Geoscience Reference
In-Depth Information
Earthquake Size Estimates
and Negative Earthquake
Magnitudes
The original and arguably the best-known magnitude scale for measuring the size of
an earthquake is the Richter scale, derived by Charles Richter in 1935 at the California
Institute of Technology to measure earthquake size in Southern California. Using an early
seismograph he defined local magnitude M
L
to be
M
L
= Log
A
- Log
A
o
where A is the maximum amplitude of deflection of a needle on a chart, in millimeters,
measured on the seismograph. A
o
is an empirical distance correction appropriate for the
region (Richter, 1936). Richter assigned a magnitude 3 to an event with amplitude of 1 mm
recorded on a Wood Anderson seismograph at 100 km distance from the source, and a
magnitude 0 with amplitude 0.001 mm at 100 km, thought to be the smallest possible
instrumentally recorded earthquake (Shemeta, 2010).
Since the 1930s advancements in equipment design such as more sensitive geophones
and digital recording equipment and closer proximity to earthquake sources dramatically
advanced the ability to record and analyze data from small earthquakes. Using borehole
seismic arrays located within a few hundred meters of an earthquake source, very small
earthquakes can be recorded. These events are smaller than the baseline magnitude of “0”
originally designed by Richter, therefore the range of event sizes continues into the negative
magnitude range (Figure E.1).
Because the Richter scale was designed for the Wood Anderson seismograph measure-
ments, its routine use in modern seismology is now quite limited; however, most modern
earthquake magnitudes are based on scales that relate back to the Richter scale.
OTHER SIZE ESTIMATES FOR EARTHQUAKES
In practice Richter's method for estimating earthquake magnitude has been largely
supplanted by other more flexible and robust measures of magnitude. The moment magni-
tude, which is scaled to agree with the Richter magnitude, is in wide use because it can be
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