rock so that it becomes thinner in the middle. • Tension occurs where two plates are moving apart, such as mid-ocean ridges, or rift valleys. Shearing • Stress that pushes a mass of rock in two opposite, horizontal directions is called shearing. • Shearing can cause rock to break and slip apart. • Shearing occurs at transform boundaries. Folding • The bending of rock layers because of stress in the Earth’s crust is called folding. • Undisturbed rock layers are horizontal, so when we see a fold we know that deformation has taken place. Folds • The two most common types of folds are: – Anticlines – Synclines • Anticlines are upward-arching folds. • Synclines are downward, trough-like folds. • Another type of fold is a monocline. • In a monocline, both ends of the fold are horizontal. Faults • When the stress on rocks causes them to break and slip past each other, a fault is formed. • The blocks of crust on each side of the fault are called fault blocks. • When faults are not vertical one side of the fault block will be called a hanging wall and the other the footwall. • The position of the fault block will determine which it is. Faults • There are 3 main types of faults: – Normal fault – Reverse, or thrust fault – Strike-slip Reverse Fault Normal Faults • Tension forces cause normal faults. • The hanging wall lies above the fault and the footwall lies below the fault. • When movement occurs along the fault line, the hanging wall slips downward. • Normal faults are found at divergent plate boundaries, where plates pull apart. Reverse Faults • Compression forces produce reverse faults. • Reverse faults have the same basic structure as a normal fault, but the blocks move in the opposite direction. • When movement occurs along the fault line, the hanging wall slides up and over the footwall. • Reverse faults are found at convergent plate boundaries, where plates are pushed together. Strike Slip Faults • Shearing creates strike-slip faults. • The rocks on either side of the fault slip past each other sideways with little up or down motion. • A strike-slip fault that forms the boundary between two plates is called a transform boundary. Fault Block Mountain • When the tension in a normal fault uplifts a block of rock, a fault-block mountain forms. • The Grand Tetons in Wyoming are an example of a faultblock mountain range. Folded Mountains • Folded mountains form at convergent boundaries where continents have collided. • The Appalachian Mountains, the Alps, and the Himalayas are examples of folded mountains. Earthquakes • An earthquake is the shaking and trembling that results from the movement of rock beneath Earth’s surface. • Not all Earthquakes occur at plate boundaries. Sometimes they happen in the middle of a tectonic plate. • Earthquakes can happen both near the Earth’s surface or far below it. • Earthquakes always begin in rock below the surface. • Most earthquakes begin in the lithosphere within 100 kilometers of the surface. • The focus is the point beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake. • The point on the surface directly above the focus is called the epicenter. Seismic Waves • Seismic waves are vibrations that travel through Earth carrying the energy released during an earthquake. • Seismic waves that travel through the Earth are called body waves. • Seismic waves that travel along Earth’s surface are called surface waves. • Each type of seismic waves travels through Earth’s layers in a different way and at a different speed. Body Waves • There are two types of body waves: – P waves – S waves P-waves • The first waves detected in an earthquake are p waves, or pressure waves. • P waves compress and expand the ground like an accordion. • P waves can travel through solids, liquids, or gases. S waves • After P waves come secondary waves, or S waves. • S waves are earthquake waves that vibrate from side to side and thrust the ground up and down, or back and forth. • When S waves reach the surface, they shake structures violently. • S waves cannot move through liquids. Surface Waves • When P and S waves reach the surface, some of them are transformed into surface waves. • Surface waves move more slowly than P and S waves, but they produce the most severe ground movements. • They can actually make the ground roll like ocean waves. • Other surface waves shake the ground from side to side. All 3 waves Detecting Seismic Waves • Geologists use a seismograph to record and measure the vibrations of seismic waves. Seismograph • When the waves reach a seismograph, the instruments creates a seismogram. • A seismogram is a tracing of earthquake motion. • Until recently, scientists used mechanical seismographs, like the one in the picture. • Today they use electronic seismographs that convert ground movements into a signal that can be recorded and printed. Seismogram Finding the Epicenter • Scientists use seismograms to find the earthquakes epicenter. • One method they use is called the S-P Time Method. • They collect readings for the same earthquake from seismographs stations at different locations. • They then use this data to determine the distance each station is from the earthquake. • They can then triangulate the results to find the epicenter. • It takes a minimum of 3 seismograph readings to find the epicenter of and earthquake. Richter Scale • There are many ways that scientists can measure an earthquake • Magnitude is a measurement of earthquake strength based on seismic waves and movement along faults. • Charles Richter created the Richter magnitude scale in the 1930s to compare earthquakes by measuring ground motion and adjusting for distance to find their strength. • When magnitude increases by one unit the measured ground motion becomes 10 times larger on the Richter scale. • The Richter scale provides accurate measurements for small, nearby earthquakes, but the scale does not work well for large, or distant earthquakes. Richter Scale no. No.of earthquakes per year Typical effects of this magnitude <¾ 800,000 Detected only by seismometers 3.5 - 4.2 30,000 Just about noticeable indoors 4.3 - 4.8 4.800 Most people notice the, windows rattle. 4.9 – 5.4 1,400 Everyone notices the, dishes may break, open doors swing. 5.5 – 6.1 500 Slight damage to buildings, plaster cracks, bricks fall. 6.2 – 6.9 100 Much damage to buildings; chimneys fall, houses move on foundations. 7.0 – 7.3 15 Serious damage; bridges twist, walls fracture, buildings may collapse. 7.4 – 7.9 4 Great damage, most buildings collapse. > 8/0 One every 5 to 10 Total damage, surface waves seen, objects years thrown in the air. Mercalli Scale • Seismologists can also measure the intensity of an earthquake. • The Modified Mercalli Intensity Scale is used to measure the degree to which an earthquake is felt by people and the amount of damage done by it. • The Mercalli scale uses Roman numbers from I to XII to describe increasing earth quake intensity levels.