Location

Location ANSS

2019/07/03 22:29:26 60.330 -151.310 59.4 4.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/07/03 22:29:26:0  60.33 -151.31  59.4 4.7 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.DIV AK.EYAK AK.FIRE AK.GHO AK.HIN 
   AK.HOM AK.KLU AK.KNK AK.PPLA AK.PWL AK.RC01 AK.SKN AK.SLK 
   AK.SWD AT.PMR AV.ILSW AV.RED AV.SPU AV.STLK II.KDAK TA.K20K 
   TA.L19K TA.M20K TA.M24K TA.N19K TA.O18K TA.O19K TA.O22K 
   TA.P18K TA.P19K TA.P23K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 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 = 1.27e+23 dyne-cm
  Mw = 4.67 
  Z  = 76 km
  Plane   Strike  Dip  Rake
   NP1      324    76   154
   NP2       60    65    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.27e+23     28     280
    N   0.00e+00     61     118
    P  -1.27e+23      8      14

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.15e+23
       Mxy    -4.48e+22
       Mxz    -7.65e+21
       Myy     9.02e+22
       Myz    -5.56e+22
       Mzz     2.52e+22
                                                     
                                                     
                                                     
                                                     
                     ---------- P -                  
                 --------------   -----              
              ###-------------------------           
             #######-----------------------          
           ###########-----------------------        
          ##############----------------------       
         #################-------------------##      
        ###################-----------------####     
        ###   ###############--------------#####     
       #### T ################-----------########    
       ####   ##################--------#########    
       ##########################-----###########    
       ###########################-##############    
        #########################--#############     
        #####################-------############     
         ################------------##########      
          #########-------------------########       
           ----------------------------######        
             ---------------------------###          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.52e+22  -7.65e+21   5.56e+22 
 -7.65e+21  -1.15e+23   4.48e+22 
  5.56e+22   4.48e+22   9.02e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190703222926/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 = 60
      DIP = 65
     RAKE = 15
       MW = 4.67
       HS = 76.0

The NDK file is 20190703222926.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  2019/07/03 22:29:26:0  60.33 -151.31  59.4 4.7 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.DIV AK.EYAK AK.FIRE AK.GHO AK.HIN 
   AK.HOM AK.KLU AK.KNK AK.PPLA AK.PWL AK.RC01 AK.SKN AK.SLK 
   AK.SWD AT.PMR AV.ILSW AV.RED AV.SPU AV.STLK II.KDAK TA.K20K 
   TA.L19K TA.M20K TA.M24K TA.N19K TA.O18K TA.O19K TA.O22K 
   TA.P18K TA.P19K TA.P23K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 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 = 1.27e+23 dyne-cm
  Mw = 4.67 
  Z  = 76 km
  Plane   Strike  Dip  Rake
   NP1      324    76   154
   NP2       60    65    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.27e+23     28     280
    N   0.00e+00     61     118
    P  -1.27e+23      8      14

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.15e+23
       Mxy    -4.48e+22
       Mxz    -7.65e+21
       Myy     9.02e+22
       Myz    -5.56e+22
       Mzz     2.52e+22
                                                     
                                                     
                                                     
                                                     
                     ---------- P -                  
                 --------------   -----              
              ###-------------------------           
             #######-----------------------          
           ###########-----------------------        
          ##############----------------------       
         #################-------------------##      
        ###################-----------------####     
        ###   ###############--------------#####     
       #### T ################-----------########    
       ####   ##################--------#########    
       ##########################-----###########    
       ###########################-##############    
        #########################--#############     
        #####################-------############     
         ################------------##########      
          #########-------------------########       
           ----------------------------######        
             ---------------------------###          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.52e+22  -7.65e+21   5.56e+22 
 -7.65e+21  -1.15e+23   4.48e+22 
  5.56e+22   4.48e+22   9.02e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190703222926/index.html
	
Regional Moment Tensor (Mwr)
Moment 1.137e+16 N-m
Magnitude 4.64 Mwr
Depth 67.0 km
Percent DC 82%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3

Nodal Planes
Plane Strike Dip Rake
NP1 330 79 159
NP2 64 69 12

Principal Axes
Axis Value Plunge Azimuth
T 1.186e+16 N-m 23 286
N -0.106e+16 N-m 66 125
P -1.081e+16 N-m 7 19

        

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 0.03 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    2.0   315    55   -50   3.85 0.2396
WVFGRD96    4.0   140    90    15   3.89 0.2613
WVFGRD96    6.0   325    70    20   3.97 0.2829
WVFGRD96    8.0   340    55    15   4.04 0.3061
WVFGRD96   10.0   340    60    15   4.08 0.3184
WVFGRD96   12.0   340    65    15   4.11 0.3216
WVFGRD96   14.0   335    75    15   4.14 0.3162
WVFGRD96   16.0   245    75    20   4.18 0.3213
WVFGRD96   18.0   245    75    10   4.20 0.3321
WVFGRD96   20.0   245    75     5   4.23 0.3453
WVFGRD96   22.0   245    75     0   4.25 0.3614
WVFGRD96   24.0   245    75     0   4.28 0.3766
WVFGRD96   26.0   245    80     0   4.29 0.3878
WVFGRD96   28.0   245    80     5   4.31 0.3956
WVFGRD96   30.0   245    75    -5   4.32 0.4010
WVFGRD96   32.0    65    85     0   4.33 0.4076
WVFGRD96   34.0    65    75     0   4.35 0.4210
WVFGRD96   36.0    65    75    -5   4.37 0.4369
WVFGRD96   38.0    55    75     5   4.42 0.4589
WVFGRD96   40.0    55    70    15   4.49 0.4907
WVFGRD96   42.0    55    70    15   4.52 0.5000
WVFGRD96   44.0    55    70    15   4.54 0.5063
WVFGRD96   46.0    55    70    15   4.56 0.5138
WVFGRD96   48.0    55    70    10   4.57 0.5221
WVFGRD96   50.0    55    70    15   4.58 0.5316
WVFGRD96   52.0    55    70    15   4.60 0.5422
WVFGRD96   54.0    60    65    15   4.60 0.5552
WVFGRD96   56.0    60    65    15   4.61 0.5671
WVFGRD96   58.0    60    65    15   4.62 0.5775
WVFGRD96   60.0    60    65    15   4.63 0.5876
WVFGRD96   62.0    60    65    15   4.63 0.5945
WVFGRD96   64.0    60    65    15   4.64 0.6023
WVFGRD96   66.0    60    65    15   4.65 0.6070
WVFGRD96   68.0    60    65    15   4.65 0.6121
WVFGRD96   70.0    60    65    15   4.66 0.6156
WVFGRD96   72.0    60    65    15   4.66 0.6185
WVFGRD96   74.0    60    65    15   4.67 0.6206
WVFGRD96   76.0    60    65    15   4.67 0.6226
WVFGRD96   78.0    60    65    15   4.68 0.6221
WVFGRD96   80.0    60    65    15   4.68 0.6218
WVFGRD96   82.0    60    65    15   4.69 0.6208
WVFGRD96   84.0    60    65    15   4.69 0.6189
WVFGRD96   86.0    60    65    15   4.69 0.6179
WVFGRD96   88.0    60    65    15   4.70 0.6150
WVFGRD96   90.0    60    65    15   4.70 0.6109
WVFGRD96   92.0    60    65    15   4.70 0.6087
WVFGRD96   94.0    60    65    15   4.71 0.6052
WVFGRD96   96.0    60    70    20   4.70 0.6012
WVFGRD96   98.0    60    70    20   4.71 0.5986

The best solution is

WVFGRD96   76.0    60    65    15   4.67 0.6226

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 0.03 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 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 Wed Jul 3 18:43:55 CDT 2019