The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 90 75 -5 3.30 0.5134 WVFGRD96 1.0 90 75 -10 3.33 0.5374 WVFGRD96 2.0 85 70 -20 3.38 0.5615 WVFGRD96 3.0 90 70 -15 3.40 0.5588 WVFGRD96 4.0 90 70 -15 3.41 0.5489 WVFGRD96 5.0 90 70 -15 3.41 0.5357 WVFGRD96 6.0 270 85 25 3.43 0.5339 WVFGRD96 7.0 270 70 10 3.43 0.5351 WVFGRD96 8.0 275 80 25 3.45 0.5361 WVFGRD96 9.0 275 80 25 3.46 0.5369 WVFGRD96 10.0 275 80 25 3.47 0.5372 WVFGRD96 11.0 275 80 25 3.48 0.5343 WVFGRD96 12.0 275 80 25 3.49 0.5317 WVFGRD96 13.0 275 80 25 3.50 0.5296 WVFGRD96 14.0 275 80 25 3.51 0.5268 WVFGRD96 15.0 275 80 25 3.52 0.5227 WVFGRD96 16.0 270 70 10 3.51 0.5184 WVFGRD96 17.0 270 70 10 3.52 0.5155 WVFGRD96 18.0 270 70 10 3.52 0.5121 WVFGRD96 19.0 270 70 10 3.53 0.5079 WVFGRD96 20.0 270 65 10 3.55 0.5046 WVFGRD96 21.0 275 60 15 3.57 0.5012 WVFGRD96 22.0 275 60 15 3.57 0.4973 WVFGRD96 23.0 270 60 10 3.57 0.4948 WVFGRD96 24.0 270 60 5 3.58 0.4926 WVFGRD96 25.0 270 60 10 3.59 0.4895 WVFGRD96 26.0 270 55 10 3.60 0.4883 WVFGRD96 27.0 270 55 10 3.61 0.4855 WVFGRD96 28.0 270 55 10 3.61 0.4839 WVFGRD96 29.0 260 65 -20 3.59 0.4804
The best solution is
WVFGRD96 2.0 85 70 -20 3.38 0.5615
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
hp c 0.03 n 3 lp c 0.10 n 3
The plots below start 30 seconds before the theoretical S arrival and continue until 60 seconds after the theoretical S arrival.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.