The ANSS event ID is usp000gx7w and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000gx7w/executive.
2009/05/17 06:45:18 42.540 -108.116 5.0 3.7 Wyoming
USGS/SLU Moment Tensor Solution ENS 2009/05/17 06:45:18:0 42.54 -108.12 5.0 3.7 Wyoming Stations used: IW.LOHW IW.MOOW IW.PHWY TA.G18A TA.G20A TA.H16A TA.H18A TA.H20A TA.H21A TA.I18A TA.I19A TA.I20A TA.I21A TA.I22A TA.J19A TA.J20A TA.J21A TA.J22A TA.K19A TA.K20A TA.L18A TA.L19A TA.L20A TA.L21A TA.L22A TA.M17A TA.M20A TA.M21A TA.M22A TA.M23A TA.N23A TA.N24A TA.O19A TA.O22A TA.O23A TA.P22A Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.47e+21 dyne-cm Mw = 3.70 Z = 17 km Plane Strike Dip Rake NP1 71 73 -115 NP2 310 30 -35 Principal Axes: Axis Value Plunge Azimuth T 4.47e+21 24 181 N 0.00e+00 24 79 P -4.47e+21 55 310 Moment Tensor: (dyne-cm) Component Value Mxx 3.10e+21 Mxy 7.75e+20 Mxz -3.02e+21 Myy -8.85e+20 Myz 1.60e+21 Mzz -2.22e+21 ############## ###################### ###------------############# ---------------------######### -------------------------######### -----------------------------####### ---------- ------------------####### ----------- P --------------------#####- ----------- ---------------------###-- ------------------------------------#----- ---------------------------------####----- ------------------------------########---- --------------------------############---- --------------------##################-- ------------##########################-- ####################################-- ###################################- #################################- ############# ############## ############ T ############# ######### ########## ############## Global CMT Convention Moment Tensor: R T P -2.22e+21 -3.02e+21 -1.60e+21 -3.02e+21 3.10e+21 -7.75e+20 -1.60e+21 -7.75e+20 -8.85e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090517064518/index.html |
STK = 310 DIP = 30 RAKE = -35 MW = 3.70 HS = 17.0
The NDK file is 20090517064518.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 0.5 80 45 -90 3.20 0.2242 WVFGRD96 1.0 80 45 -90 3.23 0.1962 WVFGRD96 2.0 260 45 90 3.38 0.2572 WVFGRD96 3.0 70 65 -55 3.40 0.1908 WVFGRD96 4.0 185 30 10 3.41 0.2199 WVFGRD96 5.0 190 25 10 3.43 0.2638 WVFGRD96 6.0 85 85 60 3.46 0.3046 WVFGRD96 7.0 85 85 60 3.48 0.3426 WVFGRD96 8.0 85 85 65 3.56 0.3694 WVFGRD96 9.0 85 80 65 3.58 0.4004 WVFGRD96 10.0 90 75 60 3.61 0.4247 WVFGRD96 11.0 85 75 60 3.62 0.4444 WVFGRD96 12.0 90 70 60 3.65 0.4585 WVFGRD96 13.0 305 25 -45 3.64 0.4696 WVFGRD96 14.0 305 25 -45 3.65 0.4817 WVFGRD96 15.0 305 25 -45 3.67 0.4896 WVFGRD96 16.0 310 30 -35 3.69 0.4940 WVFGRD96 17.0 310 30 -35 3.70 0.4951 WVFGRD96 18.0 305 30 -40 3.71 0.4933 WVFGRD96 19.0 310 30 -35 3.73 0.4889 WVFGRD96 20.0 310 30 -35 3.74 0.4810 WVFGRD96 21.0 305 30 -35 3.76 0.4711 WVFGRD96 22.0 305 25 -35 3.76 0.4585 WVFGRD96 23.0 330 25 -15 3.78 0.4446 WVFGRD96 24.0 335 25 -15 3.79 0.4311 WVFGRD96 25.0 335 25 -10 3.79 0.4161 WVFGRD96 26.0 345 20 0 3.80 0.4006 WVFGRD96 27.0 345 20 0 3.81 0.3850 WVFGRD96 28.0 355 20 10 3.81 0.3682 WVFGRD96 29.0 355 20 10 3.82 0.3516
The best solution is
WVFGRD96 17.0 310 30 -35 3.70 0.4951
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