Location

Location ANSS

The ANSS event ID is ak0176w3wvpo and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0176w3wvpo/executive.

2017/05/30 02:18:45 60.834 -151.815 81.2 5.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/30 02:18:45:0  60.83 -151.82  81.2 5.2 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM 
   AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K 
   TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K 
   TA.P18K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 9.44e+23 dyne-cm
  Mw = 5.25 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1      185    75   -85
   NP2      346    16   -108
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.44e+23     30     271
    N   0.00e+00      5       4
    P  -9.44e+23     60     102

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.02e+22
       Mxy     3.74e+22
       Mxz     9.22e+22
       Myy     4.80e+23
       Myz    -8.10e+23
       Mzz    -4.70e+23
                                                     
                                                     
                                                     
                                                     
                     ########--####                  
                 ###########-------####              
              #############-----------####           
             ##############-------------###          
           ###############---------------####        
          ################-----------------###       
         ################-------------------###      
        #################--------------------###     
        #################--------------------###     
       #####   #########----------------------###    
       ##### T #########----------   ---------###    
       #####   #########---------- P ---------###    
       #################----------   ---------###    
        ################---------------------###     
        ################---------------------###     
         ###############--------------------###      
          ##############--------------------##       
           #############-------------------##        
             ###########-----------------##          
              ###########---------------##           
                 ########-------------#              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.70e+23   9.22e+22   8.10e+23 
  9.22e+22  -1.02e+22  -3.74e+22 
  8.10e+23  -3.74e+22   4.80e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530021845/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 = 185
      DIP = 75
     RAKE = -85
       MW = 5.25
       HS = 96.0

The NDK file is 20170530021845.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
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/30 02:18:45:0  60.83 -151.82  81.2 5.2 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM 
   AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K 
   TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K 
   TA.P18K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 9.44e+23 dyne-cm
  Mw = 5.25 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1      185    75   -85
   NP2      346    16   -108
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.44e+23     30     271
    N   0.00e+00      5       4
    P  -9.44e+23     60     102

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.02e+22
       Mxy     3.74e+22
       Mxz     9.22e+22
       Myy     4.80e+23
       Myz    -8.10e+23
       Mzz    -4.70e+23
                                                     
                                                     
                                                     
                                                     
                     ########--####                  
                 ###########-------####              
              #############-----------####           
             ##############-------------###          
           ###############---------------####        
          ################-----------------###       
         ################-------------------###      
        #################--------------------###     
        #################--------------------###     
       #####   #########----------------------###    
       ##### T #########----------   ---------###    
       #####   #########---------- P ---------###    
       #################----------   ---------###    
        ################---------------------###     
        ################---------------------###     
         ###############--------------------###      
          ##############--------------------##       
           #############-------------------##        
             ###########-----------------##          
              ###########---------------##           
                 ########-------------#              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.70e+23   9.22e+22   8.10e+23 
  9.22e+22  -1.02e+22  -3.74e+22 
  8.10e+23  -3.74e+22   4.80e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530021845/index.html
	
Regional Moment Tensor (Mwr)
Moment	8.549e+16 N-m
Magnitude	5.2 Mwr
Depth	91.0 km
Percent DC	80 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	342	21	-112
NP2	186	70	-82
Principal Axes
Axis	Value	Plunge	Azimuth
T	8.071e+16 N-m	25	269
N	0.888e+16 N-m	8	3
P	-8.959e+16 N-m	64	109

        

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 -50 o DIST/3.3 +40
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     5    55    95   4.33 0.1208
WVFGRD96    4.0   300    55   -50   4.36 0.1269
WVFGRD96    6.0   265    35   -40   4.44 0.1631
WVFGRD96    8.0   265    35   -40   4.53 0.1810
WVFGRD96   10.0   300    50   -45   4.55 0.1983
WVFGRD96   12.0   300    50   -40   4.58 0.2049
WVFGRD96   14.0   305    55   -40   4.60 0.2058
WVFGRD96   16.0   310    60   -40   4.62 0.2037
WVFGRD96   18.0   310    60   -40   4.65 0.1986
WVFGRD96   20.0   315    65   -35   4.67 0.1975
WVFGRD96   22.0   320    70   -35   4.69 0.1952
WVFGRD96   24.0   325    75   -40   4.71 0.1943
WVFGRD96   26.0   335    85   -45   4.73 0.1954
WVFGRD96   28.0   190    85    60   4.78 0.2011
WVFGRD96   30.0   190    85    60   4.81 0.2025
WVFGRD96   32.0     5    90   -60   4.82 0.1987
WVFGRD96   34.0    25    80   -70   4.83 0.1995
WVFGRD96   36.0    30    75   -65   4.83 0.2049
WVFGRD96   38.0    30    70   -65   4.84 0.2171
WVFGRD96   40.0    30    65   -65   4.97 0.2580
WVFGRD96   42.0    25    55   -70   5.00 0.3001
WVFGRD96   44.0    25    50   -70   5.04 0.3404
WVFGRD96   46.0    20    45   -75   5.07 0.3704
WVFGRD96   48.0    25    45   -70   5.09 0.3840
WVFGRD96   50.0    20    40   -75   5.11 0.3967
WVFGRD96   52.0    15    35   -80   5.13 0.4096
WVFGRD96   54.0   200    55   -80   5.13 0.4270
WVFGRD96   56.0   185    55   -90   5.14 0.4420
WVFGRD96   58.0   185    55   -90   5.15 0.4573
WVFGRD96   60.0   185    60   -90   5.17 0.4747
WVFGRD96   62.0   185    60   -90   5.17 0.4892
WVFGRD96   64.0   185    60   -90   5.18 0.5010
WVFGRD96   66.0     5    30   -90   5.18 0.5139
WVFGRD96   68.0   185    65   -90   5.19 0.5216
WVFGRD96   70.0     0    25   -95   5.20 0.5319
WVFGRD96   72.0   185    65   -90   5.20 0.5394
WVFGRD96   74.0     0    25   -95   5.20 0.5452
WVFGRD96   76.0   185    65   -90   5.20 0.5507
WVFGRD96   78.0   185    65   -90   5.20 0.5536
WVFGRD96   80.0   185    65   -90   5.20 0.5575
WVFGRD96   82.0   185    70   -85   5.22 0.5592
WVFGRD96   84.0    -5    20  -100   5.22 0.5660
WVFGRD96   86.0    -5    20  -100   5.22 0.5669
WVFGRD96   88.0   -10    20  -105   5.23 0.5711
WVFGRD96   90.0   -10    20  -105   5.23 0.5711
WVFGRD96   92.0   185    70   -85   5.23 0.5730
WVFGRD96   94.0   -15    20  -110   5.23 0.5743
WVFGRD96   96.0   185    75   -85   5.25 0.5751
WVFGRD96   98.0   -15    20  -110   5.23 0.5749
WVFGRD96  100.0   -10    15  -100   5.25 0.5732
WVFGRD96  102.0   -10    15  -100   5.25 0.5736
WVFGRD96  104.0   -15    15  -105   5.26 0.5706
WVFGRD96  106.0   -15    15  -105   5.26 0.5717
WVFGRD96  108.0   -15    15  -105   5.26 0.5695
WVFGRD96  110.0   -20    15  -110   5.26 0.5668
WVFGRD96  112.0   185    75   -80   5.25 0.5651
WVFGRD96  114.0   185    75   -80   5.25 0.5607
WVFGRD96  116.0   185    75   -80   5.25 0.5573
WVFGRD96  118.0   185    75   -80   5.25 0.5531

The best solution is

WVFGRD96   96.0   185    75   -85   5.25 0.5751

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 -50 o DIST/3.3 +40
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 Sat Apr 27 01:26:37 PM CDT 2024