Mechanical Waves Waves A wave is a disturbance that propagates through space or spacetime, often transferring energy. A mechanical wave exists in a medium (which on deformation is capable of producing elastic restoring forces Waves travel and transfer energy from one point to another, often with little or no permanent displacement of the particles of the medium Instead there are oscillations around almost fixed positions. Periodic waves are characterized by crests (highs) and troughs (lows) Transversal vs. Longitudinal Waves Transverse waves are those with vibrations perpendicular to the direction of the propagation of the wave (waves on a string or ripples in water) Longitudinal waves are those with vibrations parallel to the direction of the propagation of the wave (sound waves). Sound Sound is the movement of air particles created by a vibrating source. Air particles are in constant random motion, exerting very small pressure variations around the steady-state atmospheric pressure. Each particle is subject to both an inertial force (due to its mass and acceleration) and a force which tends to restore the particle to its resting position (due to the elasticity of the medium). When an object - a sound source - is set into vibration, each air particle moves to and from about its average position along an axis parallel to the direction in which the wave propagates. Air particles themselves do no move very far, they simply transfer pressure changes by what is referred to as sound propagation. This is a 'sound wave' which moves away from the sound source at a speed determined by the medium. The speed of sound wave in air is about 344 m/s, (in water it is 1437 m/s). The Sea Transversal Waves Introductory Question You float motionless in an inner tube, just far enough from the shore that the waves aren’t breaking on top of you. You will A. drift shoreward at the speed of the waves drift gradually but steadily shoreward move in a circle as each wave passes, but make little or no progress toward shore B. C. Observations about the Sea The sea is rarely calm—it has ripples on it The broadest ripples (waves) travel fastest Waves seem to get steeper near shore Waves break or crumble near shore Waves bend after passing over sandbars You can sometimes ride waves The Tides, Part 1 The moon’s gravity acts on the earth The moon’s gravity isn’t uniform The earth’s oceans are pulled out of round The Tides, Part 2 There are two tidal bulges in the oceans As the earth rotates, these bulges moves Almost 2 high and 2 low tides per day Strongest tides are near equator Weakest tides are near poles The Sun’s Influence Sun’s gravity affects tides Strongest tides are when moon and sun are aligned Weakest tides are when moon and sun are at right angles Tidal Resonance Water in a confined channel can slosh back and forth It’s another harmonic oscillator Period depends on inertia and stiffness of the restoring force If the sloshing time matches the tidal period, resonance occurs Standing and Traveling Waves Sloshing involves standing waves Water exhibits fixed nodes and antinodes Open water surf involves traveling waves Wave crests and troughs shift continuously Water Waves Sloshing involves deep water waves: the whole liquid moves back and forth Surface waves only affect the liquid’s top Water’s Motion Surface water circles as the wave passes Circling is strongest at surface Motion is weak about 1/2 wavelength deep Question: You float motionless in an inner tube, just far enough from the shore that the waves aren’t breaking on top of you. You will drift shoreward at the speed of the waves drift gradually but steadily shoreward move in a circle as each wave passes, but make little or no progress toward shore Wavelength Longer the wavelength of surface wave, faster it travels Velociy = wave length x frequency deeper water moves as it passes more energy it contains for a given amplitude Tsunamis are very long wavelength, very deep, very high energy waves (and not strictly surface waves, either) Water in a Wave Only the wave travels, the water circles Crests are formed from local water Breaking Waves Surface waves slow down in shallow water Waves bunch as the water gets shallower Waves get taller as water gets shallower Waves break when water can’t form crest Gradually sloping bottom: rolling surf Steeply sloping bottom: plunging breakers Changing Wave Speeds Reflection Wave speed change causes partial reflection The bigger the change, the more reflection Refraction Wave speed change can redirect wave Waves bend toward shore as they slow Summary of the Sea The moon’s gravity causes the tides The tides can cause resonant motion Tidal resonances are standing waves The open sea supports traveling waves Water moves in circles in those waves Waves break when water gets too shallow Surfing – a fun way to experience ocean Nose Tail Tsunami Wave Tsunami Introduction The word "tsunami" comes from the Japanese words tsu (harbor) and nami (waves). A tsunami is a wave or series of waves in the ocean that can be hundreds of miles long and have been known to reach heights of up to 34 ft (10.5 m). These "walls of water" travel as fast or faster than a commercial jet. The massive December 26, 2004 tsunami traveled 375 miles (600 km) in 75 minutes. That's 300 mph (480 kph). These walls of water are capable of inflicting massive damage along coastal lands. December 26th 2004 Tsunami A massive underwater earthquake off the coast of Indonesia's Sumatra Island rattled the Earth in its orbit. The quake, measuring 9.0 on the Richter scale, was the largest one since 1964. This devastating earthquake was the tsunami that it caused. The death toll reached higher than 220,000 Many communities suffered devastating property damage. The shore of Banda Aceh, Sumatra, before and after the 2004 tsunami Banda Aceh northern shore detail, 2004, before and after the tsunami Typical Tsunami Wave vs. Typical Wind-generated Wave Wave Feature Wind-generated Wave Tsunami Wave 5-60 mph 500-600 mph Wave Speed 10 minutes to 2 hours apart Wave Period 5 to 20 seconds apart Wave Length 300-600 feet apart 60-300 miles apart A huge difference in size and speed! Creating a Tsunami Wave Waves in the ocean are the most often created by the wind. The most common causes of tsunamis are underwater earthquakes. Underwater earthquakes can be explained by plate tectonics. The theory of plate tectonics suggests that the lithosphere, or top layer of the Earth, is made up of a series of huge plates. These plates make up the continents and seafloor. They rest on an underlying viscous layer called the asthenosphere. So, how does it start? And then… Once the water has been pushed upward, gravity acts on it, forcing the energy out horizontally along the surface of the water. The tremendous force created by the seismic disturbance generates the tsunami's incredible speed. A tsunami's ability to maintain speed is directly influenced by the depth of the water. A tsunami moves faster in deeper water and slower in shallower water. So unlike a normal wave, the driving energy of a tsunami moves through the water as opposed to on top of it. As a result, as a tsunami moves though deep water at hundreds of miles an hour, it is barely noticeable above the waterline. A tsunami is typically no more than 3 feet (1 meter) high until it gets close to shore. Once a tsunami gets close to shore, it takes its more recognizable and deadly form. At the Shore… When a tsunami reaches land, it hits shallower water. The shallow water and coastal land acts to compress the energy traveling through the water. This starts the transformation of the tsunami. The topography of the seafloor and shape of the shore begins to affect the tsunami's appearance and behavior. As the velocity of the wave diminishes, the wave height increases considerably -- the compressed energy forces the water upward. A typical tsunami approaching land will slow down to speeds around 30 miles per hour (50 kph), and the wave heights can reach up to 90 feet (30 meters) above sea level. As the wave heights increase during this process, the wave lengths shorten considerably. (Think of squeezing an accordion.) Tsunamis most often arrive as a series of strong and fast floods of water, not one single, enormous wave. Surviving the Tsunami Wave Danger zone: 1 mile from the shore Heed Natural Warnings: An earthquake may serve as a warning that a tsunami is coming, and so may a rapid fall or rise in coastal waters Heed Official Warnings: Play it safe, even if warnings seem ambiguous or you think the danger has passed Expect Many Waves : The next wave may be bigger, and the tsunami may last for hours Head for High Ground and Stay There : Move uphill or at least inland, away from the coast Abandon Belongings : Save your life, not your possessions Don’t Count on the Roads : When fleeing a tsunami caused by a nearby earthquake, you may find roads broken or blocked Go to an Upper Floor or Roof of a Building : Only if trapped and unable to reach high ground, go to an upper story of a sturdy building or get on its roof Climb a Tree: As a last resort, climb up a strong tree if trapped on low ground Climb onto Something that Floats: If swept up by a tsunami, look for something to use as a raft Expect the Waves to Leave Debris: A tsunami will leave behind sand, the remains of houses, and bodies Expect Quakes to Lower Coastal Land: A large earthquake can leave nearby coastal areas lowered, allowing tidal water to flood them Earthquakes An earthquake is a vibration that travels through the earth's crust. Caused by: majority of naturally-occurring earthquakes are caused by movements of the earth's plates volcanic eruptions meteor impacts underground explosions (an underground nuclear test, for example) collapsing structures (such as a collapsing mine) Earthquake frequency An everyday occurrence on our planet. According to the United States Geological Survey, more than three million earthquakes occur every year. That's about 8,000 a day, or one every 11 seconds! The vast majority of these 3 million quakes are extremely weak. The law of probability also causes a good number of stronger quakes to happen in uninhabited places where no one feels them. It is the big quakes that occur in highly populated areas that get our attention. Seismic waves When a sudden break or shift occurs in the earth's crust, the energy radiates out as seismic waves . In every earthquake, there are several different types of seismic waves. Body waves move through the inner part of the earth Primary waves, also called P waves or compressional waves Secondary waves, also called S waves or shear waves Surface waves travel over the surface of the earth Ù long waves, or simply L waves -- are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations. Surface waves stem from body waves that reach the surface. Rating Magnitude and Intensity The Richter Scale is used to rate the magnitude of an earthquake -- the amount of energy it released. This is calculated using information gathered by a seismograph. The Richter Scale is logarithmic, meaning that whole-number jumps indicate a tenfold increase. In this case, The increase is in wave amplitude. The largest earthquake on record registered an 9.5 on the currently used Richter Scale, though there have certainly been stronger quakes in Earth's history. The majority of earthquakes register less than 3 on the Richter Scale. These tremors, which aren't usually felt by humans, are called microquakes. Generally, you won't see much damage from earthquakes that rate below 4 on the Richter Scale. Major earthquakes generally register at 7 or above. Mercalli Scale An earthquake's destructive power varies depending on the composition of the ground in an area and the design and placement of manmade structures. The extent of damage is rated on the Mercalli Scale. Mercalli ratings, which are given as Roman numerals, are based on largely subjective interpretations. A low intensity earthquake, one in which only some people feel the vibration and there is no significant property damage, is rated as a II. The highest rating, a XII, is applied only to earthquakes in which structures are destroyed, the ground is cracked and other natural disasters, such as landslides or Tsunamis, are initiated.
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