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.
![]() |
|
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 0.03 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 1.0 240 85 90 4.10 0.3487 WVFGRD96 2.0 30 5 55 3.96 0.4096 WVFGRD96 3.0 40 10 65 3.90 0.4386 WVFGRD96 4.0 50 10 75 3.87 0.4468 WVFGRD96 5.0 50 10 75 3.85 0.4414 WVFGRD96 6.0 65 10 85 3.84 0.4292 WVFGRD96 7.0 240 15 -100 3.85 0.4207 WVFGRD96 8.0 50 60 -80 3.89 0.4185 WVFGRD96 9.0 45 55 -80 3.91 0.4152 WVFGRD96 10.0 50 60 -80 3.91 0.3944 WVFGRD96 11.0 40 55 -85 3.92 0.3844 WVFGRD96 12.0 40 55 -85 3.92 0.3702 WVFGRD96 13.0 40 55 -85 3.91 0.3537 WVFGRD96 14.0 215 35 -90 3.91 0.3361 WVFGRD96 15.0 215 35 -90 3.91 0.3181 WVFGRD96 16.0 220 40 -80 3.92 0.3013 WVFGRD96 17.0 225 45 -75 3.92 0.2852 WVFGRD96 18.0 225 45 -75 3.92 0.2719 WVFGRD96 19.0 225 45 -75 3.93 0.2588 WVFGRD96 20.0 225 50 -75 3.94 0.2412 WVFGRD96 21.0 225 50 -75 3.95 0.2312 WVFGRD96 22.0 35 65 -90 3.95 0.2230 WVFGRD96 23.0 215 30 -85 3.96 0.2168 WVFGRD96 24.0 35 60 -90 3.96 0.2101 WVFGRD96 25.0 35 60 -90 3.97 0.2029 WVFGRD96 26.0 215 30 -85 3.97 0.1957 WVFGRD96 27.0 215 30 -85 3.98 0.1879 WVFGRD96 28.0 295 20 -85 4.02 0.1879 WVFGRD96 29.0 300 20 -75 4.03 0.1885
The best solution is
WVFGRD96 4.0 50 10 75 3.87 0.4468
The mechanism correspond to the best fit is
![]() |
|
The best fit as a function of depth is given in the following figure:
![]() |
|
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 0.03 n 3 lp c 0.10 n 3
![]() |
|
![]() |
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.