Location

2013/08/20 18:41:30 47.362 -122.734 11.7 3.6 Washington

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2013/08/20 18:41:30:0  47.36 -122.73  11.7 3.6 Washington
 
 Stations used:
   IU.COR US.HAWA UW.DOSE UW.LEBA UW.PHIN UW.RADR UW.RATT 
   UW.TOLT UW.WOLL 
 
 Filtering commands used:
   cut a -30 a 140
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 9.12e+20 dyne-cm
  Mw = 3.24 
  Z  = 2 km
  Plane   Strike  Dip  Rake
   NP1      169    58   138
   NP2      285    55    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.12e+20     51     135
    N   0.00e+00     39     319
    P  -9.12e+20      2     228

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.28e+20
       Mxy    -6.33e+20
       Mxz    -2.97e+20
       Myy    -3.23e+20
       Myz     3.35e+20
       Mzz     5.51e+20
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #####-----------------              
              #######---------------------           
             ########----------------------          
           #########-------------------------        
          ##########--------------------------       
         ####-------###########----------------      
        #----------#################------------     
        -----------####################---------     
       -------------######################-------    
       -------------########################-----    
       -------------##########################---    
       -------------###########################--    
        -------------############   ############     
        -------------############ T ############     
         -------------###########   ###########      
          -------------#######################       
           -   ---------#####################        
             P ----------##################          
               -----------################           
                 -----------###########              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.51e+20  -2.97e+20  -3.35e+20 
 -2.97e+20  -2.28e+20   6.33e+20 
 -3.35e+20   6.33e+20  -3.23e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130820184130/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 = 285
      DIP = 55
     RAKE = 40
       MW = 3.24
       HS = 2.0

The NDK file is 20130820184130.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2013/08/20 18:41:30:0  47.36 -122.73  11.7 3.6 Washington
 
 Stations used:
   IU.COR US.HAWA UW.DOSE UW.LEBA UW.PHIN UW.RADR UW.RATT 
   UW.TOLT UW.WOLL 
 
 Filtering commands used:
   cut a -30 a 140
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 9.12e+20 dyne-cm
  Mw = 3.24 
  Z  = 2 km
  Plane   Strike  Dip  Rake
   NP1      169    58   138
   NP2      285    55    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.12e+20     51     135
    N   0.00e+00     39     319
    P  -9.12e+20      2     228

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.28e+20
       Mxy    -6.33e+20
       Mxz    -2.97e+20
       Myy    -3.23e+20
       Myz     3.35e+20
       Mzz     5.51e+20
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #####-----------------              
              #######---------------------           
             ########----------------------          
           #########-------------------------        
          ##########--------------------------       
         ####-------###########----------------      
        #----------#################------------     
        -----------####################---------     
       -------------######################-------    
       -------------########################-----    
       -------------##########################---    
       -------------###########################--    
        -------------############   ############     
        -------------############ T ############     
         -------------###########   ###########      
          -------------#######################       
           -   ---------#####################        
             P ----------##################          
               -----------################           
                 -----------###########              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.51e+20  -2.97e+20  -3.35e+20 
 -2.97e+20  -2.28e+20   6.33e+20 
 -3.35e+20   6.33e+20  -3.23e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130820184130/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

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 a -30 a 140
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
br c 0.12 0.25 n 4 p 2
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   120    50    65   3.12 0.4897
WVFGRD96    1.0   285    55    35   3.16 0.4735
WVFGRD96    2.0   285    55    40   3.24 0.5535
WVFGRD96    3.0   285    55    35   3.30 0.5513
WVFGRD96    4.0   285    50    35   3.35 0.5241
WVFGRD96    5.0   280    45    25   3.37 0.4837
WVFGRD96    6.0   275    50     5   3.38 0.4562
WVFGRD96    7.0   275    65   -10   3.39 0.4570
WVFGRD96    8.0   130    65    85   3.40 0.4516
WVFGRD96    9.0   270    60   -20   3.42 0.4499
WVFGRD96   10.0   270    65   -25   3.42 0.4518
WVFGRD96   11.0   270    65   -25   3.43 0.4529
WVFGRD96   12.0   270    65   -25   3.43 0.4512
WVFGRD96   13.0   270    65   -25   3.43 0.4492
WVFGRD96   14.0    85    65   -45   3.42 0.4451
WVFGRD96   15.0    85    60   -40   3.43 0.4466
WVFGRD96   16.0    80    60   -50   3.43 0.4482
WVFGRD96   17.0   115    70   -40   3.42 0.4511
WVFGRD96   18.0   115    70   -45   3.41 0.4534
WVFGRD96   19.0   115    70   -45   3.41 0.4546
WVFGRD96   20.0   120    70   -45   3.41 0.4551
WVFGRD96   21.0   120    70   -50   3.41 0.4533
WVFGRD96   22.0   120    70   -50   3.42 0.4535
WVFGRD96   23.0   120    70   -50   3.43 0.4527
WVFGRD96   24.0    25    45   -35   3.44 0.4545
WVFGRD96   25.0    20    45   -40   3.46 0.4560
WVFGRD96   26.0    20    45   -40   3.47 0.4564
WVFGRD96   27.0    20    45   -40   3.48 0.4555
WVFGRD96   28.0    20    45   -40   3.49 0.4532
WVFGRD96   29.0    20    45   -40   3.50 0.4495

The best solution is

WVFGRD96    2.0   285    55    40   3.24 0.5535

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 a -30 a 140
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
br c 0.12 0.25 n 4 p 2
Figure 3. Waveform comparison for selected depth
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 Dec 7 00:22:17 CST 2015