GES 1101 Lecture : Earthquake (3)
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).
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).
