Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/09/24 01:11:37:0 36.37 -97.14 2.8 3.8 Oklahoma Stations used: GM.IWM01 GS.KAN01 GS.KAN05 GS.KAN06 GS.KAN08 GS.KAN10 GS.KAN12 GS.KAN13 GS.KAN14 GS.KAN17 GS.KS20 GS.KS21 GS.OK029 GS.OK031 GS.OK032 GS.OK033 GS.OK035 GS.OK038 GS.OK048 GS.OK051 GS.OK052 N4.R32B N4.T35B N4.U38B N4.Z35B OK.CROK OK.DEOK OK.FNO OK.GORE OK.MOOR OK.QUOK OK.U32A OK.X37A US.CBKS US.KSU1 US.MIAR Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.05e+21 dyne-cm Mw = 3.59 Z = 4 km Plane Strike Dip Rake NP1 105 90 -5 NP2 195 85 -180 Principal Axes: Axis Value Plunge Azimuth T 3.05e+21 4 150 N 0.00e+00 85 285 P -3.05e+21 4 60 Moment Tensor: (dyne-cm) Component Value Mxx 1.52e+21 Mxy -2.64e+21 Mxz -2.57e+20 Myy -1.52e+21 Myz -6.89e+19 Mzz 2.33e+13 ############-- ###############------- ##################---------- ##################------------ ###################-------------- ####################-------------- P ####################--------------- #####################------------------- ---#################-------------------- -----------#########---------------------- ------------------##---------------------- -------------------#####------------------ -------------------############----------- -----------------####################--- -----------------####################### ----------------###################### --------------###################### -------------##################### -----------################### ---------############# ### ------############# T --############ Global CMT Convention Moment Tensor: R T P 2.33e+13 -2.57e+20 6.89e+19 -2.57e+20 1.52e+21 2.64e+21 6.89e+19 2.64e+21 -1.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170924011137/index.html

Preferred Solution

      STK = 105
      DIP = 90
     RAKE = -5
       MW = 3.59
       HS = 4.0

The NDK file is 20170924011137.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGSMWR

USGS/SLU Moment Tensor Solution ENS 2017/09/24 01:11:37:0 36.37 -97.14 2.8 3.8 Oklahoma Stations used: GM.IWM01 GS.KAN01 GS.KAN05 GS.KAN06 GS.KAN08 GS.KAN10 GS.KAN12 GS.KAN13 GS.KAN14 GS.KAN17 GS.KS20 GS.KS21 GS.OK029 GS.OK031 GS.OK032 GS.OK033 GS.OK035 GS.OK038 GS.OK048 GS.OK051 GS.OK052 N4.R32B N4.T35B N4.U38B N4.Z35B OK.CROK OK.DEOK OK.FNO OK.GORE OK.MOOR OK.QUOK OK.U32A OK.X37A US.CBKS US.KSU1 US.MIAR Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.05e+21 dyne-cm Mw = 3.59 Z = 4 km Plane Strike Dip Rake NP1 105 90 -5 NP2 195 85 -180 Principal Axes: Axis Value Plunge Azimuth T 3.05e+21 4 150 N 0.00e+00 85 285 P -3.05e+21 4 60 Moment Tensor: (dyne-cm) Component Value Mxx 1.52e+21 Mxy -2.64e+21 Mxz -2.57e+20 Myy -1.52e+21 Myz -6.89e+19 Mzz 2.33e+13 ############-- ###############------- ##################---------- ##################------------ ###################-------------- ####################-------------- P ####################--------------- #####################------------------- ---#################-------------------- -----------#########---------------------- ------------------##---------------------- -------------------#####------------------ -------------------############----------- -----------------####################--- -----------------####################### ----------------###################### --------------###################### -------------##################### -----------################### ---------############# ### ------############# T --############ Global CMT Convention Moment Tensor: R T P 2.33e+13 -2.57e+20 6.89e+19 -2.57e+20 1.52e+21 2.64e+21 6.89e+19 2.64e+21 -1.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170924011137/index.html

Regional Moment Tensor (Mwr) Moment 6.474e+14 N-m Magnitude 3.8 Mwr Depth 5.0 km Percent DC 88 % Half Duration – Catalog US Data Source US1 Contributor US1 Nodal Planes Plane Strike Dip Rake NP1 291 89 18 NP2 200 72 178 Principal Axes Axis Value Plunge Azimuth T 6.661e+14 N-m 14 157 N -0.392e+14 N-m 72 295 P -6.269e+14 N-m 12 64

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 -20 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   200    80     0   3.34 0.3832
WVFGRD96    2.0   105    90    -5   3.47 0.4434
WVFGRD96    3.0   105    90    -5   3.54 0.4826
WVFGRD96    4.0   105    90    -5   3.59 0.4900
WVFGRD96    5.0   105    90   -10   3.62 0.4778
WVFGRD96    6.0   285    85    15   3.65 0.4626
WVFGRD96    7.0   285    85    15   3.68 0.4511
WVFGRD96    8.0   285    85    20   3.72 0.4423
WVFGRD96    9.0   285    85    20   3.74 0.4242
WVFGRD96   10.0   290    80    25   3.75 0.4081
WVFGRD96   11.0   290    80    25   3.77 0.3929
WVFGRD96   12.0   290    80    25   3.78 0.3784
WVFGRD96   13.0   290    80    25   3.79 0.3633
WVFGRD96   14.0   290    80    25   3.80 0.3487
WVFGRD96   15.0   290    80    25   3.81 0.3342
WVFGRD96   16.0   290    80    25   3.82 0.3204
WVFGRD96   17.0   290    80    25   3.83 0.3068
WVFGRD96   18.0   290    80    25   3.83 0.2936
WVFGRD96   19.0   290    80    25   3.84 0.2815
WVFGRD96   20.0   290    80    25   3.84 0.2702
WVFGRD96   21.0   295    75    25   3.84 0.2606
WVFGRD96   22.0   295    75    25   3.85 0.2524
WVFGRD96   23.0   295    75    25   3.85 0.2453
WVFGRD96   24.0   295    70    20   3.85 0.2411
WVFGRD96   25.0   195    80   -15   3.86 0.2468
WVFGRD96   26.0   195    85   -15   3.86 0.2552
WVFGRD96   27.0   195    85   -15   3.87 0.2639
WVFGRD96   28.0   195    85   -10   3.88 0.2724
WVFGRD96   29.0   195    85   -10   3.89 0.2818

The best solution is

WVFGRD96    4.0   105    90    -5   3.59 0.4900

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 -20 o DIST/3.3 +40
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:

• 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: