GES 1101 Lecture : Earthquake (3)
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Earthquake Size: Magnitude
Side note: Have you ever heard of the Richter Scale?
- Richter scale developed 1936, first measurement of EQ size.
- Based on a specific brand of seismometer, a specific distance from an
earthquake.
- We don’t typically use this scale anymore, but many continue to refer to
earthquake sizes as being on the Richter scale.
Earthquake Size: Magnitude vs. Energy
Energy released depends on three things:
1. The size of the fault (fault surface area, A).
2. How much it slips (distance the fault slips, D).
3. How strong the fault is (shear modulus, μ).
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Energy released is called Seismic Moment: Mo= A x D x μ
What we actually measure is something called Moment Magnitude:
Mw= 2/3 x log10(Mo) – 6.0
…this seems hard!
Important points:
1. We can measure Magnitude.
2. We can relate Magnitude to Energy.
3. We cab relate Energy to real geologic things (i.e. SIZE, SLIP, STRENGTH).