Earthquakes

1. Earthquakes • Seismology – the study of Earthquakes • Strain energy – crustal rocks store stress until they fail or rupture • Sudden release of pent-up energy in the Earth’s crust • Focus – precise spot BELOW the Earth’s surface where rupture occurs • Epicenter – point ON the Earth’s surface directly above the focus • Foreshocks – possible tremors foreshadowing a coming major event (often associated w/magmatic movement) • Aftershocks – tremors associated as rocks adjust to new positions

2. Focus and Epicenter

3. Seismic Waves • Earthquake energy transmitted through the Earth • Waves travel and are deflected/reflected along internal boundaries of the Earth • Body Waves • P waves – Primary waves, fastest (4mi/sec), arrive first at seismic stations, compression waves, parallel to direction of wave propagation, change in shape and volume of rock • S waves – Secondary waves, slower (2mi/sec), arrive next at seismic stations, shearing waves, perpendicular to direction of wave propagation (up and down), change in shape but not volume • Cannot travel through liquid medium • Indirect evidence for liquid outer core • Surface Waves • Rolling • Side-to-side • Travel in upper few km of crust, slowest (1.5mi/sec), cause most damage to rigid structures

4. Measuring Earthquakes • Richter Scale – defined magnitude of largest peak traced on a seismograph • Logarithmic (i.e. each successive unit is 10 times greater than previous one • Actually a 33 fold increase • Not accurate for events over 7.0 magnitude • Moment –Magnitude Scale • Length of fault rupture X depth X slip X strength of rock • More accurate because it can be calculated directly

5. Cylindrical Seismograph

6. Seismograms

7. Relative time lag between seismic waves • Single station recording

8. Seismic Waves • Delay of shock arrival at different stations • Note attenuation of surface waves

9. Depth and Magnitude • Shallow • < 70 km • 90% of all quakes occur at depths of < 100 km • Can accumulate large amounts of strain energy • Are where most large-scale EQ’s occur • Brittle failure • Intermediate • 70 – 300 km • Deep • > 300 km • Heat weakens rocks ability to store strain energy • Less brittle failure • Weaker EQ magnitudes

10. Earthquake Effects • Ground Displacement – along fault plane • Strike-slip • Dip-slip • Can be meters at a time • Can account for 1000’s meters over time • Landslides • Rock fragments detach from bedrock • Sed rx slip along bedding planes • Meta rx shift along foliation planes • Loose sediment moves down-slope • Liquefaction • Converts saturated ground with some cohesiveness into sediment that can flow like water (mud slurry – water pressure forces grains out of contact) • Seiches • The back and forth movement of water in enclosed areas

11. Chuetsu Earthquake, 2004 Ojiya, Niigata, Japan Soil liquefaction took place on this road

12. Effects cont’d • Tsunamis • Caused by submarine landslides and faulting • Can travel at speeds > 500mph • About 1m in height • Can travel across large distances • Speed, shallow water and departure of water near coast responsible for most damage • Fires • Gas mains, electrical power lines, oil/gas storage

13. Tsunami

14. Banda Ache shoreline before Dec. 26, 2004

15. Banda Ache shoreline after Dec. 26, 2004

17. Earthquake Zones • Majority occur along plate boundaries (fig. 11-13) • Shallow at mid-ocean ridges (thinner crust) • Deeper at subduction zones (thicker crust) • Benioff-Wadati Zone – progression of quake depth along the descending plate • Japan, Mexico and Central America, Western N.A. and S.A.

18. Benioff Zone

19. > M 6 earthquakes in the Indian Ocean Region

20. Sumatra-Andaman Earthquake2004/12/26 • 9.15 Magnitude • Indian/Burma Plate boundary • 1200km of faultline slipped 15m • Two phases • 400km (250mi) x 100km (60mi) rupture (largest ever known) • Traveled NW @ 2.8km/s (6,300mph) for 100 secs • 100 sec pause and then second pulse • 21 km/s (4,700mph) to plate boundary • Sea-floor rose several meters • Displacing 30km3 of ocean water

21. Sumatra-Andaman Earthquake2004/12/26 • Increased possibility of Toba Eruption (some activity recorded since in area) • Activated two other volcanoes in the Aceh province • 0.8 gigatons of TNT • As much total energy as the U.S. uses in 11 days • Earth’s surface oscillated 20-30cm • Vertical movement of 3mm as far away as Oklahoma • Entire Earth’s surface estimated to have risen 1cm • Shortened our day by ~2.68 microseconds • Caused 5-6cm wobble in rotation due to decrease in Earth’s oblateness

22. 2010 Haitian Earthquake • 7.0 M with/epicenter near Léogâne, ~25km (16 miles) west of Port-au-Prince • EQ occurred at 16:53 local time, 12 January 2010 • 52 aftershocks >4.5 • ~3,000,000 people were affected by the quake • ~316,000 died • ~300,000 injured and • 1,000,000 homeless

23. Tectonic Setting

25. March 11, 2001 Japanese EQ & Tsunami • Mag 9.0 (USGS NEIC) • Depth 32km (19.9mi) • Tectonic setting : Subduction of Pacific Plate beneath N. American Plate • Rate of convergence : 8.9cm/yr (3.5in/yr) • Size of rupture along plate boundary : 290 km (180 mi) long, 80 km (48mi) across

26. March 11, 2011 Tohoku, Japan EQ • Duration of strong shaking reported from Japan: three to five minutes • Distance that the island of Honshu appears to have moved after the quake: 2.4 meters • Change in length of a day caused by the earthquake's redistribution of Earth's mass: 1.8 microseconds shorter • Length of warning time Sendai residents had before tsunami hit: eight to 10 minutes • Number of confirmed foreshocks to the main shock: four • Magnitudes of the confirmed foreshocks: 6.0, 6.1, 6.1 and 7.2 • Number of confirmed aftershocks: 401

27. Color-coded tsunami wave height

31. Prediction • Seismic gaps – accumulation of pent-up strain as opposed to tectonic creep • Micro-quake swarms – micro cracks cause rocks to dilate • Tilt or Bulges – can be measured by tiltmeters or lasers • Change is seismic velocity – due to air pockets in micro-cracks • Variations in electrical conductivity – air lowers, water increases • Changes in ground water – level and chemistry • Animal behavior