Location

2014/04/27 12:46:55 63.812 -149.174 117.4 4.9 Alaska

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  2014/04/27 12:46:55:0  63.81 -149.17 117.4 4.9 Alaska
 
 Stations used:
   AK.BPAW AK.CCB AK.HDA AK.KTH AK.MCK AK.NEA AK.PPD AK.RND 
   AK.SAW AK.SCM AK.TRF AK.WRH AT.PMR IM.IL31 IU.COLA 
 
 Filtering commands used:
   cut a -30 a 80
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 126 km
  Plane   Strike  Dip  Rake
   NP1      306    54   110
   NP2       95    40    65
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22     72     268
    N   0.00e+00     16     115
    P  -1.10e+22      7      23

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.19e+21
       Mxy    -3.78e+21
       Mxz    -1.41e+21
       Myy    -5.92e+20
       Myz    -3.69e+21
       Mzz     9.79e+21
                                                     
                                                     
                                                     
                                                     
                     -------------                   
                 ----------------- P --              
              --------------------   -----           
             ------------------------------          
           #############---------------------        
          ###################-----------------       
         #######################---------------      
        ##########################--------------     
        ############################------------     
       ###############################-----------    
       -##############   ###############---------    
       -############## T ################--------    
       ---############   #################------#    
        ---################################----#     
        -----###############################-###     
         ------############################-###      
          ---------#####################-----#       
           -------------##########-----------        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.79e+21  -1.41e+21   3.69e+21 
 -1.41e+21  -9.19e+21   3.78e+21 
  3.69e+21   3.78e+21  -5.92e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140427124655/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 = 95
      DIP = 40
     RAKE = 65
       MW = 3.96
       HS = 126.0

The NDK file is 20140427124655.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2014/04/27 12:46:55:0  63.81 -149.17 117.4 4.9 Alaska
 
 Stations used:
   AK.BPAW AK.CCB AK.HDA AK.KTH AK.MCK AK.NEA AK.PPD AK.RND 
   AK.SAW AK.SCM AK.TRF AK.WRH AT.PMR IM.IL31 IU.COLA 
 
 Filtering commands used:
   cut a -30 a 80
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 126 km
  Plane   Strike  Dip  Rake
   NP1      306    54   110
   NP2       95    40    65
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22     72     268
    N   0.00e+00     16     115
    P  -1.10e+22      7      23

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.19e+21
       Mxy    -3.78e+21
       Mxz    -1.41e+21
       Myy    -5.92e+20
       Myz    -3.69e+21
       Mzz     9.79e+21
                                                     
                                                     
                                                     
                                                     
                     -------------                   
                 ----------------- P --              
              --------------------   -----           
             ------------------------------          
           #############---------------------        
          ###################-----------------       
         #######################---------------      
        ##########################--------------     
        ############################------------     
       ###############################-----------    
       -##############   ###############---------    
       -############## T ################--------    
       ---############   #################------#    
        ---################################----#     
        -----###############################-###     
         ------############################-###      
          ---------#####################-----#       
           -------------##########-----------        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.79e+21  -1.41e+21   3.69e+21 
 -1.41e+21  -9.19e+21   3.78e+21 
  3.69e+21   3.78e+21  -5.92e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140427124655/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 80
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.06 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   140    45   -85   3.25 0.4127
WVFGRD96    4.0   340    40   -55   3.32 0.3227
WVFGRD96    6.0   360    35   -20   3.30 0.3073
WVFGRD96    8.0    25    30   -30   3.35 0.3374
WVFGRD96   10.0    35    35   -15   3.35 0.3654
WVFGRD96   12.0    40    35   -10   3.36 0.3925
WVFGRD96   14.0    40    35   -10   3.38 0.4176
WVFGRD96   16.0    45    40    -5   3.40 0.4388
WVFGRD96   18.0    60    35    25   3.42 0.4577
WVFGRD96   20.0    65    30    30   3.43 0.4701
WVFGRD96   22.0    70    30    35   3.46 0.4816
WVFGRD96   24.0    70    30    35   3.48 0.4860
WVFGRD96   26.0    75    30    45   3.50 0.4902
WVFGRD96   28.0    75    30    45   3.51 0.4842
WVFGRD96   30.0    75    30    45   3.52 0.4779
WVFGRD96   32.0    75    30    45   3.53 0.4638
WVFGRD96   34.0    70    30    40   3.54 0.4501
WVFGRD96   36.0    30    40   -45   3.56 0.4335
WVFGRD96   38.0    35    45   -35   3.56 0.4235
WVFGRD96   40.0    30    40   -40   3.68 0.4166
WVFGRD96   42.0    30    40   -35   3.68 0.4086
WVFGRD96   44.0    30    40   -35   3.69 0.4039
WVFGRD96   46.0    35    45   -30   3.70 0.3978
WVFGRD96   48.0    45    30     0   3.69 0.3989
WVFGRD96   50.0    45    30     0   3.70 0.3984
WVFGRD96   52.0    45    30     0   3.71 0.3982
WVFGRD96   54.0    45    30     0   3.72 0.3988
WVFGRD96   56.0    45    30     0   3.73 0.3985
WVFGRD96   58.0    40    30    -5   3.74 0.3972
WVFGRD96   60.0    40    30    -5   3.75 0.3959
WVFGRD96   62.0    85    35    45   3.77 0.4018
WVFGRD96   64.0    90    35    50   3.78 0.4090
WVFGRD96   66.0    90    35    50   3.79 0.4148
WVFGRD96   68.0    90    35    50   3.79 0.4202
WVFGRD96   70.0    85    35    50   3.80 0.4302
WVFGRD96   72.0    85    35    50   3.81 0.4391
WVFGRD96   74.0   105    35    80   3.83 0.4549
WVFGRD96   76.0   295    55    95   3.84 0.4727
WVFGRD96   78.0   105    35    80   3.84 0.4905
WVFGRD96   80.0   105    35    80   3.85 0.5071
WVFGRD96   82.0   105    35    80   3.86 0.5228
WVFGRD96   84.0   105    35    80   3.87 0.5385
WVFGRD96   86.0   105    35    80   3.87 0.5527
WVFGRD96   88.0   105    35    80   3.88 0.5661
WVFGRD96   90.0   100    35    75   3.88 0.5789
WVFGRD96   92.0   100    35    75   3.89 0.5907
WVFGRD96   94.0   100    35    75   3.89 0.6011
WVFGRD96   96.0   100    35    75   3.90 0.6114
WVFGRD96   98.0   100    40    70   3.91 0.6204
WVFGRD96  100.0   100    40    70   3.92 0.6290
WVFGRD96  102.0   100    40    70   3.92 0.6371
WVFGRD96  104.0   100    40    70   3.92 0.6444
WVFGRD96  106.0   100    40    70   3.93 0.6503
WVFGRD96  108.0   100    40    70   3.93 0.6556
WVFGRD96  110.0   100    40    70   3.94 0.6601
WVFGRD96  112.0   100    40    70   3.94 0.6645
WVFGRD96  114.0    95    40    65   3.94 0.6677
WVFGRD96  116.0    95    40    65   3.95 0.6708
WVFGRD96  118.0    95    40    65   3.95 0.6725
WVFGRD96  120.0    95    40    65   3.95 0.6746
WVFGRD96  122.0    95    40    65   3.96 0.6754
WVFGRD96  124.0    95    40    65   3.96 0.6763
WVFGRD96  126.0    95    40    65   3.96 0.6766
WVFGRD96  128.0    95    40    65   3.97 0.6765
WVFGRD96  130.0    95    40    65   3.97 0.6755
WVFGRD96  132.0    95    40    65   3.97 0.6751
WVFGRD96  134.0    95    40    65   3.98 0.6742
WVFGRD96  136.0    95    40    65   3.98 0.6726
WVFGRD96  138.0    95    40    65   3.98 0.6711

The best solution is

WVFGRD96  126.0    95    40    65   3.96 0.6766

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 80
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.06 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 Dec 7 00:11:31 CST 2015