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Earthquakes

Earthquakes. Margaret Johnson Physical Geography. What causes earthquakes?. Earth shakes to release seismic energy Follows rapid movement of large blocks of crust along a fault. Faults. Breaks in the crust May be hundreds of km long and 10-20 km deep San Andreas fault is 1200 km long

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Earthquakes

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  1. Earthquakes Margaret Johnson Physical Geography

  2. What causes earthquakes? • Earth shakes to release seismic energy • Follows rapid movement of large blocks of crust along a fault

  3. Faults • Breaks in the crust • May be hundreds of km long and 10-20 km deep • San Andreas fault is 1200 km long • between North American and Pacific plates • most active fault in contiguous US

  4. North Anatolian Fault • Site of Izmit earthquake • Similar to San Andreas fault • length • movement

  5. The Deadly Izmit Earthquake • NW Turkey, • Aug. 17, 1999 at 3 a.m. • Over 14,000 killed • poorly constructed apartment complexes pancaked • each floor collapsed on the one below • More killed than average annual total for all earthquakes in the world

  6. Focus (Hypocenter) • Point on fault surface where movement begins • The earthquake source

  7. Epicenter Point on Earth’s surface directly above focus

  8. Earthquakes Named for Epicenter • Example: 1994 Northridge earthquake occurred several km below city of Northridge in metro L.A. There was structural damage, but few casualties • Example: Izmit was epicenter of Izmit earthquake, and site of greatest casualties

  9. Earthquake Distribution • Earthquakes occur on faults along plate boundaries • Largest quakes are on convergent plate boundaries

  10. World Map of earthquake center locations. Center locations of all earthquakes originating at depths 62 miles during a 6-year period are shown by red dots. Each dot represents a single location or a cluster of centers. Black circles identify centers of earthquakes of Richter magnitude 8.0 or greater during an 80-year period. The map clearly shows the pattern of earthquakes occurring at subduction boundaries.

  11. fault line scarp fault line

  12. Seismic Waves • Carry energy released from earthquake focus • 2 types of seismic waves: • Surface waves: • travel on Earth’s surface; cause much destruction • Rayleigh waves • Love waves • Body waves • travel through Earth’s interior • P (primary) waves • S (secondary) waves • Based on vibration direction and velocity

  13. P Waves (type of Body wave) • Vibrate parallel to travel direction, like a Slinky • Travel 4-6 km per second in uppermost crust

  14. S Waves (type of Body wave) • Vibrate perpendicular to travel direction, like wave in a rope • Travel 3-4 km per second in shallow crust

  15. Seismic Waves • Surface waves travel on Earth’s surface: • vertical motion (Rayleigh waves) • like water waves • sideways motions (Love waves) • no vertical movement

  16. Sudden Changes on/near Earth’s Surface Due to Earthquakes • Ground shaking • Rapid horizontal movements shift homes off foundations, collapse tall buildings • Shaking exaggerated where sediment weak or saturated with water • Fault rupture and uplift • Break in ground surface form a step called a fault scarp • Liquefaction • Water-saturated sediment reorganized by violent shaking • Sediment collapses, expelling water causes surface to subside • Landslides • Earthquakes often near mountains at convergent plate boundaries • Steep slopes prone to landslides when shaken • Tsunamis • Giant sea waves caused by submarine earthquakes, esp. in Pacific Ocean • Waves may reach height of 15 m (50 ft), speeds up to 600 mph

  17. Measurement of Earthquakes • 3 methods of measurement • Modified Mercalli Scale: measures intensity • often used to rank cultural effects of historical earthquakes • values vary with distance from epicenter, building materials, population density • Richter Scale: measures magnitude; is most well known • magnitude based on height (amplitude) of S wave on a seismogram • each division is a 10-fold increase in amplitude, 30-fold increase in energy • Moment-Magnitude Scale: more accurately measures energy released on large faults • recently found favor

  18. Modified Mercalli Scale • Useful in ranking historical earthquakes • Occurred before widespread use of seismographs (after WW II)

  19. Modified Mercalli Scale I--Not felt by people II--Felt by people at rest on upper floors of buildings III--May be felt by people indoors; similar to passing truck IV--Felt indoors by many, outdoors by few. Dishes, windows, doors disturbed; walls make cracking sound. V--Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. VI--Felt by all; many frightened. Some heavy furniture moves; some fallen plaster. Damage slight. VII--Slight to moderate damage in ordinary structures; considerable in poorly-built structures; some chimneys broken. VIII--Slight damage in earthquake resistant buildings, heavy damage in poorly constructed buildings. Chimneys, columns, walls may fall. IX--Considerable damage in specially designed structures. Damage great in substantial buildings, partial collapse. Buildings shifted off foundations. X--Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations XI--Few, if any masonry structures standing. Bridges destroyed, rails bent greatly. XII--Total damage, objects thrown into air.

  20. Richter Scale • Measures magnitude, the amplitude of seismic waves recorded on a seismograph • Developed by Charles Richter in 1930s, to measure shallow earthquakes in Calif. • Used 2 factors • difference in P-wave and S-wave arrival time • S-wave amplitude • As more seismograph stations available, developed more complex formulae • Help measure quakes at different depths

  21. Mb=log10(A/T)+Q • Formula to determine magnitude from body waves (Mb) • where A is amplitude of ground motion in microns • T is time taken for motion in seconds • Q is a correction for distance from the epicenter and the focal depth in kilometers

  22. Moment-Magnitude Scale • Moment-magnitude (Mw) scale measures energy released, more accurately than Richter scale • Energy release is related to rock rigidity, area of fault surface, amount of movement on fault • Most accurate way to compare large earthquakes

  23. Mw=2/3 log10(Mo)-10.7 • Formula to determine magnitude where Mo=mSd, • where m is shear length of faulted rock • S is area of the fault • d is fault displacement.

  24. Earthquake Distribution • Strong correlation between earthquake foci and plate boundaries • Many quakes, result of collisions of continental plates, form belt across central Asia, Middle East, and southern Europe • Hot spots such as Hawaiian Islands in central Pacific Ocean • Largest earthquakes associated with convergent plate boundaries

  25. Generalized world map of continental shields, exposed and covered. Continental centers of early Precambrian age (Archean) lie with the area encircled by a broken red line. Heavy brown lines show mountain roots of Caledonian and Hercynian orogenies.

  26. Where? How Big? • Earthquakes occur on faults • Many active faults have been discovered • Earthquake magnitude directly related to fault length • longer fault, bigger earthquake • 1906 San Francisco earthquake: Mw=7.7, 400 km of fault, lasted 2 minutes • 1994 Northridge earthquake: Mw=6,7, 14 km of fault, lasted 7 seconds

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  28. SUMMARY

  29. What is an Earthquake? • Vibration of the Earth due to rapid release of energy. Energy released because of rapid movement on a fault.

  30. What is the Earthquake Focus? • Point on fault where movement begins, the earthquake source. • Seismic waves radiate from focus • Foci occur at range of depths: • shallow (0-70 km), • intermediate (70-300 km) • deep (300-700 km). • Shallow is most common.

  31. What Are Principal Effects of an Earthquake? • Ground shaking: rapid horizontal movements. Shaking exaggerated where underlying sediment weak or saturated with water. • Fault uplift: large sections of Earth’s surface (1000s of km2) may change elevation. • Liquefaction: Water-saturated sediment collapses due to shaking. • Landslides: Earthquakes often near mountains on convergent plate boundaries. Steep slopes are prone to landslides when shaken. • Tsunamis: Giant sea waves caused by submarine earthquakes, esp.... in Pacific Ocean.

  32. What Methods Can be Used to Measure an Earthquake? • 3 methods: • Modified Mercalli Scale measures intensity based on damage caused. • Richter Scale measures magnitude using amplitude (height) of S-waves on seismogram. • Moment-Magnitude Scale most accurate measure of energy released on large faults; recently found favor

  33. How is Modified Mercalli Scale Used? • Measures intensity: level of destruction and effects on people. • Scale is I-XII (1-12 in Roman numerals) • I-VI are increasing awareness of people • VI-XII involve increasing damage. • Not widely used for modern earthquakes, inaccurate in sparsely populated areas and areas with poor building practices, and value varies with distance from epicenter

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