Location

SLU Location

My initial solution with the CUS or WUS model wanted a surce depth of 2 km but with a mechansim with nodal planes below, but the P- and T-axes reversed. So P-wave first motions were read as well as P and S times and the program elocate was used to locate the event. The free depth was 21 km for the WUS model and 13 km forthe CUS model. The location for a fixed depth of 12 for the WUS model gives agreement between the MT solution and the first motions. The output of the relocation run with fixed depth is in the file elocate.txt.

Because of the dispersion measurements, the WUS model was used for the moment tensor solution. In addition only lower frequencies were used, e.g., the 0.03 - 0.06 Hz band, because the true model seems to be intermediate between the WUS and CUS models.

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/03/21 16:33:03:0 66.69 -130.44 2.6 4.6 Northwest Territories Stations used: CN.DAWY CN.INK NY.FARO NY.MAYO NY.WGLY TA.C36M TA.D28M TA.E27K TA.E28M TA.E29M TA.EPYK TA.F30M TA.F31M TA.G27K TA.G29M TA.G30M TA.G31M TA.H27K TA.H29M TA.H31M TA.I27K TA.I28M TA.I29M TA.I30M TA.J29N TA.J30M TA.L29M TA.M30M TA.M31M 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 0.06 n 3 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 12 km Plane Strike Dip Rake NP1 245 80 88 NP2 75 10 100 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 55 153 N 0.00e+00 2 245 P -1.26e+22 35 336 Moment Tensor: (dyne-cm) Component Value Mxx -3.77e+21 Mxy 1.39e+21 Mxz -1.07e+22 Myy -4.74e+20 Myz 5.09e+21 Mzz 4.24e+21 -------------- ---------------------- ---------------------------- -------- ------------------- ---------- P --------------------- ----------- ---------------------- -----------------------------------### -----------------------------########### ------------------------################ ---------------------####################- -----------------########################- --------------###########################- ----------###############################- -------################################- ----################### #############- -##################### T ############- ##################### ###########- -###############################-- -###########################-- --#######################--- ---###############---- -------------- Global CMT Convention Moment Tensor: R T P 4.24e+21 -1.07e+22 -5.09e+21 -1.07e+22 -3.77e+21 -1.39e+21 -5.09e+21 -1.39e+21 -4.74e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190321163303/index.html

Preferred Solution

      STK = 75
      DIP = 10
     RAKE = 100
       MW = 4.00
       HS = 12.0

The NDK file is 20190321163303.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

SLUFM

USGS/SLU Moment Tensor Solution ENS 2019/03/21 16:33:03:0 66.69 -130.44 2.6 4.6 Northwest Territories Stations used: CN.DAWY CN.INK NY.FARO NY.MAYO NY.WGLY TA.C36M TA.D28M TA.E27K TA.E28M TA.E29M TA.EPYK TA.F30M TA.F31M TA.G27K TA.G29M TA.G30M TA.G31M TA.H27K TA.H29M TA.H31M TA.I27K TA.I28M TA.I29M TA.I30M TA.J29N TA.J30M TA.L29M TA.M30M TA.M31M 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 0.06 n 3 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 12 km Plane Strike Dip Rake NP1 245 80 88 NP2 75 10 100 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 55 153 N 0.00e+00 2 245 P -1.26e+22 35 336 Moment Tensor: (dyne-cm) Component Value Mxx -3.77e+21 Mxy 1.39e+21 Mxz -1.07e+22 Myy -4.74e+20 Myz 5.09e+21 Mzz 4.24e+21 -------------- ---------------------- ---------------------------- -------- ------------------- ---------- P --------------------- ----------- ---------------------- -----------------------------------### -----------------------------########### ------------------------################ ---------------------####################- -----------------########################- --------------###########################- ----------###############################- -------################################- ----################### #############- -##################### T ############- ##################### ###########- -###############################-- -###########################-- --#######################--- ---###############---- -------------- Global CMT Convention Moment Tensor: R T P 4.24e+21 -1.07e+22 -5.09e+21 -1.07e+22 -3.77e+21 -1.39e+21 -5.09e+21 -1.39e+21 -4.74e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190321163303/index.html

First motions and takeoff angles from an elocate run.

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

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 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   225    45   -85   3.73 0.2914
WVFGRD96    2.0   225    40   -90   3.79 0.2881
WVFGRD96    3.0   210    35    70   3.85 0.2567
WVFGRD96    4.0    75    15   -65   3.95 0.2931
WVFGRD96    5.0   235    75   -90   3.94 0.3189
WVFGRD96    6.0    65    90   -85   3.93 0.3401
WVFGRD96    7.0   245    85    85   3.92 0.3632
WVFGRD96    8.0    10     0    35   4.00 0.3752
WVFGRD96    9.0   310    -5   -25   4.00 0.3928
WVFGRD96   10.0    85    10   110   4.00 0.4032
WVFGRD96   11.0   245    80    85   4.00 0.4076
WVFGRD96   12.0    75    10   100   4.00 0.4078
WVFGRD96   13.0    70    10    95   4.00 0.4040
WVFGRD96   14.0   245    80    90   4.00 0.3976
WVFGRD96   15.0    90    10   110   4.01 0.3892
WVFGRD96   16.0    90    10   110   4.01 0.3796
WVFGRD96   17.0    90    10   110   4.01 0.3690
WVFGRD96   18.0   250    85    85   4.01 0.3578
WVFGRD96   19.0   250    85    85   4.01 0.3467
WVFGRD96   20.0   250    85    85   4.01 0.3354
WVFGRD96   21.0    90     5   110   4.03 0.3242
WVFGRD96   22.0    90     5   110   4.03 0.3123
WVFGRD96   23.0   330    -5   -10   4.03 0.2998
WVFGRD96   24.0   290    -5   -50   4.03 0.2888
WVFGRD96   25.0   300    -5   -40   4.04 0.2768
WVFGRD96   26.0    70    90   -85   4.04 0.2646
WVFGRD96   27.0   315    -5   -25   4.04 0.2533
WVFGRD96   28.0   315    -5   -25   4.04 0.2418
WVFGRD96   29.0    60     0    80   4.04 0.2296

The best solution is

WVFGRD96   12.0    75    10   100   4.00 0.4078

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 0.06 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:

• 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)

 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: