The ANSS event ID is nm608412 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm608412/executive.
2011/02/19 23:04:58 35.268 -92.374 6.7 3.4 Arkansas
USGS/SLU Moment Tensor Solution ENS 2011/02/19 23:04:58:0 35.27 -92.37 6.7 3.4 Arkansas Stations used: AG.WHAR NM.X102 NM.X201 TA.V39A TA.Y40A Filtering commands used: hp c 0.05 n 3 lp c 0.10 n 3 br c 0.12 0.20 n 4 p 2 Best Fitting Double Couple Mo = 1.01e+21 dyne-cm Mw = 3.27 Z = 4 km Plane Strike Dip Rake NP1 224 76 -164 NP2 130 75 -15 Principal Axes: Axis Value Plunge Azimuth T 1.01e+21 0 357 N 0.00e+00 69 266 P -1.01e+21 21 87 Moment Tensor: (dyne-cm) Component Value Mxx 1.01e+21 Mxy -9.94e+19 Mxz -1.11e+19 Myy -8.75e+20 Myz -3.39e+20 Mzz -1.31e+20 #### T ####### ######## ########### ############################ ###########################--- --########################-------- ----####################------------ ------#################--------------- --------#############------------------- ----------#########--------------------- ------------#######----------------- --- --------------###------------------- P --- ---------------#-------------------- --- --------------####------------------------ -----------#########-------------------- ----------############------------------ --------################-------------- ------####################---------- ----#########################----- -############################# ############################ ###################### ############## Global CMT Convention Moment Tensor: R T P -1.31e+20 -1.11e+19 3.39e+20 -1.11e+19 1.01e+21 9.94e+19 3.39e+20 9.94e+19 -8.75e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110219230458/index.html |
STK = 130 DIP = 75 RAKE = -15 MW = 3.27 HS = 4.0
The NDK file is 20110219230458.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:
hp c 0.05 n 3 lp c 0.10 n 3 br c 0.12 0.20 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 135 20 5 3.33 0.5449 WVFGRD96 1.0 305 75 -60 3.23 0.5755 WVFGRD96 2.0 285 45 -55 3.25 0.6204 WVFGRD96 3.0 125 60 -20 3.27 0.6680 WVFGRD96 4.0 130 75 -15 3.27 0.6691 WVFGRD96 5.0 310 90 15 3.26 0.6488 WVFGRD96 6.0 310 85 15 3.29 0.6265 WVFGRD96 7.0 310 75 15 3.31 0.6027 WVFGRD96 8.0 310 65 15 3.32 0.5807 WVFGRD96 9.0 310 60 15 3.35 0.5634 WVFGRD96 10.0 310 55 15 3.39 0.5460 WVFGRD96 11.0 310 50 20 3.41 0.5284 WVFGRD96 12.0 315 40 20 3.43 0.5127 WVFGRD96 13.0 315 35 25 3.46 0.4989 WVFGRD96 14.0 315 35 20 3.47 0.4885 WVFGRD96 15.0 315 35 20 3.48 0.4766 WVFGRD96 16.0 315 30 25 3.50 0.4659 WVFGRD96 17.0 315 30 30 3.51 0.4619 WVFGRD96 18.0 315 30 30 3.52 0.4557 WVFGRD96 19.0 325 25 40 3.53 0.4485 WVFGRD96 20.0 335 20 50 3.57 0.4403 WVFGRD96 21.0 320 25 35 3.57 0.4339 WVFGRD96 22.0 335 20 55 3.59 0.4288 WVFGRD96 23.0 165 75 30 3.88 0.4454 WVFGRD96 24.0 165 75 30 3.89 0.4453 WVFGRD96 25.0 165 75 30 3.90 0.4415 WVFGRD96 26.0 165 80 25 3.95 0.4436 WVFGRD96 27.0 165 80 25 3.96 0.4525 WVFGRD96 28.0 165 80 25 3.97 0.4583 WVFGRD96 29.0 165 80 25 3.98 0.4589
The best solution is
WVFGRD96 4.0 130 75 -15 3.27 0.6691
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 0.05 n 3 lp c 0.10 n 3 br c 0.12 0.20 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 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