The ANSS event ID is usp000gzgy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000gzgy/executive.
2009/07/21 14:20:55 49.725 -65.706 13.8 3.5 Quebec, Canada
USGS/SLU Moment Tensor Solution ENS 2009/07/21 14:20:55:0 49.72 -65.71 13.8 3.5 Quebec, Canada Stations used: CN.A11 CN.A16 CN.A21 CN.A54 CN.A64 CN.DRLN CN.GGN CN.ICQ CN.LMN CN.LMQ CN.SCHQ IU.HRV Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.57e+21 dyne-cm Mw = 3.54 Z = 16 km Plane Strike Dip Rake NP1 360 60 75 NP2 208 33 114 Principal Axes: Axis Value Plunge Azimuth T 2.57e+21 71 236 N 0.00e+00 13 8 P -2.57e+21 14 101 Moment Tensor: (dyne-cm) Component Value Mxx -2.01e+14 Mxy 5.76e+20 Mxz -3.33e+20 Myy -2.15e+21 Myz -1.24e+21 Mzz 2.15e+21 --------###### -----------###-------- ----------########---------- ---------###########---------- ---------##############----------- --------################------------ --------##################------------ --------###################------------- -------####################------------- -------######################------------- -------######################------------- ------######### ###########-------- -- ------######### T ###########-------- P -- -----######### ##########--------- - -----######################------------- ----######################------------ ----####################------------ ---####################----------- --##################---------- --################---------- -#############-------- ########------ Global CMT Convention Moment Tensor: R T P 2.15e+21 -3.33e+20 1.24e+21 -3.33e+20 -2.01e+14 -5.76e+20 1.24e+21 -5.76e+20 -2.15e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090721142055/index.html |
STK = 360 DIP = 60 RAKE = 75 MW = 3.54 HS = 16.0
The NDK file is 20090721142055.ndk The waveform inversion is preferred.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 -40 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 are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 135 55 -85 3.32 0.4987 WVFGRD96 1.0 140 50 -85 3.35 0.5109 WVFGRD96 2.0 185 60 -45 3.44 0.4698 WVFGRD96 3.0 25 75 30 3.48 0.4236 WVFGRD96 4.0 30 60 35 3.49 0.4406 WVFGRD96 5.0 30 55 35 3.50 0.4733 WVFGRD96 6.0 30 60 45 3.50 0.5054 WVFGRD96 7.0 30 65 60 3.49 0.5372 WVFGRD96 8.0 10 70 70 3.43 0.5709 WVFGRD96 9.0 10 65 75 3.46 0.6045 WVFGRD96 10.0 0 65 75 3.48 0.6312 WVFGRD96 11.0 5 60 80 3.51 0.6584 WVFGRD96 12.0 5 60 80 3.52 0.6810 WVFGRD96 13.0 5 60 80 3.53 0.6966 WVFGRD96 14.0 355 60 75 3.53 0.7062 WVFGRD96 15.0 355 60 75 3.53 0.7108 WVFGRD96 16.0 360 60 75 3.54 0.7111 WVFGRD96 17.0 360 60 75 3.55 0.7078 WVFGRD96 18.0 360 65 75 3.55 0.7016 WVFGRD96 19.0 0 65 75 3.56 0.6925 WVFGRD96 20.0 -5 65 75 3.59 0.6805 WVFGRD96 21.0 -5 65 75 3.60 0.6657 WVFGRD96 22.0 -5 65 75 3.61 0.6487 WVFGRD96 23.0 -5 65 75 3.61 0.6295 WVFGRD96 24.0 5 65 80 3.63 0.6085 WVFGRD96 25.0 5 65 80 3.64 0.5858 WVFGRD96 26.0 5 65 80 3.64 0.5615 WVFGRD96 27.0 5 65 80 3.65 0.5357 WVFGRD96 28.0 10 65 85 3.66 0.5097 WVFGRD96 29.0 10 65 85 3.66 0.4833
The best solution is
WVFGRD96 16.0 360 60 75 3.54 0.7111
The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.
The CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01 CUS Model with Q from simple gamma values ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00