Location

2010/09/25 12:06:00 62.851 -149.453 76.9 5.60 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  2010/09/25 12:06:00:0  62.85 -149.45  76.9 5.6 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CHUM AK.CNP AK.DHY 
   AK.DIV AK.DOT AK.EYAK AK.FIB AK.GLI AK.HDA AK.KLU AK.MCK 
   AK.MDM AK.MLY AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN 
   AK.SSN AK.TRF AK.WRH IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 1.43e+24 dyne-cm
  Mw = 5.37 
  Z  = 89 km
  Plane   Strike  Dip  Rake
   NP1        7    69   148
   NP2      110    60    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.43e+24     38     326
    N   0.00e+00     52     157
    P  -1.43e+24      5      60

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.59e+23
       Mxy    -1.03e+24
       Mxz     5.05e+23
       Myy    -7.82e+23
       Myz    -5.05e+23
       Mzz     5.23e+23
                                                     
                                                     
                                                     
                                                     
                     ##########----                  
                 ###############-------              
              ###################---------           
             ####################----------          
           ########   ###########-----------         
          ######### T ############---------- P       
         ##########   ############----------         
        -#########################--------------     
        --########################--------------     
       ----#######################---------------    
       ------#####################---------------    
       -------###################----------------    
       ----------################----------------    
        ------------#############---------------     
        ----------------########----------------     
         --------------------###-------------##      
          ---------------------###############       
           --------------------##############        
             -----------------#############          
              ---------------#############           
                 -----------###########              
                     ------########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.23e+23   5.05e+23   5.05e+23 
  5.05e+23   2.59e+23   1.03e+24 
  5.05e+23   1.03e+24  -7.82e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100925120600/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 = 110
      DIP = 60
     RAKE = 25
       MW = 5.37
       HS = 89.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
        
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2010/09/25 12:06:00:0  62.85 -149.45  76.9 5.6 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CHUM AK.CNP AK.DHY 
   AK.DIV AK.DOT AK.EYAK AK.FIB AK.GLI AK.HDA AK.KLU AK.MCK 
   AK.MDM AK.MLY AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN 
   AK.SSN AK.TRF AK.WRH IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 1.43e+24 dyne-cm
  Mw = 5.37 
  Z  = 89 km
  Plane   Strike  Dip  Rake
   NP1        7    69   148
   NP2      110    60    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.43e+24     38     326
    N   0.00e+00     52     157
    P  -1.43e+24      5      60

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.59e+23
       Mxy    -1.03e+24
       Mxz     5.05e+23
       Myy    -7.82e+23
       Myz    -5.05e+23
       Mzz     5.23e+23
                                                     
                                                     
                                                     
                                                     
                     ##########----                  
                 ###############-------              
              ###################---------           
             ####################----------          
           ########   ###########-----------         
          ######### T ############---------- P       
         ##########   ############----------         
        -#########################--------------     
        --########################--------------     
       ----#######################---------------    
       ------#####################---------------    
       -------###################----------------    
       ----------################----------------    
        ------------#############---------------     
        ----------------########----------------     
         --------------------###-------------##      
          ---------------------###############       
           --------------------##############        
             -----------------#############          
              ---------------#############           
                 -----------###########              
                     ------########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.23e+23   5.05e+23   5.05e+23 
  5.05e+23   2.59e+23   1.03e+24 
  5.05e+23   1.03e+24  -7.82e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100925120600/index.html
	
USGS Body-Wave Moment Tensor Solution

 10/09/25 12:06:00.00
 CENTRAL ALASKA                  
 Epicenter:  62.855 -149.467
 MW 5.4

 USGS MOMENT TENSOR SOLUTION
 Depth  89         No. of sta: 11
 Moment Tensor;   Scale 10**17 Nm
   Mrr= 0.53       Mtt= 0.29
   Mpp=-0.82       Mrt= 0.57
   Mrp= 0.87       Mtp= 1.18
  Principal axes:
   T  Val=  1.79  Plg=38  Azm=322
   N       -0.12      49      168
   P       -1.67      13       62

 Best Double Couple:Mo=1.7*10**17
  NP1:Strike=  7 Dip=74 Slip= 142
  NP2:       109     53        20
                                      
               ######-                
          ###########------           
        #############--------         
      ################---------       
    #######   ########--------        
   ######## T ########-------- P -    
   ########   ########--------   -    
  -###################-------------   
  ---#################-------------   
  ----################-------------   
  ------#############--------------   
  ---------##########--------------   
   -----------#######-------------    
   ----------------#----------####    
    ----------------#############     
      -------------############       
        ----------###########         
          --------#########           
               --#####                
                                      


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   60.0   105    50    20   5.30 0.4494
WVFGRD96   61.0   105    50    20   5.30 0.4586
WVFGRD96   62.0   110    50    25   5.31 0.4669
WVFGRD96   63.0   110    50    25   5.31 0.4761
WVFGRD96   64.0   110    50    25   5.32 0.4832
WVFGRD96   65.0   110    50    25   5.32 0.4916
WVFGRD96   66.0   110    50    25   5.32 0.4980
WVFGRD96   67.0   110    50    25   5.33 0.5032
WVFGRD96   68.0   110    50    25   5.33 0.5106
WVFGRD96   69.0   110    55    25   5.33 0.5163
WVFGRD96   70.0   110    55    25   5.33 0.5214
WVFGRD96   71.0   110    55    25   5.34 0.5289
WVFGRD96   72.0   110    55    25   5.34 0.5342
WVFGRD96   73.0   110    55    25   5.34 0.5383
WVFGRD96   74.0   110    55    25   5.34 0.5430
WVFGRD96   75.0   110    55    25   5.35 0.5466
WVFGRD96   76.0   110    55    25   5.35 0.5499
WVFGRD96   77.0   110    55    25   5.35 0.5531
WVFGRD96   78.0   110    55    25   5.35 0.5554
WVFGRD96   79.0   110    55    25   5.36 0.5568
WVFGRD96   80.0   110    55    25   5.36 0.5596
WVFGRD96   81.0   110    55    20   5.36 0.5597
WVFGRD96   82.0   110    55    20   5.37 0.5618
WVFGRD96   83.0   110    60    25   5.36 0.5629
WVFGRD96   84.0   110    60    25   5.36 0.5639
WVFGRD96   85.0   110    60    25   5.36 0.5661
WVFGRD96   86.0   110    60    25   5.36 0.5656
WVFGRD96   87.0   110    60    25   5.36 0.5672
WVFGRD96   88.0   110    60    25   5.37 0.5674
WVFGRD96   89.0   110    60    25   5.37 0.5698
WVFGRD96   90.0   110    60    25   5.37 0.5657
WVFGRD96   91.0   110    60    25   5.37 0.5675
WVFGRD96   92.0   110    60    25   5.37 0.5667
WVFGRD96   93.0   110    60    25   5.37 0.5671
WVFGRD96   94.0   110    60    25   5.37 0.5669
WVFGRD96   95.0   110    60    25   5.37 0.5681
WVFGRD96   96.0   110    60    25   5.37 0.5616
WVFGRD96   97.0   110    60    25   5.37 0.5635
WVFGRD96   98.0   110    60    20   5.38 0.5617
WVFGRD96   99.0   110    60    20   5.38 0.5615
WVFGRD96  100.0   110    60    20   5.38 0.5603
WVFGRD96  101.0   110    60    20   5.39 0.5583
WVFGRD96  102.0   110    60    20   5.39 0.5555
WVFGRD96  103.0   110    60    20   5.39 0.5560
WVFGRD96  104.0   110    60    20   5.39 0.5540
WVFGRD96  105.0   110    60    20   5.39 0.5528
WVFGRD96  106.0   110    60    20   5.39 0.5498
WVFGRD96  107.0   110    60    20   5.39 0.5481
WVFGRD96  108.0   110    60    20   5.39 0.5468
WVFGRD96  109.0   110    60    20   5.39 0.5449
WVFGRD96  110.0   110    60    20   5.39 0.5431
WVFGRD96  112.0   110    60    20   5.39 0.5378
WVFGRD96  113.0   110    60    20   5.39 0.5361
WVFGRD96  114.0   110    65    20   5.39 0.5351
WVFGRD96  115.0   110    65    20   5.39 0.5313
WVFGRD96  116.0   110    65    20   5.39 0.5305
WVFGRD96  117.0   110    65    20   5.39 0.5287
WVFGRD96  118.0   110    65    20   5.39 0.5263
WVFGRD96  119.0   110    65    20   5.39 0.5255
WVFGRD96  120.0   110    65    20   5.39 0.5225
WVFGRD96  122.0   110    65    20   5.39 0.5191
WVFGRD96  123.0   110    65    20   5.40 0.5165
WVFGRD96  124.0   110    65    20   5.40 0.5146
WVFGRD96  125.0   110    65    20   5.40 0.5130
WVFGRD96  126.0   110    65    20   5.40 0.5099
WVFGRD96  127.0   110    65    20   5.40 0.5097
WVFGRD96  128.0   110    65    20   5.40 0.5055
WVFGRD96  129.0   110    65    20   5.40 0.5050

The best solution is

WVFGRD96   89.0   110    60    25   5.37 0.5698

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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of 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 Sep 25 08:38:12 CDT 2010

Last Changed 2010/09/25