Location

Location ANSS

2017/03/02 02:11:30 59.586 -152.612 76.9 5.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/03/02 02:11:30:0  59.59 -152.61  76.9 5.5 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM 
   AK.PWL AK.RC01 AK.SKN AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 
   AV.ILSW II.KDAK TA.M22K TA.N19K TA.O19K TA.O22K TA.P18K 
   TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.66e+24 dyne-cm
  Mw = 5.55 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      302    66   141
   NP2       50    55    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.66e+24     44     262
    N   0.00e+00     45      95
    P  -2.66e+24      7     358

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.59e+24
       Mxy     2.88e+23
       Mxz    -5.01e+23
       Myy     1.34e+24
       Myz    -1.30e+24
       Mzz     1.25e+24
                                                     
                                                     
                                                     
                                                     
                     ----- P ------                  
                 ---------   ----------              
              ----------------------------           
             ------------------------------          
           ---------------------------------#        
          ##########------------------------##       
         ################-------------------###      
        #####################--------------#####     
        ########################----------######     
       ############################------########    
       ########   ###################---#########    
       ######## T ###############################    
       ########   ####################---########    
        ############################------######     
        ##########################----------####     
         #######################-------------##      
          ###################----------------#       
           ##############--------------------        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.25e+24  -5.01e+23   1.30e+24 
 -5.01e+23  -2.59e+24  -2.88e+23 
  1.30e+24  -2.88e+23   1.34e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170302021130/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 = 55
     RAKE = 30
       MW = 5.55
       HS = 90.0

The NDK file is 20170302021130.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
USGSMWR
GCMT
 USGS/SLU Moment Tensor Solution
 ENS  2017/03/02 02:11:30:0  59.59 -152.61  76.9 5.5 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM 
   AK.PWL AK.RC01 AK.SKN AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 
   AV.ILSW II.KDAK TA.M22K TA.N19K TA.O19K TA.O22K TA.P18K 
   TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.66e+24 dyne-cm
  Mw = 5.55 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      302    66   141
   NP2       50    55    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.66e+24     44     262
    N   0.00e+00     45      95
    P  -2.66e+24      7     358

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.59e+24
       Mxy     2.88e+23
       Mxz    -5.01e+23
       Myy     1.34e+24
       Myz    -1.30e+24
       Mzz     1.25e+24
                                                     
                                                     
                                                     
                                                     
                     ----- P ------                  
                 ---------   ----------              
              ----------------------------           
             ------------------------------          
           ---------------------------------#        
          ##########------------------------##       
         ################-------------------###      
        #####################--------------#####     
        ########################----------######     
       ############################------########    
       ########   ###################---#########    
       ######## T ###############################    
       ########   ####################---########    
        ############################------######     
        ##########################----------####     
         #######################-------------##      
          ###################----------------#       
           ##############--------------------        
             ------------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.25e+24  -5.01e+23   1.30e+24 
 -5.01e+23  -2.59e+24  -2.88e+23 
  1.30e+24  -2.88e+23   1.34e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170302021130/index.html
	

Nody-wave Moment Tensor (Mwb)
Moment	2.921e+17 N-m
Magnitude	5.6 Mwb
Depth	76.0 km
Percent DC	94 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	304	64	137
NP2	56	52	33
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.968e+17 N-m	48	264
N	-0.096e+17 N-m	41	99
P	-2.871e+17 N-m	7	3

        
Regional Moment Tensor (Mwr)
Moment	2.663e+17 N-m
Magnitude	5.6 Mwr
Depth	80.0 km
Percent DC	98 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	297	55	141
NP2	53	59	42
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.676e+17 N-m	51	267
N	-0.028e+17 N-m	39	82
P	-2.649e+17 N-m	2	174

        
March 2, 2017, SOUTHERN ALASKA, MW=5.6

Howard Koss

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     C201703020211A
DATA: II IU DK CU MN G  IC LD GE
 KP
L.P.BODY WAVES:132S, 217C, T= 40
MANTLE WAVES:   37S,  39C, T=125
SURFACE WAVES: 157S, 298C, T= 50
TIMESTAMP:      Q-20170302140519
CENTROID LOCATION:
ORIGIN TIME:      02:11:32.9 0.1
LAT:59.62N 0.01;LON:152.65W 0.01
DEP: 82.3  1.1;TRIANG HDUR:  1.5
MOMENT TENSOR: SCALE 10**24 D-CM
RR= 0.982 0.030; TT=-2.730 0.033
PP= 1.750 0.035; RT=-0.393 0.026
RP= 1.620 0.022; TP=-0.428 0.032
PRINCIPAL AXES:
1.(T) VAL=  3.089;PLG=38;AZM=263
2.(N)      -0.298;    51;     92
3.(P)      -2.788;     4;    356
BEST DBLE.COUPLE:M0= 2.94*10**24
NP1: STRIKE= 47;DIP=61;SLIP=  26
NP2: STRIKE=303;DIP=67;SLIP= 148

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

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 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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    80    40   -85   4.68 0.1511
WVFGRD96    4.0   300    70    30   4.69 0.1717
WVFGRD96    6.0   290    55   -25   4.73 0.1921
WVFGRD96    8.0   290    55   -30   4.82 0.2145
WVFGRD96   10.0   300    65    25   4.87 0.2287
WVFGRD96   12.0   300    65    25   4.90 0.2368
WVFGRD96   14.0   300    65    25   4.92 0.2335
WVFGRD96   16.0    35    65    25   4.94 0.2294
WVFGRD96   18.0    25    55    -5   4.98 0.2317
WVFGRD96   20.0    25    60    -5   5.02 0.2469
WVFGRD96   22.0    25    55    -5   5.05 0.2605
WVFGRD96   24.0    25    55    -5   5.07 0.2713
WVFGRD96   26.0    25    55    -5   5.10 0.2776
WVFGRD96   28.0    25    55    -5   5.11 0.2795
WVFGRD96   30.0    30    55     0   5.12 0.2809
WVFGRD96   32.0    30    60     5   5.14 0.2820
WVFGRD96   34.0    30    60     5   5.15 0.2809
WVFGRD96   36.0    35    65    10   5.16 0.2807
WVFGRD96   38.0    35    65    10   5.19 0.2868
WVFGRD96   40.0    35    55    10   5.27 0.2992
WVFGRD96   42.0    35    60    10   5.28 0.3022
WVFGRD96   44.0    35    60    15   5.31 0.3057
WVFGRD96   46.0    40    60    20   5.33 0.3090
WVFGRD96   48.0    40    60    20   5.35 0.3126
WVFGRD96   50.0    40    60    20   5.36 0.3158
WVFGRD96   52.0    40    60    20   5.37 0.3192
WVFGRD96   54.0    40    60    25   5.39 0.3237
WVFGRD96   56.0    45    60    30   5.41 0.3309
WVFGRD96   58.0    45    60    30   5.42 0.3387
WVFGRD96   60.0    45    60    30   5.43 0.3491
WVFGRD96   62.0    45    60    30   5.44 0.3597
WVFGRD96   64.0    50    55    30   5.46 0.3706
WVFGRD96   66.0    50    55    30   5.47 0.3818
WVFGRD96   68.0    50    55    30   5.48 0.3920
WVFGRD96   70.0    50    55    30   5.49 0.4020
WVFGRD96   72.0    50    55    30   5.50 0.4104
WVFGRD96   74.0    50    55    30   5.51 0.4182
WVFGRD96   76.0    50    60    30   5.52 0.4252
WVFGRD96   78.0    50    60    30   5.53 0.4299
WVFGRD96   80.0    50    60    30   5.53 0.4339
WVFGRD96   82.0    50    55    30   5.53 0.4364
WVFGRD96   84.0    50    55    30   5.54 0.4385
WVFGRD96   86.0    50    55    30   5.54 0.4407
WVFGRD96   88.0    50    55    30   5.55 0.4410
WVFGRD96   90.0    50    55    30   5.55 0.4415
WVFGRD96   92.0    50    55    30   5.55 0.4403
WVFGRD96   94.0    50    55    30   5.55 0.4382
WVFGRD96   96.0    50    55    30   5.56 0.4361
WVFGRD96   98.0    50    55    30   5.56 0.4339

The best solution is

WVFGRD96   90.0    50    55    30   5.55 0.4415

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 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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 Thu Mar 2 14:19:08 CST 2017