Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/06/07 04:49:46:0 39.26 -119.72 10.9 3.7 Nevada Stations used: BK.DANT BK.HATC BK.PACP BK.SCOT CI.GRA LB.TPH NC.AFD NC.KHBB NC.LDH NC.LMC NC.MDPB NN.DSP NN.GMN NN.LHV NN.MPK NN.MZPB NN.PAH NN.PNT NN.Q09A NN.R11B NN.RYN NN.S11A NN.WAK NN.WDEM SN.HEL UO.CHIL US.ELK US.TPNV 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 0.08 n 3 Best Fitting Double Couple Mo = 2.75e+21 dyne-cm Mw = 3.56 Z = 14 km Plane Strike Dip Rake NP1 230 65 -40 NP2 340 54 -149 Principal Axes: Axis Value Plunge Azimuth T 2.75e+21 6 287 N 0.00e+00 44 23 P -2.75e+21 45 190 Moment Tensor: (dyne-cm) Component Value Mxx -1.09e+21 Mxy -1.00e+21 Mxz 1.44e+21 Myy 2.44e+21 Myz -4.84e+19 Mzz -1.36e+21 -------------- ########-------------- ##############-------------- #################------------- ####################-----######### ######################-############# ##################-----############# T ###############---------############# #############------------############ ###############---------------############ #############-----------------############ ###########--------------------########### ##########---------------------########### #######-----------------------########## ######------------------------########## #####----------- ----------######### ###------------ P ----------######## #------------- ----------####### -------------------------##### -----------------------##### -------------------### -------------- Global CMT Convention Moment Tensor: R T P -1.36e+21 1.44e+21 4.84e+19 1.44e+21 -1.09e+21 1.00e+21 4.84e+19 1.00e+21 2.44e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190607044946/index.html

Preferred Solution

      STK = 230
      DIP = 65
     RAKE = -40
       MW = 3.56
       HS = 14.0

The NDK file is 20190607044946.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

UNR

USGS/SLU Moment Tensor Solution ENS 2019/06/07 04:49:46:0 39.26 -119.72 10.9 3.7 Nevada Stations used: BK.DANT BK.HATC BK.PACP BK.SCOT CI.GRA LB.TPH NC.AFD NC.KHBB NC.LDH NC.LMC NC.MDPB NN.DSP NN.GMN NN.LHV NN.MPK NN.MZPB NN.PAH NN.PNT NN.Q09A NN.R11B NN.RYN NN.S11A NN.WAK NN.WDEM SN.HEL UO.CHIL US.ELK US.TPNV 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 0.08 n 3 Best Fitting Double Couple Mo = 2.75e+21 dyne-cm Mw = 3.56 Z = 14 km Plane Strike Dip Rake NP1 230 65 -40 NP2 340 54 -149 Principal Axes: Axis Value Plunge Azimuth T 2.75e+21 6 287 N 0.00e+00 44 23 P -2.75e+21 45 190 Moment Tensor: (dyne-cm) Component Value Mxx -1.09e+21 Mxy -1.00e+21 Mxz 1.44e+21 Myy 2.44e+21 Myz -4.84e+19 Mzz -1.36e+21 -------------- ########-------------- ##############-------------- #################------------- ####################-----######### ######################-############# ##################-----############# T ###############---------############# #############------------############ ###############---------------############ #############-----------------############ ###########--------------------########### ##########---------------------########### #######-----------------------########## ######------------------------########## #####----------- ----------######### ###------------ P ----------######## #------------- ----------####### -------------------------##### -----------------------##### -------------------### -------------- Global CMT Convention Moment Tensor: R T P -1.36e+21 1.44e+21 4.84e+19 1.44e+21 -1.09e+21 1.00e+21 4.84e+19 1.00e+21 2.44e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190607044946/index.html

Regional Moment Tensor (Mwr) Moment 3.104e+14 N-m Magnitude 3.59 Mwr Depth 8.0 km Percent DC 100% Half Duration - Catalog NN Data Source NN 1 Contributor NN 1 Nodal Planes Plane Strike Dip Rake NP1 215 43 -81 NP2 22 48 -99 Principal Axes Axis Value Plunge Azimuth T 3.110e+14 N-m 2 118 N -0.002e+14 N-m 6 28 P -3.098e+14 N-m 83 228

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 0.08 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   200    45   -85   3.17 0.3119
WVFGRD96    2.0   195    45   -90   3.31 0.4197
WVFGRD96    3.0   240    85    30   3.27 0.4057
WVFGRD96    4.0   235    80   -45   3.34 0.4356
WVFGRD96    5.0   235    80   -50   3.37 0.4847
WVFGRD96    6.0   235    80   -45   3.39 0.5303
WVFGRD96    7.0   230    75   -45   3.41 0.5667
WVFGRD96    8.0   230    75   -50   3.48 0.5986
WVFGRD96    9.0   230    70   -50   3.50 0.6236
WVFGRD96   10.0   230    70   -45   3.51 0.6446
WVFGRD96   11.0   230    65   -45   3.53 0.6604
WVFGRD96   12.0   230    65   -40   3.54 0.6721
WVFGRD96   13.0   230    65   -40   3.55 0.6788
WVFGRD96   14.0   230    65   -40   3.56 0.6814
WVFGRD96   15.0   235    75   -35   3.56 0.6803
WVFGRD96   16.0   235    75   -35   3.57 0.6780
WVFGRD96   17.0   235    75   -35   3.58 0.6720
WVFGRD96   18.0   235    75   -35   3.59 0.6650
WVFGRD96   19.0   235    80   -35   3.60 0.6554
WVFGRD96   20.0   235    80   -35   3.60 0.6469
WVFGRD96   21.0   235    80   -35   3.61 0.6363
WVFGRD96   22.0   235    80   -35   3.62 0.6243
WVFGRD96   23.0   235    80   -35   3.63 0.6131
WVFGRD96   24.0   235    80   -35   3.63 0.6002
WVFGRD96   25.0   235    80   -35   3.64 0.5864
WVFGRD96   26.0   235    80   -35   3.64 0.5730
WVFGRD96   27.0   235    80   -35   3.65 0.5590
WVFGRD96   28.0   235    80   -35   3.65 0.5443
WVFGRD96   29.0   235    75   -35   3.66 0.5301

The best solution is

WVFGRD96   14.0   230    65   -40   3.56 0.6814

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 0.08 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 Bureau 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: