St. Rognvaldsson et R. Slunga, ROUTINE FAULT PLANE SOLUTIONS FOR LOCAL NETWORKS - A TEST WITH SYNTHETIC DATA, Bulletin of the Seismological Society of America, 83(4), 1993, pp. 1232-1247
For local networks aimed at analysis of microseismicity, the classic m
ethod for focal mechanism determination by use of first motion directi
ons can be of limited value because often very few clear first motions
are observed. A number of alternative methods making use of amplitude
s have been proposed. The radiated far field, low-frequency spectral a
mplitudes of body waves depend on the seismic moment and orientation o
f the earthquake source. This can be used to obtain fault plane soluti
ons from amplitude observations. The observed spectral amplitudes are
corrected for the free surface effect, attenuation, instrument respons
e and geometrical spreading. Searching over the entire parameter space
for strike, dip and rake, the source orientations that satisfy the po
larity observations and give ''sufficiently'' small least squares misf
its between observed and predicted amplitudes are accepted as possible
focal mechanisms for the earthquake. The reflectivity method was used
to generate synthetic seismograms for earthquake sources within a loc
al network in Iceland. The amplitude inversion algorithm was applied t
o the synthetic seismograms. Inverting noise free amplitudes at eight
stations, the solutions obtained deviate less than 10-degrees from the
correct mechanism in all source angles for all mechanisms tested. Add
ing noise and scaling the synthetic data to simulate events of varying
magnitude indicates that the method gives fault plane solutions for l
ocal events down to magnitude M(L) = 0.5 correct to within +/- 15-degr
ees in all three source angles.