* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download EarthquakesHnrs2
Survey
Document related concepts
Seismic anisotropy wikipedia , lookup
Geochemistry wikipedia , lookup
Large igneous province wikipedia , lookup
Seismic inversion wikipedia , lookup
Ionospheric dynamo region wikipedia , lookup
Physical oceanography wikipedia , lookup
Seismic communication wikipedia , lookup
Earthquake engineering wikipedia , lookup
Transcript
Earthquakes Parkland High School What is an Earthquake? Vibration of the Earth produced by the rapid release of energy or movement of the fractures in the crust Focus---the place within the Earth where the rock breaks, energy is released and radiates in the form of seismic waves Epicenter---the point on the ground's surface directly above the focus Earthquakes and Faults Vertical and horizontal movements are associated with large fractures in Earth’s crust called faults Movement along these faults is explained by plate tectonics (faults associated with plate boundaries which strain rocks) Three types of faults Reverse Fault Caused by compression • Stress that decreases the volume of material Shortens the crust http://www.calstatela.edu/faculty/acol vil/struct/blind_animation.gif Normal Fault Caused by tension – Stress that pulls a material apart Extends the crust http://www.calstatela.edu/faculty/acol vil/struct/normal_animation.gif Strike-Slip Fault Caused by shear – Stress that causes a material to twist Ex-San Andreas Fault http://www.calstatela.edu/faculty/acol vil/struct/rightlateral_animation.gif Cause of Earthquakes Stress – Force per unit area or pressure acting on a rock – When force exceeds strength of rock it fractures – Three types (compression, tension, and shear) Strain – Deformation of materials in response to stress; the result of the stress Strain/Deformation – Elastic rebound or deformation - when stresses are removed, rock returns to original shape (think of a rubber band) – Ductile or Plastic deformation - permanent deformation; when stresses are removed, rock stays bent (results in increase in size) – Rupture or Failure - breakage and fracturing of the rock, causing an earthquake Cause Continued… Can also be looked at as in Figure 6.5 on pg 167 and compared to a limber stick – Original position – Buildup of strain – Slippage (earthquake) – Strain released Stress-Strain Curve Has two segments: straight and curved – Low stress Straight segment Elastic deformation – High stress Curved segment Ductile deformation – Most brittle materials (glass, wood) fail before much ductile deformation occurs, while most ductile materials (rubber, metals) can undergo a great deal of deformation before failure begins or they never fail at all – Most rocks are brittle under low temps in Earth's crust and ductile under high temps at greater depths Foreshocks and Aftershocks Foreshocks – Small earthquakes that precede a major earthquake by days or possibly years – Can be used in prediction Aftershocks – Smaller earthquakes that occur after a major earthquake as the fault adjusts and moves – Weaker but can still cause damage Seismology The study of earthquake waves Began with the Chinese about 2000 years ago A seismograph or seismometer detects/records seismic waves Seismograph How it works: – Suspended mass hanging from a support that is attached to (and moves with) the ground. – Inertia keeps the suspended mass stationary while the ground moves below it. – The movement is recorded on a rotating drum or magnetic tape. – Prints seismogram (shows that waves are elastic energy) Seismic Waves Types of waves – Body • Travel through Earth’s interior • Primary or P waves • Secondary or S waves – Surface • Travel along Earth’s outer layer P Waves Also called compressional waves Push and pull waves (compress and expand the rocks in the same direction the wave is traveling) Fastest of the seismic waves Travel through solids, liquids, and gases (temporarily change the volume of the material by compressing and expanding) http://www.eserc.stonybrook.edu/wise/WSE187sp r2002/PrimaryWaves.html http://wwwrohan.sdsu.edu/~rmellors/lab8/l8pwav2.htm S Waves Move rocks from side to side or at right angles to the direction the wave is traveling Travel through solids only (temporarily change the shape of the material) http://www.eserc.stonybrook.edu/wise/WSE187sp r2002/SecondaryWaves.html http://wwwrohan.sdsu.edu/~rmellors/lab8/l8swav2.htm http://www.classzone.com/books/earth_science/t erc/content/visualizations/es1002/es1002page01.c fm?chapter_no=visualization Surface Waves Complex motion: up-and-down and side-toside (two directions) Slowest Causes damage to structures during an earthquake http://wwwrohan.sdsu.edu/~rmellors/lab8/l8rwav2.htm http://wwwrohan.sdsu.edu/~rmellors/lab8/l8awav2.htm Seismic Record P waves arrive first, followed by S waves, and then surface waves Results from their speeds Velocity of P waves through granite is 6 km/s Velocity of S waves through granite is 3.5 km/s On average, P waves travel about 1.7 times faster than S waves Surface waves travel at 90% of the velocity of S waves Earth’s Interior Please read this section in text on pg. 182-187 and take notes/outline Layers defined by composition: crust, mantle, core Layers defined by physical properties: lithosphere, asthenosphere, mesosphere, inner and outer core Moho Shadow zone Discovering Earth’s composition See figure 6.25 and 6.26 Locating an Earthquake The greater the interval measured on a seismogram between the arrival of the first P wave and the first S wave, the greater the distance to the earthquake source So, first examine the seismograph and determine the elapsed time between the arrival of the first P-wave and the first Swave Locating an Earthquake Continued… Next, use a time-distance graph: Knowing the S - P time, you can determine the distance to the epicenter from the seismic station – See figure 6.9 on pg. 172 Then, on a map, draw a circle around the seismic station (Radius of circle = distance to epicenter) Repeat this for two other seismic stations (Triangulation) The three circles will meet at a point; that point is the epicenter. Locations of Earthquakes Most occur along tectonic plate boundaries: – Around Pacific Ocean (Circum-Pacific Belt) – Mediterranean-Asia Belt (Indonesia, Himalayan region) – Mid-ocean ridges Some occur far from plate boundaries Measuring Earthquakes Two different types to describe the size – Intensity • Measure of the degree of the shaking based on amount of damage • Modified Mercalli Scale – Magnitude • Measure of the amount of energy released at the source • Relies on seismic records • Richter and Moment Magnitude Scales Modified Mercalli Scale Developed in 1902 by Guiseppe Mercalli Intensity scale Describes damage to structures. Ranges from I (felt by only a few) to XII (total destruction) Modified using California buildings as its standard so usable throughout US and Canada Disadvantages: based on effects, so not only ground shaking, but also population density, building design, and the nature of surface materials Richter Scale Developed by Charles Richter in 1935 at CIT Based on amplitude of largest seismic wave (P, S, or surface) recorded on the seismogram--measures magnitude Logarithmic scale: each number on the Richter Scale is ten times greater in wave amplitude (an 8 on scale is 10 times larger than a 7 and 100 times larger than a 6) and each number on the Richter Scale involves an energy release about 32 times as great Richter Continued… Wood-Anderson seismograph is the standard recording device Largest magnitude ever recorded was 8.9 Saturated for large earthquakes because they cannot distinguish between the size of the events See Table 6.2 on pg. 175 Moment Magnitude Scale Also measures magnitude of earthquakes Takes into account the size of the fault rupture (area), the amount of movement/ displacement along the fault, and the rocks’ stiffness/strength Estimated from size of several types of waves Better to describe very large earthquakes Strongest on record---moment magnitude of 9.6 (1960 Chile) Destruction Seismic Vibrations Tsunamis Landslides and Ground Subsidence Fire Seismic Vibrations Structural damage from earthquake waves depends on: – Intensity – Duration – Nature of material upon which structure sits – Design of structure Structural Damage Most damage occurs to unreinforced building made of stone, concrete, etc. Wooden structures are resilient and sustain less damage High-rise, steel-frame buildings are often reinforced and sustain less damage Buildings may rest on rubber structures to absorb vibrations Soft sediments amplify vibrations more than solid bedrock Liquefaction: soil turns into a fluid (saturated with water) Tsunamis Earthquakes under the ocean caused by seismic sea waves (caused by displacement of sea floor along a fault) Waves travel at 500 - 950 km/hr Fast, high energy waves, but not tall (less than 1 m) in deep sea Not distinguishable in open ocean (height less than 1 m) When they enter shallow water, they slow down, the water stacks up (30 m) Tsunamis Continued… First sign = withdrawal of water from coast 5 - 30 min later, a BIG wall of water arrives (100 ft)…can extend hundreds of meters inland Each surge is followed by a retreat of water Waves are separated by intervals of 10-60 minutes Tsunami Warning System Put in place by the U.S. Coast and Geodetic Survey for coastal areas of the Pacific after 1946 tsunami struck Hawaii Large earthquakes reported to the Tsunami Warning Center in Honolulu and scientists use tidal gauges to determine if tsunami has formed Within an hour a warning is issued Landslides and Ground Subsidence Caused by liquefaction Can cause great disaster Fire Begin when gas and electrical lines sever Can be compounded when water lines are broken as well Can be contained by creating a fire break: buildings are dynamited along a boulevard Can cause destruction and death Earthquake Prediction Short-range – Goal: to provide a warning of the location and magnitude of a large quake within a narrow time-frame (hours or days) Long-range – Goal: to predict the probability of a certain magnitude quake within a time scale of 30-100 years or more Short Range Efforts occurring in countries with greatest risk: US, Japan, China, and Russia Must be accurate and reliable to be utilized for evacuation Monitor possible precursors – Ground tilt (lasers, creep meters, tilt meters, strain gauges, etc.) on rocks near faults – Animal behavior – Foreshocks Long Range Based on the idea that quakes are cyclical or repetitive (as soon as one is over, the plates begin to build strain on rocks again) Monitor to look for patterns – Parkfield, California had earthquakes every 22 years – In anticipation of the next one, seismographs were put into place, but it has not yet occurred (overdue by 16 years since last one was in 1966) Seismic gaps: sections of active faults that have not experienced significant earthquakes for a long period of time – San Andreas---San Francisco will probably experience an earthquake of magnitude 7 or higher in next 30 years (1988 to 2018)