As indicated above, Richter (1935) designed the magnitude scale so that a magnitude of
0 corresponds to approximately the smallest earthquakes then being recorded. There is no
upper limit to the Richter magnitude scale, although earthquakes over an ML of 8 are rare.
Often the data from Wood-Anderson siesmographs located at different distances from the
epicenter provide different values of the Richter magnitude. This is to be expected because
of the different soil and rock conditions that the seismic waves travel through and because
the fault rupture will not release the same amount of energy in all directions.
Since the Richter magnitude scale is based on the logarithm of the maximum trace
amplitude, there is a 10-times increase in the amplitude for an increase in 1 unit of magnitude.
In terms of the energy released during the earthquake, Yeats et al. (1997) indicate that
the increase in energy for an increase of 1 unit of magnitude is roughly 30-fold and is different
for different magnitude intervals.
For the case of small earthquakes (that is, ML 6), the center of energy release and the
point where the fault rupture begins are not far apart. But in the case of large earthquakes,
these points may be very far apart. For example, the Chilean earthquake of 1960 had a fault
rupture length of about 600 mi (970 km), and the epicenter was at the northern end of the
ruptured zone which was about 300 mi (480 km) from the center of the energy release
(Housner 1963, 1970). This increased release of energy over a longer rupture distance
resulted in both a higher peak ground acceleration amax and a longer duration of shaking.
For example, Table 2.2 presents approximate correlations between the local magnitude ML
and the peak ground acceleration amax, duration of shaking, and modified Mercalli intensity
level (discussed in Sec. 2.5) near the vicinity of the fault rupture. At distances farther from
the epicenter or location of fault rupture, the intensity will decrease but the duration of
ground shaking will increase.