The ANSS event ID is usp000jafh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000jafh/executive.
2011/11/06 17:52:34 35.494 -96.828 3.1 3.4 Oklahoma
USGS/SLU Moment Tensor Solution ENS 2011/11/06 17:52:34:0 35.49 -96.83 3.1 3.4 Oklahoma Stations used: TA.136A TA.Q35A TA.R35A TA.R36A TA.R37A TA.S35A TA.S36A TA.T34A TA.T35A TA.T36A TA.T37A TA.T38A TA.TUL1 TA.U32A TA.U35A TA.U36A TA.U38A TA.U39A TA.V35A TA.V36A TA.V37A TA.V38A TA.V40A TA.W35A TA.W36A TA.W37B TA.W38A TA.W39A TA.W40A TA.X35A TA.X36A TA.X37A TA.X39A TA.Y35A TA.Y36A TA.Y37A US.KSU1 US.MIAR Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.76e+21 dyne-cm Mw = 3.43 Z = 3 km Plane Strike Dip Rake NP1 54 85 165 NP2 145 75 5 Principal Axes: Axis Value Plunge Azimuth T 1.76e+21 14 8 N 0.00e+00 74 216 P -1.76e+21 7 100 Moment Tensor: (dyne-cm) Component Value Mxx 1.56e+21 Mxy 5.43e+20 Mxz 4.47e+20 Myy -1.64e+21 Myz -1.51e+20 Mzz 7.66e+19 ######## ### ############ T ####### --############# ########## ----########################## ------###########################- --------#########################--- ----------#####################------- ------------##################---------- -------------###############------------ ---------------############--------------- ----------------########------------------ -----------------#####----------------- ------------------#-------------------- P ----------------###------------------- -------------#######-------------------- ---------############----------------- -----################--------------- -#####################------------ ######################-------- ########################---- ###################### ############## Global CMT Convention Moment Tensor: R T P 7.66e+19 4.47e+20 1.51e+20 4.47e+20 1.56e+21 -5.43e+20 1.51e+20 -5.43e+20 -1.64e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111106175234/index.html |
STK = 145 DIP = 75 RAKE = 5 MW = 3.43 HS = 3.0
The NDK file is 20111106175234.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.
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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:
hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 145 85 -10 3.29 0.4586 WVFGRD96 1.0 145 90 0 3.32 0.4840 WVFGRD96 2.0 145 85 5 3.39 0.5494 WVFGRD96 3.0 145 75 5 3.43 0.5519 WVFGRD96 4.0 145 65 5 3.47 0.5454 WVFGRD96 5.0 140 55 -5 3.52 0.5396 WVFGRD96 6.0 140 55 0 3.53 0.5335 WVFGRD96 7.0 140 60 0 3.54 0.5285 WVFGRD96 8.0 140 55 0 3.57 0.5221 WVFGRD96 9.0 140 55 5 3.59 0.5140 WVFGRD96 10.0 140 55 5 3.59 0.5076 WVFGRD96 11.0 145 60 25 3.61 0.5026 WVFGRD96 12.0 145 60 25 3.61 0.5001 WVFGRD96 13.0 145 60 25 3.62 0.4969 WVFGRD96 14.0 150 55 25 3.62 0.4934 WVFGRD96 15.0 150 55 25 3.62 0.4890 WVFGRD96 16.0 140 80 -30 3.59 0.4848 WVFGRD96 17.0 140 80 -30 3.60 0.4852 WVFGRD96 18.0 140 80 -30 3.61 0.4843 WVFGRD96 19.0 140 80 -30 3.61 0.4826 WVFGRD96 20.0 140 80 -30 3.62 0.4797 WVFGRD96 21.0 145 85 -30 3.63 0.4766 WVFGRD96 22.0 145 90 -30 3.63 0.4737 WVFGRD96 23.0 325 90 30 3.64 0.4704 WVFGRD96 24.0 325 90 30 3.65 0.4667 WVFGRD96 25.0 325 90 30 3.65 0.4623 WVFGRD96 26.0 325 85 25 3.66 0.4577 WVFGRD96 27.0 325 85 25 3.66 0.4534 WVFGRD96 28.0 325 85 25 3.67 0.4484 WVFGRD96 29.0 330 80 25 3.68 0.4434
The best solution is
WVFGRD96 3.0 145 75 5 3.43 0.5519
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
hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2
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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