The ANSS event ID is uu50364795 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/uu50364795/executive.
2008/08/30 22:06:15 41.681 -111.150 0.3 3.31 Utah
USGS/SLU Moment Tensor Solution ENS 2008/08/30 22:06:15:0 41.68 -111.15 0.3 3.3 Utah Stations used: IW.DCID1 IW.REDW TA.K17A TA.K18A TA.K19A TA.L15A TA.L16A TA.L17A TA.L18A TA.M15A TA.M16A TA.M17A TA.M18A TA.M19A TA.N15A TA.N16A TA.N17A TA.O16A US.AHID US.HWUT UU.BGU UU.SPU Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.13 0.2 n 4 p 2 Best Fitting Double Couple Mo = 1.08e+21 dyne-cm Mw = 3.29 Z = 10 km Plane Strike Dip Rake NP1 10 60 -125 NP2 245 45 -45 Principal Axes: Axis Value Plunge Azimuth T 1.08e+21 8 125 N 0.00e+00 30 30 P -1.08e+21 59 229 Moment Tensor: (dyne-cm) Component Value Mxx 2.14e+20 Mxy -6.42e+20 Mxz 2.29e+20 Myy 5.52e+20 Myz 4.91e+20 Mzz -7.66e+20 ###########--- #################----- ####################-------- ######################-------- #################-------#####----- #############-------------#########- ###########----------------########### #########-------------------############ #######---------------------############ #######----------------------############# #####------------------------############# ####-------------------------############# ###----------- -----------############## ##----------- P -----------############# #------------ ----------############## -------------------------######### # -----------------------########## T ---------------------########### ------------------############ ---------------############# -----------########### -----######### Global CMT Convention Moment Tensor: R T P -7.66e+20 2.29e+20 -4.91e+20 2.29e+20 2.14e+20 6.42e+20 -4.91e+20 6.42e+20 5.52e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080830220615/index.html |
STK = 245 DIP = 45 RAKE = -45 MW = 3.29 HS = 10.0
The NDK file is 20080830220615.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 3 br c 0.13 0.2 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 40 50 -90 2.87 0.1599 WVFGRD96 1.0 270 75 15 2.77 0.1482 WVFGRD96 2.0 20 45 -90 3.03 0.2297 WVFGRD96 3.0 90 60 20 3.03 0.2534 WVFGRD96 4.0 255 45 -25 3.11 0.2798 WVFGRD96 5.0 255 45 -25 3.15 0.3167 WVFGRD96 6.0 255 45 -25 3.18 0.3464 WVFGRD96 7.0 250 45 -35 3.21 0.3648 WVFGRD96 8.0 245 40 -45 3.28 0.3779 WVFGRD96 9.0 245 40 -45 3.29 0.3805 WVFGRD96 10.0 245 45 -45 3.29 0.3812 WVFGRD96 11.0 250 50 -35 3.28 0.3797 WVFGRD96 12.0 250 50 -35 3.29 0.3774 WVFGRD96 13.0 250 50 -35 3.30 0.3735 WVFGRD96 14.0 255 55 -30 3.30 0.3687 WVFGRD96 15.0 255 55 -30 3.31 0.3634 WVFGRD96 16.0 255 55 -30 3.32 0.3574 WVFGRD96 17.0 255 55 -30 3.32 0.3510 WVFGRD96 18.0 255 60 -30 3.32 0.3451 WVFGRD96 19.0 255 60 -30 3.33 0.3388 WVFGRD96 20.0 255 60 -30 3.34 0.3323 WVFGRD96 21.0 255 60 -30 3.35 0.3263 WVFGRD96 22.0 255 60 -35 3.36 0.3199 WVFGRD96 23.0 255 60 -35 3.36 0.3132 WVFGRD96 24.0 255 65 -35 3.36 0.3060 WVFGRD96 25.0 255 65 -35 3.36 0.2988 WVFGRD96 26.0 255 65 -35 3.37 0.2913 WVFGRD96 27.0 75 55 20 3.37 0.2840 WVFGRD96 28.0 75 55 20 3.38 0.2783 WVFGRD96 29.0 70 60 10 3.37 0.2737 WVFGRD96 30.0 70 60 10 3.37 0.2690 WVFGRD96 31.0 70 60 5 3.38 0.2640 WVFGRD96 32.0 70 60 10 3.39 0.2595 WVFGRD96 33.0 70 60 10 3.39 0.2553 WVFGRD96 34.0 70 60 10 3.40 0.2517 WVFGRD96 35.0 70 60 10 3.40 0.2490 WVFGRD96 36.0 70 60 15 3.41 0.2463 WVFGRD96 37.0 70 60 15 3.42 0.2449 WVFGRD96 38.0 70 60 20 3.44 0.2460 WVFGRD96 39.0 70 65 30 3.46 0.2484 WVFGRD96 40.0 85 50 50 3.55 0.2522 WVFGRD96 41.0 80 55 45 3.55 0.2556 WVFGRD96 42.0 80 55 45 3.56 0.2581 WVFGRD96 43.0 80 55 45 3.57 0.2598 WVFGRD96 44.0 80 55 45 3.58 0.2607 WVFGRD96 45.0 80 55 45 3.59 0.2619 WVFGRD96 46.0 80 55 45 3.60 0.2621 WVFGRD96 47.0 80 55 45 3.60 0.2622 WVFGRD96 48.0 80 55 50 3.62 0.2614 WVFGRD96 49.0 185 45 65 3.64 0.2604 WVFGRD96 50.0 185 45 65 3.65 0.2610
The best solution is
WVFGRD96 10.0 245 45 -45 3.29 0.3812
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 br c 0.13 0.2 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