Earthquakes
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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

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Earthquakes

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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


Focus and Epicenter


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


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


Cylindrical Seismograph


Seismograms


  • Relative time lag between seismic waves

  • Single station recording


Seismic Waves

  • Delay of shock arrival at different stations

  • Note attenuation of surface waves


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


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


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


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


Tsunami


Banda Ache shoreline before Dec. 26, 2004


Banda Ache shoreline after Dec. 26, 2004


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.


Benioff Zone


  • > M 6 earthquakes in the Indian Ocean Region


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


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


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


Tectonic Setting


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


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


Color-coded tsunami wave height


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


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