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:
cut o DIST/3.3 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.06 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 1.0 230 15 90 3.95 0.3836 WVFGRD96 2.0 30 5 55 3.96 0.4577 WVFGRD96 3.0 30 10 55 3.90 0.4904 WVFGRD96 4.0 35 10 60 3.87 0.5010 WVFGRD96 5.0 30 5 50 3.85 0.4963 WVFGRD96 6.0 260 10 -80 3.84 0.4858 WVFGRD96 7.0 235 20 -100 3.86 0.4800 WVFGRD96 8.0 50 60 -80 3.90 0.4789 WVFGRD96 9.0 45 55 -80 3.91 0.4729 WVFGRD96 10.0 45 60 -85 3.92 0.4484 WVFGRD96 11.0 40 55 -85 3.93 0.4347 WVFGRD96 12.0 40 55 -85 3.93 0.4173 WVFGRD96 13.0 40 55 -90 3.92 0.3972 WVFGRD96 14.0 30 55 -95 3.92 0.3758 WVFGRD96 15.0 220 40 -80 3.93 0.3545 WVFGRD96 16.0 225 45 -80 3.93 0.3348 WVFGRD96 17.0 225 50 -80 3.93 0.3178 WVFGRD96 18.0 225 50 -80 3.93 0.3022 WVFGRD96 19.0 225 50 -75 3.94 0.2862 WVFGRD96 20.0 225 50 -80 3.96 0.2649 WVFGRD96 21.0 225 50 -80 3.96 0.2523 WVFGRD96 22.0 35 65 -90 3.97 0.2429 WVFGRD96 23.0 215 25 -85 3.98 0.2350 WVFGRD96 24.0 215 25 -85 3.99 0.2279 WVFGRD96 25.0 215 25 -85 4.00 0.2202 WVFGRD96 26.0 215 25 -85 4.00 0.2121 WVFGRD96 27.0 70 80 -85 4.01 0.2049 WVFGRD96 28.0 75 80 -85 4.02 0.2032 WVFGRD96 29.0 305 20 -70 4.06 0.2023
The best solution is
WVFGRD96 4.0 35 10 60 3.87 0.5010
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
cut o DIST/3.3 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.06 n 3 lp c 0.10 n 3
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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.