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

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

Focus and Epicenter


Seismic waves

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

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

Cylindrical Seismograph


Seismograms

Seismograms


Earthquakes

  • Relative time lag between seismic waves

  • Single station recording


Seismic waves1

Seismic Waves

  • Delay of shock arrival at different stations

  • Note attenuation of surface waves


Depth and magnitude

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

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

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


Effects cont d

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

Tsunami


Banda ache shoreline before dec 26 2004

Banda Ache shoreline before Dec. 26, 2004


Banda ache shoreline after dec 26 2004

Banda Ache shoreline after Dec. 26, 2004


Earthquake zones

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

Benioff Zone


Earthquakes

  • > M 6 earthquakes in the Indian Ocean Region


Sumatra andaman earthquake 2004 12 26

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 earthquake 2004 12 261

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

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

Tectonic Setting


March 11 2001 japanese eq tsunami

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

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

Color-coded tsunami wave height


Prediction

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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