Location

Location ANSS

The ANSS event ID is ak011fra2x6x and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak011fra2x6x/executive.

2011/12/09 01:25:41 62.253 -145.650 16.1 4.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2011/12/09 01:25:41:0  62.25 -145.65  16.1 4.5 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CHUM AK.CNP 
   AK.COLD AK.DHY AK.DIV AK.EYAK AK.FID AK.FYU AK.GHO AK.HDA 
   AK.HIN AK.HMT AK.HOM AK.KAI AK.KLU AK.KNK AK.KTH AK.MCK 
   AK.MDM AK.MLY AK.PAX AK.PPLA AK.RAG AK.RC01 AK.RND AK.SAW 
   AK.SCM AK.SGA AK.SKN AK.SSN AK.SWD AK.TRF AK.WAX AT.MENT 
   AT.MID AT.PMR AT.SVW2 CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.07 n 3
 
 Best Fitting Double Couple
  Mo = 4.68e+22 dyne-cm
  Mw = 4.38 
  Z  = 19 km
  Plane   Strike  Dip  Rake
   NP1      324    69   112
   NP2       95    30    45
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.68e+22     60     266
    N   0.00e+00     21     136
    P  -4.68e+22     21      37

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.56e+22
       Mxy    -1.88e+22
       Mxz    -1.40e+22
       Myy    -3.09e+21
       Myz    -3.00e+22
       Mzz     2.86e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #####-----------------   ---           
             #########-------------- P ----          
           #############------------   ------        
          ################--------------------       
         ###################-------------------      
        ######################------------------     
        #######################-----------------     
       ##########################----------------    
       -###########   ############---------------    
       -########### T #############--------------    
       --##########   ##############------------#    
        --###########################----------#     
        ---###########################--------##     
         ----##########################-----###      
          -----########################---####       
           -------###########################        
             ----------############-----###          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.86e+22  -1.40e+22   3.00e+22 
 -1.40e+22  -2.56e+22   1.88e+22 
  3.00e+22   1.88e+22  -3.09e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111209012541/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 = 95
      DIP = 30
     RAKE = 45
       MW = 4.38
       HS = 19.0

The NDK file is 20111209012541.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:

hp c 0.02 n 3
lp c 0.07 n 3
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   310    45   -90   3.93 0.3030
WVFGRD96    1.0   305    45   -90   3.97 0.2868
WVFGRD96    2.0   310    40    90   4.08 0.3567
WVFGRD96    3.0   130    50    90   4.13 0.3085
WVFGRD96    4.0   130    75    45   4.10 0.2633
WVFGRD96    5.0    55    20   -10   4.16 0.3138
WVFGRD96    6.0    60    20     0   4.16 0.3702
WVFGRD96    7.0    65    20     5   4.17 0.4177
WVFGRD96    8.0    70    20    10   4.25 0.4542
WVFGRD96    9.0    85    20    30   4.27 0.5040
WVFGRD96   10.0    95    20    40   4.28 0.5483
WVFGRD96   11.0    95    25    40   4.30 0.5881
WVFGRD96   12.0   100    25    50   4.31 0.6242
WVFGRD96   13.0   100    25    50   4.32 0.6536
WVFGRD96   14.0   100    30    50   4.34 0.6778
WVFGRD96   15.0   100    30    50   4.35 0.6975
WVFGRD96   16.0   100    30    50   4.36 0.7119
WVFGRD96   17.0   100    30    50   4.37 0.7217
WVFGRD96   18.0   100    30    50   4.37 0.7276
WVFGRD96   19.0    95    30    45   4.38 0.7300
WVFGRD96   20.0    95    30    40   4.39 0.7298
WVFGRD96   21.0    95    30    40   4.41 0.7277
WVFGRD96   22.0    95    30    40   4.42 0.7228
WVFGRD96   23.0    90    30    35   4.42 0.7157
WVFGRD96   24.0    90    30    35   4.43 0.7070
WVFGRD96   25.0    90    30    35   4.44 0.6965
WVFGRD96   26.0    90    30    35   4.44 0.6844
WVFGRD96   27.0    90    30    35   4.45 0.6711
WVFGRD96   28.0    90    30    35   4.46 0.6563
WVFGRD96   29.0    90    30    35   4.46 0.6402

The best solution is

WVFGRD96   19.0    95    30    45   4.38 0.7300

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

hp c 0.02 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 06:30:36 PM CDT 2024