Location

Location ANSS

2019/02/06 20:27:59 61.377 -150.026 42.8 3.7 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/02/06 20:27:59:0 61.38 -150.03 42.8 3.7 Alaska Stations used: AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SSN AT.PMR AV.STLK GM.AD09 TA.M22K TA.O22K Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 3.51e+21 dyne-cm Mw = 3.63 Z = 39 km Plane Strike Dip Rake NP1 230 75 -55 NP2 340 38 -155 Principal Axes: Axis Value Plunge Azimuth T 3.51e+21 22 294 N 0.00e+00 34 40 P -3.51e+21 48 177 Moment Tensor: (dyne-cm) Component Value Mxx -1.07e+21 Mxy -1.04e+21 Mxz 2.24e+21 Myy 2.51e+21 Myz -1.20e+21 Mzz -1.44e+21 ##------------ ############---------- ##################---------- #####################--------- #########################----##### ##########################-######### ### ##################-----######### #### T ###############---------######### #### #############------------######## ##################----------------######## ################------------------######## ##############--------------------######## ############-----------------------####### ##########------------------------###### ########--------------------------###### ######------------ -----------###### ####------------- P -----------##### ##-------------- ----------##### --------------------------#### ------------------------#### --------------------## -------------- Global CMT Convention Moment Tensor: R T P -1.44e+21 2.24e+21 1.20e+21 2.24e+21 -1.07e+21 1.04e+21 1.20e+21 1.04e+21 2.51e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190206202759/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 = 230
      DIP = 75
     RAKE = -55
       MW = 3.63
       HS = 39.0

The NDK file is 20190206202759.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/02/06 20:27:59:0 61.38 -150.03 42.8 3.7 Alaska Stations used: AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW AK.SSN AT.PMR AV.STLK GM.AD09 TA.M22K TA.O22K Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 3.51e+21 dyne-cm Mw = 3.63 Z = 39 km Plane Strike Dip Rake NP1 230 75 -55 NP2 340 38 -155 Principal Axes: Axis Value Plunge Azimuth T 3.51e+21 22 294 N 0.00e+00 34 40 P -3.51e+21 48 177 Moment Tensor: (dyne-cm) Component Value Mxx -1.07e+21 Mxy -1.04e+21 Mxz 2.24e+21 Myy 2.51e+21 Myz -1.20e+21 Mzz -1.44e+21 ##------------ ############---------- ##################---------- #####################--------- #########################----##### ##########################-######### ### ##################-----######### #### T ###############---------######### #### #############------------######## ##################----------------######## ################------------------######## ##############--------------------######## ############-----------------------####### ##########------------------------###### ########--------------------------###### ######------------ -----------###### ####------------- P -----------##### ##-------------- ----------##### --------------------------#### ------------------------#### --------------------## -------------- Global CMT Convention Moment Tensor: R T P -1.44e+21 2.24e+21 1.20e+21 2.24e+21 -1.07e+21 1.04e+21 1.20e+21 1.04e+21 2.51e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190206202759/index.html

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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    1.0   320    75    15   2.90 0.1887
WVFGRD96    2.0   320    75    15   3.12 0.3323
WVFGRD96    3.0   140    75    10   3.19 0.3819
WVFGRD96    4.0   140    70    10   3.24 0.4152
WVFGRD96    5.0   135    80   -35   3.30 0.4473
WVFGRD96    6.0   130    55   -25   3.34 0.4766
WVFGRD96    7.0   135    45   -10   3.35 0.4970
WVFGRD96    8.0   135    40   -10   3.40 0.5065
WVFGRD96    9.0   130    45   -20   3.41 0.5145
WVFGRD96   10.0   130    45   -25   3.42 0.5193
WVFGRD96   11.0   130    45   -25   3.43 0.5220
WVFGRD96   12.0   130    50   -25   3.43 0.5245
WVFGRD96   13.0   130    50   -25   3.44 0.5262
WVFGRD96   14.0   130    50   -25   3.45 0.5269
WVFGRD96   15.0   130    50   -25   3.46 0.5275
WVFGRD96   16.0   130    50   -25   3.46 0.5279
WVFGRD96   17.0   130    50   -25   3.47 0.5279
WVFGRD96   18.0   130    50   -25   3.48 0.5276
WVFGRD96   19.0   130    50   -25   3.49 0.5276
WVFGRD96   20.0   130    50   -25   3.50 0.5272
WVFGRD96   21.0   135    50   -10   3.50 0.5279
WVFGRD96   22.0   135    50   -10   3.51 0.5291
WVFGRD96   23.0   135    50   -10   3.52 0.5301
WVFGRD96   24.0   160    40    15   3.52 0.5321
WVFGRD96   25.0   160    40    15   3.53 0.5356
WVFGRD96   26.0   160    40    15   3.54 0.5383
WVFGRD96   27.0   160    40    15   3.55 0.5416
WVFGRD96   28.0    35    85    50   3.59 0.5479
WVFGRD96   29.0    35    85    50   3.60 0.5549
WVFGRD96   30.0   210    90   -45   3.61 0.5553
WVFGRD96   31.0    35    85    50   3.62 0.5635
WVFGRD96   32.0    35    85    50   3.62 0.5663
WVFGRD96   33.0   210    90   -45   3.62 0.5652
WVFGRD96   34.0    35    85    50   3.63 0.5680
WVFGRD96   35.0    35    85    45   3.63 0.5680
WVFGRD96   36.0   235    80   -60   3.63 0.5698
WVFGRD96   37.0   230    75   -60   3.63 0.5742
WVFGRD96   38.0   230    75   -55   3.63 0.5793
WVFGRD96   39.0   230    75   -55   3.63 0.5825
WVFGRD96   40.0   220    80   -65   3.74 0.5804
WVFGRD96   41.0   220    80   -65   3.74 0.5795
WVFGRD96   42.0   215    75   -65   3.74 0.5783
WVFGRD96   43.0   215    75   -65   3.75 0.5780
WVFGRD96   44.0   215    75   -65   3.76 0.5769
WVFGRD96   45.0   215    75   -65   3.76 0.5761
WVFGRD96   46.0   215    75   -65   3.76 0.5740
WVFGRD96   47.0   210    70   -65   3.77 0.5741
WVFGRD96   48.0   210    70   -65   3.77 0.5727
WVFGRD96   49.0   210    70   -65   3.78 0.5729
WVFGRD96   50.0   210    70   -65   3.78 0.5713
WVFGRD96   51.0   210    70   -65   3.79 0.5707
WVFGRD96   52.0   210    70   -65   3.79 0.5694
WVFGRD96   53.0   210    70   -65   3.79 0.5690
WVFGRD96   54.0   210    70   -65   3.80 0.5686
WVFGRD96   55.0   210    70   -65   3.80 0.5670
WVFGRD96   56.0   210    70   -65   3.80 0.5682
WVFGRD96   57.0   210    70   -65   3.81 0.5668
WVFGRD96   58.0   210    70   -65   3.81 0.5663
WVFGRD96   59.0   210    70   -65   3.81 0.5647

The best solution is

WVFGRD96   39.0   230    75   -55   3.63 0.5825

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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:

The origin time and epicentral distance are incorrect
The velocity model used for the inversion is incorrect
The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

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 Wed Feb 6 16:48:55 CST 2019