Location

2010/07/08 03:15:49 61.850 -150.473 20 5.00 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/07/08 03:15:49:0  61.85 -150.47  20.0 5.0 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CHUM AK.CNP AK.COLD 
   AK.DHY AK.DIV AK.EYAK AK.FYU AK.HDA AK.MCK AK.MDM AK.MLY 
   AK.PAX AK.RAG AK.RC01 AK.RND AK.SCM AK.SKN AK.SSN AK.TRF 
   AK.WRH IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.32e+23 dyne-cm
  Mw = 4.68 
  Z  = 22 km
  Plane   Strike  Dip  Rake
   NP1      210    75    70
   NP2       85    25   142
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.32e+23     56      95
    N   0.00e+00     19     215
    P  -1.32e+23     27     316

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.32e+22
       Mxy     4.86e+22
       Mxz    -4.35e+22
       Myy    -8.74e+21
       Myz     9.87e+22
       Mzz     6.19e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ---------------------#######           
             --------------------##########          
           ----   --------------#############        
          ----- P ------------################       
         ------   -----------##################      
        --------------------####################     
        ------------------######################     
       ------------------########################    
       -----------------###########   ###########    
       ----------------############ T ##########-    
       #--------------#############   #########--    
        -------------##########################-     
        ##-----------########################---     
         ##---------########################---      
          ###------#######################----       
           ####----#####################-----        
             ########################------          
              ####-------#####------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.19e+22  -4.35e+22  -9.87e+22 
 -4.35e+22  -5.32e+22  -4.86e+22 
 -9.87e+22  -4.86e+22  -8.74e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100708031549/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 = 75
     RAKE = 70
       MW = 4.68
       HS = 22.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/07/08 03:15:49:0  61.85 -150.47  20.0 5.0 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CHUM AK.CNP AK.COLD 
   AK.DHY AK.DIV AK.EYAK AK.FYU AK.HDA AK.MCK AK.MDM AK.MLY 
   AK.PAX AK.RAG AK.RC01 AK.RND AK.SCM AK.SKN AK.SSN AK.TRF 
   AK.WRH IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.32e+23 dyne-cm
  Mw = 4.68 
  Z  = 22 km
  Plane   Strike  Dip  Rake
   NP1      210    75    70
   NP2       85    25   142
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.32e+23     56      95
    N   0.00e+00     19     215
    P  -1.32e+23     27     316

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.32e+22
       Mxy     4.86e+22
       Mxz    -4.35e+22
       Myy    -8.74e+21
       Myz     9.87e+22
       Mzz     6.19e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ---------------------#######           
             --------------------##########          
           ----   --------------#############        
          ----- P ------------################       
         ------   -----------##################      
        --------------------####################     
        ------------------######################     
       ------------------########################    
       -----------------###########   ###########    
       ----------------############ T ##########-    
       #--------------#############   #########--    
        -------------##########################-     
        ##-----------########################---     
         ##---------########################---      
          ###------#######################----       
           ####----#####################-----        
             ########################------          
              ####-------#####------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.19e+22  -4.35e+22  -9.87e+22 
 -4.35e+22  -5.32e+22  -4.86e+22 
 -9.87e+22  -4.86e+22  -8.74e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100708031549/index.html
	

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 0.06 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   215    45    90   4.28 0.3340
WVFGRD96    1.0   215    45    90   4.32 0.3321
WVFGRD96    2.0   215    45    90   4.42 0.4181
WVFGRD96    3.0   215    45    85   4.47 0.3875
WVFGRD96    4.0    35    55    80   4.46 0.3023
WVFGRD96    5.0   320    20    15   4.46 0.2894
WVFGRD96    6.0   325    15    20   4.47 0.3328
WVFGRD96    7.0   210    85    75   4.48 0.3728
WVFGRD96    8.0   210    80    80   4.57 0.4108
WVFGRD96    9.0   210    80    80   4.58 0.4585
WVFGRD96   10.0   210    80    75   4.58 0.5020
WVFGRD96   11.0   210    80    75   4.59 0.5391
WVFGRD96   12.0   210    75    75   4.61 0.5730
WVFGRD96   13.0   210    75    75   4.61 0.6023
WVFGRD96   14.0   210    75    75   4.62 0.6272
WVFGRD96   15.0   210    75    70   4.63 0.6483
WVFGRD96   16.0   210    75    70   4.64 0.6655
WVFGRD96   17.0   210    75    70   4.64 0.6793
WVFGRD96   18.0   210    75    70   4.65 0.6900
WVFGRD96   19.0   210    75    70   4.66 0.6979
WVFGRD96   20.0   210    75    70   4.66 0.7029
WVFGRD96   21.0   210    75    70   4.68 0.7057
WVFGRD96   22.0   210    75    70   4.68 0.7060
WVFGRD96   23.0   210    75    75   4.69 0.7057
WVFGRD96   24.0   210    80    70   4.69 0.7036
WVFGRD96   25.0   210    80    70   4.70 0.6996
WVFGRD96   26.0   210    80    75   4.70 0.6949
WVFGRD96   27.0   210    80    75   4.71 0.6889
WVFGRD96   28.0   210    80    75   4.71 0.6825
WVFGRD96   29.0   210    80    75   4.72 0.6742

The best solution is

WVFGRD96   22.0   210    75    70   4.68 0.7060

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 0.06 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 WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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    

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=Thu Jul 8 05:51:36 MDT 2010

Last Changed 2010/07/08