principles and applications of microearthquake networks

8.3. Temporal Vcrridoris of‘ Other Seisrnic. Pcimmeters 2 19
8.3.3. Changes in Source Characteristics
It is important in earthquake prediction to distinguish a foreshock sequence
from the background seismicity of a region, or from a swarm that
will not culminate in a much larger earthquake. An obvious hypothesis to
be tested is whether genuine foreshocks have different source characteristics
than background seismicity or swarms. Changes in source characteristics
may be inferred, for example, from studies of waveforms, amplitudes,
and spectra of seismic waves, and from fault-plane solutions. In
this subsection, we summarize a few examples of different approaches to
investigate changes in source characteristics.
There is some evidence that variations in the ratio of P to S amplitudes
may be useful. The amplitude ratio (P/S) for an earthquake at a given
station is usually determined from the maximum P amplitude and the
maximum S amplitude recorded on a given component of the station’s
seismogram. It depends mainly on the following factors: (1) the orientation
of the fault plane at the source; (2) the direction of rupture during the
earthquake; (3) the propagation path of the seismic waves; and (4) the
position of the recording station with respect to the earthquake hypocenter.
If the earthquakes being studied occur relatively close to one
another, then the amplitude ratio should depend mostly on the first two
factors. If these earthquakes have the same source characteristics, then
the amplitude ratio at a given station should be constant.
Chin et af. (1978) studied amplitude ratios from vertical-component recordings
of P and SV phases from foreshocks and aftershocks of the
magnitude 7.3 Haicheng, China, earthquake of February 4, 1975. The
amplitude ratios (P/SV) for the foreshocks were found to be quite stable
with time, although their values differed from station to station. However,
the amplitude ratios (P/SV) for the aftershocks were found to be highly
variable at all stations. In order to distinguish foreshocks from swarms,
Chin et al. also studied amplitude ratios (P/SV) for five earthquake
swarms occurring elsewhere in China. They found that these ratios for the
swarms were unstable with time.
The amplitude ratios (P/SV) for three main shocks in California ranging
in magnitude from 4.3 to 5.7 were examined by Lindh et al. (1978a).
Because of low background seismicity in the focal regions prior to the
foreshock sequences, they were restricted to comparing amplitude ratios
between foreshocks and aftershocks. The amplitude ratios (P/SV) were
measured from short-period, vertical-component seismograms at one station
near each of the three focal regions. They found that the amplitude
ratios showed significant decreases from foreshocks to aftershocks in all
cases. These amplitude ratio changes were attributed to a slight rotation