Location

2015/07/21 03:08:32 62.344 -149.704 53.0 4.4 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  2015/07/21 03:08:32:0  62.34 -149.70  53.0 4.4 Alaska
 
 Stations used:
   AK.BWN AK.CCB AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH 
   AK.MCK AK.MDM AK.NEA2 AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW 
   AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH IM.IL31 TA.M24K TA.N25K 
   TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 2.60e+22 dyne-cm
  Mw = 4.21 
  Z  = 58 km
  Plane   Strike  Dip  Rake
   NP1      317    69   -131
   NP2      205    45   -30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.60e+22     14      76
    N   0.00e+00     38     334
    P  -2.60e+22     49     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.88e+21
       Mxy     5.26e+21
       Mxz     1.44e+22
       Myy     2.29e+22
       Myz     6.73e+21
       Mzz    -1.30e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 --------------########              
              -------------###############           
             #####-------##################          
           ###########-######################        
          ###########----#####################       
         ###########-------####################      
        ###########----------###############   #     
        ##########-------------############# T #     
       ##########----------------###########   ##    
       #########------------------###############    
       #########--------------------#############    
       #########---------------------############    
        #######-----------------------##########     
        #######------------------------#########     
         ######-----------   -----------#######      
          ######---------- P ------------#####       
           #####----------   ------------####        
             ###--------------------------#          
              ###-------------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.30e+22   1.44e+22  -6.73e+21 
  1.44e+22  -9.88e+21  -5.26e+21 
 -6.73e+21  -5.26e+21   2.29e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150721030832/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 = 205
      DIP = 45
     RAKE = -30
       MW = 4.21
       HS = 58.0

The NDK file is 20150721030832.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  2015/07/21 03:08:32:0  62.34 -149.70  53.0 4.4 Alaska
 
 Stations used:
   AK.BWN AK.CCB AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.KTH 
   AK.MCK AK.MDM AK.NEA2 AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW 
   AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH IM.IL31 TA.M24K TA.N25K 
   TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 2.60e+22 dyne-cm
  Mw = 4.21 
  Z  = 58 km
  Plane   Strike  Dip  Rake
   NP1      317    69   -131
   NP2      205    45   -30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.60e+22     14      76
    N   0.00e+00     38     334
    P  -2.60e+22     49     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.88e+21
       Mxy     5.26e+21
       Mxz     1.44e+22
       Myy     2.29e+22
       Myz     6.73e+21
       Mzz    -1.30e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 --------------########              
              -------------###############           
             #####-------##################          
           ###########-######################        
          ###########----#####################       
         ###########-------####################      
        ###########----------###############   #     
        ##########-------------############# T #     
       ##########----------------###########   ##    
       #########------------------###############    
       #########--------------------#############    
       #########---------------------############    
        #######-----------------------##########     
        #######------------------------#########     
         ######-----------   -----------#######      
          ######---------- P ------------#####       
           #####----------   ------------####        
             ###--------------------------#          
              ###-------------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.30e+22   1.44e+22  -6.73e+21 
  1.44e+22  -9.88e+21  -5.26e+21 
 -6.73e+21  -5.26e+21   2.29e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150721030832/index.html
	
Regional Moment Tensor (Mwr)
Moment	2.378e+15 N-m
Magnitude	4.18
Depth	57.0 km
Percent DC	87%
Half Duration	–
Catalog	AK (ak11651501)
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	208	53	-30
NP2	317	66	-139
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.295	8	79
N	0.158	43	341
P	-2.453	45	178

        

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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:

cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.05 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   160    45   -90   3.54 0.1797
WVFGRD96    4.0   260    45    85   3.62 0.1911
WVFGRD96    6.0   230    65    20   3.60 0.1911
WVFGRD96    8.0   230    50    30   3.67 0.2065
WVFGRD96   10.0   215    50   -10   3.67 0.2208
WVFGRD96   12.0   215    50   -10   3.70 0.2422
WVFGRD96   14.0   210    55   -20   3.72 0.2694
WVFGRD96   16.0   210    50   -20   3.75 0.2973
WVFGRD96   18.0   210    50   -20   3.77 0.3237
WVFGRD96   20.0   210    50   -20   3.79 0.3471
WVFGRD96   22.0   210    50   -20   3.82 0.3685
WVFGRD96   24.0   210    50   -20   3.84 0.3885
WVFGRD96   26.0   210    50   -20   3.86 0.4068
WVFGRD96   28.0   210    50   -20   3.88 0.4232
WVFGRD96   30.0   210    50   -20   3.90 0.4379
WVFGRD96   32.0   210    50   -20   3.91 0.4507
WVFGRD96   34.0   210    50   -20   3.93 0.4615
WVFGRD96   36.0   210    50   -20   3.95 0.4705
WVFGRD96   38.0   210    55   -20   3.97 0.4776
WVFGRD96   40.0   205    40   -25   4.08 0.4891
WVFGRD96   42.0   205    40   -30   4.10 0.5019
WVFGRD96   44.0   205    45   -30   4.11 0.5129
WVFGRD96   46.0   205    45   -30   4.13 0.5238
WVFGRD96   48.0   205    45   -30   4.14 0.5325
WVFGRD96   50.0   205    45   -30   4.16 0.5398
WVFGRD96   52.0   205    45   -30   4.17 0.5451
WVFGRD96   54.0   205    45   -30   4.18 0.5492
WVFGRD96   56.0   205    45   -30   4.20 0.5519
WVFGRD96   58.0   205    45   -30   4.21 0.5531
WVFGRD96   60.0   205    45   -30   4.22 0.5529
WVFGRD96   62.0   205    45   -30   4.23 0.5516
WVFGRD96   64.0   205    45   -30   4.24 0.5488
WVFGRD96   66.0   205    45   -30   4.25 0.5444
WVFGRD96   68.0   210    50   -25   4.26 0.5392
WVFGRD96   70.0   210    50   -25   4.27 0.5339
WVFGRD96   72.0   210    50   -25   4.28 0.5276
WVFGRD96   74.0   210    50   -25   4.28 0.5198
WVFGRD96   76.0   210    50   -25   4.29 0.5111
WVFGRD96   78.0   215    50   -20   4.30 0.5025
WVFGRD96   80.0   215    50   -20   4.31 0.4939
WVFGRD96   82.0   215    50   -15   4.31 0.4862
WVFGRD96   84.0   215    50   -15   4.32 0.4787
WVFGRD96   86.0   215    50   -15   4.33 0.4706
WVFGRD96   88.0   215    50   -15   4.33 0.4617
WVFGRD96   90.0   215    50   -15   4.34 0.4520
WVFGRD96   92.0   215    50   -15   4.34 0.4420
WVFGRD96   94.0   215    50   -15   4.35 0.4313
WVFGRD96   96.0   215    50   -15   4.35 0.4204
WVFGRD96   98.0   215    50   -10   4.35 0.4095

The best solution is

WVFGRD96   58.0   205    45   -30   4.21 0.5531

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

cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.05 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
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 00:05:45 CST 2015