principles and applications of microearthquake networks

156 6. Methods of Datu Analysls
magnitude from the latter. Indeed, Bakun et ul. (1978a) verified that a
Wood-Anderson equivalent from a modern high-gain seismograph can be
obtained that matches closely the Wood-Anderson seismogram observed
at the same site. It takes some effort, however, to calibrate and maintain a
microearthquake network so that the ground motion can be calculated
(see Section 2.2.5). In the USGS Central California Microearthquake
Network, the maximum amplitude and corresponding period of seismic
signals can be measured from low-gain stations at small epicentral distances,
and from high-gain stations at larger epicentral distances. In this
way it is possible to extend Richter's method of calculating magnitude for
local earthquakes recorded in microearthquake networks. However, it
may be difficult to relate this type of local magnitude scale to the original
Richter scale as discussed by Thatcher (1973).
Another approach is to use signal duration instead of maximum amplitude.
This idea appears to originate from Bisztricsany (1958) who determined
the relationship between earthquakes with magnitudes 5 to 8 and
durations of their surface waves at epicentral distances between 4 and
160". Solov'ev (1965) applied this technique in the study of the seismicity of
Sakhalin Island, but used the total signal duration instead. Tsumura (1967)
studied the determination of magnitude from total signal duration for local
earthquakes recorded by the Wakayama Microearthquake Network in Japan.
He derived an empirical relationship between total signal duration
and the magnitude determined by the Japan Meteorological Agency using
amplitudes.
Lee ct ul. (1972) established an empirical formula for estimating magnitudes
of local earthquakes recorded by the USGS Central California
Microearthquake Network using signal durations. For a set of 351 earthquakes,
they computed the local magnitudes (as defined by Richter,
1935) from Wood-Anderson seismograms or their equivalents. Correlating
these local magnitudes with the signal durations measured from seismograms
recorded by the USGS network, they obtained the following
empirical formula:
(6.43) Q = -0.87 + 2.00 log 7 + 0.0035 A
where h? is an estimate of Richter magnitude, T is signal duration in
seconds, and A is epicentral distance in kilometers. In an independent
work, Crosson (1972) obtained a similar formula using 23 events recorded
by his microearthquake network in the Puget Sound region of Washington
state.
Since 1972, the use of signal duration to determine magnitude and also
seismic moment of local earthquakes has been investigated by several