ROUTINE FAULT PLANE SOLUTIONS FOR LOCAL NETWORKS - A TEST WITH SYNTHETIC DATA

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.