Location

2009/04/07 20:12:55 61.4690 -149.7580 25.0 4.60 Southern 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  2009/04/07 20:12:55:0  61.47 -149.76  25.0 4.6 Southern Alaska
 
 Stations used:
   AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 8.71e+22 dyne-cm
  Mw = 4.56 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      216    55   -87
   NP2       30    35   -95
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.71e+22     10     304
    N   0.00e+00      3      34
    P  -8.71e+22     80     140

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.42e+22
       Mxy    -3.75e+22
       Mxz     2.02e+22
       Myy     5.74e+22
       Myz    -2.26e+22
       Mzz    -8.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ####################------##           
             #################-----------##          
              ##############--------------###        
          # T ###########-----------------####       
         ##   #########--------------------####      
        ##############---------------------#####     
        ############-----------------------#####     
       ############------------------------######    
       ###########-------------------------######    
       ##########------------   ----------#######    
       ##########------------ P ----------#######    
        ########-------------   ---------#######     
        #######-------------------------########     
         ######------------------------########      
          #####----------------------#########       
           ####---------------------#########        
             ##-------------------#########          
              #----------------###########           
                 ---------#############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.15e+22   2.02e+22   2.26e+22 
  2.02e+22   2.42e+22   3.75e+22 
  2.26e+22   3.75e+22   5.74e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/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 = 30
      DIP = 35
     RAKE = -95
       MW = 4.56
       HS = 50.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 USGS/SLU Moment Tensor Solution
 ENS  2009/04/07 20:12:55:0  61.47 -149.76  25.0 4.6 Southern Alaska
 
 Stations used:
   AK.BMR AK.DIV AK.EYAK AK.PAX AK.SAW AK.SWD AK.TRF AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 8.71e+22 dyne-cm
  Mw = 4.56 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      216    55   -87
   NP2       30    35   -95
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.71e+22     10     304
    N   0.00e+00      3      34
    P  -8.71e+22     80     140

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.42e+22
       Mxy    -3.75e+22
       Mxz     2.02e+22
       Myy     5.74e+22
       Myz    -2.26e+22
       Mzz    -8.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ####################------##           
             #################-----------##          
              ##############--------------###        
          # T ###########-----------------####       
         ##   #########--------------------####      
        ##############---------------------#####     
        ############-----------------------#####     
       ############------------------------######    
       ###########-------------------------######    
       ##########------------   ----------#######    
       ##########------------ P ----------#######    
        ########-------------   ---------#######     
        #######-------------------------########     
         ######------------------------########      
          #####----------------------#########       
           ####---------------------#########        
             ##-------------------#########          
              #----------------###########           
                 ---------#############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.15e+22   2.02e+22   2.26e+22 
  2.02e+22   2.42e+22   3.75e+22 
  2.26e+22   3.75e+22   5.74e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090407201255/index.html
	


Date: 2009/04/07
Time: 20:12 (UTC)
Region: Cook Inlet Region of Alaska
Mw=4.6

Location:

Lat.  61.4686;  Lon.  -149.7585; Depth   45 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 8

Best Double Couple:

         strike    dip    rake 
Plane 1:    7.8   48.2  -106.5
Plane 2:  211.7   44.3   -72.4

Moment Tensor Parameters:

Mo = 8.83406e+22 dyn-cm
Mxx =  0.51; Mxy = -3.01; Mxz =  1.47
Myy =  7.83; Myz =  1.15; Mzz = -8.34


Principal Axes:

     value   azimuth   plunge
T:    8.93   109.35    1.98
N:   -0.18    18.92   12.19
P:   -8.74   208.44   77.64


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   30.0   235    55   -65   4.27 0.6456
WVFGRD96   31.0   235    55   -70   4.28 0.6538
WVFGRD96   32.0   230    55   -70   4.29 0.6635
WVFGRD96   33.0   230    55   -70   4.30 0.6723
WVFGRD96   34.0   230    55   -70   4.31 0.6799
WVFGRD96   35.0   230    55   -75   4.32 0.6874
WVFGRD96   36.0   230    55   -75   4.33 0.6935
WVFGRD96   37.0   225    55   -75   4.35 0.7003
WVFGRD96   38.0   225    55   -75   4.36 0.7064
WVFGRD96   39.0   225    55   -75   4.37 0.7112
WVFGRD96   40.0   220    50   -80   4.47 0.7049
WVFGRD96   41.0   215    50   -85   4.49 0.7136
WVFGRD96   42.0   215    50   -85   4.50 0.7223
WVFGRD96   43.0   215    50   -85   4.50 0.7283
WVFGRD96   44.0    30    40   -95   4.51 0.7329
WVFGRD96   45.0   215    50   -85   4.52 0.7362
WVFGRD96   46.0    25    40   -95   4.52 0.7388
WVFGRD96   47.0   220    55   -85   4.54 0.7419
WVFGRD96   48.0    35    35   -95   4.55 0.7424
WVFGRD96   49.0   220    55   -85   4.55 0.7438
WVFGRD96   50.0    30    35   -95   4.56 0.7440
WVFGRD96   51.0   215    55   -90   4.57 0.7432
WVFGRD96   52.0    35    35   -90   4.57 0.7418
WVFGRD96   53.0   215    55   -90   4.57 0.7390
WVFGRD96   54.0    35    35   -90   4.58 0.7359
WVFGRD96   55.0   215    55   -90   4.58 0.7308
WVFGRD96   56.0    40    35   -85   4.59 0.7258
WVFGRD96   57.0   215    55   -90   4.59 0.7193
WVFGRD96   58.0   210    55   -90   4.59 0.7119
WVFGRD96   59.0   215    55   -95   4.60 0.7044

The best solution is

WVFGRD96   50.0    30    35   -95   4.56 0.7440

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 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=Wed Apr 8 09:00:39 CDT 2009

Last Changed 2009/04/07