Location

2011/03/25 14:19:38 62.659 -151.454 115.0 4.30 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  2011/03/25 14:19:38:0  62.66 -151.45 115.0 4.3 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.DHY AK.KTH AK.MCK AK.MDM AK.MLY AK.PPLA 
   AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD AK.TRF AK.WRH AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.08 n 3
 
 Best Fitting Double Couple
  Mo = 4.22e+22 dyne-cm
  Mw = 4.35 
  Z  = 108 km
  Plane   Strike  Dip  Rake
   NP1       80    80   -50
   NP2      182    41   -165
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.22e+22     24     140
    N   0.00e+00     39     252
    P  -4.22e+22     41      27

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.59e+21
       Mxy    -2.70e+22
       Mxz    -3.07e+22
       Myy     9.46e+21
       Myz     6.36e+20
       Mzz    -1.10e+22
                                                     
                                                     
                                                     
                                                     
                     #####---------                  
                 ######----------------              
              ########--------------------           
             #######-----------------------          
           ########-------------   ----------        
          ########-------------- P -----------       
         ########---------------   ------------      
        #########-------------------------------     
        ########--------------------------------     
       #########-------------------------------##    
       #########--------------------------#######    
       #########--------------------#############    
       #########-----------######################    
        --------################################     
        ---------###############################     
         --------##############################      
          --------###################   ######       
           --------################## T #####        
             -------#################   ###          
              -------#####################           
                 ------################              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.10e+22  -3.07e+22  -6.36e+20 
 -3.07e+22   1.59e+21   2.70e+22 
 -6.36e+20   2.70e+22   9.46e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110325141938/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 = 80
      DIP = 80
     RAKE = -50
       MW = 4.35
       HS = 108.0

The NDK file is 20110325141938.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2011/03/25 14:19:38:0  62.66 -151.45 115.0 4.3 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.DHY AK.KTH AK.MCK AK.MDM AK.MLY AK.PPLA 
   AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD AK.TRF AK.WRH AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.08 n 3
 
 Best Fitting Double Couple
  Mo = 4.22e+22 dyne-cm
  Mw = 4.35 
  Z  = 108 km
  Plane   Strike  Dip  Rake
   NP1       80    80   -50
   NP2      182    41   -165
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.22e+22     24     140
    N   0.00e+00     39     252
    P  -4.22e+22     41      27

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.59e+21
       Mxy    -2.70e+22
       Mxz    -3.07e+22
       Myy     9.46e+21
       Myz     6.36e+20
       Mzz    -1.10e+22
                                                     
                                                     
                                                     
                                                     
                     #####---------                  
                 ######----------------              
              ########--------------------           
             #######-----------------------          
           ########-------------   ----------        
          ########-------------- P -----------       
         ########---------------   ------------      
        #########-------------------------------     
        ########--------------------------------     
       #########-------------------------------##    
       #########--------------------------#######    
       #########--------------------#############    
       #########-----------######################    
        --------################################     
        ---------###############################     
         --------##############################      
          --------###################   ######       
           --------################## T #####        
             -------#################   ###          
              -------#####################           
                 ------################              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.10e+22  -3.07e+22  -6.36e+20 
 -3.07e+22   1.59e+21   2.70e+22 
 -6.36e+20   2.70e+22   9.46e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110325141938/index.html
	

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 0.08 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    2.0   195    70    20   3.42 0.2491
WVFGRD96    4.0    10    70   -10   3.50 0.2574
WVFGRD96    6.0   315    60    20   3.57 0.2759
WVFGRD96    8.0   315    60    20   3.64 0.2959
WVFGRD96   10.0   315    60    20   3.67 0.3066
WVFGRD96   12.0   135    65    25   3.71 0.3137
WVFGRD96   14.0   130    65    20   3.73 0.3227
WVFGRD96   16.0   130    65    20   3.76 0.3289
WVFGRD96   18.0   105    65     5   3.78 0.3364
WVFGRD96   20.0   105    65     5   3.80 0.3481
WVFGRD96   22.0   105    65     5   3.82 0.3587
WVFGRD96   24.0   105    70     5   3.84 0.3694
WVFGRD96   26.0   105    65     5   3.86 0.3794
WVFGRD96   28.0   105    65     5   3.88 0.3875
WVFGRD96   30.0   110    60    15   3.91 0.3939
WVFGRD96   32.0   110    65    15   3.92 0.4005
WVFGRD96   34.0   110    65    15   3.94 0.4063
WVFGRD96   36.0   100    80   -20   3.94 0.4117
WVFGRD96   38.0   100    80   -20   3.97 0.4185
WVFGRD96   40.0    95    75   -30   4.04 0.4260
WVFGRD96   42.0    95    70   -30   4.06 0.4284
WVFGRD96   44.0    95    70   -25   4.08 0.4284
WVFGRD96   46.0    95    70   -25   4.09 0.4285
WVFGRD96   48.0    95    60   -20   4.12 0.4318
WVFGRD96   50.0    95    60   -15   4.14 0.4345
WVFGRD96   52.0    95    60   -20   4.15 0.4382
WVFGRD96   54.0    95    60   -20   4.16 0.4418
WVFGRD96   56.0    95    60   -15   4.18 0.4472
WVFGRD96   58.0    95    65   -20   4.18 0.4523
WVFGRD96   60.0    95    60   -15   4.21 0.4589
WVFGRD96   62.0    95    60   -15   4.22 0.4643
WVFGRD96   64.0    90    75   -35   4.20 0.4703
WVFGRD96   66.0    90    75   -35   4.21 0.4790
WVFGRD96   68.0    90    75   -40   4.22 0.4878
WVFGRD96   70.0    85    75   -45   4.23 0.4953
WVFGRD96   72.0    85    75   -45   4.24 0.5052
WVFGRD96   74.0    85    75   -45   4.25 0.5149
WVFGRD96   76.0    85    70   -40   4.26 0.5231
WVFGRD96   78.0    85    70   -40   4.27 0.5326
WVFGRD96   80.0    85    75   -45   4.27 0.5406
WVFGRD96   82.0    85    75   -40   4.28 0.5491
WVFGRD96   84.0    85    75   -40   4.29 0.5579
WVFGRD96   86.0    85    75   -40   4.30 0.5640
WVFGRD96   88.0    80    75   -45   4.31 0.5722
WVFGRD96   90.0    80    75   -45   4.32 0.5779
WVFGRD96   92.0    80    75   -45   4.32 0.5842
WVFGRD96   94.0    80    75   -45   4.33 0.5875
WVFGRD96   96.0    80    75   -45   4.33 0.5910
WVFGRD96   98.0    80    80   -50   4.33 0.5933
WVFGRD96  100.0    80    80   -50   4.34 0.5972
WVFGRD96  101.0    80    80   -50   4.34 0.5979
WVFGRD96  102.0    80    80   -50   4.34 0.5980
WVFGRD96  103.0    80    80   -50   4.34 0.6002
WVFGRD96  104.0    80    80   -50   4.34 0.6003
WVFGRD96  105.0    80    80   -50   4.35 0.6004
WVFGRD96  106.0    80    80   -50   4.35 0.5999
WVFGRD96  107.0    80    80   -50   4.35 0.6005
WVFGRD96  108.0    80    80   -50   4.35 0.6006
WVFGRD96  109.0    80    80   -45   4.35 0.6004
WVFGRD96  110.0    80    80   -45   4.36 0.5997
WVFGRD96  111.0    80    80   -45   4.36 0.5991
WVFGRD96  112.0    80    80   -45   4.36 0.5994
WVFGRD96  113.0    85    85   -45   4.35 0.5989
WVFGRD96  114.0    85    85   -45   4.35 0.5986
WVFGRD96  115.0    85    85   -45   4.35 0.5975
WVFGRD96  116.0    85    85   -45   4.36 0.5974
WVFGRD96  117.0    85    85   -45   4.36 0.5979
WVFGRD96  118.0    85    85   -45   4.36 0.5965
WVFGRD96  119.0    85    85   -45   4.36 0.5958
WVFGRD96  120.0    85    85   -45   4.36 0.5940
WVFGRD96  121.0    85    85   -45   4.36 0.5938
WVFGRD96  122.0    85    85   -45   4.36 0.5934
WVFGRD96  123.0    85    85   -45   4.36 0.5913
WVFGRD96  124.0    85    85   -45   4.37 0.5902
WVFGRD96  125.0    85    85   -45   4.37 0.5882

The best solution is

WVFGRD96  108.0    80    80   -50   4.35 0.6006

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 0.08 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 Sun Dec 6 19:37:28 CST 2015