Application of waveform modeling to determine focal mechanisms of four 1982 Miramichi aftershocks

Abstract

The objective of this study is to determine the focal mechanisms of small earthquakes at very near epicentral distances and use them to predict the observed waveforms. To achieve our objectives, the following steps were adopted before attempting to match the entire waveform: (a) obtain an appropriate velocity model; (b) calculate the Green's functions and predict ground motions due to an earthquake specified by dip (δ), slip (λ), and strike ( ϕ ) of a fault; (c) select all mechanisms which satisfy the first-motion polarity at all stations; (d) select a subset of focal mechanisms from those obtained in step (c) which produce P and S amplitudes and P/SV and SV/SH amplitude ratios correctly; and (e) use focal mechanisms of step (d) to match waveforms. As an alternative approach, a least-squares inversion for the moment-tensor elements was performed and decomposition of moment tensor matrix to two double couples (major and minor) was performed. Green's functions were computed using the Cagniard-de-Hoop theory (Helmberger and Harkrider, 1978). We applied the above approaches to four aftershocks ( mbLg ≦ 3.5) of the New Brunswick earthquake of 9 January 1982 and succeeded in obtaining focal mechanisms and producing synthetic waveforms very similar to the observed waveforms. Two groups of focal mechanisms are found. Two of the earthquakes are found to have reverse fault geometry similar to that of the main event of 9 January 1982 ( mb = 5.7), while the other two have significant components of strike-slip motion on a dipping plane. The results obtained from the least-squares analysis support the results of the search method. The fault geometries determined by the search method correspond well to the results of the major double couple of the least-square inversion. For completeness of the source characteristics investigation, spectral analysis of the seismograms was performed to determine the source parameters: corner frequencies, fc , seismic moments, source dimensions, and stress drops.