Location

2009/04/10 05:50:10 63.4540 -151.7210 16.0 4.70 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  2009/04/10 05:50:10:0  63.45 -151.72  16.0 4.7 Alaska
 
 Stations used:
   AK.BMR AK.CHUM AK.COLD AK.DIV AK.DOT AK.EYAK AK.PAX AK.SAW 
   AK.SWD AK.TRF AT.PMR IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 6.84e+22 dyne-cm
  Mw = 4.49 
  Z  = 16 km
  Plane   Strike  Dip  Rake
   NP1       10    65    55
   NP2      249    42   141
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.84e+22     55     234
    N   0.00e+00     31      26
    P  -6.84e+22     13     125

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.35e+22
       Mxy     4.07e+22
       Mxz    -1.01e+22
       Myy    -2.95e+22
       Myz    -3.83e+22
       Mzz     4.29e+22
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 ----------------######              
              --------------------########           
             ---------------------#########          
           ---------------#########----######        
          ------------#############---------##       
         ---------#################-----------#      
        --------###################-------------     
        ------#####################-------------     
       -----#######################--------------    
       ----########################--------------    
       ---########################---------------    
       ---##########   ###########---------------    
        -########### T ##########---------------     
        -###########   ##########---------------     
         #######################----------   --      
          ######################---------- P -       
           ####################-----------           
             #################-------------          
              ###############-------------           
                 ##########------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.29e+22  -1.01e+22   3.83e+22 
 -1.01e+22  -1.35e+22  -4.07e+22 
  3.83e+22  -4.07e+22  -2.95e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090410055010/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 = 10
      DIP = 65
     RAKE = 55
       MW = 4.49
       HS = 16.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
AEIC
 USGS/SLU Moment Tensor Solution
 ENS  2009/04/10 05:50:10:0  63.45 -151.72  16.0 4.7 Alaska
 
 Stations used:
   AK.BMR AK.CHUM AK.COLD AK.DIV AK.DOT AK.EYAK AK.PAX AK.SAW 
   AK.SWD AK.TRF AT.PMR IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 6.84e+22 dyne-cm
  Mw = 4.49 
  Z  = 16 km
  Plane   Strike  Dip  Rake
   NP1       10    65    55
   NP2      249    42   141
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.84e+22     55     234
    N   0.00e+00     31      26
    P  -6.84e+22     13     125

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.35e+22
       Mxy     4.07e+22
       Mxz    -1.01e+22
       Myy    -2.95e+22
       Myz    -3.83e+22
       Mzz     4.29e+22
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 ----------------######              
              --------------------########           
             ---------------------#########          
           ---------------#########----######        
          ------------#############---------##       
         ---------#################-----------#      
        --------###################-------------     
        ------#####################-------------     
       -----#######################--------------    
       ----########################--------------    
       ---########################---------------    
       ---##########   ###########---------------    
        -########### T ##########---------------     
        -###########   ##########---------------     
         #######################----------   --      
          ######################---------- P -       
           ####################-----------           
             #################-------------          
              ###############-------------           
                 ##########------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.29e+22  -1.01e+22   3.83e+22 
 -1.01e+22  -1.35e+22  -4.07e+22 
  3.83e+22  -4.07e+22  -2.95e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090410055010/index.html
	

Moment tensor inversion summary for event 2009/04/10 05:50
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2009/04/10 05:50

Date 2009/04/10
Region: Central Region of Alaska
Mw=4.5

Centroid Location:
Time 05:50; Lat. 63.54N; Lon. 208.21W; Depth 15 km

Best Double Couple:
Plane 1: strike = 21; dip = 56; rake = 54
Plane 2: strike = 254; dip = 48; rake = 132

Moment Tensor:
Mo = 6.63781e+22 dyn-cm
Mxx = -289.996; Mxy = 396.588; Mxz = -136.563
Myy = -205.982; Myz = -280.544; Mzz = 495.978

Principal Axes:
T: value = 84.000; azimuth = 235; plunge = 60
N: value = 80.000; azimuth = 136; plunge = 5
P: value = 78.000; azimuth = 44; plunge = 30 

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 0.10 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   175    65    40   4.24 0.4324
WVFGRD96    1.0   185    50    55   4.27 0.4514
WVFGRD96    2.0   200    45    75   4.36 0.4681
WVFGRD96    3.0   190    50    60   4.38 0.4104
WVFGRD96    4.0   160    60   -40   4.36 0.4355
WVFGRD96    5.0   175    85   -40   4.34 0.4594
WVFGRD96    6.0   175    90   -40   4.35 0.4889
WVFGRD96    7.0    -5    90    40   4.36 0.5160
WVFGRD96    8.0     0    85    40   4.38 0.5402
WVFGRD96    9.0    10    65    55   4.40 0.5660
WVFGRD96   10.0    10    65    55   4.43 0.5967
WVFGRD96   11.0    10    65    55   4.44 0.6260
WVFGRD96   12.0    10    65    55   4.45 0.6479
WVFGRD96   13.0    10    65    55   4.46 0.6640
WVFGRD96   14.0    10    65    55   4.47 0.6757
WVFGRD96   15.0    10    65    55   4.48 0.6824
WVFGRD96   16.0    10    65    55   4.49 0.6862
WVFGRD96   17.0    10    65    60   4.49 0.6856
WVFGRD96   18.0    10    65    60   4.50 0.6826
WVFGRD96   19.0    10    65    60   4.51 0.6762
WVFGRD96   20.0    10    65    60   4.53 0.6706
WVFGRD96   21.0    10    65    60   4.54 0.6610
WVFGRD96   22.0    15    60    70   4.54 0.6490
WVFGRD96   23.0    10    65    65   4.55 0.6346
WVFGRD96   24.0    10    65    65   4.55 0.6193
WVFGRD96   25.0    10    70    60   4.55 0.6041
WVFGRD96   26.0   165    70   -50   4.56 0.5998
WVFGRD96   27.0   165    65   -50   4.58 0.5986
WVFGRD96   28.0   165    65   -50   4.58 0.5987
WVFGRD96   29.0   165    65   -50   4.59 0.5989

The best solution is

WVFGRD96   16.0    10    65    55   4.49 0.6862

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 0.10 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=Fri Apr 10 12:01:31 CDT 2009

Last Changed 2009/04/10