Location

2012/08/24 21:58:09 63.931 -148.416 12.7 3.70 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  2012/08/24 21:58:09:0  63.93 -148.42  12.7 3.7 Alaska
 
 Stations used:
   AK.BAL AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CTG AK.DHY 
   AK.FID AK.FYU AK.GHO AK.GLI AK.GLM AK.HDA AK.HIN AK.HMT 
   AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX 
   AK.PPD AK.PPLA AK.RAG AK.RIDG AK.RND AK.SAW AK.SCM AK.SGA 
   AK.SKN AK.SSN AK.TGL AK.WAX AK.WRH AK.YAH AT.MID AT.SVW2 
   CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 6.53e+21 dyne-cm
  Mw = 3.81 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1       82    67   101
   NP2      235    25    65
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.53e+21     66      12
    N   0.00e+00     10     258
    P  -6.53e+21     22     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.14e+21
       Mxy     1.73e+21
       Mxz     4.55e+21
       Myy    -3.96e+20
       Myz    -1.33e+20
       Mzz     4.53e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------###############---           
             -------######################-          
           -------###########################        
          ------##############################       
         -----##############   ################      
        -----############### T #################     
        ----################   #################     
       ----######################################    
       ---#####################################--    
       ---##################################-----    
       ---##############################---------    
        ##-#######################--------------     
        ##--------------------------------------     
         #-------------------------------------      
          ------------------------------------       
           ----------------------------------        
             ------------------   ---------          
              ----------------- P --------           
                 --------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.53e+21   4.55e+21   1.33e+20 
  4.55e+21  -4.14e+21  -1.73e+21 
  1.33e+20  -1.73e+21  -3.96e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120824215809/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 = 235
      DIP = 25
     RAKE = 65
       MW = 3.81
       HS = 17.0

The NDK file is 20120824215809.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  2012/08/24 21:58:09:0  63.93 -148.42  12.7 3.7 Alaska
 
 Stations used:
   AK.BAL AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CTG AK.DHY 
   AK.FID AK.FYU AK.GHO AK.GLI AK.GLM AK.HDA AK.HIN AK.HMT 
   AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX 
   AK.PPD AK.PPLA AK.RAG AK.RIDG AK.RND AK.SAW AK.SCM AK.SGA 
   AK.SKN AK.SSN AK.TGL AK.WAX AK.WRH AK.YAH AT.MID AT.SVW2 
   CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 6.53e+21 dyne-cm
  Mw = 3.81 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1       82    67   101
   NP2      235    25    65
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.53e+21     66      12
    N   0.00e+00     10     258
    P  -6.53e+21     22     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.14e+21
       Mxy     1.73e+21
       Mxz     4.55e+21
       Myy    -3.96e+20
       Myz    -1.33e+20
       Mzz     4.53e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------###############---           
             -------######################-          
           -------###########################        
          ------##############################       
         -----##############   ################      
        -----############### T #################     
        ----################   #################     
       ----######################################    
       ---#####################################--    
       ---##################################-----    
       ---##############################---------    
        ##-#######################--------------     
        ##--------------------------------------     
         #-------------------------------------      
          ------------------------------------       
           ----------------------------------        
             ------------------   ---------          
              ----------------- P --------           
                 --------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.53e+21   4.55e+21   1.33e+20 
  4.55e+21  -4.14e+21  -1.73e+21 
  1.33e+20  -1.73e+21  -3.96e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120824215809/index.html
	
NEIC Mwr

Moment
    6.34e+14 N-m
Magnitude
    3.8
Percent DC
    89%
Depth
    14.0 km

Principal Axes
Axis	Value	Plunge	Azimuth
T	6.506	62	11
N	-0.336	15	249
P	-6.170	22	153
Nodal Planes
Plane	Strike	Dip	Rake
NP1	225	34	61
NP2	78	64	118


        

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   275    40   -90   3.48 0.3567
WVFGRD96    1.0   275    40   -90   3.51 0.3353
WVFGRD96    2.0   115    50   -95   3.60 0.3929
WVFGRD96    3.0   330    55   -30   3.68 0.3513
WVFGRD96    4.0   335    60   -15   3.71 0.3298
WVFGRD96    5.0   175    20    -5   3.68 0.3725
WVFGRD96    6.0   180    20     0   3.68 0.4237
WVFGRD96    7.0   180    25     0   3.68 0.4595
WVFGRD96    8.0   185    20     5   3.75 0.4849
WVFGRD96    9.0   190    20    15   3.75 0.5144
WVFGRD96   10.0   200    20    25   3.75 0.5382
WVFGRD96   11.0   210    20    35   3.76 0.5579
WVFGRD96   12.0   210    25    40   3.77 0.5725
WVFGRD96   13.0   220    25    50   3.78 0.5847
WVFGRD96   14.0   220    25    50   3.79 0.5931
WVFGRD96   15.0   230    25    60   3.80 0.5975
WVFGRD96   16.0   230    25    60   3.80 0.5994
WVFGRD96   17.0   235    25    65   3.81 0.5994
WVFGRD96   18.0   230    25    60   3.81 0.5966
WVFGRD96   19.0   230    25    60   3.81 0.5918
WVFGRD96   20.0   230    25    60   3.81 0.5860
WVFGRD96   21.0   235    25    65   3.83 0.5799
WVFGRD96   22.0   235    25    65   3.83 0.5716
WVFGRD96   23.0   230    25    60   3.83 0.5624
WVFGRD96   24.0   230    25    60   3.84 0.5529
WVFGRD96   25.0   230    25    60   3.84 0.5427
WVFGRD96   26.0   230    25    60   3.84 0.5320
WVFGRD96   27.0   230    25    60   3.85 0.5211
WVFGRD96   28.0   225    25    55   3.85 0.5096
WVFGRD96   29.0   225    25    55   3.85 0.4975

The best solution is

WVFGRD96   17.0   235    25    65   3.81 0.5994

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 00:25:26 CST 2015