Location

Location ANSS

2021/03/29 01:05:52 61.238 -146.337 6.4 3.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2021/03/29 01:05:52:0  61.24 -146.34   6.4 3.7 Alaska
 
 Stations used:
   AK.BMR AK.BRLK AK.CAST AK.CRQ AK.DHY AK.DIV AK.DOT AK.EYAK 
   AK.GHO AK.GLB AK.GLI AK.GRNC AK.HIN AK.J26L AK.KAI AK.KLU 
   AK.KNK AK.L20K AK.M20K AK.M26K AK.M27K AK.MCAR AK.MCK 
   AK.MESA AK.P23K AK.PAX AK.POKR AK.PPLA AK.PWL AK.RAG 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN 
   AK.TGL AK.VRDI AK.WAX AK.WRH AT.PMR AV.SPCP IM.IL31 TA.L27K 
   TA.M22K TA.M29M TA.O22K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 8.91e+21 dyne-cm
  Mw = 3.90 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      245    65    65
   NP2      113    35   132
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.91e+21     62     116
    N   0.00e+00     23     256
    P  -8.91e+21     16     353

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.70e+21
       Mxy     1.76e+20
       Mxz    -4.03e+21
       Myy     1.51e+21
       Myz     3.64e+21
       Mzz     6.19e+21
                                                     
                                                     
                                                     
                                                     
                     ---   --------                  
                 ------- P ------------              
              ----------   ---------------           
             ------------------------------          
           ----------------------------------        
          ------------------------------------       
         -----------------------###############      
        #------------------#####################     
        #--------------#########################     
       ##-----------#############################    
       ###-------################################    
       ####----##################   #############    
       ####--#################### T #############    
        ####-####################   ############     
        ##----##################################     
         -------###############################      
          --------###########################-       
           -----------####################---        
             --------------##########------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.19e+21  -4.03e+21  -3.64e+21 
 -4.03e+21  -7.70e+21  -1.76e+20 
 -3.64e+21  -1.76e+20   1.51e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210329010552/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 245
      DIP = 65
     RAKE = 65
       MW = 3.90
       HS = 18.0

The NDK file is 20210329010552.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2021/03/29 01:05:52:0  61.24 -146.34   6.4 3.7 Alaska
 
 Stations used:
   AK.BMR AK.BRLK AK.CAST AK.CRQ AK.DHY AK.DIV AK.DOT AK.EYAK 
   AK.GHO AK.GLB AK.GLI AK.GRNC AK.HIN AK.J26L AK.KAI AK.KLU 
   AK.KNK AK.L20K AK.M20K AK.M26K AK.M27K AK.MCAR AK.MCK 
   AK.MESA AK.P23K AK.PAX AK.POKR AK.PPLA AK.PWL AK.RAG 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN 
   AK.TGL AK.VRDI AK.WAX AK.WRH AT.PMR AV.SPCP IM.IL31 TA.L27K 
   TA.M22K TA.M29M TA.O22K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 8.91e+21 dyne-cm
  Mw = 3.90 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      245    65    65
   NP2      113    35   132
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.91e+21     62     116
    N   0.00e+00     23     256
    P  -8.91e+21     16     353

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.70e+21
       Mxy     1.76e+20
       Mxz    -4.03e+21
       Myy     1.51e+21
       Myz     3.64e+21
       Mzz     6.19e+21
                                                     
                                                     
                                                     
                                                     
                     ---   --------                  
                 ------- P ------------              
              ----------   ---------------           
             ------------------------------          
           ----------------------------------        
          ------------------------------------       
         -----------------------###############      
        #------------------#####################     
        #--------------#########################     
       ##-----------#############################    
       ###-------################################    
       ####----##################   #############    
       ####--#################### T #############    
        ####-####################   ############     
        ##----##################################     
         -------###############################      
          --------###########################-       
           -----------####################---        
             --------------##########------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.19e+21  -4.03e+21  -3.64e+21 
 -4.03e+21  -7.70e+21  -1.76e+20 
 -3.64e+21  -1.76e+20   1.51e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210329010552/index.html
	
Regional Moment Tensor (Mwr)
Moment 9.680e+14 N-m
Magnitude 3.92 Mwr
Depth 19.0 km
Percent DC 73%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2

Nodal Planes
Plane Strike Dip Rake
NP1 243 72 64
NP2 120 31 143

Principal Axes
Axis Value Plunge Azimuth
T 8.906e+14 N-m 56 120
N 1.397e+14 N-m 25 252
P -10.303e+14 N-m 22 353


        

Magnitudes

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. 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).

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 0.08 n 3 
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   240    45   -90   3.46 0.2601
WVFGRD96    2.0    60    40   -90   3.59 0.3327
WVFGRD96    3.0   230    55   -70   3.64 0.2750
WVFGRD96    4.0   235    85   -50   3.61 0.2884
WVFGRD96    5.0   240    90    60   3.65 0.3248
WVFGRD96    6.0   245    85    60   3.66 0.3723
WVFGRD96    7.0   245    85    60   3.67 0.4131
WVFGRD96    8.0   245    85    65   3.75 0.4441
WVFGRD96    9.0   245    80    65   3.77 0.4822
WVFGRD96   10.0   245    75    65   3.78 0.5152
WVFGRD96   11.0   245    75    65   3.80 0.5445
WVFGRD96   12.0   245    70    65   3.82 0.5702
WVFGRD96   13.0   245    70    65   3.83 0.5916
WVFGRD96   14.0   245    70    65   3.84 0.6081
WVFGRD96   15.0   245    70    65   3.86 0.6205
WVFGRD96   16.0   245    65    70   3.87 0.6295
WVFGRD96   17.0   245    65    65   3.88 0.6353
WVFGRD96   18.0   245    65    65   3.90 0.6377
WVFGRD96   19.0   245    65    65   3.91 0.6366
WVFGRD96   20.0   240    70    60   3.91 0.6333
WVFGRD96   21.0   240    70    60   3.93 0.6288
WVFGRD96   22.0   240    70    60   3.94 0.6210
WVFGRD96   23.0   240    70    60   3.94 0.6109
WVFGRD96   24.0   240    70    60   3.95 0.5992
WVFGRD96   25.0   240    70    65   3.96 0.5868
WVFGRD96   26.0   240    70    65   3.96 0.5740
WVFGRD96   27.0   240    70    65   3.97 0.5606
WVFGRD96   28.0   240    65    65   3.97 0.5480
WVFGRD96   29.0   240    65    65   3.98 0.5375

The best solution is

WVFGRD96   18.0   245    65    65   3.90 0.6377

The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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 0.08 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.
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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    

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Sun Mar 28 21:06:27 CDT 2021