Lecture 16 Earthquakes

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Lecture 16 Earthquakes

• What are earthquakes?
• Elastic rebound theory
• Waves generated by earthquakes
• P waves, S waves, Surface waves
• Locating earthquakes
• Earthquake magnitude
• Earthquake distributions

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Denali Earthquake 2002/11/03, 5km, M7.9
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• Northridge earthquake, Jan 1994. The parking lot
  at Cal State Univ. Northridge campus collapsed
  during the earthquake.

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What are earthquakes?

• Earthquakes are vibrations of earth, produced
  by the rupture and sudden movement of rocks,
  which are caused by stresses beyond the elastic
  limits of the rocks.

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• The point inside the Earth where the rupture
  generates earthquake energy is called focus or
  hypocenter. The point at the Earth's surface
  directly above the focus is the epicenter.

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Elastic rebound theory

• Reid in early 20th century suggested that
  earthquakes are caused by "spring back" of
  deformed rocks (termed elastic rebound).
• Example The displacement in the 1906 San
  Francisco earthquake was as much as 15 feet.

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• Elastic rebound. As rock is deformed, it bends,
  storing elastic energy. Once strained beyond its
  breaking point, the rock cracks, releasing the
  stored-up energy, which generates earthquake
  waves.

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• This fence was offset 2.5 m in the 1906 San
  Francisco earthquake (G.K. Gilbert)

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• Fault scarp produced by vertical displacement in
  the 1964 Alaska earthquake. (USGS)

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Waves generated by earthquakes

• Three types of waves are generated by an
  earthquake
• P waves (primary waves),
• S waves (secondary waves)
• Surface waves (Rayleigh waves, and Love waves)

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• P waves are compressional waves, which push
  (compress) and pull (expand) rock particles in
  the direction the waves are traveling.

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• S waves are shear waves. The particles in S waves
  moves at right angles to the direction the waves
  are traveling.

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• Surface waves
• The motions of surface waves are more complex.
  There are two types of surface waves that
  propagate along Earth's surface Rayleigh waves
  and Love waves.
• Surface waves are generally strongest waves.
  Their amplitudes decay strongly with depth (i.e.
  they are generally confined near the surface).

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• The particle motions of Love waves (top) are
  perpendicular to the propagation direction and
  parallel to the surface. The particle motions of
  Rayleigh waves (bottom) are along the propagation
  direction and the vertical plane.

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• Wave speeds
• P waves are faster than S waves.
• S waves are faster than surface waves.
• Thus, in a typical seismogram, P wave arrives
  first, then S wave, and then surface wave.

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• Typical seismogram, showing P wave, S wave, and
  surface wave.

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• Seismograph. The inertia of the suspended mass
  tends to keep it motionless, while the recording
  instrument vibrates with the earth.

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Locating earthquakes

• Locating an earthquake requires four basic
  parameters origin time, and latitude, longitude,
  and depth of the hypocenter.
• A basic method for locating an earthquake is
  using travel times of P waves and S waves
  recorded at seismic stations.
• For example, epicenter can be quickly estimated
  from the time interval between the P wave and the
  S wave recorded at 3 or more stations
• (1) Using one station, the distance between
  the epicenter and the station can be estimated
  from the S-P time at the station.
• (2) The epicenter can then located using
  distance estimates from 3 or more stations.

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• The distance to the epicenter can be determined
  using S-P time. Here, the S-P time is 5 minutes,
  the epicenter is about 3800 km (2350 miles) away.

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• Earthquake epicenter can be located using the
  distances obtained from travel times of three or
  more seismic stations.

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Earthquake magnitude

• Earthquake magnitude measures the amount of
  energy released by an earthquake.
• The best-known magnitude scale is Richter scale,
  developed by Charles Ricther of Caltech.
• Richter magnitude is determined by (1) the
  largest amplitude of the seismogram recorded at
  the Wood-Anderson instrument and (2) the distance
  from the focus.

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• Richter scale can be determined graphically using
  a seismogram recorded at a Wood-Anderson
  instrument.

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• Richter scale is logarithmic. Each unit increase
  corresponds to tenfold increase in amplitude, and
  32-fold increase in energy.
• Thus, the largest amplitude of a magnitude 6
  earthquake is 10 times as great as that of
  magnitude 5. The energy released by a magnitude 6
  earthquake is 32 times that of a magnitude 5
  earthquake.

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Earthquake distributions

• A great majority of earthquakes originates in a
  few narrow zone of the globe along plate
  boundaries.

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• Earthquake depths
• Earthquakes occur at depths from near the surface
  to nearly 700 km, although a great majority of
  earthquakes are shallow.
• Shallow earthquakes 0-70 km
• intermediate earthquakes 70 - 300 km
• deep earthquakes 300-700 km

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• Wadati-Benioff zone at Tonga subduction zone.
  Almost all deep earthquakes are associated with
  subduction zones. The focal depths increase with
  the distances from the trench. These subduction
  seismic zones are called Wadati-Benioff zones.