Location

2010/04/07 16:19:15 61.5800 -149.6520 35.0 4.60 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  2010/04/07 16:19:15:0  61.58 -149.65  35.0 4.6 Alaska
 
 Stations used:
   AK.BMR AK.BRLK AK.CAST AK.CCB AK.DDM AK.DIV AK.KLU AK.KTH 
   AK.MCK AK.MDM AK.MLY AK.PPLA AK.RAG AK.RC01 AK.RND AK.SCM 
   AK.SCRK AK.SSN AK.SWD AK.TRF AK.WRH IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 7.85e+22 dyne-cm
  Mw = 4.53 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      210    75   -75
   NP2      344    21   -134
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.85e+22     28     288
    N   0.00e+00     14      26
    P  -7.85e+22     57     140

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.52e+21
       Mxy    -6.61e+21
       Mxz     3.74e+22
       Myy     4.54e+22
       Myz    -5.43e+22
       Mzz    -3.79e+22
                                                     
                                                     
                                                     
                                                     
                     ########------                  
                 #################----#              
              #####################-######           
             ####################------####          
           #####################--------#####        
          ####################-----------#####       
         ####################--------------####      
        ####   #############---------------#####     
        #### T ###########------------------####     
       #####   ##########-------------------#####    
       #################---------------------####    
       ################----------------------####    
       ###############-----------------------####    
        #############-----------   ----------###     
        #############----------- P ---------####     
         ###########------------   ---------###      
          #########------------------------###       
           ########-----------------------###        
             #####-----------------------##          
              ####---------------------###           
                 #-------------------##              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.79e+22   3.74e+22   5.43e+22 
  3.74e+22  -7.52e+21   6.61e+21 
  5.43e+22   6.61e+21   4.54e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100407161915/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 = -75
       MW = 4.53
       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  2010/04/07 16:19:15:0  61.58 -149.65  35.0 4.6 Alaska
 
 Stations used:
   AK.BMR AK.BRLK AK.CAST AK.CCB AK.DDM AK.DIV AK.KLU AK.KTH 
   AK.MCK AK.MDM AK.MLY AK.PPLA AK.RAG AK.RC01 AK.RND AK.SCM 
   AK.SCRK AK.SSN AK.SWD AK.TRF AK.WRH IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.05 n 3
 
 Best Fitting Double Couple
  Mo = 7.85e+22 dyne-cm
  Mw = 4.53 
  Z  = 45 km
  Plane   Strike  Dip  Rake
   NP1      210    75   -75
   NP2      344    21   -134
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.85e+22     28     288
    N   0.00e+00     14      26
    P  -7.85e+22     57     140

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.52e+21
       Mxy    -6.61e+21
       Mxz     3.74e+22
       Myy     4.54e+22
       Myz    -5.43e+22
       Mzz    -3.79e+22
                                                     
                                                     
                                                     
                                                     
                     ########------                  
                 #################----#              
              #####################-######           
             ####################------####          
           #####################--------#####        
          ####################-----------#####       
         ####################--------------####      
        ####   #############---------------#####     
        #### T ###########------------------####     
       #####   ##########-------------------#####    
       #################---------------------####    
       ################----------------------####    
       ###############-----------------------####    
        #############-----------   ----------###     
        #############----------- P ---------####     
         ###########------------   ---------###      
          #########------------------------###       
           ########-----------------------###        
             #####-----------------------##          
              ####---------------------###           
                 #-------------------##              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.79e+22   3.74e+22   5.43e+22 
  3.74e+22  -7.52e+21   6.61e+21 
  5.43e+22   6.61e+21   4.54e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100407161915/index.html
	

Moment tensor inversion summary for event 2010/04/07 16:19

Date: 2010/04/07
Time: 16:19 (UTC)
Region: Cook Inlet Region of Alaska
Mw=4.6

Location:

Lat.  61.5795;  Lon.  -149.6519; Depth   35 km 
(Best-fitting depth from moment tensor inversion)

Solution quality: good;
Number of stations = 12

Best Double Couple:

         strike    dip    rake 
Plane 1:  204.0   68.6   -84.5
Plane 2:    9.3   22.0  -103.7

Moment Tensor Parameters:

Mo = 8.41584e+22 dyn-cm
Mxx =  1.25; Mxy = -0.92; Mxz =  2.73
Myy =  4.61; Myz = -5.07; Mzz = -5.85


Principal Axes:

     value   azimuth   plunge
T:    7.28   289.81   23.46
N:    1.14    22.03    5.10
P:   -8.42   123.56   65.93

	

        

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   20.0   205    80   -80   4.20 0.5087
WVFGRD96   21.0   205    80   -80   4.22 0.5236
WVFGRD96   22.0   205    80   -80   4.23 0.5386
WVFGRD96   23.0   205    80   -75   4.24 0.5530
WVFGRD96   24.0   205    80   -75   4.24 0.5670
WVFGRD96   25.0   205    80   -75   4.25 0.5805
WVFGRD96   26.0   205    80   -75   4.26 0.5939
WVFGRD96   27.0   205    80   -75   4.27 0.6062
WVFGRD96   28.0   205    80   -75   4.28 0.6178
WVFGRD96   29.0   205    80   -75   4.29 0.6292
WVFGRD96   30.0   210    75   -75   4.29 0.6395
WVFGRD96   31.0   205    75   -75   4.30 0.6494
WVFGRD96   32.0   205    75   -75   4.31 0.6580
WVFGRD96   33.0   205    75   -70   4.32 0.6657
WVFGRD96   34.0   205    75   -70   4.33 0.6734
WVFGRD96   35.0   205    75   -70   4.33 0.6793
WVFGRD96   36.0   205    75   -70   4.34 0.6842
WVFGRD96   37.0   205    75   -70   4.34 0.6885
WVFGRD96   38.0   205    75   -70   4.35 0.6912
WVFGRD96   39.0   205    75   -70   4.35 0.6931
WVFGRD96   40.0   210    75   -75   4.49 0.6960
WVFGRD96   41.0   210    75   -75   4.50 0.6996
WVFGRD96   42.0   210    75   -75   4.51 0.7028
WVFGRD96   43.0   210    75   -75   4.51 0.7047
WVFGRD96   44.0   210    75   -75   4.52 0.7056
WVFGRD96   45.0   210    75   -75   4.53 0.7061
WVFGRD96   46.0   210    75   -75   4.53 0.7057
WVFGRD96   47.0   210    75   -75   4.54 0.7042
WVFGRD96   48.0   210    75   -75   4.55 0.7024
WVFGRD96   49.0   210    75   -75   4.55 0.6997

The best solution is

WVFGRD96   45.0   210    75   -75   4.53 0.7061

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=Sat Apr 10 16:23:38 CDT 2010

Last Changed 2010/04/07