Location

Location ANSS

The ANSS event ID is ak017cjub2q2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017cjub2q2/executive.

2017/09/30 21:15:06 59.621 -152.242 83.6 4.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/30 21:15:06:0  59.62 -152.24  83.6 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SSN AV.ILSW II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.4 -30 o DIST/3.4 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 80 km
  Plane   Strike  Dip  Rake
   NP1       35    50    95
   NP2      207    40    84
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22     84     340
    N   0.00e+00      4     212
    P  -2.79e+22      5     121

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.24e+21
       Mxy     1.22e+22
       Mxz     4.04e+21
       Myy    -2.01e+22
       Myz    -3.05e+21
       Mzz     2.73e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------#########              
              ------------##############--           
             -----------#################--          
           -----------###################----        
          ----------#####################-----       
         ----------######################------      
        ---------########################-------     
        ---------########################-------     
       ---------##########   ###########---------    
       --------########### T ###########---------    
       --------###########   ##########----------    
       -------########################-----------    
        ------#######################-----------     
        ------######################------------     
         -----####################---------   -      
          -----#################----------- P        
           ----###############-------------          
             ---############---------------          
              ---#######------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.73e+22   4.04e+21   3.05e+21 
  4.04e+21  -7.24e+21  -1.22e+22 
  3.05e+21  -1.22e+22  -2.01e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170930211506/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 35
      DIP = 50
     RAKE = 95
       MW = 4.23
       HS = 80.0

The NDK file is 20170930211506.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/30 21:15:06:0  59.62 -152.24  83.6 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SSN AV.ILSW II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.4 -30 o DIST/3.4 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 80 km
  Plane   Strike  Dip  Rake
   NP1       35    50    95
   NP2      207    40    84
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22     84     340
    N   0.00e+00      4     212
    P  -2.79e+22      5     121

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.24e+21
       Mxy     1.22e+22
       Mxz     4.04e+21
       Myy    -2.01e+22
       Myz    -3.05e+21
       Mzz     2.73e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------#########              
              ------------##############--           
             -----------#################--          
           -----------###################----        
          ----------#####################-----       
         ----------######################------      
        ---------########################-------     
        ---------########################-------     
       ---------##########   ###########---------    
       --------########### T ###########---------    
       --------###########   ##########----------    
       -------########################-----------    
        ------#######################-----------     
        ------######################------------     
         -----####################---------   -      
          -----#################----------- P        
           ----###############-------------          
             ---############---------------          
              ---#######------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.73e+22   4.04e+21   3.05e+21 
  4.04e+21  -7.24e+21  -1.22e+22 
  3.05e+21  -1.22e+22  -2.01e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170930211506/index.html
	
Moment	3.376e+15 N-m
Magnitude	4.3 Mwr
Depth	88.0 km
Percent DC	94 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	210	39	88
NP2	32	51	91
Principal Axes
Axis	Value	Plunge	Azimuth
T	3.429e+15 N-m	84	311
N	-0.110e+15 N-m	1	211
P	-3.319e+15 N-m	6	121

        

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude


Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.


Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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/3.4 -30 o DIST/3.4 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0    20    50   -90   3.49 0.3142
WVFGRD96    4.0    65    30   -15   3.48 0.2448
WVFGRD96    6.0    75    30     0   3.50 0.2985
WVFGRD96    8.0    75    25    -5   3.59 0.3407
WVFGRD96   10.0    65    30   -25   3.63 0.3810
WVFGRD96   12.0    60    30   -40   3.66 0.4113
WVFGRD96   14.0    50    35   -50   3.71 0.4332
WVFGRD96   16.0    45    35   -60   3.74 0.4484
WVFGRD96   18.0    45    40   -55   3.77 0.4573
WVFGRD96   20.0    45    40   -55   3.80 0.4610
WVFGRD96   22.0    45    40   -50   3.82 0.4525
WVFGRD96   24.0    45    40   -50   3.84 0.4300
WVFGRD96   26.0    50    45   -40   3.86 0.3998
WVFGRD96   28.0   170    80   -65   3.84 0.3745
WVFGRD96   30.0   170    80   -65   3.86 0.3628
WVFGRD96   32.0   180    80   -65   3.87 0.3511
WVFGRD96   34.0   180    80   -65   3.88 0.3448
WVFGRD96   36.0   185    80   -65   3.89 0.3411
WVFGRD96   38.0    25    50    85   3.91 0.3413
WVFGRD96   40.0    25    50    90   4.03 0.4307
WVFGRD96   42.0    25    50    85   4.07 0.4551
WVFGRD96   44.0    25    50    85   4.09 0.4760
WVFGRD96   46.0   200    40    80   4.11 0.4994
WVFGRD96   48.0   200    40    80   4.13 0.5239
WVFGRD96   50.0   200    45    80   4.14 0.5484
WVFGRD96   52.0   200    45    80   4.16 0.5741
WVFGRD96   54.0   200    45    80   4.16 0.5991
WVFGRD96   56.0   200    45    80   4.17 0.6210
WVFGRD96   58.0   200    45    80   4.18 0.6432
WVFGRD96   60.0   200    45    80   4.18 0.6622
WVFGRD96   62.0   200    45    80   4.19 0.6781
WVFGRD96   64.0   200    45    80   4.19 0.6939
WVFGRD96   66.0   200    45    80   4.20 0.7047
WVFGRD96   68.0   200    45    80   4.20 0.7145
WVFGRD96   70.0   200    45    80   4.20 0.7234
WVFGRD96   72.0   200    45    80   4.21 0.7322
WVFGRD96   74.0   200    45    80   4.21 0.7378
WVFGRD96   76.0   205    40    85   4.22 0.7422
WVFGRD96   78.0   205    40    85   4.23 0.7444
WVFGRD96   80.0    35    50    95   4.23 0.7485
WVFGRD96   82.0    35    50    95   4.23 0.7481
WVFGRD96   84.0    30    50    95   4.23 0.7465
WVFGRD96   86.0    30    45    95   4.23 0.7465
WVFGRD96   88.0    30    50    95   4.24 0.7463
WVFGRD96   90.0    30    50    95   4.24 0.7405
WVFGRD96   92.0    30    45    95   4.24 0.7406
WVFGRD96   94.0   190    45    75   4.24 0.7361
WVFGRD96   96.0   190    45    75   4.24 0.7314
WVFGRD96   98.0    30    45    95   4.25 0.7277

The best solution is

WVFGRD96   80.0    35    50    95   4.23 0.7485

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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/3.4 -30 o DIST/3.4 +60
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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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    
Last Changed Sat Apr 27 07:43:32 PM CDT 2024