Location

Location ANSS

The ANSS event ID is ak0169byh1zh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0169byh1zh/executive.

2016/07/21 14:07:16 61.625 -150.321 60.2 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/07/21 14:07:16:0  61.62 -150.32  60.2 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CUT AK.DIV AK.FID AK.FIRE 
   AK.GHO AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR 
   AK.MCK AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.TRF 
   AT.PMR AV.ILSW TA.K20K TA.M19K TA.M20K TA.M22K TA.N19K 
   TA.O19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.11e+22 dyne-cm
  Mw = 4.15 
  Z  = 64 km
  Plane   Strike  Dip  Rake
   NP1       60    85    45
   NP2      325    45   173
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.11e+22     34     293
    N   0.00e+00     45      65
    P  -2.11e+22     26     184

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.48e+22
       Mxy    -6.32e+21
       Mxz     1.21e+22
       Myy     1.22e+22
       Myz    -8.49e+21
       Mzz     2.60e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ############----------------           
             ################--------------          
           #####################-------------        
          ########################------------       
         ###########################-------####      
        ######   ####################----#######     
        ###### T ###############################     
       #######   ##################----##########    
       #########################--------#########    
       ######################------------########    
       ###################---------------########    
        ##############--------------------######     
        ###########-----------------------######     
         ######---------------------------#####      
          #-------------------------------####       
           -------------------------------###        
             -------------   ------------##          
              ------------ P ------------#           
                 ---------   ----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.60e+21   1.21e+22   8.49e+21 
  1.21e+22  -1.48e+22   6.32e+21 
  8.49e+21   6.32e+21   1.22e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160721140716/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 = 60
      DIP = 85
     RAKE = 45
       MW = 4.15
       HS = 64.0

The NDK file is 20160721140716.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
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2016/07/21 14:07:16:0  61.62 -150.32  60.2 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CUT AK.DIV AK.FID AK.FIRE 
   AK.GHO AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR 
   AK.MCK AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.TRF 
   AT.PMR AV.ILSW TA.K20K TA.M19K TA.M20K TA.M22K TA.N19K 
   TA.O19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.11e+22 dyne-cm
  Mw = 4.15 
  Z  = 64 km
  Plane   Strike  Dip  Rake
   NP1       60    85    45
   NP2      325    45   173
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.11e+22     34     293
    N   0.00e+00     45      65
    P  -2.11e+22     26     184

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.48e+22
       Mxy    -6.32e+21
       Mxz     1.21e+22
       Myy     1.22e+22
       Myz    -8.49e+21
       Mzz     2.60e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ############----------------           
             ################--------------          
           #####################-------------        
          ########################------------       
         ###########################-------####      
        ######   ####################----#######     
        ###### T ###############################     
       #######   ##################----##########    
       #########################--------#########    
       ######################------------########    
       ###################---------------########    
        ##############--------------------######     
        ###########-----------------------######     
         ######---------------------------#####      
          #-------------------------------####       
           -------------------------------###        
             -------------   ------------##          
              ------------ P ------------#           
                 ---------   ----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.60e+21   1.21e+22   8.49e+21 
  1.21e+22  -1.48e+22   6.32e+21 
  8.49e+21   6.32e+21   1.22e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160721140716/index.html
	
Regional Moment Tensor (Mwr)
Moment	1.691e+15 N-m
Magnitude	4.1 Mwr
Depth	56.0 km
Percent DC	56 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	334	37	-168
NP2	235	83	-54
Principal Axes
Axis	Value	Plunge	Azimuth
T	1.858e+15 N-m	29	296
N	-0.411e+15 N-m	36	50
P	-1.447e+15 N-m	41	178

        

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/3.5 -40 o DIST/3.5 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0    55    30   -55   3.45 0.1900
WVFGRD96    4.0    55    65   -30   3.47 0.2182
WVFGRD96    6.0    60    70   -20   3.52 0.2407
WVFGRD96    8.0    60    65   -25   3.59 0.2655
WVFGRD96   10.0    65    70   -20   3.62 0.2752
WVFGRD96   12.0    70    70    20   3.65 0.2875
WVFGRD96   14.0    70    75    20   3.67 0.2974
WVFGRD96   16.0    70    75    20   3.70 0.3058
WVFGRD96   18.0    70    75    20   3.72 0.3125
WVFGRD96   20.0    75    75    20   3.74 0.3201
WVFGRD96   22.0    75    75    25   3.76 0.3300
WVFGRD96   24.0    75    75    25   3.78 0.3402
WVFGRD96   26.0    80    75    25   3.80 0.3511
WVFGRD96   28.0    75    80    25   3.82 0.3610
WVFGRD96   30.0    75    80    25   3.84 0.3697
WVFGRD96   32.0    75    80    25   3.85 0.3771
WVFGRD96   34.0    75    80    25   3.87 0.3827
WVFGRD96   36.0    70    80    25   3.89 0.3896
WVFGRD96   38.0    70    80    20   3.92 0.4037
WVFGRD96   40.0    70    80    30   3.99 0.4100
WVFGRD96   42.0    70    80    35   4.01 0.4164
WVFGRD96   44.0    65    85    35   4.03 0.4248
WVFGRD96   46.0    65    85    35   4.05 0.4345
WVFGRD96   48.0    65    85    35   4.07 0.4426
WVFGRD96   50.0    65    85    35   4.08 0.4490
WVFGRD96   52.0   240    90   -40   4.10 0.4532
WVFGRD96   54.0    65    85    40   4.10 0.4598
WVFGRD96   56.0    65    85    40   4.11 0.4632
WVFGRD96   58.0    60    85    40   4.13 0.4666
WVFGRD96   60.0   235    90   -45   4.15 0.4657
WVFGRD96   62.0    60    85    45   4.15 0.4712
WVFGRD96   64.0    60    85    45   4.15 0.4719
WVFGRD96   66.0    60    85    45   4.16 0.4714
WVFGRD96   68.0    60    85    45   4.17 0.4700
WVFGRD96   70.0    55    85    50   4.18 0.4675
WVFGRD96   72.0    60    80    45   4.18 0.4706
WVFGRD96   74.0    60    80    45   4.19 0.4708
WVFGRD96   76.0    60    80    45   4.19 0.4684
WVFGRD96   78.0    80    20   -30   4.20 0.4661
WVFGRD96   80.0    85    20   -25   4.20 0.4660
WVFGRD96   82.0    80    15   -30   4.21 0.4657
WVFGRD96   84.0    85    15   -25   4.22 0.4649
WVFGRD96   86.0    85    15   -25   4.22 0.4644
WVFGRD96   88.0    90    15   -20   4.22 0.4632
WVFGRD96   90.0    95    15   -15   4.23 0.4621
WVFGRD96   92.0    95    15   -15   4.23 0.4609
WVFGRD96   94.0   100    15   -10   4.24 0.4589
WVFGRD96   96.0   105    15    -5   4.25 0.4592
WVFGRD96   98.0   105    10   -10   4.26 0.4636

The best solution is

WVFGRD96   64.0    60    85    45   4.15 0.4719

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/3.5 -40 o DIST/3.5 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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 Fri Apr 26 06:56:21 PM CDT 2024