Location

Location ANSS

The ANSS event ID is ak02556vvgv8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02556vvgv8/executive.

2025/04/23 00:24:51 61.807 -150.061 26.1 4.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2025/04/23 00:24:51:0  61.81 -150.06  26.1 4.5 Alaska
 
 Stations used:
   AK.BAE AK.BMR AK.BPAW AK.CAPN AK.CAST AK.FID AK.FIRE AK.GHO 
   AK.HIN AK.KNK AK.L22K AK.MCK AK.O19K AK.PAX AK.PWL AK.RAG 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN 
   AK.SWD AT.PMR AT.TTA AV.RED AV.SPCL AV.STLK 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.75e+22 dyne-cm
  Mw = 4.44 
  Z  = 46 km
  Plane   Strike  Dip  Rake
   NP1       30    65   -50
   NP2      147    46   -144
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.75e+22     11      92
    N   0.00e+00     36     190
    P  -5.75e+22     52     348

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.06e+22
       Mxy     2.14e+21
       Mxz    -2.77e+22
       Myy     5.44e+22
       Myz     1.67e+22
       Mzz    -3.38e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ##-----------------------###           
             ##------------------------####          
           ####---------   ------------######        
          ####---------- P -----------########       
         #####----------   -----------#########      
        ######------------------------##########     
        #######----------------------###########     
       ########---------------------#############    
       ########---------------------#########   #    
       #########-------------------########## T #    
       ##########-----------------###########   #    
        ##########---------------###############     
        ###########------------#################     
         ###########----------#################      
          ############-------#################       
           #############---##################        
             ############--################          
              ########--------############           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.38e+22  -2.77e+22  -1.67e+22 
 -2.77e+22  -2.06e+22  -2.14e+21 
 -1.67e+22  -2.14e+21   5.44e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250423002451/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 = 30
      DIP = 65
     RAKE = -50
       MW = 4.44
       HS = 46.0

The NDK file is 20250423002451.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  2025/04/23 00:24:51:0  61.81 -150.06  26.1 4.5 Alaska
 
 Stations used:
   AK.BAE AK.BMR AK.BPAW AK.CAPN AK.CAST AK.FID AK.FIRE AK.GHO 
   AK.HIN AK.KNK AK.L22K AK.MCK AK.O19K AK.PAX AK.PWL AK.RAG 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN 
   AK.SWD AT.PMR AT.TTA AV.RED AV.SPCL AV.STLK 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.75e+22 dyne-cm
  Mw = 4.44 
  Z  = 46 km
  Plane   Strike  Dip  Rake
   NP1       30    65   -50
   NP2      147    46   -144
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.75e+22     11      92
    N   0.00e+00     36     190
    P  -5.75e+22     52     348

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.06e+22
       Mxy     2.14e+21
       Mxz    -2.77e+22
       Myy     5.44e+22
       Myz     1.67e+22
       Mzz    -3.38e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ##-----------------------###           
             ##------------------------####          
           ####---------   ------------######        
          ####---------- P -----------########       
         #####----------   -----------#########      
        ######------------------------##########     
        #######----------------------###########     
       ########---------------------#############    
       ########---------------------#########   #    
       #########-------------------########## T #    
       ##########-----------------###########   #    
        ##########---------------###############     
        ###########------------#################     
         ###########----------#################      
          ############-------#################       
           #############---##################        
             ############--################          
              ########--------############           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.38e+22  -2.77e+22  -1.67e+22 
 -2.77e+22  -2.06e+22  -2.14e+21 
 -1.67e+22  -2.14e+21   5.44e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250423002451/index.html
	
Regional Moment Tensor (Mwr)
Moment 6.556e+15 N-m
Magnitude 4.48 Mwr
Depth 51.0 km
Percent DC 90%
Half Duration -
Catalog US
Data Source US
Contributor US
Nodal Planes
Plane	Strike	Dip	Rake
NP1	21	51	-63
NP2	162	47	-119
Principal Axes
Axis	Value	Plunge	Azimuth
T	6.714e+15	2	92
N	-0.328e+15	21	183
P	-6.386e+15	69	356

        

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 +50
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    1.0   145    50    40   3.58 0.1482
WVFGRD96    2.0   340    45    70   3.76 0.1951
WVFGRD96    3.0   320    55    30   3.76 0.1908
WVFGRD96    4.0   125    50   -10   3.78 0.2032
WVFGRD96    5.0   125    55   -15   3.81 0.2190
WVFGRD96    6.0   125    55   -15   3.84 0.2310
WVFGRD96    7.0   125    60   -20   3.87 0.2397
WVFGRD96    8.0   125    55   -15   3.92 0.2431
WVFGRD96    9.0    50    65    35   3.95 0.2481
WVFGRD96   10.0    50    65    30   3.98 0.2563
WVFGRD96   11.0    50    65    30   4.00 0.2630
WVFGRD96   12.0    50    65    30   4.01 0.2682
WVFGRD96   13.0    50    65    30   4.03 0.2717
WVFGRD96   14.0   215    70   -30   4.04 0.2771
WVFGRD96   15.0   215    70   -25   4.06 0.2813
WVFGRD96   16.0   215    70   -25   4.08 0.2844
WVFGRD96   17.0   215    70   -25   4.09 0.2872
WVFGRD96   18.0   215    70   -25   4.10 0.2895
WVFGRD96   19.0   215    70   -25   4.11 0.2911
WVFGRD96   20.0   215    70   -25   4.13 0.2916
WVFGRD96   21.0   215    70   -25   4.14 0.2913
WVFGRD96   22.0   215    70   -25   4.15 0.2902
WVFGRD96   23.0   220    75   -20   4.16 0.2888
WVFGRD96   24.0   225    80    25   4.18 0.2922
WVFGRD96   25.0   225    75    25   4.19 0.2977
WVFGRD96   26.0   225    80    30   4.20 0.3043
WVFGRD96   27.0    40    80   -30   4.20 0.3095
WVFGRD96   28.0    40    80   -30   4.21 0.3187
WVFGRD96   29.0    40    80   -30   4.22 0.3300
WVFGRD96   30.0    40    80   -35   4.23 0.3407
WVFGRD96   31.0    40    75   -35   4.24 0.3515
WVFGRD96   32.0    40    75   -35   4.25 0.3639
WVFGRD96   33.0    40    75   -35   4.26 0.3745
WVFGRD96   34.0    35    70   -40   4.27 0.3857
WVFGRD96   35.0    35    70   -40   4.28 0.3936
WVFGRD96   36.0    35    70   -40   4.29 0.4016
WVFGRD96   37.0    35    70   -40   4.29 0.4068
WVFGRD96   38.0    35    70   -40   4.30 0.4119
WVFGRD96   39.0    35    70   -35   4.32 0.4168
WVFGRD96   40.0    35    70   -45   4.39 0.4225
WVFGRD96   41.0    35    70   -45   4.40 0.4274
WVFGRD96   42.0    35    70   -45   4.41 0.4289
WVFGRD96   43.0    30    65   -50   4.42 0.4326
WVFGRD96   44.0    30    65   -50   4.43 0.4347
WVFGRD96   45.0    30    65   -50   4.43 0.4350
WVFGRD96   46.0    30    65   -50   4.44 0.4364
WVFGRD96   47.0    30    65   -50   4.45 0.4359
WVFGRD96   48.0    30    65   -50   4.45 0.4348
WVFGRD96   49.0    30    65   -50   4.45 0.4344
WVFGRD96   50.0    30    60   -45   4.46 0.4316
WVFGRD96   51.0    30    60   -45   4.46 0.4319
WVFGRD96   52.0    30    60   -45   4.47 0.4292
WVFGRD96   53.0    30    60   -45   4.47 0.4292
WVFGRD96   54.0    30    65   -45   4.47 0.4266
WVFGRD96   55.0    30    65   -45   4.47 0.4260
WVFGRD96   56.0    30    65   -45   4.47 0.4233
WVFGRD96   57.0    30    65   -45   4.47 0.4224
WVFGRD96   58.0    30    65   -45   4.47 0.4195
WVFGRD96   59.0    35    70   -45   4.48 0.4190

The best solution is

WVFGRD96   46.0    30    65   -50   4.44 0.4364

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 +50
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 Wed Apr 23 05:43:12 CDT 2025