Location

2008/10/28 14:30:12 66.3810 -157.7640 5.0 4.70 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
 2008/10/28 14:30:12  66.3810  -157.7640  5.0  4.70 Alaska
 
 Best Fitting Double Couple
    Mo = 1.46e+23 dyne-cm
    Mw = 4.71 
    Z  = 16 km
     Plane   Strike  Dip  Rake
      NP1      345    75   -25
      NP2       82    66   -164
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.46e+23      6      35
     N   0.00e+00     61     136
     P  -1.46e+23     28     302



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     6.61e+22
       Mxy     1.19e+23
       Mxz    -1.93e+22
       Myy    -3.52e+22
       Myz     6.06e+22
       Mzz    -3.09e+22
                                                     
                                                     
                                                     
                                                     
                     --############                  
                 --------#############               
              ------------############ T #           
             --------------###########   ##          
           -----------------#################        
          ----   ------------#################       
         ----- P ------------##################      
        ------   -------------##################     
        -----------------------#################     
       ------------------------#################-    
       -------------------------###############--    
       -------------------------############-----    
       --------------------------########--------    
        ###----------------------###------------     
        ############-----########---------------     
         #########################-------------      
          ########################------------       
           #######################-----------        
             #####################---------          
              ####################--------           
                 #################-----              
                     #############-                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -3.09e+22  -1.93e+22  -6.06e+22 
 -1.93e+22   6.61e+22  -1.19e+23 
 -6.06e+22  -1.19e+23  -3.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/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 = 345
      DIP = 75
     RAKE = -25
       MW = 4.71
       HS = 16.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
        
SLU
AEIC
 SLU Moment Tensor Solution
 2008/10/28 14:30:12  66.3810  -157.7640  5.0  4.70 Alaska
 
 Best Fitting Double Couple
    Mo = 1.46e+23 dyne-cm
    Mw = 4.71 
    Z  = 16 km
     Plane   Strike  Dip  Rake
      NP1      345    75   -25
      NP2       82    66   -164
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.46e+23      6      35
     N   0.00e+00     61     136
     P  -1.46e+23     28     302



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     6.61e+22
       Mxy     1.19e+23
       Mxz    -1.93e+22
       Myy    -3.52e+22
       Myz     6.06e+22
       Mzz    -3.09e+22
                                                     
                                                     
                                                     
                                                     
                     --############                  
                 --------#############               
              ------------############ T #           
             --------------###########   ##          
           -----------------#################        
          ----   ------------#################       
         ----- P ------------##################      
        ------   -------------##################     
        -----------------------#################     
       ------------------------#################-    
       -------------------------###############--    
       -------------------------############-----    
       --------------------------########--------    
        ###----------------------###------------     
        ############-----########---------------     
         #########################-------------      
          ########################------------       
           #######################-----------        
             #####################---------          
              ####################--------           
                 #################-----              
                     #############-                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -3.09e+22  -1.93e+22  -6.06e+22 
 -1.93e+22   6.61e+22  -1.19e+23 
 -6.06e+22  -1.19e+23  -3.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081028143012/index.html
	

Moment tensor inversion summary for event 2008/10/28 14:30

Date: 2008/10/28
Time: 14:30 (UTC)
Region: North-central Region of Alaska
Mw=4.8

Location:

Lat.  66.4123;  Lon.  -157.7271; Depth    5 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 8

Best Double Couple:

         strike    dip    rake 
Plane 1:  265.5   74.1  -135.6
Plane 2:  160.6   47.7   -21.7

Moment Tensor Parameters:

Mo = 1.96293e+23 dyn-cm
Mxx =  0.99; Mxy =  1.30; Mxz =  1.11
Myy = -0.24; Myz = -0.37; Mzz = -0.75


Principal Axes:

     value   azimuth   plunge
T:    2.03    27.33   16.42
N:   -0.14   281.13   43.42
P:   -1.89   132.71   41.99



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   160    70   -40   4.65 0.5142
WVFGRD96    1.0   160    70   -40   4.65 0.5199
WVFGRD96    2.0   165    70   -30   4.63 0.5243
WVFGRD96    3.0   330    35   -40   4.75 0.5301
WVFGRD96    4.0   335    45   -40   4.71 0.5318
WVFGRD96    5.0   335    65   -45   4.68 0.5493
WVFGRD96    6.0   335    65   -40   4.67 0.5640
WVFGRD96    7.0   335    65   -40   4.67 0.5749
WVFGRD96    8.0   340    65   -35   4.67 0.5845
WVFGRD96    9.0   340    65   -35   4.67 0.5922
WVFGRD96   10.0   340    65   -35   4.69 0.6004
WVFGRD96   11.0   340    65   -35   4.69 0.6071
WVFGRD96   12.0   340    65   -30   4.69 0.6111
WVFGRD96   13.0   340    70   -30   4.69 0.6152
WVFGRD96   14.0   345    70   -25   4.70 0.6175
WVFGRD96   15.0   345    75   -25   4.70 0.6190
WVFGRD96   16.0   345    75   -25   4.71 0.6200
WVFGRD96   17.0   345    75   -25   4.71 0.6193
WVFGRD96   18.0   345    75   -25   4.72 0.6175
WVFGRD96   19.0   345    75   -20   4.73 0.6150
WVFGRD96   20.0   345    75   -25   4.74 0.6130
WVFGRD96   21.0   345    75   -25   4.75 0.6094
WVFGRD96   22.0   345    75   -25   4.75 0.6049
WVFGRD96   23.0   345    75   -20   4.76 0.6004
WVFGRD96   24.0   345    75   -20   4.77 0.5954
WVFGRD96   25.0   345    75   -20   4.77 0.5896
WVFGRD96   26.0   345    75   -20   4.78 0.5832
WVFGRD96   27.0   345    75   -20   4.78 0.5762
WVFGRD96   28.0   345    80   -20   4.79 0.5694
WVFGRD96   29.0   345    80   -20   4.80 0.5622
WVFGRD96   30.0   345    80   -20   4.80 0.5546
WVFGRD96   31.0   170    85    20   4.81 0.5449
WVFGRD96   32.0   170    85    20   4.82 0.5379
WVFGRD96   33.0   170    85    15   4.84 0.5315
WVFGRD96   34.0   170    85    15   4.85 0.5254
WVFGRD96   35.0   170    85    15   4.86 0.5192
WVFGRD96   36.0   345    85   -15   4.86 0.5116
WVFGRD96   37.0   170    85    15   4.88 0.5057
WVFGRD96   38.0   350    85   -10   4.90 0.4966
WVFGRD96   39.0   170    85    15   4.91 0.4888
WVFGRD96   40.0   170    80    20   4.94 0.4797
WVFGRD96   41.0   170    80    20   4.95 0.4746
WVFGRD96   42.0   170    80    20   4.96 0.4687
WVFGRD96   43.0   170    80    20   4.96 0.4623
WVFGRD96   44.0   170    80    20   4.97 0.4553
WVFGRD96   45.0   170    80    20   4.98 0.4481
WVFGRD96   46.0   170    80    20   4.98 0.4405
WVFGRD96   47.0   170    80    15   4.99 0.4330
WVFGRD96   48.0   170    80    15   5.00 0.4254
WVFGRD96   49.0   170    80    15   5.00 0.4175
WVFGRD96   50.0   170    80    15   5.01 0.4093

The best solution is

WVFGRD96   16.0   345    75   -25   4.71 0.6200

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=Mon Nov 3 17:53:06 CST 2008

Last Changed 2008/10/28