Location

Location ANSS

The ANSS event ID is ak020at0jk8z and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020at0jk8z/executive.

2020/08/22 22:25:53 59.473 -152.563 73.3 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/08/22 22:25:53:0  59.47 -152.56  73.3 3.9 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.GLI AK.HOM AK.KNK AK.N18K AK.N19K 
   AK.O18K AK.O19K AK.P17K AK.Q19K AK.RC01 AK.SKN AK.SLK 
   AK.SWD AT.PMR AV.ILSW AV.SPU AV.STLK II.KDAK 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.55e+22 dyne-cm
  Mw = 4.06 
  Z  = 82 km
  Plane   Strike  Dip  Rake
   NP1      303    71   159
   NP2       40    70    20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.55e+22     28     261
    N   0.00e+00     62      83
    P  -1.55e+22      1     352

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.49e+22
       Mxy     4.05e+21
       Mxz    -1.20e+21
       Myy     1.15e+22
       Myz    -6.31e+21
       Mzz     3.41e+21
                                                     
                                                     
                                                     
                                                     
                     --- P --------                  
                 -------   ------------              
              ---------------------------#           
             ----------------------------##          
           ------------------------------####        
          ########----------------------######       
         ##############----------------########      
        ##################------------##########     
        ######################-------###########     
       #########################----#############    
       ##########################################    
       #####   ###################---############    
       ##### T #################-------##########    
        ####   ################----------#######     
        #####################--------------#####     
         ##################-----------------###      
          ###############--------------------#       
           ############----------------------        
             #######-----------------------          
              #---------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.41e+21  -1.20e+21   6.31e+21 
 -1.20e+21  -1.49e+22  -4.05e+21 
  6.31e+21  -4.05e+21   1.15e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200822222553/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 = 40
      DIP = 70
     RAKE = 20
       MW = 4.06
       HS = 82.0

The NDK file is 20200822222553.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 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   305    90   -10   3.05 0.1614
WVFGRD96    4.0   125    90    25   3.18 0.1966
WVFGRD96    6.0   130    60    20   3.27 0.2231
WVFGRD96    8.0   130    60    20   3.36 0.2512
WVFGRD96   10.0   130    65    20   3.40 0.2631
WVFGRD96   12.0   125    65    15   3.45 0.2612
WVFGRD96   14.0    40    75   -30   3.45 0.2549
WVFGRD96   16.0    40    75   -30   3.49 0.2619
WVFGRD96   18.0    40    75   -30   3.52 0.2708
WVFGRD96   20.0    35    70   -25   3.55 0.2822
WVFGRD96   22.0    35    70   -25   3.58 0.2943
WVFGRD96   24.0    35    70   -20   3.61 0.3063
WVFGRD96   26.0    35    70   -20   3.63 0.3172
WVFGRD96   28.0    35    70   -10   3.64 0.3268
WVFGRD96   30.0    40    70    -5   3.65 0.3345
WVFGRD96   32.0    40    70     0   3.67 0.3408
WVFGRD96   34.0    40    65     5   3.69 0.3452
WVFGRD96   36.0    40    70     5   3.71 0.3480
WVFGRD96   38.0    40    70     5   3.74 0.3524
WVFGRD96   40.0    45    55    15   3.82 0.3678
WVFGRD96   42.0    40    65    10   3.83 0.3748
WVFGRD96   44.0    40    65    10   3.86 0.3858
WVFGRD96   46.0    40    65    10   3.88 0.3960
WVFGRD96   48.0    40    65    10   3.90 0.4074
WVFGRD96   50.0    40    65    10   3.92 0.4180
WVFGRD96   52.0    40    70    15   3.93 0.4302
WVFGRD96   54.0    40    70    15   3.95 0.4416
WVFGRD96   56.0    40    70    15   3.96 0.4548
WVFGRD96   58.0    40    70    20   3.98 0.4661
WVFGRD96   60.0    40    70    20   3.99 0.4795
WVFGRD96   62.0    40    70    20   4.00 0.4896
WVFGRD96   64.0    40    70    20   4.01 0.4992
WVFGRD96   66.0    40    70    20   4.02 0.5092
WVFGRD96   68.0    40    70    20   4.03 0.5163
WVFGRD96   70.0    40    70    20   4.03 0.5212
WVFGRD96   72.0    40    70    20   4.04 0.5260
WVFGRD96   74.0    40    70    20   4.05 0.5308
WVFGRD96   76.0    40    70    20   4.05 0.5321
WVFGRD96   78.0    40    70    20   4.06 0.5351
WVFGRD96   80.0    40    70    20   4.06 0.5350
WVFGRD96   82.0    40    70    20   4.06 0.5353
WVFGRD96   84.0    40    70    20   4.07 0.5341
WVFGRD96   86.0    40    70    20   4.07 0.5332
WVFGRD96   88.0    40    70    20   4.07 0.5313
WVFGRD96   90.0    40    70    20   4.08 0.5286
WVFGRD96   92.0    45    65    20   4.08 0.5267
WVFGRD96   94.0    45    65    20   4.09 0.5246
WVFGRD96   96.0    45    65    20   4.09 0.5217
WVFGRD96   98.0    45    65    20   4.09 0.5196
WVFGRD96  100.0    45    65    20   4.09 0.5165
WVFGRD96  102.0    45    65    20   4.10 0.5139
WVFGRD96  104.0    45    65    20   4.10 0.5110
WVFGRD96  106.0    45    65    20   4.10 0.5088
WVFGRD96  108.0    45    70    20   4.09 0.5065

The best solution is

WVFGRD96   82.0    40    70    20   4.06 0.5353

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 
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 08:56:43 PM CDT 2024