Transcript earthquake
WHAT IS A EARTHQUAKE? An earthquake is the shaking and vibration of the ground caused by the movement of rocky plates within the earth. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. This usually happens when two blocks of the earth suddenly slip past one another, or break apart from each other as a results of tension caused by prolonged energy build. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter. The point underground where the actual breaking of the rock occurs is called the focus. This is where the main event occurs underground. The point directly above the focus on the surface of Earth is called the epicenter. This is where the ground shaking is usually the strongest. Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. The main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. What exactly causes the rock underground to break? This is the result of stress along plate boundaries on Earth. The plates are dynamic, so they are always moving. Sometimes they move enough that they push into each other or pull apart. There may be compressional stress, tensional stress and shear stress. Compressional stress Compression is a type of stress that causes the rocks to push or squeeze against one another. Tensional stress Tensional stress occurs when rocks are pulled apart - they're being stretched farther than they would be otherwise. Shear stress Shear stress is when rocks slide past each other in opposite directions - it's like rubbing your hands together; they don't push or pull, but there's a lot of friction there! How are earthquakes measured? An earthquake is measured using a seismometer. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph. The strength, or magnitude, of an earthquake is measured using the Richter scale. The Richter scale is numbered 0-10. What Is Seismology? Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves. What Are Seismic Waves? Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves. BODY WAVES P WAVES The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air. P waves are also known as compressional waves, because of the pushing and pulling they do. S WAVES The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock particles up and down, or side-to-side--perpendicular to the direction that the wave is traveling in. SURFACE WAVES Travelling only through the crust, surface waves are of a lower frequency than body waves, and are easily distinguished on a seismogram as a result. Though they arrive after body waves, it is surface waves that are almost entirely responsible for the damage and destruction associated with earthquakes. This damage and the strength of the surface waves are reduced in deeper earthquakes. LOVE WAVES The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion. RAYLEIGH WAVES The other kind of surface wave is the Rayleigh wave, a Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. Before an Earthquake • Store heavy items or glassware in lower cabinets so they do not become dangerous projectiles. • Update home insurance policies to adequately cover building costs, possession replacement, and injury deductibles. • Secure large appliances such as refrigerators, water heaters, air conditioners, and other bulky items with straps, bolts, and other stabilizing methods. • Be sure both old and new buildings meet earthquake construction requirements. • Do not put heavy artwork, mirrors, or shelves over beds. • Prepare an earthquake emergency kit with non-perishable food, bottled water, copies of important documents (birth certificates, prescriptions, insurance papers, etc.), flashlights, first aid materials, blankets, spare glasses, and other essential items and store it where it will be easily accessible in case of a quake. • Keep cell phones charged and replace emergency kit supplies as necessary to keep them usable. • Plan alternative commuting routes in case an earthquake damages roads. • Set up a family meeting location in a safe area. • Teach all family members basic first aid, how to behave during a quake, and what to do after a quake. During an Earthquake • Immediately seek a safe location such as in a doorway, a table or desk, or along an interior wall away from windows or hazardous objects. • Cover the back of your head and your eyes to minimize injury from flying debris. • If cooking, turn off heating elements immediately. • If outdoors, stay in open areas away from buildings, power lines, trees, and other potential hazards. • If driving, stop quickly but safely and stay in the vehicle. Do not stop near power lines, bridges, overpasses, or other potentially dangerous locations. • Stay calm and brace yourself to keep your balance, sitting if possible. Here is a short clip https://www.youtube.com/watch?v=FSnWIcNRzKc After an Earthquake • Be prepared for aftershocks, which may be stronger than the initial jolt. • Tend injuries immediately and summon emergency assistance if necessary. • Check for structural damage, but do not enter a building that shows damage or has visible cracks in the walls or foundation. • Wear shoes at all times to avoid stepping on broken glass. • Be cautious opening cabinets, cupboards, and closets in case items may be poised to fall. • Keep phone lines clear for emergency use. References http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/earthquakes _rev2.shtml http://geology.com/articles/earthquake-safety.shtml http://safety.lovetoknow.com/Earthquake_Safety_Precautions http://www.geo.mtu.edu/UPSeis/waves.html http://www.usgs.gov/faq/categories/9827/3343 http://earthquake.usgs.gov/learn/kids/eqscience.php http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_an d_interior.html https://msnucleus.org/membership/html/k-6/pt/plate/5/ptpt5_2a.html https://www.youtube.com/watch?v=FSnWIcNRzKc https://www.youtube.com/watch?v=yOGoKCK17a4