Location

2008/11/15 14:23:34 66.3420 -156.6280 14.0 4.60 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
 2008/11/15 14:23:34  66.3420  -156.6280  14.0  4.60 Alaska
 
 Best Fitting Double Couple
    Mo = 1.04e+23 dyne-cm
    Mw = 4.61 
    Z  = 17 km
     Plane   Strike  Dip  Rake
      NP1      350    85   -20
      NP2       82    70   -175
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.04e+23     10      37
     N   0.00e+00     69     157
     P  -1.04e+23     18     304



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     3.33e+22
       Mxy     9.21e+22
       Mxz    -2.29e+21
       Myy    -2.72e+22
       Myz     3.58e+22
       Mzz    -6.15e+21
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 --------############                
              ------------########### T ##           
             -------------###########   ###          
           -   ------------##################        
          -- P ------------###################       
         ---   -------------###################      
        --------------------####################     
        --------------------####################     
       ----------------------###################-    
       ----------------------#################---    
       ----------------------##############------    
       -----------------------########-----------    
        ####------------------#-----------------     
        ######################------------------     
         ######################----------------      
          #####################---------------       
           ####################--------------        
             ##################------------          
              #################-----------           
                 ###############-------              
                     ###########---                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -6.15e+21  -2.29e+21  -3.58e+22 
 -2.29e+21   3.33e+22  -9.21e+22 
 -3.58e+22  -9.21e+22  -2.72e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/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 = 350
      DIP = 85
     RAKE = -20
       MW = 4.61
       HS = 17.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 SLU Moment Tensor Solution
 2008/11/15 14:23:34  66.3420  -156.6280  14.0  4.60 Alaska
 
 Best Fitting Double Couple
    Mo = 1.04e+23 dyne-cm
    Mw = 4.61 
    Z  = 17 km
     Plane   Strike  Dip  Rake
      NP1      350    85   -20
      NP2       82    70   -175
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.04e+23     10      37
     N   0.00e+00     69     157
     P  -1.04e+23     18     304



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     3.33e+22
       Mxy     9.21e+22
       Mxz    -2.29e+21
       Myy    -2.72e+22
       Myz     3.58e+22
       Mzz    -6.15e+21
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 --------############                
              ------------########### T ##           
             -------------###########   ###          
           -   ------------##################        
          -- P ------------###################       
         ---   -------------###################      
        --------------------####################     
        --------------------####################     
       ----------------------###################-    
       ----------------------#################---    
       ----------------------##############------    
       -----------------------########-----------    
        ####------------------#-----------------     
        ######################------------------     
         ######################----------------      
          #####################---------------       
           ####################--------------        
             ##################------------          
              #################-----------           
                 ###############-------              
                     ###########---                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -6.15e+21  -2.29e+21  -3.58e+22 
 -2.29e+21   3.33e+22  -9.21e+22 
 -3.58e+22  -9.21e+22  -2.72e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081115142334/index.html
	
Moment tensor inversion summary for event 2008/11/15 14:23
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/11/15 14:23

Date 2008/11/15
Region: North-central Region of Alaska
Mw=4.7

Centroid Location:
Time 14:23; Lat. 66.29N; Lon. 203.09W; Depth 20 km

Best Double Couple:
Plane 1: strike = 155; dip = 58; rake = -42
Plane 2: strike = 270; dip = 56; rake = -140

Moment Tensor:
Mo = 1.23456e+23 dyn-cm
Mxx = 835.143; Mxy = 967.439; Mxz = 107.083
Myy = -52.712; Myz = -262.199; Mzz = -782.432

Principal Axes:
T: value = 84.000; azimuth = 213; plunge = 1
N: value = 80.000; azimuth = 121; plunge = 51
P: value = 78.000; azimuth = 304; plunge = 39

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   165    65   -25   4.29 0.4312
WVFGRD96    1.0   165    70   -30   4.32 0.4472
WVFGRD96    2.0   165    70   -30   4.39 0.5197
WVFGRD96    3.0   165    70   -30   4.42 0.5430
WVFGRD96    4.0   165    75   -30   4.43 0.5624
WVFGRD96    5.0   170    85   -25   4.44 0.5851
WVFGRD96    6.0   170    85   -25   4.46 0.6080
WVFGRD96    7.0   170    85   -20   4.48 0.6307
WVFGRD96    8.0   170    85   -25   4.51 0.6524
WVFGRD96    9.0   170    85   -25   4.53 0.6688
WVFGRD96   10.0   170    85   -20   4.54 0.6818
WVFGRD96   11.0   170    85   -20   4.55 0.6910
WVFGRD96   12.0   350    90    20   4.56 0.6939
WVFGRD96   13.0   350    90    20   4.57 0.6934
WVFGRD96   14.0   350    80   -20   4.58 0.6932
WVFGRD96   15.0   350    80   -20   4.59 0.6991
WVFGRD96   16.0   350    80   -20   4.60 0.7026
WVFGRD96   17.0   350    85   -20   4.61 0.7045
WVFGRD96   18.0   170    90    20   4.61 0.7026
WVFGRD96   19.0   170    90    15   4.63 0.7013
WVFGRD96   20.0   170    90    15   4.64 0.7001
WVFGRD96   21.0   170    90    15   4.64 0.6965
WVFGRD96   22.0   350    90   -15   4.65 0.6916
WVFGRD96   23.0   350    90   -15   4.66 0.6850
WVFGRD96   24.0   170    85    15   4.66 0.6774
WVFGRD96   25.0   350    90   -15   4.67 0.6677
WVFGRD96   26.0   350    90   -15   4.68 0.6577
WVFGRD96   27.0   170    85    10   4.69 0.6481
WVFGRD96   28.0   350    90   -10   4.69 0.6369
WVFGRD96   29.0   170    85    10   4.70 0.6255

The best solution is

WVFGRD96   17.0   350    85   -20   4.61 0.7045

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=Sat Nov 15 12:26:36 CST 2008

Last Changed 2008/11/15