Location

2008/08/03 09:44:28 58.7840 -137.7270 10.0 4.10 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
 2008/08/03 09:44:28  58.7840  -137.7270  10.0  4.10 Alaska
 
 Best Fitting Double Couple
    Mo = 4.84e+22 dyne-cm
    Mw = 4.39 
    Z  = 15 km
     Plane   Strike  Dip  Rake
      NP1      115    65    60
      NP2      349    38   137
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   4.84e+22     59     342
     N   0.00e+00     27     129
     P  -4.84e+22     15     226



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.58e+21
       Mxy    -2.64e+22
       Mxz     2.88e+22
       Myy    -2.25e+22
       Myz     2.12e+21
       Mzz     3.21e+22
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ##############--------              
              ###################---------           
             ######################--------          
           ##########################--------        
          ##############   ###########--------       
         ############### T ############--------      
        --##############   ############---------     
        ----############################--------     
       ------############################--------    
       --------##########################--------    
       ----------########################--------    
       -------------#####################--------    
        ---------------##################-------     
        -------------------##############-------     
         -----------------------########-------      
          ---   ------------------------######       
           -- P -----------------------######        
                ----------------------#####          
              -----------------------#####           
                 ------------------####              
                     ------------##                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  3.21e+22   2.88e+22  -2.12e+21 
  2.88e+22  -9.58e+21   2.64e+22 
 -2.12e+21   2.64e+22  -2.25e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080803094428/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 = 115
      DIP = 65
     RAKE = 60
       MW = 4.39
       HS = 15.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 SLU Moment Tensor Solution
 2008/08/03 09:44:28  58.7840  -137.7270  10.0  4.10 Alaska
 
 Best Fitting Double Couple
    Mo = 4.84e+22 dyne-cm
    Mw = 4.39 
    Z  = 15 km
     Plane   Strike  Dip  Rake
      NP1      115    65    60
      NP2      349    38   137
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   4.84e+22     59     342
     N   0.00e+00     27     129
     P  -4.84e+22     15     226



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.58e+21
       Mxy    -2.64e+22
       Mxz     2.88e+22
       Myy    -2.25e+22
       Myz     2.12e+21
       Mzz     3.21e+22
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ##############--------              
              ###################---------           
             ######################--------          
           ##########################--------        
          ##############   ###########--------       
         ############### T ############--------      
        --##############   ############---------     
        ----############################--------     
       ------############################--------    
       --------##########################--------    
       ----------########################--------    
       -------------#####################--------    
        ---------------##################-------     
        -------------------##############-------     
         -----------------------########-------      
          ---   ------------------------######       
           -- P -----------------------######        
                ----------------------#####          
              -----------------------#####           
                 ------------------####              
                     ------------##                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  3.21e+22   2.88e+22  -2.12e+21 
  2.88e+22  -9.58e+21   2.64e+22 
 -2.12e+21   2.64e+22  -2.25e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080803094428/index.html
	

Moment tensor inversion summary for event 2008/08/03 09:44
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/08/03 09:44

Date 2008/08/03
Region: Southeastern Alaska
Mw=4.4

Centroid Location:
Time 09:44; Lat. 58.66N; Lon. 222.52W; Depth 15 km

Best Double Couple:
Plane 1: strike = 105; dip = 53; rake = 54
Plane 2: strike = 336; dip = 50; rake = 128

Moment Tensor:
Mo = 4.41826e+22 dyn-cm
Mxx = -213.932; Mxy = -263.874; Mxz = 135.043
Myy = -130.654; Myz = -128.710; Mzz = 344.586

Principal Axes:
T: value = 84.000; azimuth = 313; plunge = 62
N: value = 80.000; azimuth = 220; plunge = 2
P: value = 78.000; azimuth = 129; plunge = 28
	
        

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   190    25   -85   4.20 0.4988
WVFGRD96    1.0   190    20   -85   4.27 0.4931
WVFGRD96    2.0   230    15   -30   4.40 0.5194
WVFGRD96    3.0   240    20   -20   4.35 0.5480
WVFGRD96    4.0   240    25   -25   4.32 0.5705
WVFGRD96    5.0   240    25   -25   4.31 0.5848
WVFGRD96    6.0   245    25   -15   4.30 0.6002
WVFGRD96    7.0   290    15    90   4.34 0.6289
WVFGRD96    8.0   110    70    85   4.34 0.6607
WVFGRD96    9.0   110    65    80   4.35 0.6898
WVFGRD96   10.0   110    65    80   4.38 0.7109
WVFGRD96   11.0   115    65    65   4.40 0.7366
WVFGRD96   12.0   120    60    65   4.41 0.7588
WVFGRD96   13.0   115    65    60   4.40 0.7723
WVFGRD96   14.0   115    65    60   4.39 0.7798
WVFGRD96   15.0   115    65    60   4.39 0.7815
WVFGRD96   16.0   115    65    60   4.39 0.7788
WVFGRD96   17.0   115    65    55   4.39 0.7748
WVFGRD96   18.0   115    65    55   4.39 0.7679
WVFGRD96   19.0   115    65    55   4.39 0.7583
WVFGRD96   20.0   105    55    60   4.41 0.7569
WVFGRD96   21.0   100    60    55   4.41 0.7479
WVFGRD96   22.0   100    60    55   4.41 0.7380
WVFGRD96   23.0    95    60    45   4.41 0.7268
WVFGRD96   24.0    95    60    45   4.42 0.7173
WVFGRD96   25.0    95    60    45   4.42 0.7067
WVFGRD96   26.0    95    60    45   4.42 0.6952
WVFGRD96   27.0    95    60    45   4.42 0.6829
WVFGRD96   28.0    95    60    45   4.43 0.6699
WVFGRD96   29.0    95    60    40   4.44 0.6563

The best solution is

WVFGRD96   15.0   115    65    60   4.39 0.7815

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 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=Sun Aug 3 21:53:58 CDT 2008

Last Changed 2008/08/03