Seismology Basics

Earthquake locations and types

• most large earthquakes occur on tectonic plate boundaries (fig.)
• 3 types of boundaries
  • spreading (divergent) boundaries (ex. Mid-Atlantic ridge)
  • convergent boundaries (ex. south Pacific subduction zones)
  • transform boundaries (ex. San Andreas)
  
• Earthquake maps: global, U.S.A., eastern U.S.
• but active faults can exist far away from plate boundaries as well
• earthquakes can happen anywhere stresses in the Earth are released
  • near mountain ranges, under reservoirs, and places where no feature has yet been identified
• most earthquakes occur within tens of kilometers of surface (in the crust) but occasionally as deep as 600 km

The creation of seismic waves

• 3 types of faults, analogous to three types of plate boundaries
  • normal faults result from "stretching" a region
  • thrust (or reverse) faults result from compressing a region
  • strike-slip faults result from sliding one region past another (shearing)
  • in reality, most faults are oblique combinations: strike-slip/normal or strike-slip/thrust
  • animations demonstrating types of fault motion (at www.iris.edu)
• when fault ruptures, energy radiates out from the hypocenter
• complex pattern of vibrations can be reduced to 4 types of motion
  • compressional waves (animation)
  • shear waves (animation)

    wave type	common velocities
    compressional	8-11 km/sec
    shear	5-7 km/sec
    love	3.5-4.5 km/sec
    rayleigh	3-4 km/sec

  • rayleigh waves
  • love waves
• compressional (P) and shear (S) waves travel many paths through Earth's interior and called body waves
• Each path through the earth has an associated traveltime
• rayleigh and love waves "roll" along the surface so they are known as surface waves
• each of the four wave types travels at different speeds, shown at right
• in rare cases, these velocities may vary from typical values

Recording seismic waves and earthquakes

• though technologically advanced, basic principle of most seismometers is the same
• seismometer oscillates around a loosely connected mass (animation)
• many methods translate this motion into a paper or digital recording
• most seismometers record vibrations in all 3 dimensions - up-down, north-south, east-west (fig.)
• generally, compressional & rayleigh waves oscillate more in vertical direction, shear & love waves in the horizontal directions
• following a quake, waves arrive minutes to hours later depending on distance and wave type (example from Turkey, Aug. 17, 1999)
  • 1st: P wave family, most visible on vertical (or Z) component
  • 2nd: S wave family, most visible on N and E components (known as the horizontal components)
  • 3rd: love waves, most visible on N and E components
  • 4th: rayleigh waves, strongest on Z, but usually visible on all 3 components