Location

2015/11/17 20:42:18 61.680 -147.583 13.8 3.8 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  2015/11/17 20:42:18:0  61.68 -147.58  13.8 3.8 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CRQ AK.DHY AK.EYAK AK.GLB AK.GLI AK.HDA 
   AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX 
   AK.PPD AK.PWL AK.RIDG AK.RND AK.SAW AK.SCM AK.VRDI AK.WRH 
   AT.PMR TA.H24K TA.J20K TA.K20K TA.L19K TA.L26K TA.M24K 
   TA.M27K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 38 km
  Plane   Strike  Dip  Rake
   NP1      260    80   -60
   NP2        7    31   -161
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21     29     326
    N   0.00e+00     29      74
    P  -5.13e+21     47     201

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.10e+20
       Mxy    -2.63e+21
       Mxz     4.19e+21
       Myy     9.10e+20
       Myz    -2.86e+20
       Mzz    -1.52e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ##################----              
              #######################-----           
             #####   ##################----          
           ####### T ###################-----        
          ########   ####################-----       
         ################################------      
        ##################################------     
        ##################################------     
       ######################-------------#######    
       #############----------------------#######    
       #######----------------------------#######    
       ##---------------------------------#######    
        ----------------------------------######     
        ---------------------------------#######     
         --------------   --------------#######      
          ------------- P --------------######       
           ------------   ------------#######        
             ------------------------######          
              ---------------------#######           
                 ----------------######              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.52e+21   4.19e+21   2.86e+20 
  4.19e+21   6.10e+20   2.63e+21 
  2.86e+20   2.63e+21   9.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151117204218/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 = 260
      DIP = 80
     RAKE = -60
       MW = 3.74
       HS = 38.0

The NDK file is 20151117204218.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  2015/11/17 20:42:18:0  61.68 -147.58  13.8 3.8 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CRQ AK.DHY AK.EYAK AK.GLB AK.GLI AK.HDA 
   AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX 
   AK.PPD AK.PWL AK.RIDG AK.RND AK.SAW AK.SCM AK.VRDI AK.WRH 
   AT.PMR TA.H24K TA.J20K TA.K20K TA.L19K TA.L26K TA.M24K 
   TA.M27K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 38 km
  Plane   Strike  Dip  Rake
   NP1      260    80   -60
   NP2        7    31   -161
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21     29     326
    N   0.00e+00     29      74
    P  -5.13e+21     47     201

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.10e+20
       Mxy    -2.63e+21
       Mxz     4.19e+21
       Myy     9.10e+20
       Myz    -2.86e+20
       Mzz    -1.52e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ##################----              
              #######################-----           
             #####   ##################----          
           ####### T ###################-----        
          ########   ####################-----       
         ################################------      
        ##################################------     
        ##################################------     
       ######################-------------#######    
       #############----------------------#######    
       #######----------------------------#######    
       ##---------------------------------#######    
        ----------------------------------######     
        ---------------------------------#######     
         --------------   --------------#######      
          ------------- P --------------######       
           ------------   ------------#######        
             ------------------------######          
              ---------------------#######           
                 ----------------######              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.52e+21   4.19e+21   2.86e+20 
  4.19e+21   6.10e+20   2.63e+21 
  2.86e+20   2.63e+21   9.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151117204218/index.html
	
Regional Moment Tensor (Mwr)
Moment	9.077e+14 N-m
Magnitude	3.91
Depth	42.0 km
Percent DC	93%
Half Duration	–
Catalog	US (us10003z3w)
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	11	26	-162
NP2	264	82	-65
Principal Axes
Axis	Value	Plunge	Azimuth
T	8.910	32	334
N	0.326	25	81
P	-9.236	47	200

        

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 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 
br c 0.12 0.25 n 4 p 2
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    80    45    90   3.39 0.4417
WVFGRD96    4.0   260    50    85   3.52 0.4495
WVFGRD96    6.0   295    75   -55   3.47 0.4067
WVFGRD96    8.0   265    70   -80   3.54 0.4307
WVFGRD96   10.0   260    70   -80   3.51 0.4502
WVFGRD96   12.0   175    20    -5   3.48 0.4746
WVFGRD96   14.0    75    90    65   3.48 0.5054
WVFGRD96   16.0    80    85    60   3.50 0.5399
WVFGRD96   18.0   260    90   -60   3.52 0.5710
WVFGRD96   20.0   260    90   -60   3.55 0.6018
WVFGRD96   22.0    80    90    60   3.58 0.6307
WVFGRD96   24.0   260    90   -60   3.61 0.6580
WVFGRD96   26.0   260    90   -60   3.64 0.6821
WVFGRD96   28.0    85    90    60   3.66 0.7032
WVFGRD96   30.0    85    90    60   3.69 0.7212
WVFGRD96   32.0   260    85   -60   3.70 0.7352
WVFGRD96   34.0   265    85   -60   3.72 0.7438
WVFGRD96   36.0   265    85   -60   3.73 0.7480
WVFGRD96   38.0   260    80   -60   3.74 0.7493
WVFGRD96   40.0   265    85   -70   3.88 0.7475
WVFGRD96   42.0   265    80   -65   3.89 0.7440
WVFGRD96   44.0   265    80   -65   3.91 0.7382
WVFGRD96   46.0   265    80   -65   3.92 0.7299
WVFGRD96   48.0   265    80   -65   3.93 0.7192
WVFGRD96   50.0   265    80   -65   3.94 0.7064
WVFGRD96   52.0   265    80   -60   3.95 0.6925
WVFGRD96   54.0   265    80   -60   3.96 0.6768
WVFGRD96   56.0   265    80   -60   3.97 0.6596
WVFGRD96   58.0   265    80   -60   3.97 0.6412
WVFGRD96   60.0   265    80   -60   3.98 0.6224
WVFGRD96   62.0   265    80   -60   3.98 0.6023
WVFGRD96   64.0   265    80   -70   3.99 0.5847
WVFGRD96   66.0   265    85   -70   3.99 0.5683
WVFGRD96   68.0   265    85   -70   3.99 0.5539

The best solution is

WVFGRD96   38.0   260    80   -60   3.74 0.7493

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 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 
br c 0.12 0.25 n 4 p 2
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 00:09:07 CST 2015