Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/01/08 23:47:12:0 74.32 -92.31 18.9 5.8 Canada Stations used: CN.EUNU CN.FRB CN.INK CN.RES CN.YKAW3 DK.NEEM DK.NOR DK.TULEG NY.WGLY TA.A36M TA.C36M TA.E25K TA.E27K TA.EPYK TA.F31M TA.G26K TA.G27K TA.G30M TA.H27K TA.I29M Filtering commands used: cut o DIST/3.3 -100 o DIST/3.3 +150 rtr taper w 0.1 hp c 0.01 n 3 lp c 0.04 n 3 Best Fitting Double Couple Mo = 7.76e+24 dyne-cm Mw = 5.86 Z = 34 km Plane Strike Dip Rake NP1 312 56 97 NP2 120 35 80 Principal Axes: Axis Value Plunge Azimuth T 7.76e+24 78 247 N 0.00e+00 6 128 P -7.76e+24 10 37 Moment Tensor: (dyne-cm) Component Value Mxx -4.72e+24 Mxy -3.50e+24 Mxz -1.71e+24 Myy -2.47e+24 Myz -2.26e+24 Mzz 7.18e+24 -------------- -------------------- ----------------------- P -- #######----------------- --- ##############-------------------- ###################----------------- -######################--------------- --########################-------------- --##########################------------ ---###########################------------ ----############# ############---------- ----############# T #############--------- -----############ ##############-------- -----#############################------ -------############################----- --------##########################---- ---------########################--- -----------#####################-# -------------##############--- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 7.18e+24 -1.71e+24 2.26e+24 -1.71e+24 -4.72e+24 3.50e+24 2.26e+24 3.50e+24 -2.47e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170108234712/index.html

Preferred Solution

      STK = 120
      DIP = 35
     RAKE = 80
       MW = 5.86
       HS = 34.0

The NDK file is 20170108234712.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 2017/01/08 23:47:12:0 74.32 -92.31 18.9 5.8 Canada Stations used: CN.EUNU CN.FRB CN.INK CN.RES CN.YKAW3 DK.NEEM DK.NOR DK.TULEG NY.WGLY TA.A36M TA.C36M TA.E25K TA.E27K TA.EPYK TA.F31M TA.G26K TA.G27K TA.G30M TA.H27K TA.I29M Filtering commands used: cut o DIST/3.3 -100 o DIST/3.3 +150 rtr taper w 0.1 hp c 0.01 n 3 lp c 0.04 n 3 Best Fitting Double Couple Mo = 7.76e+24 dyne-cm Mw = 5.86 Z = 34 km Plane Strike Dip Rake NP1 312 56 97 NP2 120 35 80 Principal Axes: Axis Value Plunge Azimuth T 7.76e+24 78 247 N 0.00e+00 6 128 P -7.76e+24 10 37 Moment Tensor: (dyne-cm) Component Value Mxx -4.72e+24 Mxy -3.50e+24 Mxz -1.71e+24 Myy -2.47e+24 Myz -2.26e+24 Mzz 7.18e+24 -------------- -------------------- ----------------------- P -- #######----------------- --- ##############-------------------- ###################----------------- -######################--------------- --########################-------------- --##########################------------ ---###########################------------ ----############# ############---------- ----############# T #############--------- -----############ ##############-------- -----#############################------ -------############################----- --------##########################---- ---------########################--- -----------#####################-# -------------##############--- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 7.18e+24 -1.71e+24 2.26e+24 -1.71e+24 -4.72e+24 3.50e+24 2.26e+24 3.50e+24 -2.47e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170108234712/index.html

Body-wave Moment Tensor (Mwb) Moment 9.868e+17 N-m Magnitude 5.9 Mwb Depth 21.0 km Percent DC 58 % Half Duration – Catalog US Data Source US2 Contributor US2 Nodal Planes Plane Strike Dip Rake NP1 326 59 125 NP2 92 46 47 Principal Axes Axis Value Plunge Azimuth T 8.519e+17 N-m 60 289 N 2.294e+17 N-m 29 126 P -10.813e+17 N-m 7 32

Magnitudes

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 -100 o DIST/3.3 +150
rtr
taper w 0.1
hp c 0.01 n 3 
lp c 0.04 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   305    40   -90   5.66 0.4391
WVFGRD96    2.0   320    40   -90   5.68 0.4466
WVFGRD96    3.0   165    45   -75   5.66 0.4212
WVFGRD96    4.0   340    90   -10   5.53 0.4012
WVFGRD96    5.0   160    90    10   5.55 0.3908
WVFGRD96    6.0    70    55    -5   5.57 0.3795
WVFGRD96    7.0    70    55    -5   5.58 0.3787
WVFGRD96    8.0   320    80   -50   5.66 0.3756
WVFGRD96    9.0   320    80   -50   5.66 0.3849
WVFGRD96   10.0   325    80   -55   5.66 0.3975
WVFGRD96   11.0   325    85   -60   5.67 0.4083
WVFGRD96   12.0   325    85   -60   5.67 0.4207
WVFGRD96   13.0   285    75    70   5.76 0.4393
WVFGRD96   14.0   285    75    70   5.76 0.4600
WVFGRD96   15.0   285    75    70   5.76 0.4771
WVFGRD96   16.0   285    75    70   5.76 0.4925
WVFGRD96   17.0   290    70    65   5.76 0.5064
WVFGRD96   18.0   290    70    70   5.77 0.5234
WVFGRD96   19.0   290    70    70   5.77 0.5374
WVFGRD96   20.0   290    70    70   5.81 0.5469
WVFGRD96   21.0   290    70    70   5.81 0.5565
WVFGRD96   22.0   290    70    70   5.81 0.5636
WVFGRD96   23.0   295    65    70   5.80 0.5708
WVFGRD96   24.0   295    65    75   5.81 0.5776
WVFGRD96   25.0   295    65    75   5.81 0.5828
WVFGRD96   26.0   295    65    75   5.81 0.5860
WVFGRD96   27.0   300    65    75   5.82 0.5882
WVFGRD96   28.0   300    65    75   5.82 0.5899
WVFGRD96   29.0   135    30   100   5.83 0.5916
WVFGRD96   30.0   130    30    95   5.83 0.5932
WVFGRD96   31.0   120    35    85   5.84 0.5942
WVFGRD96   32.0   125    35    85   5.84 0.5951
WVFGRD96   33.0   120    35    80   5.85 0.5955
WVFGRD96   34.0   120    35    80   5.86 0.5957
WVFGRD96   35.0   115    40    75   5.87 0.5955
WVFGRD96   36.0   115    40    75   5.87 0.5952
WVFGRD96   37.0   115    45    75   5.88 0.5940
WVFGRD96   38.0   120    50    70   5.90 0.5938
WVFGRD96   39.0   120    50    70   5.91 0.5930

The best solution is

WVFGRD96   34.0   120    35    80   5.86 0.5957

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 -100 o DIST/3.3 +150
rtr
taper w 0.1
hp c 0.01 n 3 
lp c 0.04 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: