Location

Location ANSS

2017/05/01 12:31:54 59.844 -136.695 2.2 6.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/01 12:31:54:0  59.84 -136.70   2.2 6.2 Alaska
 
 Stations used:
   AK.BARN AK.BESE AK.CTG AK.CYK AK.DOT AK.GLB AK.JIS AK.KIAG 
   AK.LOGN AK.MCAR AK.PAX AK.PIN AK.PNL AK.PTPK AK.RIDG 
   AK.SAMH AK.SAW AK.SCRK AK.VRDI AT.CRAG AT.SIT AT.SKAG 
   AV.WACK CN.BVCY CN.DAWY CN.HYT CN.YUK2 CN.YUK3 CN.YUK4 
   CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 NY.FARO NY.MAYO NY.WTLY 
   TA.HARP TA.I29M TA.J26L TA.J29M TA.K29M TA.L26K TA.L27K 
   TA.L29M TA.M23K TA.M26K TA.M27K TA.M29M TA.M30M TA.M31M 
   TA.N25K TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M 
   TA.P32M TA.P33M TA.Q32M TA.R31K TA.R32K TA.R33M TA.S31K 
   TA.S32K TA.S34M TA.T33K TA.T35M TA.U33K TA.U35K TA.V35K 
   US.EGAK US.WRAK YO.BEAV YO.LIRD 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.50e+25 dyne-cm
  Mw = 6.05 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      170    70   142
   NP2      275    55    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.50e+25     41     127
    N   0.00e+00     48     326
    P  -1.50e+25      9     225

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.97e+24
       Mxy    -1.15e+25
       Mxz    -2.83e+24
       Myy    -1.97e+24
       Myz     7.56e+24
       Mzz     5.94e+24
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ######----------------              
              #########-------------------           
             #########---------------------          
           ###########-----------------------        
          ############------------------------       
         ##########---##########---------------      
        ######--------################----------     
        ##------------####################------     
       #--------------#######################----    
       ----------------########################--    
       ----------------#########################-    
       ----------------##########################    
        ----------------########################     
        ----------------############   #########     
         ----------------########### T ########      
          ---------------###########   #######       
           --   ----------###################        
              P -----------################          
                ------------##############           
                 ------------##########              
                     ----------####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.94e+24  -2.83e+24  -7.56e+24 
 -2.83e+24  -3.97e+24   1.15e+25 
 -7.56e+24   1.15e+25  -1.97e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170501123154/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 = 275
      DIP = 55
     RAKE = 25
       MW = 6.05
       HS = 18.0

The NDK file is 20170501123154.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
GCMT
USGSW
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/01 12:31:54:0  59.84 -136.70   2.2 6.2 Alaska
 
 Stations used:
   AK.BARN AK.BESE AK.CTG AK.CYK AK.DOT AK.GLB AK.JIS AK.KIAG 
   AK.LOGN AK.MCAR AK.PAX AK.PIN AK.PNL AK.PTPK AK.RIDG 
   AK.SAMH AK.SAW AK.SCRK AK.VRDI AT.CRAG AT.SIT AT.SKAG 
   AV.WACK CN.BVCY CN.DAWY CN.HYT CN.YUK2 CN.YUK3 CN.YUK4 
   CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 NY.FARO NY.MAYO NY.WTLY 
   TA.HARP TA.I29M TA.J26L TA.J29M TA.K29M TA.L26K TA.L27K 
   TA.L29M TA.M23K TA.M26K TA.M27K TA.M29M TA.M30M TA.M31M 
   TA.N25K TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M 
   TA.P32M TA.P33M TA.Q32M TA.R31K TA.R32K TA.R33M TA.S31K 
   TA.S32K TA.S34M TA.T33K TA.T35M TA.U33K TA.U35K TA.V35K 
   US.EGAK US.WRAK YO.BEAV YO.LIRD 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.50e+25 dyne-cm
  Mw = 6.05 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      170    70   142
   NP2      275    55    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.50e+25     41     127
    N   0.00e+00     48     326
    P  -1.50e+25      9     225

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.97e+24
       Mxy    -1.15e+25
       Mxz    -2.83e+24
       Myy    -1.97e+24
       Myz     7.56e+24
       Mzz     5.94e+24
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ######----------------              
              #########-------------------           
             #########---------------------          
           ###########-----------------------        
          ############------------------------       
         ##########---##########---------------      
        ######--------################----------     
        ##------------####################------     
       #--------------#######################----    
       ----------------########################--    
       ----------------#########################-    
       ----------------##########################    
        ----------------########################     
        ----------------############   #########     
         ----------------########### T ########      
          ---------------###########   #######       
           --   ----------###################        
              P -----------################          
                ------------##############           
                 ------------##########              
                     ----------####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.94e+24  -2.83e+24  -7.56e+24 
 -2.83e+24  -3.97e+24   1.15e+25 
 -7.56e+24   1.15e+25  -1.97e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170501123154/index.html
	
CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201705011231A
DATA: II LD IU DK CU MN G  IC GE
 KP
L.P.BODY WAVES:162S, 350C, T= 40
MANTLE WAVES:  129S, 193C, T=125
SURFACE WAVES: 166S, 407C, T= 50
TIMESTAMP:      Q-20170501130642
CENTROID LOCATION:
ORIGIN TIME:      12:31:59.7 0.1
LAT:59.94N 0.00;LON:136.56W 0.01
DEP: 17.4  0.3;TRIANG HDUR:  3.1
MOMENT TENSOR: SCALE 10**25 D-CM
RR= 1.710 0.021; TT=-0.923 0.018
PP=-0.790 0.017; RT= 1.030 0.054
RP=-1.630 0.059; TP= 2.050 0.015
PRINCIPAL AXES:
1.(T) VAL=  2.514;PLG=64;AZM= 89
2.(N)       1.060;    16;    322
3.(P)      -3.577;    20;    226
BEST DBLE.COUPLE:M0= 3.05*10**25
NP1: STRIKE=290;DIP=29;SLIP=  55
NP2: STRIKE=149;DIP=67;SLIP= 108

            -----------
        #------------------
      ###---#######----------
    ###--###############-------
   #-----##################-----
  #-------###################----
  --------####################---
 ----------##########   #######---
 -----------######### T ########--
 ------------########   #########-
 -------------###################-
  -------------##################
  --------------#################
   ---   ---------##############
    -- P ----------############
         -------------#######
        -----------------##
            -----------
        
W-phase Moment Tensor (Mww)
Moment	2.570e+18 N-m
Magnitude	6.2 Mww
Depth	15.5 km
Percent DC	88 %
Half Duration	6 s
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	282	46	35
NP2	165	66	130
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.489e+18 N-m	52	122
N	0.156e+18 N-m	36	326
P	-2.645e+18 N-m	12	228

        

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 using wvfgrd96

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 0.03 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    1.0    90    90     0   5.60 0.3317
WVFGRD96    2.0   315    45    90   5.84 0.4186
WVFGRD96    3.0    85    70   -15   5.76 0.4394
WVFGRD96    4.0    85    65   -15   5.80 0.4582
WVFGRD96    5.0    85    55   -10   5.84 0.4759
WVFGRD96    6.0    90    55     5   5.86 0.5009
WVFGRD96    7.0    90    55     5   5.88 0.5285
WVFGRD96    8.0    90    50    10   5.93 0.5539
WVFGRD96    9.0   275    50    20   5.95 0.5779
WVFGRD96   10.0   275    50    25   5.97 0.6056
WVFGRD96   11.0   275    50    25   5.98 0.6289
WVFGRD96   12.0   275    50    25   5.99 0.6473
WVFGRD96   13.0   275    50    25   6.00 0.6614
WVFGRD96   14.0   275    55    25   6.01 0.6727
WVFGRD96   15.0   275    55    25   6.02 0.6807
WVFGRD96   16.0   275    55    25   6.03 0.6860
WVFGRD96   17.0   275    55    25   6.04 0.6886
WVFGRD96   18.0   275    55    25   6.05 0.6889
WVFGRD96   19.0   275    55    25   6.05 0.6873
WVFGRD96   20.0   275    55    25   6.06 0.6836
WVFGRD96   21.0   275    55    25   6.08 0.6816
WVFGRD96   22.0   275    55    25   6.08 0.6752
WVFGRD96   23.0   275    55    25   6.09 0.6674
WVFGRD96   24.0   275    55    25   6.10 0.6584
WVFGRD96   25.0   275    55    25   6.10 0.6482
WVFGRD96   26.0   275    55    25   6.11 0.6367
WVFGRD96   27.0   275    60    20   6.12 0.6245
WVFGRD96   28.0   275    60    20   6.12 0.6117
WVFGRD96   29.0   275    60    20   6.13 0.5981

The best solution is

WVFGRD96   18.0   275    55    25   6.05 0.6889

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 0.03 n 3 
lp c 0.06 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 May 1 13:08:06 CDT 2017