Location

Location ANSS

2016/03/12 21:57:55 60.252 -152.312 99.0 4.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/03/12 21:57:55:0  60.25 -152.31  99.0 4.9 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.HOM AK.KNK AK.RC01 
   AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M22K 
   TA.N19K TA.O19K TA.O22K TA.P18K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.41e+23 dyne-cm
  Mw = 4.70 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      235    75   -20
   NP2      330    71   -164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.41e+23      3     283
    N   0.00e+00     65      20
    P  -1.41e+23     25     192

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.04e+23
       Mxy    -5.52e+22
       Mxz     5.40e+22
       Myy     1.28e+23
       Myz     4.14e+21
       Mzz    -2.42e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              #######---------------------           
             ###########-------------------          
           ##############--------------------        
          #################------------#######       
         ###################------#############      
          ####################-#################     
        T ##################---#################     
          ###############-------#################    
       ################----------################    
       ##############-------------###############    
       ###########-----------------##############    
        #########-------------------############     
        #######---------------------############     
         ####------------------------##########      
          ##-------------------------#########       
           ---------------------------#######        
             ----------   ------------#####          
              --------- P ------------####           
                 ------   ------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.42e+22   5.40e+22  -4.14e+21 
  5.40e+22  -1.04e+23   5.52e+22 
 -4.14e+21   5.52e+22   1.28e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160312215755/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 = 75
     RAKE = -20
       MW = 4.70
       HS = 90.0

The NDK file is 20160312215755.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  2016/03/12 21:57:55:0  60.25 -152.31  99.0 4.9 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.HOM AK.KNK AK.RC01 
   AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M22K 
   TA.N19K TA.O19K TA.O22K TA.P18K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.41e+23 dyne-cm
  Mw = 4.70 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      235    75   -20
   NP2      330    71   -164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.41e+23      3     283
    N   0.00e+00     65      20
    P  -1.41e+23     25     192

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.04e+23
       Mxy    -5.52e+22
       Mxz     5.40e+22
       Myy     1.28e+23
       Myz     4.14e+21
       Mzz    -2.42e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              #######---------------------           
             ###########-------------------          
           ##############--------------------        
          #################------------#######       
         ###################------#############      
          ####################-#################     
        T ##################---#################     
          ###############-------#################    
       ################----------################    
       ##############-------------###############    
       ###########-----------------##############    
        #########-------------------############     
        #######---------------------############     
         ####------------------------##########      
          ##-------------------------#########       
           ---------------------------#######        
             ----------   ------------#####          
              --------- P ------------####           
                 ------   ------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.42e+22   5.40e+22  -4.14e+21 
  5.40e+22  -1.04e+23   5.52e+22 
 -4.14e+21   5.52e+22   1.28e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160312215755/index.html
	

Regional Moment Tensor (Mwr)
Moment	1.328e+16 N-m
Magnitude	4.68
Depth	92.0 km
Percent DC	64%
Half Duration	–
Catalog	US (us10004x3k)
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	235	74	-9
NP2	327	81	-164
Principal Axes
Axis	Value	Plunge	Azimuth
T	1.182	5	100
N	0.255	72	355
P	-1.437	18	192

        

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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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    55    75    15   3.81 0.2638
WVFGRD96    4.0    50    90   -10   3.88 0.3245
WVFGRD96    6.0   235    85    15   3.96 0.3643
WVFGRD96    8.0   235    85    15   4.02 0.3973
WVFGRD96   10.0    50    85   -15   4.06 0.4146
WVFGRD96   12.0    50    80   -10   4.09 0.4237
WVFGRD96   14.0    50    85   -10   4.12 0.4264
WVFGRD96   16.0    50    85   -10   4.14 0.4265
WVFGRD96   18.0    50    85   -10   4.16 0.4240
WVFGRD96   20.0   230    90    10   4.18 0.4215
WVFGRD96   22.0   230    90    10   4.20 0.4210
WVFGRD96   24.0    50    90   -10   4.21 0.4218
WVFGRD96   26.0    50    90   -10   4.23 0.4244
WVFGRD96   28.0   230    90     5   4.25 0.4294
WVFGRD96   30.0    50    85     0   4.26 0.4354
WVFGRD96   32.0   230    90     0   4.28 0.4408
WVFGRD96   34.0   230    90    -5   4.30 0.4456
WVFGRD96   36.0    50    90     5   4.33 0.4496
WVFGRD96   38.0    50    90     5   4.35 0.4542
WVFGRD96   40.0   230    85   -10   4.40 0.4634
WVFGRD96   42.0   230    85   -10   4.42 0.4689
WVFGRD96   44.0   230    85   -15   4.44 0.4746
WVFGRD96   46.0   230    80   -15   4.47 0.4795
WVFGRD96   48.0   230    80   -20   4.49 0.4860
WVFGRD96   50.0   230    80   -20   4.51 0.4934
WVFGRD96   52.0   230    80   -20   4.52 0.5005
WVFGRD96   54.0   230    75   -25   4.55 0.5095
WVFGRD96   56.0   230    75   -20   4.55 0.5187
WVFGRD96   58.0   230    70   -25   4.59 0.5286
WVFGRD96   60.0   230    70   -25   4.60 0.5375
WVFGRD96   62.0   230    70   -25   4.61 0.5477
WVFGRD96   64.0   230    70   -25   4.62 0.5563
WVFGRD96   66.0   230    70   -25   4.63 0.5637
WVFGRD96   68.0   230    70   -25   4.64 0.5712
WVFGRD96   70.0   230    70   -20   4.64 0.5784
WVFGRD96   72.0   235    75   -25   4.65 0.5842
WVFGRD96   74.0   235    75   -25   4.66 0.5895
WVFGRD96   76.0   235    75   -20   4.66 0.5946
WVFGRD96   78.0   235    75   -20   4.66 0.5987
WVFGRD96   80.0   235    75   -20   4.67 0.6021
WVFGRD96   82.0   235    75   -20   4.68 0.6047
WVFGRD96   84.0   235    75   -20   4.68 0.6069
WVFGRD96   86.0   235    75   -20   4.69 0.6075
WVFGRD96   88.0   235    75   -20   4.69 0.6087
WVFGRD96   90.0   235    75   -20   4.70 0.6087
WVFGRD96   92.0   235    75   -20   4.70 0.6078
WVFGRD96   94.0   235    75   -15   4.70 0.6066
WVFGRD96   96.0   235    75   -15   4.71 0.6054
WVFGRD96   98.0   235    75   -15   4.71 0.6046
WVFGRD96  100.0   235    80   -15   4.70 0.6039
WVFGRD96  102.0   235    80   -15   4.70 0.6024
WVFGRD96  104.0   235    80   -15   4.71 0.6004
WVFGRD96  106.0    60    80    45   4.72 0.5998
WVFGRD96  108.0    60    80    45   4.72 0.6012

The best solution is

WVFGRD96   90.0   235    75   -20   4.70 0.6087

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 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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 Sat Mar 12 18:20:09 CST 2016