Location

Location ANSS

The ANSS event ID is ak0252ksopkj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0252ksopkj/executive.

2025/02/25 11:07:03 62.846 -150.798 101.2 3.6 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2025/02/25 11:07:03:0  62.85 -150.80 101.2 3.6 Alaska
 
 Stations used:
   AK.BAE AK.BPAW AK.CAST AK.GHO AK.KNK AK.L22K AK.MCK AK.RC01 
   AK.RND AK.SLK AT.PMR AV.STLK 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.53e+21 dyne-cm
  Mw = 3.81 
  Z  = 102 km
  Plane   Strike  Dip  Rake
   NP1      333    81   124
   NP2       75    35    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.53e+21     43     276
    N   0.00e+00     34     147
    P  -6.53e+21     28      36

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.29e+21
       Mxy    -2.74e+21
       Mxz    -1.90e+21
       Myy     1.70e+21
       Myz    -4.84e+21
       Mzz     1.59e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ##--------------------              
              #######---------------------           
             #########-------------   -----          
           ############------------ P -------        
          ###############----------   --------       
         #################---------------------      
        ###################---------------------     
        ####################--------------------     
       ########   ###########-------------------#    
       ######## T ############-----------------##    
       ########   #############---------------###    
       #########################-------------####    
        #########################-----------####     
        -#########################---------#####     
         -########################-------######      
          ---######################---########       
           -----###################-#########        
             ----------####---------#######          
              -----------------------#####           
                 --------------------##              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.59e+21  -1.90e+21   4.84e+21 
 -1.90e+21  -3.29e+21   2.74e+21 
  4.84e+21   2.74e+21   1.70e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250225110703/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 = 75
      DIP = 35
     RAKE = 15
       MW = 3.81
       HS = 102.0

The NDK file is 20250225110703.ndk The waveform inversion is preferred.

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/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   165    85   -10   2.97 0.3253
WVFGRD96    4.0   165    85   -25   3.09 0.3873
WVFGRD96    6.0   345    90    45   3.19 0.4297
WVFGRD96    8.0   165    90   -45   3.25 0.4477
WVFGRD96   10.0   350    80    40   3.27 0.4536
WVFGRD96   12.0   160    75   -40   3.30 0.4563
WVFGRD96   14.0   160    75   -40   3.33 0.4527
WVFGRD96   16.0   160    75   -35   3.34 0.4477
WVFGRD96   18.0   160    75   -35   3.36 0.4402
WVFGRD96   20.0   160    75   -40   3.39 0.4334
WVFGRD96   22.0   160    75   -40   3.42 0.4258
WVFGRD96   24.0   165    80   -40   3.44 0.4129
WVFGRD96   26.0    -5    75    45   3.47 0.3949
WVFGRD96   28.0   250    45   -15   3.49 0.3866
WVFGRD96   30.0   250    45   -15   3.51 0.3881
WVFGRD96   32.0   250    45   -15   3.53 0.3842
WVFGRD96   34.0   260    50    10   3.53 0.3787
WVFGRD96   36.0   255    70   -20   3.53 0.3828
WVFGRD96   38.0   255    75   -20   3.56 0.4077
WVFGRD96   40.0   255    70   -25   3.65 0.4551
WVFGRD96   42.0   260    70   -10   3.68 0.4884
WVFGRD96   44.0   260    70   -10   3.71 0.5130
WVFGRD96   46.0   260    75   -15   3.72 0.5245
WVFGRD96   48.0   260    75   -15   3.74 0.5232
WVFGRD96   50.0   260    75   -15   3.75 0.5202
WVFGRD96   52.0    65    65   -40   3.76 0.5243
WVFGRD96   54.0    65    65   -40   3.78 0.5352
WVFGRD96   56.0    65    65   -40   3.80 0.5441
WVFGRD96   58.0    65    65   -40   3.81 0.5517
WVFGRD96   60.0    65    65   -40   3.82 0.5626
WVFGRD96   62.0    65    65   -40   3.83 0.5710
WVFGRD96   64.0    65    65   -40   3.84 0.5771
WVFGRD96   66.0    60    55   -35   3.81 0.5855
WVFGRD96   68.0    65    60   -30   3.81 0.5916
WVFGRD96   70.0    60    55   -35   3.82 0.5963
WVFGRD96   72.0    35    35   -30   3.84 0.6014
WVFGRD96   74.0    35    35   -30   3.85 0.6035
WVFGRD96   76.0    35    35   -30   3.85 0.6065
WVFGRD96   78.0    45    40   -20   3.82 0.6067
WVFGRD96   80.0    65    60   -30   3.84 0.6093
WVFGRD96   82.0    65    60   -25   3.83 0.6081
WVFGRD96   84.0    35    30   -25   3.86 0.6085
WVFGRD96   86.0    40    30   -15   3.85 0.6089
WVFGRD96   88.0    65    35     0   3.79 0.6131
WVFGRD96   90.0    70    35    10   3.79 0.6152
WVFGRD96   92.0    70    35    10   3.79 0.6169
WVFGRD96   94.0    70    35     5   3.80 0.6192
WVFGRD96   96.0    75    35    15   3.80 0.6200
WVFGRD96   98.0    75    35    15   3.80 0.6227
WVFGRD96  100.0    75    35    15   3.80 0.6221
WVFGRD96  102.0    75    35    15   3.81 0.6243
WVFGRD96  104.0    75    35    15   3.81 0.6237
WVFGRD96  106.0    75    35    15   3.81 0.6228
WVFGRD96  108.0    80    35    20   3.81 0.6224
WVFGRD96  110.0    80    35    20   3.81 0.6201
WVFGRD96  112.0    80    35    20   3.82 0.6204
WVFGRD96  114.0    80    35    20   3.82 0.6184
WVFGRD96  116.0    80    35    20   3.82 0.6159
WVFGRD96  118.0    80    35    20   3.82 0.6139
WVFGRD96  120.0    85    35    30   3.83 0.6105
WVFGRD96  122.0    85    35    30   3.83 0.6094
WVFGRD96  124.0    85    35    30   3.83 0.6074
WVFGRD96  126.0    85    40    25   3.83 0.6036
WVFGRD96  128.0    85    40    25   3.83 0.6028
WVFGRD96  130.0    85    40    30   3.84 0.6002
WVFGRD96  132.0    85    40    30   3.84 0.5969
WVFGRD96  134.0    85    40    30   3.84 0.5957
WVFGRD96  136.0    85    40    30   3.84 0.5930
WVFGRD96  138.0    85    40    30   3.84 0.5900
WVFGRD96  140.0    85    40    30   3.84 0.5878
WVFGRD96  142.0    90    40    35   3.85 0.5847
WVFGRD96  144.0    90    40    35   3.85 0.5815
WVFGRD96  146.0    90    40    35   3.85 0.5790
WVFGRD96  148.0    90    40    35   3.86 0.5758

The best solution is

WVFGRD96  102.0    75    35    15   3.81 0.6243

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/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2
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 Feb 25 06:08:25 CST 2025