Location

Location ANSS

2017/09/05 21:23:19 42.596 -111.430 5.0 4.3 Idaho

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/05 21:23:19:0  42.60 -111.43   5.0 4.3 Idaho
 
 Stations used:
   IW.DLMT IW.FXWY IW.IMW IW.MOOW IW.REDW TA.H17A TA.K22A 
   TA.O20A US.AHID US.BOZ US.BW06 US.DUG US.ELK US.HLID 
   US.HWUT US.RLMT UU.BGU UU.HVU UU.MPU UU.NLU UU.SPU UU.SWUT 
   UU.TCU UU.TMU WY.YFT WY.YHL 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      341    82   -145
   NP2      245    55   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22     18     108
    N   0.00e+00     54     352
    P  -2.69e+22     30     209

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.30e+22
       Mxy    -1.56e+22
       Mxz     7.87e+21
       Myy     1.74e+22
       Myz     1.31e+22
       Mzz    -4.39e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####-----------------              
              ##########------------------           
             ############------------------          
           ##############--------------------        
          ################----##############--       
         ################--####################      
        #############-------####################     
        ###########----------###################     
       #########-------------####################    
       #######----------------###################    
       ######------------------##################    
       #####-------------------############   ###    
        ###---------------------########### T ##     
        ##----------------------###########   ##     
         ------------------------##############      
          ----------   ----------#############       
           --------- P -----------###########        
             -------   -----------#########          
              --------------------########           
                 ------------------####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.39e+21   7.87e+21  -1.31e+22 
  7.87e+21  -1.30e+22   1.56e+22 
 -1.31e+22   1.56e+22   1.74e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170905212319/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 = 245
      DIP = 55
     RAKE = -10
       MW = 4.22
       HS = 11.0

The NDK file is 20170905212319.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/05 21:23:19:0  42.60 -111.43   5.0 4.3 Idaho
 
 Stations used:
   IW.DLMT IW.FXWY IW.IMW IW.MOOW IW.REDW TA.H17A TA.K22A 
   TA.O20A US.AHID US.BOZ US.BW06 US.DUG US.ELK US.HLID 
   US.HWUT US.RLMT UU.BGU UU.HVU UU.MPU UU.NLU UU.SPU UU.SWUT 
   UU.TCU UU.TMU WY.YFT WY.YHL 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      341    82   -145
   NP2      245    55   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22     18     108
    N   0.00e+00     54     352
    P  -2.69e+22     30     209

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.30e+22
       Mxy    -1.56e+22
       Mxz     7.87e+21
       Myy     1.74e+22
       Myz     1.31e+22
       Mzz    -4.39e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####-----------------              
              ##########------------------           
             ############------------------          
           ##############--------------------        
          ################----##############--       
         ################--####################      
        #############-------####################     
        ###########----------###################     
       #########-------------####################    
       #######----------------###################    
       ######------------------##################    
       #####-------------------############   ###    
        ###---------------------########### T ##     
        ##----------------------###########   ##     
         ------------------------##############      
          ----------   ----------#############       
           --------- P -----------###########        
             -------   -----------#########          
              --------------------########           
                 ------------------####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.39e+21   7.87e+21  -1.31e+22 
  7.87e+21  -1.30e+22   1.56e+22 
 -1.31e+22   1.56e+22   1.74e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170905212319/index.html
	
Regional Moment Tensor (Mwr)
Moment	3.590e+15 N-m
Magnitude	4.3 Mwr
Depth	11.0 km
Percent DC	53 %
Half Duration	–
Catalog	US
Data Source	US1
Contributor	US1
Nodal Planes
Plane	Strike	Dip	Rake
NP1	235	52	-33
NP2	347	64	-137
Principal Axes
Axis	Value	Plunge	Azimuth
T	3.965e+15 N-m	7	109
N	-0.937e+15 N-m	41	12
P	-3.028e+15 N-m	48	207

        

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 -20 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    85    55    40   3.76 0.3383
WVFGRD96    2.0    85    55    40   3.91 0.4341
WVFGRD96    3.0   245    55   -10   3.99 0.4781
WVFGRD96    4.0   240    50   -20   4.04 0.5405
WVFGRD96    5.0   240    50   -20   4.06 0.5952
WVFGRD96    6.0   245    55   -15   4.08 0.6387
WVFGRD96    7.0   245    55   -10   4.10 0.6741
WVFGRD96    8.0   240    50   -15   4.17 0.7029
WVFGRD96    9.0   245    55   -10   4.19 0.7209
WVFGRD96   10.0   245    55   -10   4.20 0.7318
WVFGRD96   11.0   245    55   -10   4.22 0.7346
WVFGRD96   12.0   245    60   -10   4.24 0.7331
WVFGRD96   13.0   245    60   -10   4.25 0.7276
WVFGRD96   14.0   245    60   -10   4.26 0.7183
WVFGRD96   15.0   245    60   -10   4.27 0.7048
WVFGRD96   16.0   245    60   -10   4.28 0.6882
WVFGRD96   17.0   240    55   -20   4.29 0.6705
WVFGRD96   18.0   240    55   -20   4.30 0.6525
WVFGRD96   19.0   240    55   -20   4.30 0.6325
WVFGRD96   20.0   240    55   -20   4.31 0.6118
WVFGRD96   21.0   240    55   -20   4.32 0.5913
WVFGRD96   22.0   240    55   -20   4.33 0.5687
WVFGRD96   23.0   240    50   -15   4.33 0.5480
WVFGRD96   24.0   240    50   -15   4.33 0.5269
WVFGRD96   25.0   235    50   -20   4.34 0.5071
WVFGRD96   26.0   235    45   -20   4.34 0.4876
WVFGRD96   27.0   235    45   -20   4.35 0.4695
WVFGRD96   28.0   235    45   -20   4.35 0.4514
WVFGRD96   29.0   235    45   -20   4.35 0.4336

The best solution is

WVFGRD96   11.0   245    55   -10   4.22 0.7346

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 -20 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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:

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 Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

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 Sep 5 18:23:37 CDT 2017