Location

Location ANSS

2017/09/04 22:15:47 42.567 -111.426 5.8 3.5 Idaho

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/04 22:15:47:0  42.57 -111.43   5.8 3.5 Idaho
 
 Stations used:
   IM.PD31 IW.DLMT IW.FXWY IW.IMW IW.REDW US.AHID US.BOZ 
   US.DUG US.ELK US.HWUT US.RLMT UU.BGU UU.CTU UU.HVU UU.MPU 
   UU.SPU UU.SWUT UU.TCU WY.YFT WY.YHL WY.YNR WY.YPP 
 
 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 = 4.03e+21 dyne-cm
  Mw = 3.67 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      345    67   -136
   NP2      235    50   -30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.03e+21     11     106
    N   0.00e+00     42       7
    P  -4.03e+21     46     208

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.18e+21
       Mxy    -1.85e+21
       Mxz     1.57e+21
       Myy     3.16e+21
       Myz     1.64e+21
       Mzz    -1.98e+21
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 #########-------------              
              #############---------------           
             ###############---------------          
           ##################-###############        
          ###############-----################       
         #############---------################      
        ############------------################     
        ##########---------------###############     
       #########-----------------################    
       ########-------------------###############    
       #######--------------------###############    
       ######---------------------###########   #    
        ####-----------------------########## T      
        ###------------------------##########        
         ##----------   -----------############      
          #---------- P -----------###########       
           ----------   -----------##########        
             ----------------------########          
              ---------------------#######           
                 -----------------#####              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.98e+21   1.57e+21  -1.64e+21 
  1.57e+21  -1.18e+21   1.85e+21 
 -1.64e+21   1.85e+21   3.16e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170904221547/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 = 235
      DIP = 50
     RAKE = -30
       MW = 3.67
       HS = 9.0

The NDK file is 20170904221547.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/04 22:15:47:0  42.57 -111.43   5.8 3.5 Idaho
 
 Stations used:
   IM.PD31 IW.DLMT IW.FXWY IW.IMW IW.REDW US.AHID US.BOZ 
   US.DUG US.ELK US.HWUT US.RLMT UU.BGU UU.CTU UU.HVU UU.MPU 
   UU.SPU UU.SWUT UU.TCU WY.YFT WY.YHL WY.YNR WY.YPP 
 
 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 = 4.03e+21 dyne-cm
  Mw = 3.67 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      345    67   -136
   NP2      235    50   -30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.03e+21     11     106
    N   0.00e+00     42       7
    P  -4.03e+21     46     208

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.18e+21
       Mxy    -1.85e+21
       Mxz     1.57e+21
       Myy     3.16e+21
       Myz     1.64e+21
       Mzz    -1.98e+21
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 #########-------------              
              #############---------------           
             ###############---------------          
           ##################-###############        
          ###############-----################       
         #############---------################      
        ############------------################     
        ##########---------------###############     
       #########-----------------################    
       ########-------------------###############    
       #######--------------------###############    
       ######---------------------###########   #    
        ####-----------------------########## T      
        ###------------------------##########        
         ##----------   -----------############      
          #---------- P -----------###########       
           ----------   -----------##########        
             ----------------------########          
              ---------------------#######           
                 -----------------#####              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.98e+21   1.57e+21  -1.64e+21 
  1.57e+21  -1.18e+21   1.85e+21 
 -1.64e+21   1.85e+21   3.16e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170904221547/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 -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    45    40   3.29 0.3764
WVFGRD96    2.0    80    45    35   3.43 0.4597
WVFGRD96    3.0   240    45   -15   3.51 0.4812
WVFGRD96    4.0   235    45   -25   3.54 0.5531
WVFGRD96    5.0   235    50   -25   3.56 0.6017
WVFGRD96    6.0   235    50   -25   3.57 0.6347
WVFGRD96    7.0   240    55   -20   3.59 0.6548
WVFGRD96    8.0   235    50   -25   3.66 0.6732
WVFGRD96    9.0   235    50   -30   3.67 0.6789
WVFGRD96   10.0   235    55   -30   3.69 0.6788
WVFGRD96   11.0   235    55   -25   3.70 0.6747
WVFGRD96   12.0   235    55   -25   3.71 0.6655
WVFGRD96   13.0   235    55   -25   3.72 0.6529
WVFGRD96   14.0   235    55   -25   3.73 0.6383
WVFGRD96   15.0   240    60   -20   3.74 0.6219
WVFGRD96   16.0   240    60   -20   3.75 0.6034
WVFGRD96   17.0   240    65   -20   3.77 0.5853
WVFGRD96   18.0   240    65   -20   3.78 0.5660
WVFGRD96   19.0   240    65   -20   3.78 0.5457
WVFGRD96   20.0   240    60   -15   3.78 0.5247
WVFGRD96   21.0   235    55   -20   3.79 0.5062
WVFGRD96   22.0   235    55   -15   3.79 0.4869
WVFGRD96   23.0   235    55   -15   3.79 0.4678
WVFGRD96   24.0   160    80    50   3.81 0.4498
WVFGRD96   25.0   160    80    50   3.82 0.4379
WVFGRD96   26.0   155    85    50   3.81 0.4263
WVFGRD96   27.0   155    85    50   3.82 0.4138
WVFGRD96   28.0   155    85    50   3.83 0.4005
WVFGRD96   29.0   155    85    50   3.83 0.3860

The best solution is

WVFGRD96    9.0   235    50   -30   3.67 0.6789

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 Mon Sep 4 18:57:58 CDT 2017