Measuring Earthquake Size

• Two main ways to describe EQ size
  • Severity of effects: intensity
    • indicator of degree of shaking at a given locale, based on movement and damage.
  • Ground Motion: magnitude
    • estimate of the amount of energy released by the earthquake
• Let’s talk about Magnitude now

E.g. Seismic moment (M_o) based on amount of displacement on a fault during an EQ:

• M_o= f(slip, area of rupture, modulus of rigidity)

Moment Magnitude (M_w) calculated using seismic wave amplitude

• Scale of 1-10
• Each increment is a 10x difference in wave amplitude
• Which corresponds to a 32x increase in energy released

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