Location

2012/03/08 10:57:42 61.003 -150.916 7.8 4.1 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  2012/03/08 10:57:42:0  61.00 -150.92   7.8 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.GLI AK.KLU 
   AK.KTH AK.MCK AK.MLY AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM 
   AK.SKN AK.SSN AK.SWD AK.TRF IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.55e+22 dyne-cm
  Mw = 4.06 
  Z  = 30 km
  Plane   Strike  Dip  Rake
   NP1       43    78   118
   NP2      155    30    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.55e+22     50     343
    N   0.00e+00     27     217
    P  -1.55e+22     28     111

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.36e+21
       Mxy     2.34e+21
       Mxz     9.63e+21
       Myy    -1.00e+22
       Myz    -8.10e+21
       Mzz     5.67e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              --##########################           
             --###########################-          
           ---##########   ##############----        
          ----########## T #############------       
         ----###########   ############--------      
        -----########################-----------     
        -----#######################------------     
       ------######################--------------    
       ------####################----------------    
       ------##################------------------    
       ------#################-----------   -----    
        ------##############------------- P ----     
        -------###########---------------   ----     
         -------########-----------------------      
          -------#####------------------------       
           ----------------------------------        
             ---####-----------------------          
              #########-------------------           
                 #########-------------              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.67e+21   9.63e+21   8.10e+21 
  9.63e+21   4.36e+21  -2.34e+21 
  8.10e+21  -2.34e+21  -1.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120308105742/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 = 155
      DIP = 30
     RAKE = 25
       MW = 4.06
       HS = 30.0

The NDK file is 20120308105742.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2012/03/08 10:57:42:0  61.00 -150.92   7.8 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.GLI AK.KLU 
   AK.KTH AK.MCK AK.MLY AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM 
   AK.SKN AK.SSN AK.SWD AK.TRF IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.55e+22 dyne-cm
  Mw = 4.06 
  Z  = 30 km
  Plane   Strike  Dip  Rake
   NP1       43    78   118
   NP2      155    30    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.55e+22     50     343
    N   0.00e+00     27     217
    P  -1.55e+22     28     111

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.36e+21
       Mxy     2.34e+21
       Mxz     9.63e+21
       Myy    -1.00e+22
       Myz    -8.10e+21
       Mzz     5.67e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              --##########################           
             --###########################-          
           ---##########   ##############----        
          ----########## T #############------       
         ----###########   ############--------      
        -----########################-----------     
        -----#######################------------     
       ------######################--------------    
       ------####################----------------    
       ------##################------------------    
       ------#################-----------   -----    
        ------##############------------- P ----     
        -------###########---------------   ----     
         -------########-----------------------      
          -------#####------------------------       
           ----------------------------------        
             ---####-----------------------          
              #########-------------------           
                 #########-------------              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.67e+21   9.63e+21   8.10e+21 
  9.63e+21   4.36e+21  -2.34e+21 
  8.10e+21  -2.34e+21  -1.00e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120308105742/index.html
	
USGS/SLU Regional Moment Solution

12/03/08 10:57:44.07

Epicenter:  60.996 -150.950
MW 4.1

USGS/SLU REGIONAL MOMENT TENSOR
Depth  26         No. of sta: 67
Moment Tensor;   Scale 10**15 Nm
  Mrr= 0.91       Mtt= 0.56
  Mpp=-1.46       Mrt= 0.90
  Mrp= 0.45       Mtp=-0.04
 Principal axes:
  T  Val=  1.68  Plg=51  Azm=350
  N       -0.11      36      196
  P       -1.57      13       96

Best Double Couple:Mo=1.6*10**15
 NP1:Strike=149 Dip=45 Slip=  34
 NP2:        34     67       130




        

Magnitudes

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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 0.06 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   265    45   -90   3.51 0.2877
WVFGRD96    1.0    85    45   -90   3.57 0.3106
WVFGRD96    2.0   265    45   -90   3.67 0.3877
WVFGRD96    3.0   265    45   -85   3.76 0.4133
WVFGRD96    4.0   115    60   -35   3.78 0.3850
WVFGRD96    5.0   120    75   -20   3.80 0.3546
WVFGRD96    6.0   125    90   -10   3.80 0.3296
WVFGRD96    7.0   130    50     0   3.78 0.3288
WVFGRD96    8.0   135    40     5   3.83 0.3427
WVFGRD96    9.0   140    35    15   3.84 0.3722
WVFGRD96   10.0   140    30    10   3.85 0.4032
WVFGRD96   11.0   145    30    15   3.85 0.4333
WVFGRD96   12.0   145    30    20   3.87 0.4614
WVFGRD96   13.0   150    30    25   3.88 0.4882
WVFGRD96   14.0   150    30    25   3.90 0.5131
WVFGRD96   15.0   150    30    25   3.91 0.5365
WVFGRD96   16.0   155    30    30   3.92 0.5580
WVFGRD96   17.0   155    30    30   3.93 0.5780
WVFGRD96   18.0   155    30    30   3.94 0.5962
WVFGRD96   19.0   155    30    30   3.95 0.6129
WVFGRD96   20.0   155    30    30   3.96 0.6282
WVFGRD96   21.0   160    30    30   3.97 0.6408
WVFGRD96   22.0   160    30    30   3.98 0.6538
WVFGRD96   23.0   160    30    30   3.99 0.6652
WVFGRD96   24.0   160    30    30   4.00 0.6751
WVFGRD96   25.0   160    30    30   4.01 0.6837
WVFGRD96   26.0   160    30    30   4.02 0.6906
WVFGRD96   27.0   160    30    30   4.03 0.6957
WVFGRD96   28.0   160    30    30   4.04 0.6994
WVFGRD96   29.0   155    30    25   4.05 0.7014
WVFGRD96   30.0   155    30    25   4.06 0.7019
WVFGRD96   31.0   155    30    25   4.07 0.7007
WVFGRD96   32.0   155    30    25   4.08 0.6977
WVFGRD96   33.0   155    30    25   4.08 0.6935
WVFGRD96   34.0   155    30    25   4.09 0.6880
WVFGRD96   35.0   155    30    25   4.09 0.6813
WVFGRD96   36.0   150    35    25   4.11 0.6748
WVFGRD96   37.0   150    35    25   4.12 0.6681
WVFGRD96   38.0   150    40    25   4.13 0.6617
WVFGRD96   39.0   150    40    25   4.13 0.6548
WVFGRD96   40.0   155    25    25   4.24 0.6416
WVFGRD96   41.0   155    25    25   4.24 0.6289
WVFGRD96   42.0   150    30    20   4.25 0.6154
WVFGRD96   43.0   150    30    20   4.25 0.6027
WVFGRD96   44.0   150    30    20   4.26 0.5895
WVFGRD96   45.0   150    30    20   4.26 0.5759
WVFGRD96   46.0   150    30    20   4.26 0.5620
WVFGRD96   47.0   150    30    20   4.26 0.5478
WVFGRD96   48.0   145    35    15   4.27 0.5346
WVFGRD96   49.0   145    35    15   4.27 0.5213

The best solution is

WVFGRD96   30.0   155    30    25   4.06 0.7019

The mechanism corresponding 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 0.06 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

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS model 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:

Last Changed Mon Dec 7 00:24:27 CST 2015