Question

Richter defined the magnitude of an earthquake
to be

M = log
I
S
,

where I is the intensity of the earthquake (measured by the amplitude of the seismograph wave) and S is the intensity of a “standard” earthquake, which is barely detectable.

The magnitude of a standard earthquake is

M = log
S
S
=

Answer 1

M = log(S/S) = log(1) = 0.

(Here log denotes base 10, so a standard barely-detectable quake has magnitude 0.)