Location

Location ANSS

The ANSS event ID is ak0238gji26s and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0238gji26s/executive.

2023/07/03 14:47:29 61.289 -149.590 35.6 4.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2023/07/03 14:47:29:0  61.29 -149.59  35.6 4.5 Alaska
 
 Stations used:
   AK.BARN AK.BMR AK.BPAW AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY 
   AK.DIV AK.EYAK AK.FID AK.FIRE AK.GHO AK.GLB AK.GLI AK.HDA 
   AK.HIN AK.HOM AK.I23K AK.J19K AK.J20K AK.J25K AK.K20K 
   AK.K24K AK.KLU AK.KNK AK.L19K AK.L20K AK.L22K AK.M19K 
   AK.M27K AK.MCAR AK.MCK AK.MLY AK.N18K AK.N19K AK.O18K 
   AK.P23K AK.PAX AK.POKR AK.PPLA AK.PWL AK.RND AK.SAW AK.SCM 
   AK.SCRK AK.SKN AK.SLK AK.SWD AK.VRDI AK.WAT6 AK.WRH AT.PMR 
   AV.SPCP AV.STLK IU.COLA 
 
 Filtering commands used:
   cut o DIST/3.4 -40 o DIST/3.4 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 5.96e+22 dyne-cm
  Mw = 4.45 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      190    60   -90
   NP2       10    30   -90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.96e+22     15     280
    N   0.00e+00     -0     190
    P  -5.96e+22     75     100

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.56e+21
       Mxy    -8.82e+21
       Mxz     5.17e+21
       Myy     5.00e+22
       Myz    -2.93e+22
       Mzz    -5.16e+22
                                                     
                                                     
                                                     
                                                     
                     #########---##                  
                 ###########--------###              
              ############------------####           
             ############--------------####          
           #############----------------#####        
          #############------------------#####       
         #############--------------------#####      
        ##############--------------------######     
        #   #########----------------------#####     
       ## T #########----------------------######    
       ##   ########-----------   ---------######    
       #############----------- P ---------######    
       #############-----------   --------#######    
        ############----------------------######     
        ############---------------------#######     
         ###########---------------------######      
          ##########--------------------######       
           ##########-----------------#######        
             ########----------------######          
              ########-------------#######           
                 ######----------######              
                     ###-----######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.16e+22   5.17e+21   2.93e+22 
  5.17e+21   1.56e+21   8.82e+21 
  2.93e+22   8.82e+21   5.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230703144729/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 = 10
      DIP = 30
     RAKE = -90
       MW = 4.45
       HS = 45.0

The NDK file is 20230703144729.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  2023/07/03 14:47:29:0  61.29 -149.59  35.6 4.5 Alaska
 
 Stations used:
   AK.BARN AK.BMR AK.BPAW AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY 
   AK.DIV AK.EYAK AK.FID AK.FIRE AK.GHO AK.GLB AK.GLI AK.HDA 
   AK.HIN AK.HOM AK.I23K AK.J19K AK.J20K AK.J25K AK.K20K 
   AK.K24K AK.KLU AK.KNK AK.L19K AK.L20K AK.L22K AK.M19K 
   AK.M27K AK.MCAR AK.MCK AK.MLY AK.N18K AK.N19K AK.O18K 
   AK.P23K AK.PAX AK.POKR AK.PPLA AK.PWL AK.RND AK.SAW AK.SCM 
   AK.SCRK AK.SKN AK.SLK AK.SWD AK.VRDI AK.WAT6 AK.WRH AT.PMR 
   AV.SPCP AV.STLK IU.COLA 
 
 Filtering commands used:
   cut o DIST/3.4 -40 o DIST/3.4 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 5.96e+22 dyne-cm
  Mw = 4.45 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      190    60   -90
   NP2       10    30   -90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.96e+22     15     280
    N   0.00e+00     -0     190
    P  -5.96e+22     75     100

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.56e+21
       Mxy    -8.82e+21
       Mxz     5.17e+21
       Myy     5.00e+22
       Myz    -2.93e+22
       Mzz    -5.16e+22
                                                     
                                                     
                                                     
                                                     
                     #########---##                  
                 ###########--------###              
              ############------------####           
             ############--------------####          
           #############----------------#####        
          #############------------------#####       
         #############--------------------#####      
        ##############--------------------######     
        #   #########----------------------#####     
       ## T #########----------------------######    
       ##   ########-----------   ---------######    
       #############----------- P ---------######    
       #############-----------   --------#######    
        ############----------------------######     
        ############---------------------#######     
         ###########---------------------######      
          ##########--------------------######       
           ##########-----------------#######        
             ########----------------######          
              ########-------------#######           
                 ######----------######              
                     ###-----######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.16e+22   5.17e+21   2.93e+22 
  5.17e+21   1.56e+21   8.82e+21 
  2.93e+22   8.82e+21   5.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230703144729/index.html
	
Regional Moment Tensor (Mwr)
Moment 6.840e+15 N-m
Magnitude 4.49 Mwr
Depth 44.0 km
Percent DC 95%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3

Nodal Planes
Plane Strike Dip Rake
NP1 12 28 -94
NP2 197 62 -88

Principal Axes
Axis Value Plunge Azimuth
T 6.922e+15 N-m 17 285
N -0.167e+15 N-m 2 16
P -6.755e+15 N-m 73 113

        

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 -40 o DIST/3.4 +50
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    1.0   180    50    90   3.63 0.1661
WVFGRD96    2.0     5    40    90   3.76 0.2164
WVFGRD96    3.0   185    55    85   3.83 0.2105
WVFGRD96    4.0   180    55    80   3.84 0.1976
WVFGRD96    5.0    95    30    -5   3.82 0.2121
WVFGRD96    6.0   100    30     5   3.83 0.2392
WVFGRD96    7.0   100    30     5   3.84 0.2616
WVFGRD96    8.0   105    25    15   3.91 0.2788
WVFGRD96    9.0   105    30    15   3.93 0.3002
WVFGRD96   10.0   105    30    15   3.94 0.3191
WVFGRD96   11.0   110    35    20   3.96 0.3361
WVFGRD96   12.0   110    35    20   3.97 0.3522
WVFGRD96   13.0   110    35    20   3.99 0.3658
WVFGRD96   14.0   110    35    20   4.00 0.3778
WVFGRD96   15.0   110    35    20   4.01 0.3879
WVFGRD96   16.0   110    35    20   4.03 0.3966
WVFGRD96   17.0   110    35    20   4.04 0.4038
WVFGRD96   18.0   110    40    15   4.06 0.4099
WVFGRD96   19.0   105    40    10   4.07 0.4162
WVFGRD96   20.0   105    40    10   4.09 0.4212
WVFGRD96   21.0   105    40    10   4.10 0.4247
WVFGRD96   22.0   105    40    10   4.12 0.4282
WVFGRD96   23.0   105    35     5   4.13 0.4307
WVFGRD96   24.0    65    35   -20   4.12 0.4365
WVFGRD96   25.0    65    35   -20   4.14 0.4474
WVFGRD96   26.0    65    35   -25   4.15 0.4582
WVFGRD96   27.0    65    35   -25   4.17 0.4689
WVFGRD96   28.0    60    35   -30   4.18 0.4793
WVFGRD96   29.0    55    30   -40   4.19 0.4919
WVFGRD96   30.0    50    25   -50   4.20 0.5049
WVFGRD96   31.0    45    25   -55   4.21 0.5192
WVFGRD96   32.0    40    25   -60   4.22 0.5330
WVFGRD96   33.0    35    25   -65   4.23 0.5475
WVFGRD96   34.0    30    25   -75   4.24 0.5609
WVFGRD96   35.0    10    25  -100   4.26 0.5753
WVFGRD96   36.0    10    25  -100   4.27 0.5892
WVFGRD96   37.0    10    25  -100   4.28 0.6003
WVFGRD96   38.0     5    25  -100   4.28 0.6087
WVFGRD96   39.0     5    30  -100   4.30 0.6179
WVFGRD96   40.0   195    65   -85   4.41 0.6205
WVFGRD96   41.0   195    65   -85   4.42 0.6255
WVFGRD96   42.0    10    25   -95   4.43 0.6284
WVFGRD96   43.0   190    60   -90   4.44 0.6289
WVFGRD96   44.0    10    30   -90   4.44 0.6294
WVFGRD96   45.0    10    30   -90   4.45 0.6305
WVFGRD96   46.0    10    30   -90   4.46 0.6279
WVFGRD96   47.0    10    30   -90   4.46 0.6254
WVFGRD96   48.0   190    60   -90   4.47 0.6209
WVFGRD96   49.0    10    30   -90   4.47 0.6155
WVFGRD96   50.0    10    30   -90   4.48 0.6090
WVFGRD96   51.0   190    60   -90   4.48 0.6008
WVFGRD96   52.0    15    30   -85   4.49 0.5936
WVFGRD96   53.0    15    30   -85   4.49 0.5839
WVFGRD96   54.0   190    60   -90   4.49 0.5743
WVFGRD96   55.0    15    30   -85   4.49 0.5647
WVFGRD96   56.0    20    30   -80   4.50 0.5530
WVFGRD96   57.0    15    30   -85   4.50 0.5431
WVFGRD96   58.0    15    30   -80   4.50 0.5313
WVFGRD96   59.0    15    30   -80   4.50 0.5201

The best solution is

WVFGRD96   45.0    10    30   -90   4.45 0.6305

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 -40 o DIST/3.4 +50
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 Tue Apr 23 01:00:16 AM CDT 2024