The ANSS event ID is ak0132mf6kwp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0132mf6kwp/executive.
2013/02/26 09:32:18 64.098 -149.311 139.2 4.9 Arkansas
USGS/SLU Moment Tensor Solution ENS 2013/02/26 09:32:18:0 64.10 -149.31 139.2 4.9 Arkansas Stations used: AK.CAST AK.CCB AK.COLD AK.DHY AK.DOT AK.FYU AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM AK.MLY AK.PAX AK.PPD AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SSN AK.TRF AK.WRH AT.MENT AT.PMR IU.COLA US.EGAK Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.41e+23 dyne-cm Mw = 4.70 Z = 155 km Plane Strike Dip Rake NP1 359 54 -110 NP2 210 40 -65 Principal Axes: Axis Value Plunge Azimuth T 1.41e+23 7 102 N 0.00e+00 16 10 P -1.41e+23 72 217 Moment Tensor: (dyne-cm) Component Value Mxx -1.71e+21 Mxy -3.54e+22 Mxz 2.85e+22 Myy 1.28e+23 Myz 4.21e+22 Mzz -1.26e+23 ########------ ##############-####### #############-----########## ###########----------######### ###########------------########### ##########---------------########### ##########-----------------########### #########-------------------############ ########---------------------########### #########---------------------############ ########----------------------############ #######-----------------------############ #######--------- -----------######## # ######--------- P -----------######## T ######--------- ----------######### #####----------------------########### ####----------------------########## ####--------------------########## ##-------------------######### ##-----------------######### ---------------####### ---------##### Global CMT Convention Moment Tensor: R T P -1.26e+23 2.85e+22 -4.21e+22 2.85e+22 -1.71e+21 3.54e+22 -4.21e+22 3.54e+22 1.28e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130226093218/index.html |
STK = 210 DIP = 40 RAKE = -65 MW = 4.70 HS = 155.0
The NDK file is 20130226093218.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: 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 0.02 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 5.0 10 70 85 3.84 0.1984 WVFGRD96 10.0 345 50 40 3.96 0.2636 WVFGRD96 15.0 305 45 -45 4.04 0.3001 WVFGRD96 20.0 250 20 -35 4.13 0.2931 WVFGRD96 25.0 205 30 -75 4.17 0.2794 WVFGRD96 30.0 200 35 -75 4.20 0.2615 WVFGRD96 35.0 260 55 65 4.22 0.2971 WVFGRD96 40.0 265 50 70 4.34 0.3194 WVFGRD96 45.0 265 50 70 4.41 0.3320 WVFGRD96 50.0 260 50 55 4.45 0.3329 WVFGRD96 55.0 255 55 45 4.48 0.3310 WVFGRD96 60.0 255 55 40 4.50 0.3333 WVFGRD96 65.0 200 35 -85 4.51 0.3747 WVFGRD96 70.0 205 35 -80 4.53 0.4152 WVFGRD96 75.0 210 35 -70 4.54 0.4485 WVFGRD96 80.0 210 35 -70 4.55 0.4760 WVFGRD96 85.0 215 35 -65 4.57 0.4997 WVFGRD96 90.0 215 35 -65 4.58 0.5181 WVFGRD96 95.0 220 40 -60 4.59 0.5378 WVFGRD96 100.0 220 40 -60 4.60 0.5548 WVFGRD96 105.0 220 40 -60 4.61 0.5715 WVFGRD96 110.0 220 40 -60 4.62 0.5866 WVFGRD96 115.0 210 35 -65 4.63 0.6018 WVFGRD96 120.0 210 35 -65 4.64 0.6160 WVFGRD96 125.0 215 40 -60 4.65 0.6306 WVFGRD96 130.0 215 40 -60 4.66 0.6428 WVFGRD96 135.0 215 40 -60 4.67 0.6545 WVFGRD96 140.0 215 40 -60 4.68 0.6627 WVFGRD96 145.0 210 40 -65 4.68 0.6672 WVFGRD96 150.0 210 40 -65 4.69 0.6706 WVFGRD96 155.0 210 40 -65 4.70 0.6707 WVFGRD96 160.0 210 40 -65 4.71 0.6669 WVFGRD96 165.0 210 40 -65 4.71 0.6585 WVFGRD96 170.0 210 40 -65 4.72 0.6477
The best solution is
WVFGRD96 155.0 210 40 -65 4.70 0.6707
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 0.10 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 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