Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/05/30 23:12:22:0 36.28 -97.51 4.5 3.8 Oklahoma Stations used: AG.HHAR GS.KAN01 GS.KAN05 GS.KAN08 GS.KAN09 GS.KAN13 GS.KAN14 GS.KS21 GS.OK029 GS.OK030 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.CSTR OK.FNO OK.U32A OK.W35A TX.FW06 US.CBKS US.KSU1 US.MIAR Filtering commands used: cut o DIST/3.3 -30 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 = 3.63e+21 dyne-cm Mw = 3.64 Z = 4 km Plane Strike Dip Rake NP1 130 90 -10 NP2 220 80 -180 Principal Axes: Axis Value Plunge Azimuth T 3.63e+21 7 175 N 0.00e+00 80 310 P -3.63e+21 7 85 Moment Tensor: (dyne-cm) Component Value Mxx 3.52e+21 Mxy -6.21e+20 Mxz -4.83e+20 Myy -3.52e+21 Myz -4.05e+20 Mzz 5.51e+13 ############## ###################### ###########################- #########################----- ---######################--------- ------##################------------ ---------##############--------------- ------------##########------------------ --------------#######------------------- ------------------##------------------- ------------------##------------------- P -----------------#####----------------- ----------------########------------------ -------------#############-------------- ------------################------------ ----------###################--------- -------#######################------ -----##########################--- ---########################### -########################### ########### ######## ####### T #### Global CMT Convention Moment Tensor: R T P 5.51e+13 -4.83e+20 4.05e+20 -4.83e+20 3.52e+21 6.21e+20 4.05e+20 6.21e+20 -3.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530231222/index.html

Preferred Solution

      STK = 130
      DIP = 90
     RAKE = -10
       MW = 3.64
       HS = 4.0

The NDK file is 20170530231222.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 2017/05/30 23:12:22:0 36.28 -97.51 4.5 3.8 Oklahoma Stations used: AG.HHAR GS.KAN01 GS.KAN05 GS.KAN08 GS.KAN09 GS.KAN13 GS.KAN14 GS.KS21 GS.OK029 GS.OK030 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.CSTR OK.FNO OK.U32A OK.W35A TX.FW06 US.CBKS US.KSU1 US.MIAR Filtering commands used: cut o DIST/3.3 -30 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 = 3.63e+21 dyne-cm Mw = 3.64 Z = 4 km Plane Strike Dip Rake NP1 130 90 -10 NP2 220 80 -180 Principal Axes: Axis Value Plunge Azimuth T 3.63e+21 7 175 N 0.00e+00 80 310 P -3.63e+21 7 85 Moment Tensor: (dyne-cm) Component Value Mxx 3.52e+21 Mxy -6.21e+20 Mxz -4.83e+20 Myy -3.52e+21 Myz -4.05e+20 Mzz 5.51e+13 ############## ###################### ###########################- #########################----- ---######################--------- ------##################------------ ---------##############--------------- ------------##########------------------ --------------#######------------------- ------------------##------------------- ------------------##------------------- P -----------------#####----------------- ----------------########------------------ -------------#############-------------- ------------################------------ ----------###################--------- -------#######################------ -----##########################--- ---########################### -########################### ########### ######## ####### T #### Global CMT Convention Moment Tensor: R T P 5.51e+13 -4.83e+20 4.05e+20 -4.83e+20 3.52e+21 6.21e+20 4.05e+20 6.21e+20 -3.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530231222/index.html

Regional Moment Tensor (Mwr) Moment 7.472e+14 N-m Magnitude 3.8 Mwr Depth 4.0 km Percent DC 80 % Half Duration – Catalog US Data Source US1 Contributor US1 Nodal Planes Plane Strike Dip Rake NP1 226 72 -175 NP2 135 85 -18 Principal Axes Axis Value Plunge Azimuth T 7.827e+14 N-m 9 182 N -0.769e+14 N-m 71 299 P -7.058e+14 N-m 16 89

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 +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   310    85    25   3.41 0.4734
WVFGRD96    2.0   130    90   -20   3.54 0.5769
WVFGRD96    3.0   130    90   -15   3.60 0.6227
WVFGRD96    4.0   130    90   -10   3.64 0.6339
WVFGRD96    5.0   310    90    -5   3.67 0.6300
WVFGRD96    6.0   310    90    -5   3.71 0.6160
WVFGRD96    7.0   130    90    10   3.74 0.6018
WVFGRD96    8.0   130    90    15   3.78 0.5903
WVFGRD96    9.0   130    90   -20   3.80 0.5666
WVFGRD96   10.0   130    90    15   3.81 0.5466
WVFGRD96   11.0   310    90   -15   3.83 0.5272
WVFGRD96   12.0   310    90    20   3.85 0.5087
WVFGRD96   13.0   130    85   -20   3.87 0.4930
WVFGRD96   14.0   310    90    20   3.88 0.4761
WVFGRD96   15.0   130    85   -20   3.88 0.4609
WVFGRD96   16.0   130    85   -20   3.89 0.4472
WVFGRD96   17.0   130    85   -20   3.90 0.4342
WVFGRD96   18.0   130    85   -20   3.91 0.4228
WVFGRD96   19.0   135    80    10   3.91 0.4149
WVFGRD96   20.0   135    75    10   3.92 0.4084
WVFGRD96   21.0   135    75    10   3.92 0.4050
WVFGRD96   22.0   135    75    10   3.93 0.4023
WVFGRD96   23.0   135    75    10   3.94 0.4011
WVFGRD96   24.0   135    75     5   3.95 0.4026
WVFGRD96   25.0   135    75    10   3.95 0.4051
WVFGRD96   26.0   135    75    10   3.96 0.4079
WVFGRD96   27.0   135    75    10   3.97 0.4107
WVFGRD96   28.0   135    75    10   3.97 0.4133
WVFGRD96   29.0   135    75    10   3.98 0.4146

The best solution is

WVFGRD96    4.0   130    90   -10   3.64 0.6339

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 +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: