Location

Location ANSS

2008/03/25 11:59:38 44.697 -110.049 0.2 4.1 Wyoming

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2008/03/25 11:59:38:0 44.70 -110.05 0.2 4.1 Wyoming Stations used: IW.DCID1 IW.DLMT IW.IMW IW.LOHW IW.REDW IW.RRI2 IW.SNOW IW.TPAW TA.A11A TA.A12A TA.A14A TA.A15A TA.A16A TA.A17A TA.B10A TA.B11A TA.B12A TA.B15A TA.B16A TA.B17A TA.B18A TA.C10A TA.C12B TA.C13A TA.C14A TA.C15A TA.C16A TA.C17A TA.D10A TA.D11A TA.D12A TA.D13A TA.D14A TA.D15A TA.D16A TA.D17A TA.D18A TA.E09A TA.E10A TA.E11A TA.E13A TA.E14A TA.E15A TA.E16A TA.E17A TA.E18A TA.F08A TA.F09A TA.F10A TA.F11A TA.F13A TA.F14A TA.F15A TA.F16A TA.F17A TA.F18A TA.G09A TA.G10A TA.G11A TA.G13A TA.G14A TA.G15A TA.G16A TA.G17A TA.G18A TA.H08A TA.H09A TA.H11A TA.H12A TA.H13A TA.H15A TA.H16A TA.I08A TA.I09A TA.I11A TA.I12A TA.I13A TA.I15A TA.I16A TA.I18A TA.J10A TA.J12A TA.J13A TA.J17A TA.J18A TA.K12A TA.K13A TA.K14A TA.K15A TA.K16A TA.K17A TA.K18A TA.K19A TA.K20A TA.L14A TA.L15A TA.L16A TA.L19A TA.L20A TA.L21A TA.M12A TA.M13A TA.M14A TA.M15A TA.M16A TA.M17A TA.M18A TA.M20A TA.M21A TA.M22A TA.N11A TA.N12A TA.N13A TA.N14A TA.N15A TA.N16A TA.N17A TA.N20A TA.N22A TA.O12A TA.O13A TA.O16A TA.O17A TA.O18A TA.P13A TA.P15A TA.P16A TA.P18A US.AHID US.BOZ US.BW06 US.EGMT US.HLID US.LAO US.MSO US.RLMT UU.BGU UU.SPU Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 2.69e+22 dyne-cm Mw = 4.22 Z = 14 km Plane Strike Dip Rake NP1 111 64 -146 NP2 5 60 -30 Principal Axes: Axis Value Plunge Azimuth T 2.69e+22 3 237 N 0.00e+00 49 144 P -2.69e+22 41 330 Moment Tensor: (dyne-cm) Component Value Mxx -3.42e+21 Mxy 1.89e+22 Mxz -1.22e+22 Myy 1.51e+22 Myz 5.69e+21 Mzz -1.17e+22 ----------#### ---------------####### -------------------######### ---------------------######### --------- ------------########## ---------- P ------------########### ----------- ------------############ #---------------------------############ ##--------------------------############ #####------------------------############# #######----------------------############# #########--------------------############# ############----------------############## ##############-------------############# ###################--------############# #################################-- T #####################------------- #####################------------ ###################----------- #################----------- ############---------- #######------- Global CMT Convention Moment Tensor: R T P -1.17e+22 -1.22e+22 -5.69e+21 -1.22e+22 -3.42e+21 -1.89e+22 -5.69e+21 -1.89e+22 1.51e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080325115938/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 5
      DIP = 60
     RAKE = -30
       MW = 4.22
       HS = 14.0

The NDK file is 20080325115938.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2008/03/25 11:59:38:0 44.70 -110.05 0.2 4.1 Wyoming Stations used: IW.DCID1 IW.DLMT IW.IMW IW.LOHW IW.REDW IW.RRI2 IW.SNOW IW.TPAW TA.A11A TA.A12A TA.A14A TA.A15A TA.A16A TA.A17A TA.B10A TA.B11A TA.B12A TA.B15A TA.B16A TA.B17A TA.B18A TA.C10A TA.C12B TA.C13A TA.C14A TA.C15A TA.C16A TA.C17A TA.D10A TA.D11A TA.D12A TA.D13A TA.D14A TA.D15A TA.D16A TA.D17A TA.D18A TA.E09A TA.E10A TA.E11A TA.E13A TA.E14A TA.E15A TA.E16A TA.E17A TA.E18A TA.F08A TA.F09A TA.F10A TA.F11A TA.F13A TA.F14A TA.F15A TA.F16A TA.F17A TA.F18A TA.G09A TA.G10A TA.G11A TA.G13A TA.G14A TA.G15A TA.G16A TA.G17A TA.G18A TA.H08A TA.H09A TA.H11A TA.H12A TA.H13A TA.H15A TA.H16A TA.I08A TA.I09A TA.I11A TA.I12A TA.I13A TA.I15A TA.I16A TA.I18A TA.J10A TA.J12A TA.J13A TA.J17A TA.J18A TA.K12A TA.K13A TA.K14A TA.K15A TA.K16A TA.K17A TA.K18A TA.K19A TA.K20A TA.L14A TA.L15A TA.L16A TA.L19A TA.L20A TA.L21A TA.M12A TA.M13A TA.M14A TA.M15A TA.M16A TA.M17A TA.M18A TA.M20A TA.M21A TA.M22A TA.N11A TA.N12A TA.N13A TA.N14A TA.N15A TA.N16A TA.N17A TA.N20A TA.N22A TA.O12A TA.O13A TA.O16A TA.O17A TA.O18A TA.P13A TA.P15A TA.P16A TA.P18A US.AHID US.BOZ US.BW06 US.EGMT US.HLID US.LAO US.MSO US.RLMT UU.BGU UU.SPU Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.025 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 2.69e+22 dyne-cm Mw = 4.22 Z = 14 km Plane Strike Dip Rake NP1 111 64 -146 NP2 5 60 -30 Principal Axes: Axis Value Plunge Azimuth T 2.69e+22 3 237 N 0.00e+00 49 144 P -2.69e+22 41 330 Moment Tensor: (dyne-cm) Component Value Mxx -3.42e+21 Mxy 1.89e+22 Mxz -1.22e+22 Myy 1.51e+22 Myz 5.69e+21 Mzz -1.17e+22 ----------#### ---------------####### -------------------######### ---------------------######### --------- ------------########## ---------- P ------------########### ----------- ------------############ #---------------------------############ ##--------------------------############ #####------------------------############# #######----------------------############# #########--------------------############# ############----------------############## ##############-------------############# ###################--------############# #################################-- T #####################------------- #####################------------ ###################----------- #################----------- ############---------- #######------- Global CMT Convention Moment Tensor: R T P -1.17e+22 -1.22e+22 -5.69e+21 -1.22e+22 -3.42e+21 -1.89e+22 -5.69e+21 -1.89e+22 1.51e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080325115938/index.html

Magnitudes

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.

Location of broadband stations used for waveform inversion

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 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:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.025 n 3 
lp c 0.05 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   220    45    60   3.98 0.3651
WVFGRD96    2.0   215    50    55   4.06 0.4281
WVFGRD96    3.0    30    55    50   4.10 0.4455
WVFGRD96    4.0    25    55    40   4.11 0.4417
WVFGRD96    5.0    15    55     5   4.10 0.4444
WVFGRD96    6.0    10    55   -10   4.11 0.4556
WVFGRD96    7.0    10    55   -10   4.13 0.4645
WVFGRD96    8.0    10    50   -10   4.17 0.4679
WVFGRD96    9.0     5    50   -20   4.18 0.4722
WVFGRD96   10.0     5    55   -25   4.19 0.4796
WVFGRD96   11.0     5    60   -30   4.20 0.4861
WVFGRD96   12.0     5    60   -30   4.21 0.4911
WVFGRD96   13.0     5    60   -30   4.21 0.4942
WVFGRD96   14.0     5    60   -30   4.22 0.4952
WVFGRD96   15.0     5    60   -35   4.23 0.4946
WVFGRD96   16.0     5    65   -35   4.23 0.4934
WVFGRD96   17.0     5    65   -35   4.24 0.4907
WVFGRD96   18.0     5    65   -35   4.24 0.4870
WVFGRD96   19.0     5    70   -35   4.25 0.4831
WVFGRD96   20.0     5    70   -35   4.26 0.4795
WVFGRD96   21.0     5    70   -35   4.27 0.4744
WVFGRD96   22.0     5    70   -35   4.27 0.4695
WVFGRD96   23.0     5    70   -35   4.28 0.4643
WVFGRD96   24.0     5    70   -35   4.28 0.4588
WVFGRD96   25.0     5    70   -35   4.29 0.4529
WVFGRD96   26.0     5    70   -35   4.29 0.4468
WVFGRD96   27.0    10    75   -35   4.30 0.4406
WVFGRD96   28.0    10    75   -35   4.30 0.4345
WVFGRD96   29.0    10    75   -35   4.31 0.4283

The best solution is

WVFGRD96   14.0     5    60   -30   4.22 0.4952

The mechanism correspond to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

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 and because the velocity model used in the predictions may not be perfect. 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

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.025 n 3 
lp c 0.05 n 3

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.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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:

The origin time and epicentral distance are incorrect
The velocity model used for the inversion is incorrect
The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Tue May 17 03:29:15 CDT 2022