Abstract
Waveforms of the direct P-, SV- and SH-waves of five 1983 Goodnow, New York, aftershocks (mb = 1.4–3.1), locally recorded at four hard-rock sites (epicentral distances=1.9–8.0 km) with GEOS systems, were studied to obtain their focal mechanism solutions by waveform fit using both systematic search and moment tensor inversion. Both synthetic and observed data were low-pass filtered at 10 Hz to reduce sensitivity to shallow earth structure. It was discovered that only the first cycle of P-wave and S-wave appear to have pure direct body wave characteristics. The strong P- and S-coda have no stable polarization. The five aftershocks have similar locations, identical P-first motions, but varying direct S-waveforms.
A layered velocity model with a P-wave velocity of 4.4 km/s in the surface layer was derived. Fault plane solutions of four events indicate reverse faulting mechanisms that have a near horizontal P-axis with a strike of ENE. This is similar to the fault plane solution of the mainshock (October 7, 1983, mb = 5.1) and the composite focal mechanism of the aftershocks. Four aftershocks occurred on the fault planes with the strike NW-N and dip of 52°–64° toward NE-E. The fifth event studied has significant strike-slip motion with the P axis is also nearly horizontal and oriented NE. The results of systematic search technique agree well with those of moment tensor inversion. The first motion directions, pulse widths, amplitudes, amplitude ratios and arrival times of the direct P-, SV- and SH-phases of the synthetic seismograms are consistent with those of the observed seismograms. The results of the research demonstrated that the S-wave amplitude can provide important constraints on the focal mechanism.
