Location

Location ANSS

The ANSS event ID is ak010buy7ula and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak010buy7ula/executive.

2010/09/15 16:06:41 59.861 -153.176 121.0 5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/09/15 16:06:41:0  59.86 -153.18 121.0 5.0 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.FID AK.GLI AK.RC01 AK.SAW AK.SCM AK.SII 
   AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.23e+23 dyne-cm
  Mw = 4.66 
  Z  = 114 km
  Plane   Strike  Dip  Rake
   NP1       70    75    45
   NP2      325    47   159
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.23e+23     42     298
    N   0.00e+00     43      85
    P  -1.23e+23     17     192

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.24e+22
       Mxy    -5.04e+22
       Mxz     6.31e+22
       Myy     4.89e+22
       Myz    -4.69e+22
       Mzz     4.35e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###########-----------------           
             ################--------------          
           ####################--------------        
          ########################------------       
         #######   ################------------      
        ######## T ##################----------#     
        ########   ###################-------###     
       ################################---#######    
       ################################-#########    
       #############################----#########    
       ########################----------########    
        ##################---------------#######     
        ###########-----------------------######     
         ---------------------------------#####      
          --------------------------------####       
           ------------------------------####        
             ----------------------------##          
              ---------   --------------##           
                 ------ P -------------              
                     --   ---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.35e+22   6.31e+22   4.69e+22 
  6.31e+22  -9.24e+22   5.04e+22 
  4.69e+22   5.04e+22   4.89e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100915160641/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 = 70
      DIP = 75
     RAKE = 45
       MW = 4.66
       HS = 114.0

The NDK file is 20100915160641.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  2010/09/15 16:06:41:0  59.86 -153.18 121.0 5.0 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.FID AK.GLI AK.RC01 AK.SAW AK.SCM AK.SII 
   AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.23e+23 dyne-cm
  Mw = 4.66 
  Z  = 114 km
  Plane   Strike  Dip  Rake
   NP1       70    75    45
   NP2      325    47   159
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.23e+23     42     298
    N   0.00e+00     43      85
    P  -1.23e+23     17     192

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.24e+22
       Mxy    -5.04e+22
       Mxz     6.31e+22
       Myy     4.89e+22
       Myz    -4.69e+22
       Mzz     4.35e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###########-----------------           
             ################--------------          
           ####################--------------        
          ########################------------       
         #######   ################------------      
        ######## T ##################----------#     
        ########   ###################-------###     
       ################################---#######    
       ################################-#########    
       #############################----#########    
       ########################----------########    
        ##################---------------#######     
        ###########-----------------------######     
         ---------------------------------#####      
          --------------------------------####       
           ------------------------------####        
             ----------------------------##          
              ---------   --------------##           
                 ------ P -------------              
                     --   ---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.35e+22   6.31e+22   4.69e+22 
  6.31e+22  -9.24e+22   5.04e+22 
  4.69e+22   5.04e+22   4.89e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100915160641/index.html
	


        

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 0.07 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   340    80   -20   3.75 0.2091
WVFGRD96    4.0   165    85    10   3.83 0.2575
WVFGRD96    6.0   160    90    10   3.90 0.2731
WVFGRD96    8.0   340    85   -10   3.96 0.2936
WVFGRD96   10.0   340    70    -5   4.00 0.2977
WVFGRD96   12.0   340    80   -10   4.03 0.3011
WVFGRD96   14.0   340    75   -15   4.04 0.3031
WVFGRD96   16.0   340    75   -15   4.06 0.3046
WVFGRD96   18.0   340    80   -15   4.08 0.3029
WVFGRD96   20.0   340    80   -15   4.10 0.2980
WVFGRD96   22.0    80    75    20   4.12 0.3018
WVFGRD96   24.0    80    75    20   4.14 0.3110
WVFGRD96   26.0    80    75    20   4.16 0.3188
WVFGRD96   28.0    80    75    20   4.18 0.3246
WVFGRD96   30.0    80    75    20   4.19 0.3297
WVFGRD96   32.0    80    75    20   4.21 0.3328
WVFGRD96   34.0    80    75    15   4.23 0.3347
WVFGRD96   36.0    80    75    15   4.25 0.3364
WVFGRD96   38.0    75    80    15   4.30 0.3390
WVFGRD96   40.0    80    75    25   4.36 0.3424
WVFGRD96   42.0    80    75    25   4.38 0.3436
WVFGRD96   44.0    75    80    20   4.40 0.3451
WVFGRD96   46.0    75    80    20   4.42 0.3477
WVFGRD96   48.0    75    80    20   4.44 0.3497
WVFGRD96   50.0    75    80    20   4.45 0.3525
WVFGRD96   52.0    75    75    10   4.46 0.3618
WVFGRD96   54.0    75    75    10   4.48 0.3722
WVFGRD96   56.0    75    75    15   4.49 0.3819
WVFGRD96   58.0    75    75    15   4.50 0.3924
WVFGRD96   60.0    75    75    15   4.52 0.4024
WVFGRD96   62.0    75    75    15   4.53 0.4121
WVFGRD96   64.0    75    75    15   4.54 0.4244
WVFGRD96   66.0    75    75    20   4.55 0.4377
WVFGRD96   68.0    75    75    20   4.56 0.4482
WVFGRD96   70.0    70    80    25   4.59 0.4611
WVFGRD96   72.0    70    80    25   4.59 0.4721
WVFGRD96   74.0    70    80    25   4.60 0.4830
WVFGRD96   76.0    70    80    30   4.61 0.4930
WVFGRD96   78.0    70    80    30   4.61 0.5026
WVFGRD96   80.0    70    80    30   4.62 0.5115
WVFGRD96   82.0    70    80    35   4.62 0.5190
WVFGRD96   84.0    70    80    35   4.63 0.5265
WVFGRD96   86.0    70    80    35   4.63 0.5338
WVFGRD96   88.0    70    80    35   4.63 0.5396
WVFGRD96   90.0    70    80    35   4.63 0.5442
WVFGRD96   92.0    70    80    40   4.64 0.5482
WVFGRD96   94.0    70    75    35   4.65 0.5518
WVFGRD96   96.0    70    75    35   4.65 0.5559
WVFGRD96   98.0    70    75    40   4.65 0.5600
WVFGRD96  100.0    70    75    40   4.65 0.5632
WVFGRD96  102.0    70    75    40   4.65 0.5661
WVFGRD96  104.0    70    75    40   4.65 0.5675
WVFGRD96  106.0    70    75    40   4.65 0.5693
WVFGRD96  108.0    70    75    45   4.66 0.5703
WVFGRD96  110.0    70    75    45   4.66 0.5711
WVFGRD96  112.0    70    75    45   4.66 0.5716
WVFGRD96  114.0    70    75    45   4.66 0.5717
WVFGRD96  116.0    70    75    45   4.66 0.5713
WVFGRD96  118.0    70    75    45   4.66 0.5704
WVFGRD96  120.0    70    75    50   4.66 0.5698
WVFGRD96  122.0    70    75    50   4.66 0.5688
WVFGRD96  124.0    70    75    50   4.66 0.5678
WVFGRD96  126.0    70    75    50   4.66 0.5665
WVFGRD96  128.0    70    75    50   4.66 0.5654

The best solution is

WVFGRD96  114.0    70    75    45   4.66 0.5717

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 0.07 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 01:33:04 PM CDT 2024