Geo 105, Main points for Earthquakes for Jan. 29

Earthquakes & Earthquake Hazards

• What?
  • Earthquakes are rapid slip across a fault
  • This generates seismic waves
  • Which propagate away from the slipping fault in all directions
  • They cause ground shaking in the vicinity of the epicenter
  • and, can be recorded by seismographs located far away
• How Big?
  • Intensity: a crude measure of earthquake size based on
    reports of shaking and/or damage in the epicentral area
    For example, maximum intensity of the Sept 2, 1994 Michigan EQ
    (magnitude of 3.5) was "V"
  • Magnitude: the "best" way to measure earthquake size
    Use amplitudes of seismic waves recorded by seismographs.
    At the same distance from the epicenter, an increase in the wave
    amplitude by a factor of ten translates into an increase in magnitude
    by one unit, e.g. from "4" to "5".
• How Many?
  • Every year, the various national & international agencies detect and
    locate tens of thousands of earthquakes
  • But nearly all of these events are small, too small to be felt
  • "Gutenberg & Richter Law": For any given region and time interval,
    the number of earthquakes increases by about a factor of ten as we decrease the
    magnitude by one unit.
  • For example, average annual global occurrence:
    • Magnitude=8 & larger # is about: 1
    • Magnitude=7 & larger # is about: 10
    • Magnitude=6 & larger # is about: 100
    • Magnitude=5 & larger # is about: 1000
    • etc. etc. etc.
  • This overall statisticcal "law" can be used to estimate the likelihood of
    future earthquakes
• Where?
  • Mostly at Plate Boundaries
  • Furthermore, Mostly at Subduction Zones
  • However, detailed view shows that there can be many active faults around
    the main plate boundary fault
  • For example, many earthquakes in California that are NOT on the San Andreas fault
  • Even more puzzling are the odd places in the middle of plates that have earthquakes
  • In the case of the New madrid zone in central USA, we know that a sequence
    of great earthquakes occurred in the early 1800's
    will a great earthquake happen there again? if so, when?