Earthquake Size Estimates and Negative Earthquake Magnitudes - Induced Seismicity Potential in Energy Technologies - page 190

Amplitude (mm) on Wood Anderson Seismometer at 100 km
FIGURE E.1 A plot of measured earthquake amplitude versus magnitude. The more sensitive the seismic
instruments, the smaller the measureable magnitude, reaching into the negative magnitude range.
tied to other direct measures of the size of an earthquake. The seismic moment is a routine
measurement describing the strength of an earthquake and is defined as
M o = μSd
where μ is the shear modulus, S is the surface area of the fault, and d is the average dis-
placement along the fault. The moment magnitude, M w , is related to seismic moment by
the Hanks and Kanamori (1979) equation
M w = 2 / 3 LogM o - 6
where M o is in Newton meters, valid for earthquakes ranging from magnitude 3 to 7
(Shemeta, 2010). There are a variety of methods used to calculate a seismic moment from
microseismic waveforms.
EARTHQUAKE “B VALUES”
Small earthquakes occur much more often than large earthquakes. The number of
earthquakes with respect to magnitude follows a power law distribution and is described by