Location

2012/09/18 01:44:49 56.883 -154.079 48.1 5.50 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  2012/09/18 01:44:49:0  56.88 -154.08  48.1 5.5 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.DHY AK.DIV AK.EYAK 
   AK.FALS AK.FID AK.GLI AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK 
   AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF 
   AK.UNV AT.AKUT AT.CHGN AT.OHAK AT.SDPT AT.SVW2 II.KDAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.04 n 3
 
 Best Fitting Double Couple
  Mo = 5.25e+23 dyne-cm
  Mw = 5.08 
  Z  = 42 km
  Plane   Strike  Dip  Rake
   NP1      142    86   -150
   NP2       50    60    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.25e+23     17     272
    N   0.00e+00     60     150
    P  -5.25e+23     24      10

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.23e+23
       Mxy    -9.81e+22
       Mxz    -1.86e+23
       Myy     4.62e+23
       Myz    -1.86e+23
       Mzz    -3.96e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ------------   -------              
              ##------------- P ----------           
             ####------------   -----------          
           #######--------------------------#        
          #########-------------------------##       
         ############----------------------####      
        ##############--------------------######     
        ###############------------------#######     
       ##   #############---------------#########    
       ## T ##############-------------##########    
       ##   ###############-----------###########    
       ######################-------#############    
        ######################----##############     
        ########################################     
         #####################---##############      
          #################--------###########       
           ###########--------------#########        
             -------------------------#####          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.96e+22  -1.86e+23   1.86e+23 
 -1.86e+23  -4.23e+23   9.81e+22 
  1.86e+23   9.81e+22   4.62e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120918014449/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 = 50
      DIP = 60
     RAKE = -5
       MW = 5.08
       HS = 42.0

The NDK file is 20120918014449.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
GCMT
 USGS/SLU Moment Tensor Solution
 ENS  2012/09/18 01:44:49:0  56.88 -154.08  48.1 5.5 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.DHY AK.DIV AK.EYAK 
   AK.FALS AK.FID AK.GLI AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK 
   AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF 
   AK.UNV AT.AKUT AT.CHGN AT.OHAK AT.SDPT AT.SVW2 II.KDAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.04 n 3
 
 Best Fitting Double Couple
  Mo = 5.25e+23 dyne-cm
  Mw = 5.08 
  Z  = 42 km
  Plane   Strike  Dip  Rake
   NP1      142    86   -150
   NP2       50    60    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.25e+23     17     272
    N   0.00e+00     60     150
    P  -5.25e+23     24      10

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.23e+23
       Mxy    -9.81e+22
       Mxz    -1.86e+23
       Myy     4.62e+23
       Myz    -1.86e+23
       Mzz    -3.96e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ------------   -------              
              ##------------- P ----------           
             ####------------   -----------          
           #######--------------------------#        
          #########-------------------------##       
         ############----------------------####      
        ##############--------------------######     
        ###############------------------#######     
       ##   #############---------------#########    
       ## T ##############-------------##########    
       ##   ###############-----------###########    
       ######################-------#############    
        ######################----##############     
        ########################################     
         #####################---##############      
          #################--------###########       
           ###########--------------#########        
             -------------------------#####          
              --------------------------##           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.96e+22  -1.86e+23   1.86e+23 
 -1.86e+23  -4.23e+23   9.81e+22 
  1.86e+23   9.81e+22   4.62e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120918014449/index.html
	
Global CMT Project Moment Tensor Solution


September 18, 2012, KODIAK ISLAND REGION, ALASKA, MW=5.2

Howard Koss

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201209180144A  
DATA: II IU CU MN LD G  IC DK GE
L.P.BODY WAVES: 72S, 104C, T= 40
SURFACE WAVES: 115S, 185C, T= 50
TIMESTAMP:      Q-20120918053847
CENTROID LOCATION:
ORIGIN TIME:      01:44:49.8 0.2
LAT:56.96N 0.02;LON:154.30W 0.02
DEP: 30.7  1.0;TRIANG HDUR:  1.0
MOMENT TENSOR: SCALE 10**23 D-CM
RR=-2.770 0.174; TT=-3.040 0.159
PP= 5.810 0.145; RT= 3.640 0.233
RP= 4.830 0.218; TP= 2.600 0.103
PRINCIPAL AXES:
1.(T) VAL=  9.233;PLG=26;AZM=290
2.(N)      -2.440;    29;     35
3.(P)      -6.793;    50;    165
BEST DBLE.COUPLE:M0= 8.01*10**23
NP1: STRIKE=335;DIP=32;SLIP=-154
NP2: STRIKE=223;DIP=77;SLIP= -60

            -----------           
        ###########--------       
      ################------#     
    ####################-######   
   ###################----######  
  ##   #############-------###### 
  ## T ###########----------##### 
 ###   #########-------------#####
 ##############--------------#####
 ############----------------#####
 ###########-----------------#####
  ########-------------------#### 
  #######---------   --------#### 
   #####---------- P --------###  
    ###-----------   -------###   
      ---------------------##     
        ------------------#       
            -----------           


        

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 0.04 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   190    45    80   4.62 0.3180
WVFGRD96    1.0   105    90     5   4.50 0.3342
WVFGRD96    2.0   190    45    90   4.69 0.3974
WVFGRD96    3.0   115    65    30   4.62 0.4228
WVFGRD96    4.0   290    60    20   4.66 0.4384
WVFGRD96    5.0   290    65    20   4.68 0.4418
WVFGRD96    6.0   105    75    10   4.69 0.4388
WVFGRD96    7.0   105    90     0   4.70 0.4324
WVFGRD96    8.0    65    55    10   4.75 0.4346
WVFGRD96    9.0    65    55    15   4.76 0.4564
WVFGRD96   10.0    60    55    10   4.76 0.4739
WVFGRD96   11.0    60    55    10   4.77 0.4903
WVFGRD96   12.0    60    55    10   4.78 0.5053
WVFGRD96   13.0    60    55    15   4.78 0.5189
WVFGRD96   14.0    60    60    10   4.80 0.5313
WVFGRD96   15.0    60    60    10   4.80 0.5436
WVFGRD96   16.0    55    60     5   4.80 0.5556
WVFGRD96   17.0    55    60     5   4.81 0.5684
WVFGRD96   18.0    55    60     5   4.82 0.5793
WVFGRD96   19.0    55    60     5   4.82 0.5891
WVFGRD96   20.0    55    60     5   4.83 0.5992
WVFGRD96   21.0    50    60     0   4.84 0.6085
WVFGRD96   22.0    50    60    -5   4.85 0.6188
WVFGRD96   23.0    50    60    -5   4.86 0.6279
WVFGRD96   24.0    50    60    -5   4.87 0.6364
WVFGRD96   25.0    50    60    -5   4.87 0.6449
WVFGRD96   26.0    50    60    -5   4.88 0.6519
WVFGRD96   27.0    50    60    -5   4.89 0.6583
WVFGRD96   28.0    50    60    -5   4.90 0.6648
WVFGRD96   29.0    50    60   -10   4.91 0.6697
WVFGRD96   30.0    50    65    -5   4.93 0.6747
WVFGRD96   31.0    50    65    -5   4.93 0.6801
WVFGRD96   32.0    50    65     0   4.94 0.6846
WVFGRD96   33.0    50    65     0   4.95 0.6892
WVFGRD96   34.0    50    65     0   4.95 0.6929
WVFGRD96   35.0    50    65     0   4.96 0.6964
WVFGRD96   36.0    50    70    10   4.98 0.7000
WVFGRD96   37.0    50    70    10   4.99 0.7061
WVFGRD96   38.0    50    70    10   5.00 0.7122
WVFGRD96   39.0    50    70    10   5.01 0.7180
WVFGRD96   40.0    50    60    -5   5.06 0.7187
WVFGRD96   41.0    50    60    -5   5.07 0.7198
WVFGRD96   42.0    50    60    -5   5.08 0.7199
WVFGRD96   43.0    50    60    -5   5.08 0.7193
WVFGRD96   44.0    50    60    -5   5.09 0.7180
WVFGRD96   45.0    50    60    -5   5.09 0.7160
WVFGRD96   46.0    50    60    -5   5.10 0.7134
WVFGRD96   47.0    50    60    -5   5.10 0.7103
WVFGRD96   48.0    50    65     0   5.11 0.7075
WVFGRD96   49.0    50    65     5   5.11 0.7047

The best solution is

WVFGRD96   42.0    50    60    -5   5.08 0.7199

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 0.04 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 00:25:45 CST 2015