Location

2008/09/24 12:19:52 63.411 -150.068 7.0 4.20 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for

Focal Mechanism

 SLU Moment Tensor Solution
 2008/09/24 12:19:52  63.411  -150.068  7.0  4.20 Alaska 
 
 Best Fitting Double Couple
    Mo = 1.97e+22 dyne-cm
    Mw = 4.13 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      210    55    45
      NP2       90    55   135
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.97e+22     55      60
     N   0.00e+00     35     240
     P  -1.97e+22      0     330



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.32e+22
       Mxy     1.14e+22
       Mxz     4.54e+21
       Myy     6.47e+19
       Myz     8.13e+21
       Mzz     1.31e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 P -----------------###              
              --   ------------###########           
             ----------------##############          
           ---------------###################        
          ---------------#####################       
         --------------########################      
        --------------############   ###########     
        ------------############## T ###########     
       ------------###############   ############    
       ------------##############################    
       #----------##############################-    
       ###-------#############################---    
        ####-----###########################----     
        ################################--------     
         #######------############-------------      
          ######------------------------------       
           #####-----------------------------        
             ###---------------------------          
              ###-------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  1.31e+22   4.54e+21  -8.13e+21 
  4.54e+21  -1.32e+22  -1.14e+22 
 -8.13e+21  -1.14e+22   6.47e+19 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/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 = 210
      DIP = 55
     RAKE = 45
       MW = 4.13
       HS = 9.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 SLU Moment Tensor Solution
 2008/09/24 12:19:52  63.411  -150.068  7.0  4.20 Alaska 
 
 Best Fitting Double Couple
    Mo = 1.97e+22 dyne-cm
    Mw = 4.13 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      210    55    45
      NP2       90    55   135
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.97e+22     55      60
     N   0.00e+00     35     240
     P  -1.97e+22      0     330



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.32e+22
       Mxy     1.14e+22
       Mxz     4.54e+21
       Myy     6.47e+19
       Myz     8.13e+21
       Mzz     1.31e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 P -----------------###              
              --   ------------###########           
             ----------------##############          
           ---------------###################        
          ---------------#####################       
         --------------########################      
        --------------############   ###########     
        ------------############## T ###########     
       ------------###############   ############    
       ------------##############################    
       #----------##############################-    
       ###-------#############################---    
        ####-----###########################----     
        ################################--------     
         #######------############-------------      
          ######------------------------------       
           #####-----------------------------        
             ###---------------------------          
              ###-------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  1.31e+22   4.54e+21  -8.13e+21 
  4.54e+21  -1.32e+22  -1.14e+22 
 -8.13e+21  -1.14e+22   6.47e+19 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/index.html
	

Moment tensor inversion summary for event 2008/09/24 12:19 This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/09/24 12:19 Date 2008/09/24 Region: Central Region of Alaska Mw=4.1 Centroid Location: Time 12:19; Lat. 63.46N; Lon. 209.83W; Depth 10 km Best Double Couple: Plane 1: strike = 211; dip = 69; rake = 40 Plane 2: strike = 104; dip = 53; rake = 154 Moment Tensor: Mo = 1.42074e+22 dyn-cm Mxx = -109.997; Mxy = 83.630; Mxz = -12.066 Myy = 43.179; Myz = 64.976; Mzz = 66.818 Principal Axes: T: value = 80.000; azimuth = 334; plunge = 10 N: value = 84.000; azimuth = 73; plunge = 43 P: value = 78.000; azimuth = 233; plunge = 46

        

Waveform Inversion

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:

hp c 0.02 n 3
lp c 0.05 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    0.5   185    35   -15   4.02 0.4421
WVFGRD96    1.0   185    30   -15   4.07 0.4476
WVFGRD96    2.0   190    35   -15   4.07 0.4494
WVFGRD96    3.0   195    35   -10   4.08 0.4550
WVFGRD96    4.0   200    40     5   4.07 0.4714
WVFGRD96    5.0   205    50    25   4.08 0.5027
WVFGRD96    6.0   205    55    35   4.10 0.5451
WVFGRD96    7.0   205    55    35   4.11 0.5766
WVFGRD96    8.0   210    55    40   4.12 0.5908
WVFGRD96    9.0   210    55    45   4.13 0.5953
WVFGRD96   10.0   210    55    40   4.14 0.5938
WVFGRD96   11.0   210    55    35   4.13 0.5839
WVFGRD96   12.0   205    60    35   4.13 0.5710
WVFGRD96   13.0   205    60    35   4.12 0.5559
WVFGRD96   14.0   205    60    35   4.12 0.5384
WVFGRD96   15.0   205    60    30   4.11 0.5192
WVFGRD96   16.0   205    60    30   4.11 0.5017
WVFGRD96   17.0   205    60    30   4.11 0.4844
WVFGRD96   18.0   200    70    40   4.11 0.4736
WVFGRD96   19.0   200    70    40   4.11 0.4659
WVFGRD96   20.0   200    70    40   4.13 0.4552
WVFGRD96   21.0   200    70    40   4.13 0.4462
WVFGRD96   22.0   200    70    40   4.14 0.4363
WVFGRD96   23.0   200    75    40   4.14 0.4264
WVFGRD96   24.0   200    75    40   4.14 0.4169
WVFGRD96   25.0   200    75    40   4.14 0.4071
WVFGRD96   26.0   195    80    40   4.14 0.3971
WVFGRD96   27.0   195    80    40   4.14 0.3881
WVFGRD96   28.0   195    80    40   4.15 0.3788
WVFGRD96   29.0   195    80    35   4.15 0.3702
WVFGRD96   30.0   195    80    35   4.15 0.3616
WVFGRD96   31.0   195    85    35   4.16 0.3536
WVFGRD96   32.0   195    85    35   4.16 0.3459
WVFGRD96   33.0   195    85    35   4.17 0.3379
WVFGRD96   34.0   195    85    35   4.17 0.3300
WVFGRD96   35.0    15    90   -35   4.17 0.3185
WVFGRD96   36.0    10    90   -30   4.18 0.3117
WVFGRD96   37.0   195    85    30   4.19 0.3079
WVFGRD96   38.0   195    80    40   4.17 0.3016
WVFGRD96   39.0   195    80    40   4.17 0.2960
WVFGRD96   40.0   195    80    45   4.26 0.2890
WVFGRD96   41.0    95    60   -20   4.26 0.2889
WVFGRD96   42.0    95    60   -20   4.27 0.2876
WVFGRD96   43.0    95    60   -15   4.27 0.2863
WVFGRD96   44.0    95    60   -15   4.28 0.2847
WVFGRD96   45.0    95    60   -15   4.28 0.2834
WVFGRD96   46.0    95    65   -15   4.28 0.2819
WVFGRD96   47.0    95    65   -15   4.29 0.2811
WVFGRD96   48.0    95    65   -15   4.29 0.2803
WVFGRD96   49.0    95    65   -15   4.30 0.2786
WVFGRD96   50.0    95    65   -15   4.30 0.2783

The best solution is

WVFGRD96    9.0   210    55    45   4.13 0.5953

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.05 n 3
Figure 3. Waveform comparison for selected depth
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.

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

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

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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:

DATE=Wed Sep 24 11:35:04 CDT 2008

Last Changed 2008/09/24