Location

SLU Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/08/01 10:11:13:0 35.03 -97.59 5.0 3.5 Oklahoma Stations used: GM.IWM01 GS.KAN17 GS.OK029 GS.OK031 GS.OK032 GS.OK038 GS.OK048 GS.OK052 N4.Z35B O2.ARCA O2.CHAN O2.CRES O2.DRUM O2.PERK O2.PERY O2.POCA O2.SHWN O2.SMNL OK.CROK OK.FNO OK.X37A TA.TUL3 TX.FW03 TX.WTFS US.WMOK 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 0.08 n 3 Best Fitting Double Couple Mo = 1.29e+21 dyne-cm Mw = 3.34 Z = 8 km Plane Strike Dip Rake NP1 140 85 25 NP2 48 65 174 Principal Axes: Axis Value Plunge Azimuth T 1.29e+21 21 7 N 0.00e+00 65 151 P -1.29e+21 14 271 Moment Tensor: (dyne-cm) Component Value Mxx 1.11e+21 Mxy 1.55e+20 Mxz 4.23e+20 Myy -1.20e+21 Myz 3.45e+20 Mzz 9.45e+19 ############## ########### ######## -############# T ########### ---############ ############ ------##########################-- --------#########################--- -----------######################----- -------------####################------- ---------------#################-------- - -------------###############---------- - P ---------------###########------------ - ----------------#########------------- ----------------------#####--------------- ----------------------##---------------- ----------------------##---------------- -------------------######------------- ---------------##########----------- ----------################-------- --########################---- ###########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 9.45e+19 4.23e+20 -3.45e+20 4.23e+20 1.11e+21 -1.55e+20 -3.45e+20 -1.55e+20 -1.20e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180801101113/index.html

Preferred Solution

      STK = 140
      DIP = 85
     RAKE = 25
       MW = 3.34
       HS = 8.0

The NDK file is 20180801101113.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 2018/08/01 10:11:13:0 35.03 -97.59 5.0 3.5 Oklahoma Stations used: GM.IWM01 GS.KAN17 GS.OK029 GS.OK031 GS.OK032 GS.OK038 GS.OK048 GS.OK052 N4.Z35B O2.ARCA O2.CHAN O2.CRES O2.DRUM O2.PERK O2.PERY O2.POCA O2.SHWN O2.SMNL OK.CROK OK.FNO OK.X37A TA.TUL3 TX.FW03 TX.WTFS US.WMOK 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 0.08 n 3 Best Fitting Double Couple Mo = 1.29e+21 dyne-cm Mw = 3.34 Z = 8 km Plane Strike Dip Rake NP1 140 85 25 NP2 48 65 174 Principal Axes: Axis Value Plunge Azimuth T 1.29e+21 21 7 N 0.00e+00 65 151 P -1.29e+21 14 271 Moment Tensor: (dyne-cm) Component Value Mxx 1.11e+21 Mxy 1.55e+20 Mxz 4.23e+20 Myy -1.20e+21 Myz 3.45e+20 Mzz 9.45e+19 ############## ########### ######## -############# T ########### ---############ ############ ------##########################-- --------#########################--- -----------######################----- -------------####################------- ---------------#################-------- - -------------###############---------- - P ---------------###########------------ - ----------------#########------------- ----------------------#####--------------- ----------------------##---------------- ----------------------##---------------- -------------------######------------- ---------------##########----------- ----------################-------- --########################---- ###########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 9.45e+19 4.23e+20 -3.45e+20 4.23e+20 1.11e+21 -1.55e+20 -3.45e+20 -1.55e+20 -1.20e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180801101113/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 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   320    90     0   2.97 0.3484
WVFGRD96    2.0   140    90     0   3.10 0.4582
WVFGRD96    3.0   320    85     5   3.16 0.4920
WVFGRD96    4.0   320    80    10   3.20 0.5043
WVFGRD96    5.0   140    80    20   3.24 0.5123
WVFGRD96    6.0   140    80    20   3.27 0.5200
WVFGRD96    7.0   140    80    20   3.30 0.5264
WVFGRD96    8.0   140    85    25   3.34 0.5316
WVFGRD96    9.0   140    80    20   3.35 0.5293
WVFGRD96   10.0   320    55    15   3.40 0.5312
WVFGRD96   11.0   320    60    15   3.40 0.5294
WVFGRD96   12.0   320    60    15   3.42 0.5246
WVFGRD96   13.0   320    60    10   3.43 0.5179
WVFGRD96   14.0   320    60    10   3.44 0.5098
WVFGRD96   15.0   320    60    10   3.45 0.4996
WVFGRD96   16.0   320    65    10   3.45 0.4882
WVFGRD96   17.0   320    65    10   3.46 0.4762
WVFGRD96   18.0   320    65    10   3.46 0.4639
WVFGRD96   19.0   145    65    20   3.48 0.4521
WVFGRD96   20.0   145    65    20   3.49 0.4408
WVFGRD96   21.0   145    65    20   3.50 0.4305
WVFGRD96   22.0   145    65    20   3.51 0.4196
WVFGRD96   23.0    50    75    20   3.49 0.4123
WVFGRD96   24.0    50    75    20   3.50 0.4149
WVFGRD96   25.0    50    75    20   3.51 0.4172
WVFGRD96   26.0    50    75    20   3.51 0.4180
WVFGRD96   27.0    50    75    20   3.52 0.4186
WVFGRD96   28.0    50    75    20   3.53 0.4185
WVFGRD96   29.0    50    75    20   3.53 0.4174

The best solution is

WVFGRD96    8.0   140    85    25   3.34 0.5316

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 0.08 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: