Location

2009/04/30 04:54:57 58.9830 -151.2980 50.0 4.80 KODIAK ISLAND REGION, 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  2009/04/30 04:54:57:0  58.98 -151.30  50.0 4.8 KODIAK ISLAND REGION, ALASKA
 
 Stations used:
   AK.BMR AK.BRLK AK.DIV AK.EYAK AK.MCK AK.PAX AK.RAG AK.RC01 
   AK.SWD AK.TRF AT.MID AT.OHAK AT.PMR AT.SVW2 
 
 Filtering commands used:
   hp c 0.0200 n 3
   lp c 0.050 n 3
 
 Best Fitting Double Couple
  Mo = 2.82e+23 dyne-cm
  Mw = 4.90 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      184    66   -97
   NP2       20    25   -75
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.82e+23     21     279
    N   0.00e+00      6     186
    P  -2.82e+23     68      80

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.58e+21
       Mxy    -4.34e+22
       Mxz    -2.27e+21
       Myy     2.04e+23
       Myz    -1.87e+23
       Mzz    -2.09e+23
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 ##########-----------#              
              ############-------------###           
             ############----------------##          
           #############------------------###        
          ##############-------------------###       
         ##############--------------------####      
        ###############---------------------####     
        ##   #########----------------------####     
       ### T #########----------   ---------#####    
       ###   #########---------- P ---------#####    
       ###############----------   ---------#####    
       ###############----------------------#####    
        ##############---------------------#####     
        ##############--------------------######     
         #############--------------------#####      
          ############------------------######       
           ############----------------######        
             ##########--------------######          
              ##########-----------#######           
                 ########-------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.09e+23  -2.27e+21   1.87e+23 
 -2.27e+21   4.58e+21   4.34e+22 
  1.87e+23   4.34e+22   2.04e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090430045457/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 = 20
      DIP = 25
     RAKE = -75
       MW = 4.90
       HS = 45.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 
 USGS/SLU Moment Tensor Solution
 ENS  2009/04/30 04:54:57:0  58.98 -151.30  50.0 4.8 KODIAK ISLAND REGION, ALASKA
 
 Stations used:
   AK.BMR AK.BRLK AK.DIV AK.EYAK AK.MCK AK.PAX AK.RAG AK.RC01 
   AK.SWD AK.TRF AT.MID AT.OHAK AT.PMR AT.SVW2 
 
 Filtering commands used:
   hp c 0.0200 n 3
   lp c 0.050 n 3
 
 Best Fitting Double Couple
  Mo = 2.82e+23 dyne-cm
  Mw = 4.90 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      184    66   -97
   NP2       20    25   -75
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.82e+23     21     279
    N   0.00e+00      6     186
    P  -2.82e+23     68      80

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.58e+21
       Mxy    -4.34e+22
       Mxz    -2.27e+21
       Myy     2.04e+23
       Myz    -1.87e+23
       Mzz    -2.09e+23
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 ##########-----------#              
              ############-------------###           
             ############----------------##          
           #############------------------###        
          ##############-------------------###       
         ##############--------------------####      
        ###############---------------------####     
        ##   #########----------------------####     
       ### T #########----------   ---------#####    
       ###   #########---------- P ---------#####    
       ###############----------   ---------#####    
       ###############----------------------#####    
        ##############---------------------#####     
        ##############--------------------######     
         #############--------------------#####      
          ############------------------######       
           ############----------------######        
             ##########--------------######          
              ##########-----------#######           
                 ########-------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.09e+23  -2.27e+21   1.87e+23 
 -2.27e+21   4.58e+21   4.34e+22 
  1.87e+23   4.34e+22   2.04e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090430045457/index.html
	


Moment tensor inversion summary for event 2009/04/30 04:54

Date: 2009/04/30
Time: 04:54 (UTC)
Region: Kodiak Island Region of Alaska
Mw=4.9

Location:

Lat.  58.9833;  Lon.  -151.2986; Depth   35 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 5

Best Double Couple:

         strike    dip    rake 
Plane 1:  194.5   61.2   -91.4
Plane 2:   17.5   28.8   -87.4

Moment Tensor Parameters:

Mo = 2.76043e+23 dyn-cm
Mxx =  0.40; Mxy = -0.49; Mxz =  0.31
Myy =  1.95; Myz = -1.37; Mzz = -2.35


Principal Axes:

     value   azimuth   plunge
T:    2.51   285.56   16.17
N:    0.26   195.19    1.26
P:   -2.76   100.85   73.77

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 0.0200 n 3
lp c 0.050 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   30.0     0    20   -95   4.65 0.4793
WVFGRD96   31.0     0    20   -95   4.66 0.4873
WVFGRD96   32.0     5    20   -90   4.67 0.4947
WVFGRD96   33.0     5    20   -90   4.67 0.5015
WVFGRD96   34.0    10    20   -85   4.68 0.5073
WVFGRD96   35.0     5    25   -90   4.68 0.5129
WVFGRD96   36.0    10    25   -85   4.69 0.5180
WVFGRD96   37.0    10    25   -85   4.70 0.5226
WVFGRD96   38.0    15    25   -85   4.72 0.5266
WVFGRD96   39.0    20    25   -80   4.73 0.5305
WVFGRD96   40.0    20    25   -75   4.86 0.5285
WVFGRD96   41.0    20    25   -75   4.87 0.5331
WVFGRD96   42.0    20    25   -75   4.88 0.5365
WVFGRD96   43.0    20    25   -75   4.89 0.5389
WVFGRD96   44.0    20    25   -75   4.89 0.5404
WVFGRD96   45.0    20    25   -75   4.90 0.5409
WVFGRD96   46.0    20    25   -75   4.91 0.5407
WVFGRD96   47.0    20    25   -75   4.91 0.5396
WVFGRD96   48.0    20    25   -75   4.92 0.5377
WVFGRD96   49.0    25    30   -70   4.93 0.5355
WVFGRD96   50.0    25    30   -70   4.93 0.5326
WVFGRD96   51.0    25    30   -70   4.94 0.5290
WVFGRD96   52.0    25    30   -70   4.95 0.5246
WVFGRD96   53.0    25    30   -65   4.93 0.5200
WVFGRD96   54.0    25    30   -65   4.94 0.5148
WVFGRD96   55.0    30    30   -65   4.96 0.5092
WVFGRD96   56.0    30    30   -65   4.97 0.5031
WVFGRD96   57.0    30    30   -65   4.97 0.4966
WVFGRD96   58.0    30    30   -60   4.96 0.4898
WVFGRD96   59.0    30    30   -60   4.96 0.4827

The best solution is 

WVFGRD96   45.0    20    25   -75   4.90 0.5409

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 0.0200 n 3
lp c 0.050 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=Fri May 1 10:07:33 CDT 2009

Last Changed 2009/04/30