The ANSS event ID is us20003hgz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us20003hgz/executive.
2015/09/09 03:42:49 36.182 -97.498 3.5 3.5 Oklahoma
USGS/SLU Moment Tensor Solution ENS 2015/09/09 03:42:49:0 36.18 -97.50 3.5 3.5 Oklahoma Stations used: GS.KAN05 GS.KAN08 GS.KAN11 GS.KAN12 GS.KAN13 GS.KAN14 GS.KAN17 GS.KS20 GS.KS21 GS.OK025 GS.OK029 GS.OK031 GS.OK032 N4.T35B N4.U38B N4.Z35B OK.CROK OK.FNO OK.GORE OK.X37A Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 1.38e+21 dyne-cm Mw = 3.36 Z = 3 km Plane Strike Dip Rake NP1 120 75 -25 NP2 217 66 -164 Principal Axes: Axis Value Plunge Azimuth T 1.38e+21 6 170 N 0.00e+00 61 271 P -1.38e+21 28 77 Moment Tensor: (dyne-cm) Component Value Mxx 1.27e+21 Mxy -4.78e+20 Mxz -2.76e+20 Myy -9.74e+20 Myz -5.33e+20 Mzz -2.92e+20 ############## ###################### ########################---- #####################--------- ####################-------------- ###################----------------- --################-------------------- -----############----------------------- -------#########----------------- ---- ----------#####------------------- P ----- -------------#-------------------- ----- -------------##--------------------------- ------------######------------------------ ----------##########-------------------- ---------###############---------------- --------####################---------- ------############################-- -----############################# ---########################### -########################### ############# ###### ######### T ## Global CMT Convention Moment Tensor: R T P -2.92e+20 -2.76e+20 5.33e+20 -2.76e+20 1.27e+21 4.78e+20 5.33e+20 4.78e+20 -9.74e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150909034249/index.html |
STK = 120 DIP = 75 RAKE = -25 MW = 3.36 HS = 3.0
The NDK file is 20150909034249.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 125 70 -30 3.19 0.5005 WVFGRD96 2.0 310 65 -30 3.31 0.5961 WVFGRD96 3.0 120 75 -25 3.36 0.6269 WVFGRD96 4.0 120 90 -5 3.40 0.6147 WVFGRD96 5.0 120 90 -10 3.42 0.5859 WVFGRD96 6.0 120 90 -20 3.45 0.5591 WVFGRD96 7.0 305 85 20 3.46 0.5511 WVFGRD96 8.0 130 75 25 3.48 0.5476 WVFGRD96 9.0 130 75 25 3.48 0.5391 WVFGRD96 10.0 315 75 30 3.51 0.5358 WVFGRD96 11.0 315 75 30 3.53 0.5335 WVFGRD96 12.0 315 75 30 3.54 0.5307 WVFGRD96 13.0 315 75 30 3.54 0.5273 WVFGRD96 14.0 315 75 30 3.55 0.5242 WVFGRD96 15.0 315 75 30 3.56 0.5202 WVFGRD96 16.0 315 75 25 3.57 0.5155 WVFGRD96 17.0 315 80 30 3.58 0.5104 WVFGRD96 18.0 315 80 30 3.59 0.5050 WVFGRD96 19.0 315 80 30 3.60 0.4995 WVFGRD96 20.0 315 80 30 3.60 0.4940 WVFGRD96 21.0 315 75 25 3.60 0.4870 WVFGRD96 22.0 315 75 25 3.61 0.4812 WVFGRD96 23.0 315 75 25 3.62 0.4757 WVFGRD96 24.0 310 80 25 3.62 0.4701 WVFGRD96 25.0 315 75 20 3.62 0.4656 WVFGRD96 26.0 310 80 20 3.63 0.4614 WVFGRD96 27.0 310 80 20 3.64 0.4573 WVFGRD96 28.0 315 75 10 3.64 0.4540 WVFGRD96 29.0 320 70 -5 3.65 0.4511
The best solution is
WVFGRD96 3.0 120 75 -25 3.36 0.6269
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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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 WUS.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 Model after 8 iterations 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 1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00 6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00 13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00 19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00 0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00