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EARTHQUAKES: WHY? AND HOW?

EARTHQUAKES: WHY? AND HOW?. EARTHQUAKES. sudden movement or shaking of the Earth. Caused by plate tectonic stresses. Located at plate boundaries Resulting in breakage of the Earth’s brittle crust. PLATE TECTONIC STRESSES.

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EARTHQUAKES: WHY? AND HOW?

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  1. EARTHQUAKES:WHY? AND HOW?

  2. EARTHQUAKES sudden movement or shaking of the Earth • Caused by plate tectonic stresses • Located at plate boundaries • Resulting in breakage of the Earth’s brittle crust

  3. PLATE TECTONIC STRESSES • Plate boundaries and faults (= cracks where plate sections are moving in different directions) cause friction as plates move • Plates in a fault zone have STICK-SLIP motion • Periods of no movement (stick) and fast movement (slip) • Energy stored as plates stick, • Energy released as plates slip

  4. EARTHQUAKE DAMAGE • Landsides • Building damage • Liquefaction

  5. LIQUEFACTION when a solid (sand and soil) becomes saturated with water and acts like a heavy liquid • Results in a loss of soil strength & the ability of the soil to support weight

  6. EARTHQUAKE DAMAGE Most caused by SURFACE waves (arrive last)

  7. EARTHQUAKE INTENSITY measures damage to man-made structures at certain location • Modified Mercalli scale= measurement of damage to structures • From I to XII (Roman numerals) • Descriptive, changes withdistance from epicenter • Can change from location to location • What you need: • Your senses!

  8. ISOSEISMIC MAPS • Connects areas of with the same Modified Mercalli number • Areas are colored according to Modified Mercalli number Loma Prieta Earthquake 1989 show the distribution of intensities

  9. EARTHQUAKE WAVES • FOCUS= place deep within the Earth and along the fault where rupture occurs • EPICENTER = geographic point on surface directly above focus • SEISMIC WAVES produced by the release of energy • move out in circles from the point of rupture (focus) • 2 types: surface & body (travel inside & through earth’s layers) • P waves: back and forth movement of rock; travel thru solid, liquid, gas • S waves: sideways movement of rock; travel thru solids only

  10. EARTHQUAKE WAVES • Seismographs record earthquake waves • Seismograms show: • Amplitude of seismic waves (how much rock moves or vibrates) • Distance to the epicenter • Earthquake direction

  11. EARTHQUAKE WAVES • 3 types of seismic waves show up on seismogram • P waves: shake earth in same direction as wave; travel thru solid, liquid, gas • S waves: Shake earth sideways to wave direction; travel thru solids only • Surface waves: circular movement of rock; travel on surface – cause most damage!!

  12. EARTHQUAKE WAVES P waves move through solids & liquids S waves move through solids only!!!

  13. EARTHQUAKE WAVES Primary (1st to arrive)Longitudinal, Compression Secondary (2nd to arrive - larger) Transverse, Shear all states of matter (solid, liquid, gas) Can go through solids only back and forth movement of rock • push/pull or compression/stretch out • Like slinky down stairs Vibration is same as the direction of travel Move sideways • perpendicular to direction of wave travel • Like snake

  14. EARTHQUAKE WAVES Lets test your understanding!! Is this a P or an S wave? P wave! S Wave

  15. EARTHQUAKEMAGNITUDE measures the size of seismic waves  the energy released by the earthquake • Richter scale=measurement of energy released based upon wave amplitude (size of vibration) • <2 to ~10 • Amplitude of wave goes up by 10 (Logarithmic scale) • What you need: • Amplitude (size of vibration = wave height) • Time between arrival of 1st P and 1st S waves

  16. HOW TO READ SEISMOGRAMS P & S (body waves) move through earth & arrive first • P & S waves used to calculate magnitude of earthquake • Amplitude = height of wave (how much the rock moves; size of vibration)

  17. MERCALLI VS. RICHTER

  18. Let’s try a simulation • Simplified Epicenter Triangulation • P waves generally travel between 5.95 and 6.75 kilometers per second in the crust, depending on compressibility, rigidity, uniformity, and density of the materials traversed. • S waves tend to move at velocities between 2.9 and 4.0 km/sec in the crust. • Rayleigh waves travel somewhere between 2.7 and 3.7 km/sec. • Time and distance can be graphed for each of the wave types. By comparing the time between pairs of wave types on the graph, a rough estimate of the distance to the earthquake focus can be made. In each of the following sets of problems, you need to convert the time difference given between the arrivals of each of the two wave types into kilometers. To do this, measure the difference in time on the Y axis, marking it on a ruler or piece of paper. Then, find the distance on the X axis where the two relevant curves diverge by just that time difference on your marked paper or ruler. Give your answer rounded to the nearest 250 km.

  19. In each of the following sets of problems, you need to convert the time difference given between the arrivals of each of the two wave types into kilometers. • To do this, measure the difference in time on the Y axis, marking it on a ruler or piece of paper. Then, find the distance on the X axis where the two relevant curves diverge by just that time difference on your marked paper or ruler. Give your answer rounded to the nearest 250 km.

  20. Simplified Graph of P, S, and R Waves through Time

  21. Calculate together • Notebooks…… see handout

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