Location

2011/04/25 19:29:15 59.063 -152.582 62 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  2011/04/25 19:29:15:0  59.06 -152.58  62.0 4.7 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV AK.HOM AK.KLU 
   AK.KTH AK.SAW AK.SCM AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA 
   II.KDAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 3.47e+23 dyne-cm
  Mw = 4.96 
  Z  = 63 km
  Plane   Strike  Dip  Rake
   NP1      324    67   153
   NP2       65    65    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.47e+23     35     284
    N   0.00e+00     55     107
    P  -3.47e+23      2      15

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.10e+23
       Mxy    -1.40e+23
       Mxz     2.92e+22
       Myy     1.98e+23
       Myz    -1.60e+23
       Mzz     1.12e+23
                                                     
                                                     
                                                     
                                                     
                     ----------- P                   
                 ---------------   ----              
              #####-----------------------           
             #########---------------------          
           ##############--------------------        
          #################-------------------       
         ####################------------------      
        ######################----------------##     
        #####   ################------------####     
       ###### T #################----------######    
       ######   ##################-------########    
       ############################----##########    
       ##########################################    
        #########################----###########     
        #####################---------##########     
         ###############---------------########      
          -----------------------------#######       
           -----------------------------#####        
             ---------------------------###          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.12e+23   2.92e+22   1.60e+23 
  2.92e+22  -3.10e+23   1.40e+23 
  1.60e+23   1.40e+23   1.98e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110425192915/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 = 65
      DIP = 65
     RAKE = 25
       MW = 4.96
       HS = 63.0

The NDK file is 20110425192915.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  2011/04/25 19:29:15:0  59.06 -152.58  62.0 4.7 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV AK.HOM AK.KLU 
   AK.KTH AK.SAW AK.SCM AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA 
   II.KDAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 3.47e+23 dyne-cm
  Mw = 4.96 
  Z  = 63 km
  Plane   Strike  Dip  Rake
   NP1      324    67   153
   NP2       65    65    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.47e+23     35     284
    N   0.00e+00     55     107
    P  -3.47e+23      2      15

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.10e+23
       Mxy    -1.40e+23
       Mxz     2.92e+22
       Myy     1.98e+23
       Myz    -1.60e+23
       Mzz     1.12e+23
                                                     
                                                     
                                                     
                                                     
                     ----------- P                   
                 ---------------   ----              
              #####-----------------------           
             #########---------------------          
           ##############--------------------        
          #################-------------------       
         ####################------------------      
        ######################----------------##     
        #####   ################------------####     
       ###### T #################----------######    
       ######   ##################-------########    
       ############################----##########    
       ##########################################    
        #########################----###########     
        #####################---------##########     
         ###############---------------########      
          -----------------------------#######       
           -----------------------------#####        
             ---------------------------###          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.12e+23   2.92e+22   1.60e+23 
  2.92e+22  -3.10e+23   1.40e+23 
  1.60e+23   1.40e+23   1.98e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110425192915/index.html
	
USGS/SLU Regional Moment Solution

11/04/25 19:29:15.59

Epicenter:  59.175 -152.834
MW 4.9

USGS/SLU REGIONAL MOMENT TENSOR
Depth  56         No. of sta: 43
Moment Tensor;   Scale 10**16 Nm
  Mrr= 0.48       Mtt=-1.79
  Mpp= 1.31       Mrt= 0.67
  Mrp= 0.78       Mtp= 2.14
 Principal axes:
  T  Val=  2.83  Plg=23  Azm=298
  N        0.07      66      106
  P       -2.90       4      206

Best Double Couple:Mo=2.9*10**16
 NP1:Strike= 74 Dip=77 Slip=  20
 NP2:       340     71       166


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 0.10 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   320    50   -30   4.01 0.2260
WVFGRD96    1.0   330    70   -15   4.00 0.2269
WVFGRD96    2.0   330    80   -10   4.13 0.2883
WVFGRD96    3.0   320    85   -45   4.24 0.3317
WVFGRD96    4.0   325    90   -35   4.25 0.3585
WVFGRD96    5.0   325    90   -30   4.28 0.3730
WVFGRD96    6.0   325    90   -30   4.30 0.3795
WVFGRD96    7.0   325    90   -25   4.33 0.3825
WVFGRD96    8.0   325    90   -30   4.37 0.3808
WVFGRD96    9.0   300    75   -25   4.39 0.3815
WVFGRD96   10.0   300    75   -25   4.41 0.3795
WVFGRD96   11.0   300    70   -25   4.44 0.3764
WVFGRD96   12.0   300    70   -25   4.45 0.3726
WVFGRD96   13.0   300    70   -25   4.47 0.3669
WVFGRD96   14.0   300    70   -25   4.48 0.3596
WVFGRD96   15.0   300    70   -25   4.49 0.3507
WVFGRD96   16.0   300    70   -25   4.50 0.3401
WVFGRD96   17.0   300    70   -25   4.51 0.3291
WVFGRD96   18.0   240    65    10   4.51 0.3217
WVFGRD96   19.0   240    65    10   4.53 0.3260
WVFGRD96   20.0   240    65    10   4.54 0.3305
WVFGRD96   21.0   235    65    10   4.55 0.3360
WVFGRD96   22.0   235    65     5   4.57 0.3441
WVFGRD96   23.0   235    65     0   4.58 0.3545
WVFGRD96   24.0   235    65     0   4.59 0.3640
WVFGRD96   25.0   235    65     0   4.61 0.3745
WVFGRD96   26.0   240    75    15   4.62 0.3865
WVFGRD96   27.0   240    75    15   4.64 0.4056
WVFGRD96   28.0   240    70    15   4.64 0.4246
WVFGRD96   29.0   240    70    15   4.66 0.4431
WVFGRD96   30.0   240    70    15   4.67 0.4594
WVFGRD96   31.0   240    70    15   4.68 0.4754
WVFGRD96   32.0   240    70    15   4.69 0.4904
WVFGRD96   33.0   240    75    10   4.70 0.5034
WVFGRD96   34.0   240    75    10   4.71 0.5136
WVFGRD96   35.0   240    75    10   4.72 0.5244
WVFGRD96   36.0   240    75    10   4.74 0.5343
WVFGRD96   37.0   240    75    10   4.75 0.5430
WVFGRD96   38.0    60    85     0   4.78 0.5541
WVFGRD96   39.0    60    85     0   4.80 0.5694
WVFGRD96   40.0    65    75    10   4.84 0.5839
WVFGRD96   41.0    65    75    10   4.86 0.5862
WVFGRD96   42.0    65    75    10   4.87 0.5864
WVFGRD96   43.0    65    75    10   4.88 0.5883
WVFGRD96   44.0    65    70    15   4.89 0.5925
WVFGRD96   45.0    65    70    15   4.90 0.5993
WVFGRD96   46.0    65    70    15   4.91 0.6071
WVFGRD96   47.0    65    70    15   4.92 0.6162
WVFGRD96   48.0    65    70    15   4.93 0.6249
WVFGRD96   49.0    65    70    15   4.93 0.6336
WVFGRD96   50.0    65    70    15   4.94 0.6404
WVFGRD96   51.0    65    65    20   4.94 0.6475
WVFGRD96   52.0    65    65    20   4.94 0.6551
WVFGRD96   53.0    65    65    20   4.95 0.6638
WVFGRD96   54.0    65    65    20   4.95 0.6691
WVFGRD96   55.0    65    65    20   4.95 0.6708
WVFGRD96   56.0    65    65    20   4.96 0.6772
WVFGRD96   57.0    65    65    20   4.96 0.6815
WVFGRD96   58.0    65    65    20   4.96 0.6835
WVFGRD96   59.0    65    65    20   4.96 0.6850
WVFGRD96   60.0    65    65    20   4.96 0.6877
WVFGRD96   61.0    65    65    25   4.96 0.6874
WVFGRD96   62.0    65    65    25   4.96 0.6862
WVFGRD96   63.0    65    65    25   4.96 0.6888
WVFGRD96   64.0    65    65    25   4.96 0.6867
WVFGRD96   65.0    65    65    25   4.96 0.6880
WVFGRD96   66.0    65    65    25   4.96 0.6868
WVFGRD96   67.0    65    65    25   4.96 0.6834
WVFGRD96   68.0    65    65    25   4.96 0.6848
WVFGRD96   69.0    65    65    25   4.96 0.6791
WVFGRD96   70.0    65    65    25   4.96 0.6802
WVFGRD96   71.0    65    65    25   4.96 0.6770
WVFGRD96   72.0    65    65    25   4.96 0.6772
WVFGRD96   73.0    65    65    25   4.96 0.6729
WVFGRD96   74.0    65    65    25   4.96 0.6716
WVFGRD96   75.0    65    70    25   4.96 0.6696
WVFGRD96   76.0    65    65    25   4.96 0.6669
WVFGRD96   77.0    65    70    25   4.96 0.6644
WVFGRD96   78.0    65    65    25   4.96 0.6624
WVFGRD96   79.0    65    70    25   4.96 0.6593
WVFGRD96   80.0    65    70    25   4.96 0.6581
WVFGRD96   81.0    65    70    25   4.96 0.6549
WVFGRD96   82.0    65    70    25   4.96 0.6527
WVFGRD96   83.0    65    70    25   4.96 0.6503
WVFGRD96   84.0    65    70    30   4.96 0.6494
WVFGRD96   85.0    65    70    30   4.96 0.6445
WVFGRD96   86.0    65    70    30   4.96 0.6459
WVFGRD96   87.0    65    70    30   4.96 0.6389
WVFGRD96   88.0    65    70    30   4.96 0.6416
WVFGRD96   89.0    65    70    30   4.96 0.6365
WVFGRD96   90.0    65    70    30   4.96 0.6366
WVFGRD96   91.0    65    70    30   4.96 0.6342
WVFGRD96   92.0    65    70    30   4.96 0.6313
WVFGRD96   93.0    65    70    30   4.96 0.6309
WVFGRD96   94.0    65    70    30   4.96 0.6255
WVFGRD96   95.0    65    70    30   4.96 0.6277
WVFGRD96   96.0    65    70    35   4.96 0.6235
WVFGRD96   97.0    65    70    35   4.96 0.6232
WVFGRD96   98.0    65    70    35   4.96 0.6210
WVFGRD96   99.0    65    70    35   4.96 0.6182

The best solution is

WVFGRD96   63.0    65    65    25   4.96 0.6888

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

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 19:57:31 CST 2015