Location

Location ANSS

The ANSS event ID is ak01913ozsba and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01913ozsba/executive.

2019/01/24 00:16:12 61.289 -150.081 43.7 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/01/24 00:16:12:0  61.29 -150.08  43.7 3.8 Alaska
 
 Stations used:
   AK.FIRE AK.GHO AK.HIN AK.PWL AK.RC01 AK.SKN AK.SSN AT.PMR 
   AV.SPU AV.STLK GM.AD09 TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.06e+22 dyne-cm
  Mw = 3.95 
  Z  = 48 km
  Plane   Strike  Dip  Rake
   NP1      194    78   -99
   NP2       50    15   -55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.06e+22     32     291
    N   0.00e+00      9     196
    P  -1.06e+22     56      93

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     9.97e+20
       Mxy    -2.41e+21
       Mxz     1.98e+21
       Myy     3.34e+21
       Myz    -9.33e+21
       Mzz    -4.34e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ##############--------              
              ################------------           
             ################--------------          
           ##################----------------        
          ##################------------------       
         ##################-------------------#      
        #####   ###########--------------------#     
        ##### T ##########---------------------#     
       ######   #########----------------------##    
       ##################----------   ---------##    
       ##################---------- P ---------##    
       #################-----------   --------###    
        ################----------------------##     
        ###############----------------------###     
         ##############---------------------###      
          #############--------------------###       
           ############------------------####        
             ##########----------------####          
              -########--------------#####           
                 --####----------######              
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.34e+21   1.98e+21   9.33e+21 
  1.98e+21   9.97e+20   2.41e+21 
  9.33e+21   2.41e+21   3.34e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190124001612/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 = 50
      DIP = 15
     RAKE = -55
       MW = 3.95
       HS = 48.0

The NDK file is 20190124001612.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  2019/01/24 00:16:12:0  61.29 -150.08  43.7 3.8 Alaska
 
 Stations used:
   AK.FIRE AK.GHO AK.HIN AK.PWL AK.RC01 AK.SKN AK.SSN AT.PMR 
   AV.SPU AV.STLK GM.AD09 TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.06e+22 dyne-cm
  Mw = 3.95 
  Z  = 48 km
  Plane   Strike  Dip  Rake
   NP1      194    78   -99
   NP2       50    15   -55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.06e+22     32     291
    N   0.00e+00      9     196
    P  -1.06e+22     56      93

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     9.97e+20
       Mxy    -2.41e+21
       Mxz     1.98e+21
       Myy     3.34e+21
       Myz    -9.33e+21
       Mzz    -4.34e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ##############--------              
              ################------------           
             ################--------------          
           ##################----------------        
          ##################------------------       
         ##################-------------------#      
        #####   ###########--------------------#     
        ##### T ##########---------------------#     
       ######   #########----------------------##    
       ##################----------   ---------##    
       ##################---------- P ---------##    
       #################-----------   --------###    
        ################----------------------##     
        ###############----------------------###     
         ##############---------------------###      
          #############--------------------###       
           ############------------------####        
             ##########----------------####          
              -########--------------#####           
                 --####----------######              
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.34e+21   1.98e+21   9.33e+21 
  1.98e+21   9.97e+20   2.41e+21 
  9.33e+21   2.41e+21   3.34e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190124001612/index.html
	
Regional Moment Tensor (Mwr)
Moment 1.016e+15 N-m
Magnitude 3.94 Mwr
Depth 51.0 km
Percent DC 90%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 342 17 -122
NP2 194 76 -81
Principal Axes
Axis Value Plunge Azimuth
T 1.041e+15 N-m 30 277
N -0.052e+15 N-m 9 12
P -0.990e+15 N-m 58 116

        

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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   170    90     5   3.31 0.2613
WVFGRD96    4.0   210    70    35   3.41 0.3048
WVFGRD96    6.0    15    60   -35   3.48 0.3294
WVFGRD96    8.0    15    60   -40   3.53 0.3469
WVFGRD96   10.0    15    65   -35   3.54 0.3484
WVFGRD96   12.0    90    55    25   3.58 0.3511
WVFGRD96   14.0    85    60    15   3.60 0.3589
WVFGRD96   16.0    85    55    15   3.61 0.3677
WVFGRD96   18.0    85    55    15   3.63 0.3776
WVFGRD96   20.0    85    55    15   3.64 0.3869
WVFGRD96   22.0    85    50    15   3.66 0.3954
WVFGRD96   24.0    85    50    15   3.68 0.4053
WVFGRD96   26.0    85    45    10   3.70 0.4149
WVFGRD96   28.0    85    45    10   3.72 0.4248
WVFGRD96   30.0    85    45    10   3.74 0.4361
WVFGRD96   32.0    85    45    10   3.75 0.4476
WVFGRD96   34.0    85    45    10   3.77 0.4573
WVFGRD96   36.0    75    20   -25   3.75 0.4637
WVFGRD96   38.0    60    15   -45   3.76 0.4856
WVFGRD96   40.0    55    10   -45   3.91 0.5017
WVFGRD96   42.0    55    15   -50   3.92 0.5046
WVFGRD96   44.0    50    15   -55   3.93 0.5082
WVFGRD96   46.0    50    15   -55   3.94 0.5090
WVFGRD96   48.0    50    15   -55   3.95 0.5100
WVFGRD96   50.0    50    15   -55   3.96 0.5090
WVFGRD96   52.0    55    15   -50   3.97 0.5077
WVFGRD96   54.0    65    15   -40   3.97 0.5066
WVFGRD96   56.0    70    15   -35   3.98 0.5070
WVFGRD96   58.0    70    15   -35   3.99 0.5070
WVFGRD96   60.0    75    15   -30   3.99 0.5075
WVFGRD96   62.0    75    15   -30   4.00 0.5072
WVFGRD96   64.0    80    15   -25   4.01 0.5078
WVFGRD96   66.0    80    15   -25   4.02 0.5065
WVFGRD96   68.0    85    15   -20   4.02 0.5068
WVFGRD96   70.0    85    15   -20   4.03 0.5048
WVFGRD96   72.0    85    15   -20   4.04 0.5045
WVFGRD96   74.0    90    15   -15   4.04 0.5027
WVFGRD96   76.0    90    15   -15   4.05 0.5004
WVFGRD96   78.0    90    15   -15   4.06 0.4984
WVFGRD96   80.0    90    15   -15   4.06 0.4950
WVFGRD96   82.0    95    20   -15   4.08 0.4924
WVFGRD96   84.0    95    20   -15   4.08 0.4882
WVFGRD96   86.0   100    20   -10   4.09 0.4838
WVFGRD96   88.0   100    20   -10   4.09 0.4785
WVFGRD96   90.0    95    20   -15   4.10 0.4727
WVFGRD96   92.0    95    20   -15   4.10 0.4667
WVFGRD96   94.0    90    20   -20   4.10 0.4605
WVFGRD96   96.0    90    20   -20   4.10 0.4536
WVFGRD96   98.0    90    20   -20   4.11 0.4472
WVFGRD96  100.0    90    20   -20   4.11 0.4399
WVFGRD96  102.0    95    20   -15   4.11 0.4324
WVFGRD96  104.0    95    20   -15   4.11 0.4253
WVFGRD96  106.0   120    20    15   4.12 0.4179
WVFGRD96  108.0   120    25    10   4.14 0.4116
WVFGRD96  110.0   120    25    10   4.14 0.4032
WVFGRD96  112.0   120    25    10   4.14 0.3891
WVFGRD96  114.0   120    25    10   4.13 0.3577
WVFGRD96  116.0   130    30   -15   4.17 0.3223
WVFGRD96  118.0   130    30   -20   4.17 0.2911
WVFGRD96  120.0   130    30   -20   4.15 0.2557
WVFGRD96  122.0    10    70   -80   4.08 0.2409
WVFGRD96  124.0     5    70   -75   4.09 0.2397
WVFGRD96  126.0     5    70   -75   4.09 0.2383
WVFGRD96  128.0     5    70   -75   4.09 0.2371
WVFGRD96  130.0     5    70   -75   4.09 0.2353
WVFGRD96  132.0     5    70   -75   4.10 0.2336
WVFGRD96  134.0     5    70   -75   4.10 0.2318
WVFGRD96  136.0     5    70   -75   4.10 0.2307
WVFGRD96  138.0     5    70   -75   4.10 0.2289
WVFGRD96  140.0    10    70   -75   4.11 0.2250
WVFGRD96  142.0    10    70   -75   4.11 0.2159
WVFGRD96  144.0    10    70   -75   4.10 0.2001
WVFGRD96  146.0    20    65   -90   4.10 0.1815
WVFGRD96  148.0   210    25   -85   4.08 0.1588

The best solution is

WVFGRD96   48.0    50    15   -55   3.95 0.5100

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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 Thu Apr 25 08:28:14 AM CDT 2024