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Earth Quakes chapter 19 Section 19.1 Forces within Earth -Earthquakes occur when rocks fracture or break deep within the Earth’s crust. -These fractures are caused by stress (the amount of force per unit area acting on a material) 3 types of stress 1. Compression: decreases volume of a material. 2. Tension: pulls a material apart 3. Shear: causes a material to twist Strain: The deformation of materials in response to stress. Section 19.1 page 497 Strain: The deformation of materials in response to stress. -As stress is added to rocks they deform (strain) As this stress is removed, the rocks can return to their original size and shape. (ex stretch rubber band and release) This is elastic strain. -Ductile Deformation: If stress exceeds a certain value, the material is permanently deformed. Section 19.1 page 497 Fault: Fracture or system of fractures in Earth’s crust that occurs when stress is applied too quickly or too great;can form as a result of horizontal compression (reverse fault), horizontal shear (strike slip fault), or horizontal tension (normal fault) Section 19.1 page 497 Normal Fault In a normal fault the two involved blocks are (by gravity) pulling away from one another causing one of the fault blocks to slip upward and the other downward with respect to the fault plane (it is hard to determine whether both or just one block has moved.) <------ -----> Section 19.1 page 497 Reverse Fault The reverse fault is a normal fault except the general movement of the fault blocks is toward each other, not away from each other as in the normal fault. -----> <----- Section 19.1 page 497 Strike slip Fault Probably the most well known and well studied fault is the transcurrent (strike-slip) fault known as the San Andreas fault of California. This fault marks the margin line between the Pacific and North American Plates. Movement on a strike strip fault is generally horizontal. Section 19.1 page 497 Section 19.1 page 498 3 Types of seismic waves 1. Primary or P waves: Squeeze and pull rocks together in the direction of travel. This is the fastest wave and can travel through liquids or solids. Section 19.1 page 498 2. Secondary or S waves: cause rocks to move at right angles in relation to the direction of the waves. S waves are slower than P waves and can only travel through solids. Section 19.1 page 498 3. Surface (L) waves : Up-and-down (rolling) or side-toside motion of the earth surface. Surface waves, the slowest earthquake waves, travel along the surface of the earth rather than down into the earth. Although they are the slowest of all earthquake waves, L waves usually cause more damage to society than P or S waves. Section 19.1 page 498 Make a simple chart: P wave Fastest Solids + liquids Particles move lengthwise S wave Medium Solids only Particles move up and down L wave Slowest Surface Parts move rolling Section 19.1 page 499 Focus: Point where earthquake originates. Focal depth:How far below Earth’s surface the focus is. The closer the focus is to the surface the more damage it does. Epicenter: Point on Earth’s surface directly above the focus. Section 19.2 page 500 Seismometers (seismograph)-instrument that detects and records vibrations in the Earth’s crust. Seismogram: The recorded data produced by the seismometer. Travel time curves: show the time it takes for the P waves and S-waves to travel to a given seismic station. Lab Skills: Follow these steps to calculate epicenter distance. Step 1. Find the arrival times for the P and S waves on the seismogram. Ex: the P wave arrived at 4:00 and the S wave came at 4:04 and 20 seconds. Step 2. Find the difference in arrival time between the P and S waves. Ex: If the P wave arrives at 4:00, and the S wave arrives at 4:04 and 20 seconds; the difference would then be 4 minutes and 20 seconds. Step 3. Finding the difference of travel time between the curves. Look at the graph in your reference tables on page 11. Each box going up the Y axis (side) is worth 20 seconds. Therefore every 3 boxes = a minute Going straight up and down between the two curves find 4 minutes and 20 seconds. (Hint: how many little boxes would this be?) Breaking step 3 down: First off how many boxes are in a minute again? There are 3. So if we want to figure out 4 minutes and 20 seconds lets multiply the number of minutes which is 4, by how many little boxes equal one minute which is three. 4 minutes X 3 little boxes/minute = 12 little boxes Did we have any seconds to deal with? Yes we have 20 seconds. How many boxes is 20 seconds worth? 20 seconds = 1 little box. So how many little boxes do we have all together? 12 from the minutes + 1 from the seconds = 13 little boxes. Finishing step 3: So what do we do with this 13 boxes? It is your job to find where there are 13 boxes going up and down between the P and S wave curve. If you count too many, move towards the left. If you count too few, look more towards the right. Don’t worry, the more you practice, the easier this gets. Step 4. Finding the distance Once you find the correct line, follow it straight down and find the distance to the epicenter. Here you should have come across the number 3. This is in thousands of Km’s. 3 = 3,000 Km away from the epicenter So how much are the little boxes on the X axis (bottom) worth? If you count from the origin to 1, you find 5 boxes. Now divide 1000 by 5 and you get 200. So each little box is worth 200 Km. Earthquakes II The Sequel !!! Rated E Step 5. Using a compass on the map a) Once you have the distance, use the scale on your map and set the compass to the correct distance. b) Place the point of the compass on the city where the station is located. c) Draw the circle Step 6. Triangulation. You will need to do this 3 times to find the epicenter. If you only draw 2 circles, there will be 2 intersect points representing 2 possible locations for the epicenter. When you draw 3 circles (correctly) there will only be one point where all three circles intersect. 3 stations are needed to find the location of an epicenter. Notice where all three circles intersect. This is triangulation. Section 19.2 Clues to the Earth’s Interior page 502-503 Most of our knowledge of the Earth’s interior comes from the study of seismic waves. These waves change speed and direction when they encounter different materials. This change in the waves direction is called refraction The denser the material, the faster the waves will go. -Liquids will block S waves. From this information we can determine the density and composition of the Earth’s interior. Other evidence of Earth’s interior Meteorites. But how is this so??? Meteorites that have orbited the Sun are thought to have formed the same way the Earth has formed. By this rational, the inside of a meteorite will have a similar composition to that of the Earth. 19.3 Measuring and Locating Earthquakes page 505 How many earthquakes occur each year? Over one million, you just can’t feel most of them! Magnitude: The amount of energy released by an earthquake. Richter scale: Numerical scale used to measure the magnitude of an earthquake, using values based on the size of the earthquake’s largest seismic waves. An earthquake at a number 7 is 10 times larger than a 6 and releases 32 times more energy. 19.3 Measuring and Locating Earthquakes page 506 Moment magnitude scale: Takes into account the size of the fault rupture, the amount of movement along the fault, and the rocks’ stiffness. 19.3 Measuring and Locating Earthquakes page 506 Modified Mercalli Scale: Rates the type and extent of damage and other effects of the earthquake as noted by observers. (Listed on page 507). This would be around a 9 on a scale of 12 19.3 Measuring and Locating Earthquakes page Key points about damage: -The closer a city is to the epicenter, the more damage it will sustain. -The closer the focal point is to the surface, the more damage will occur. -Earthquakes occur most often at plate boundaries -80 percent at the Circum-Pacific belt (a seismic belt) 19.4 Earthquakes and Society Earthquake Hazards Tsunami 19.4 Earthquakes and Society Earthquake Hazards -Fault Scarps Fault Scarps 19.4 Earthquakes and Society Earthquake Hazards -Structural failure Look at the map. What kind of risk do we have where we live? Earthquake predictions are based on probability. This is based on history of an area and the rate at which strain builds up. 19.4 Earthquakes and Society page 515 Seismic Gaps: Sections of active faults that haven’t experienced significant earthquakes for a long period of time. (Meaning it will happen sooner than later) Ch 19 Quiz 1. Which of the following is not a type of stress? A. Compression C. Thrust B. Tension D. Shear 2. Where do most volcanoes and earthquakes occur world wide? A. B. C. D. Hot spots Plate boundaries The Canadian shield The East Coast of North and South America Ch 19 Quiz 3. Which wave can not pass through the outer core because it is a liquid? A. P Waves B. S Waves C. L Waves D. R waves 4. What is the distance to the epicenter if the P wave arrived at 3:00 and the S wave arrived at 3:05 and 40 seconds? A. 1,000 Km B. 2,000 Km C. 3,000 Km D. 4,000 Km Ch 19 Quiz 5. If the epicenter was 2,000 Km away, how long did the p wave travel for? A. 1 minute B. 2 minutes C. 3 minutes D. 4 minutes Bonus: How many stations do you need to find the epicenter of an earthquake?