Location

Location ANSS

2016/02/28 13:48:51 61.787 -150.688 59.3 4.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/02/28 13:48:51:0  61.79 -150.69  59.3 4.3 Alaska
 
 Stations used:
   AK.CAST AK.KNK AK.KTH AK.RC01 AK.SAW AK.SCM AK.TRF AT.PMR 
   TA.M22K TA.N19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.50 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 2.99e+22 dyne-cm
  Mw = 4.25 
  Z  = 62 km
  Plane   Strike  Dip  Rake
   NP1      348    85   155
   NP2       80    65     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.99e+22     21     301
    N   0.00e+00     65     158
    P  -2.99e+22     14      37

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.12e+22
       Mxy    -2.50e+22
       Mxz    -5.35e+20
       Myy     9.16e+21
       Myz    -1.27e+22
       Mzz     1.99e+21
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ########--------------              
              ###########-------------   -           
             #############------------ P --          
           ###############------------   ----        
          ##   ############-------------------       
         ### T ############--------------------      
        ####   #############--------------------     
        ####################--------------------     
       ######################-------------------#    
       ######################-----------------###    
       ######################--------------######    
       #######################----------#########    
        -#####################------############     
        -------#############--##################     
         ----------------------################      
          ---------------------###############       
           --------------------##############        
             -------------------###########          
              ------------------##########           
                 ---------------#######              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.99e+21  -5.35e+20   1.27e+22 
 -5.35e+20  -1.12e+22   2.50e+22 
  1.27e+22   2.50e+22   9.16e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160228134851/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 = 80
      DIP = 65
     RAKE = 5
       MW = 4.25
       HS = 62.0

The NDK file is 20160228134851.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/02/28 13:48:51:0  61.79 -150.69  59.3 4.3 Alaska
 
 Stations used:
   AK.CAST AK.KNK AK.KTH AK.RC01 AK.SAW AK.SCM AK.TRF AT.PMR 
   TA.M22K TA.N19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.50 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 2.99e+22 dyne-cm
  Mw = 4.25 
  Z  = 62 km
  Plane   Strike  Dip  Rake
   NP1      348    85   155
   NP2       80    65     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.99e+22     21     301
    N   0.00e+00     65     158
    P  -2.99e+22     14      37

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.12e+22
       Mxy    -2.50e+22
       Mxz    -5.35e+20
       Myy     9.16e+21
       Myz    -1.27e+22
       Mzz     1.99e+21
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ########--------------              
              ###########-------------   -           
             #############------------ P --          
           ###############------------   ----        
          ##   ############-------------------       
         ### T ############--------------------      
        ####   #############--------------------     
        ####################--------------------     
       ######################-------------------#    
       ######################-----------------###    
       ######################--------------######    
       #######################----------#########    
        -#####################------############     
        -------#############--##################     
         ----------------------################      
          ---------------------###############       
           --------------------##############        
             -------------------###########          
              ------------------##########           
                 ---------------#######              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.99e+21  -5.35e+20   1.27e+22 
 -5.35e+20  -1.12e+22   2.50e+22 
  1.27e+22   2.50e+22   9.16e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160228134851/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 o DIST/3.3 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.50 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    2.0    75    60     0   3.60 0.5895
WVFGRD96    4.0    70    65   -15   3.63 0.5910
WVFGRD96    6.0   265    55    35   3.71 0.5940
WVFGRD96    8.0   265    55    35   3.76 0.6047
WVFGRD96   10.0   265    60    35   3.77 0.6112
WVFGRD96   12.0   265    65    40   3.81 0.6161
WVFGRD96   14.0   265    65    35   3.82 0.6185
WVFGRD96   16.0   265    65    40   3.86 0.6196
WVFGRD96   18.0   265    65    40   3.88 0.6211
WVFGRD96   20.0   265    65    40   3.91 0.6202
WVFGRD96   22.0   265    65    40   3.94 0.6183
WVFGRD96   24.0   265    65    40   3.96 0.6139
WVFGRD96   26.0   265    60    35   3.97 0.6100
WVFGRD96   28.0   265    60    35   4.00 0.6046
WVFGRD96   30.0   265    60    35   4.02 0.5970
WVFGRD96   32.0    75    75   -10   3.98 0.6090
WVFGRD96   34.0    75    75   -10   4.00 0.6309
WVFGRD96   36.0    75    75    -5   4.03 0.6549
WVFGRD96   38.0    75    80    -5   4.07 0.6796
WVFGRD96   40.0    75    70   -10   4.12 0.7059
WVFGRD96   42.0    75    70   -10   4.14 0.7182
WVFGRD96   44.0    75    70    -5   4.15 0.7275
WVFGRD96   46.0    75    70    -5   4.17 0.7351
WVFGRD96   48.0    75    70    -5   4.19 0.7412
WVFGRD96   50.0    75    70     0   4.20 0.7457
WVFGRD96   52.0    80    65     0   4.21 0.7517
WVFGRD96   54.0    80    65     0   4.22 0.7573
WVFGRD96   56.0    80    65     0   4.23 0.7615
WVFGRD96   58.0    80    65     0   4.24 0.7648
WVFGRD96   60.0    80    65     5   4.25 0.7660
WVFGRD96   62.0    80    65     5   4.25 0.7683
WVFGRD96   64.0    80    65     5   4.26 0.7678
WVFGRD96   66.0    80    65     5   4.27 0.7674
WVFGRD96   68.0    80    65    10   4.27 0.7670
WVFGRD96   70.0    80    65    10   4.28 0.7657
WVFGRD96   72.0    80    65    15   4.28 0.7639
WVFGRD96   74.0    80    65    15   4.29 0.7631
WVFGRD96   76.0    80    65    15   4.29 0.7614
WVFGRD96   78.0    85    60    15   4.30 0.7602
WVFGRD96   80.0    85    60    15   4.30 0.7593
WVFGRD96   82.0    85    60    20   4.31 0.7584
WVFGRD96   84.0    85    60    20   4.31 0.7564
WVFGRD96   86.0    85    60    25   4.32 0.7560
WVFGRD96   88.0    85    60    25   4.32 0.7550
WVFGRD96   90.0    85    60    25   4.33 0.7546
WVFGRD96   92.0    85    60    30   4.34 0.7536
WVFGRD96   94.0    85    60    30   4.34 0.7522
WVFGRD96   96.0    90    55    30   4.35 0.7518
WVFGRD96   98.0    90    55    30   4.35 0.7509

The best solution is

WVFGRD96   62.0    80    65     5   4.25 0.7683

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 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.50 n 4 p 2
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 Sun Feb 28 09:44:32 CST 2016