Location

Location ANSS

2020/05/15 18:10:32 38.178 -117.849 8.1 4.0 Nevada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/05/15 18:10:32:0  38.18 -117.85   8.1 4.0 Nevada
 
 Stations used:
   BK.DANT BK.OVRO BK.WELL CI.CLC CI.CWC CI.FUR CI.GRA CI.HAR 
   CI.ISA CI.LRL CI.MPM CI.MTP CI.TIN IM.NV31 NC.AFD NC.MDPB 
   NN.BEK NN.CMK6 NN.DSP NN.GMN NN.GWY NN.KVN NN.LHV NN.MPK 
   NN.PAH NN.PIO NN.PNT NN.PRN NN.Q09A NN.QSM NN.REDF NN.S11A 
   NN.SHP NN.WAK NN.WDEM NN.WLDB SN.HEL US.TPNV UU.PSUT 
   UU.VRUT 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 4.62e+21 dyne-cm
  Mw = 3.71 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1       41    64   -106
   NP2      255    30   -60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.62e+21     18     143
    N   0.00e+00     14      48
    P  -4.62e+21     67     282

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.66e+21
       Mxy    -1.87e+21
       Mxz    -1.42e+21
       Myy     8.10e+20
       Myz     2.45e+21
       Mzz    -3.47e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##########--------------#####-          
           ########--------------------------        
          ######-------------------------###--       
         #####---------------------------#####-      
        ####----------------------------#######-     
        ###----------------------------#########     
       ###-----------   --------------###########    
       ##------------ P -------------############    
       ##------------   ------------#############    
       #--------------------------###############    
        -------------------------###############     
        -----------------------#################     
         --------------------##################      
          -----------------###################       
           -------------#############   #####        
             --------################ T ###          
              #######################   ##           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.47e+21  -1.42e+21  -2.45e+21 
 -1.42e+21   2.66e+21   1.87e+21 
 -2.45e+21   1.87e+21   8.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200515181032/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 255
      DIP = 30
     RAKE = -60
       MW = 3.71
       HS = 9.0

The NDK file is 20200515181032.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2020/05/15 18:10:32:0  38.18 -117.85   8.1 4.0 Nevada
 
 Stations used:
   BK.DANT BK.OVRO BK.WELL CI.CLC CI.CWC CI.FUR CI.GRA CI.HAR 
   CI.ISA CI.LRL CI.MPM CI.MTP CI.TIN IM.NV31 NC.AFD NC.MDPB 
   NN.BEK NN.CMK6 NN.DSP NN.GMN NN.GWY NN.KVN NN.LHV NN.MPK 
   NN.PAH NN.PIO NN.PNT NN.PRN NN.Q09A NN.QSM NN.REDF NN.S11A 
   NN.SHP NN.WAK NN.WDEM NN.WLDB SN.HEL US.TPNV UU.PSUT 
   UU.VRUT 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 4.62e+21 dyne-cm
  Mw = 3.71 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1       41    64   -106
   NP2      255    30   -60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.62e+21     18     143
    N   0.00e+00     14      48
    P  -4.62e+21     67     282

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.66e+21
       Mxy    -1.87e+21
       Mxz    -1.42e+21
       Myy     8.10e+20
       Myz     2.45e+21
       Mzz    -3.47e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##########--------------#####-          
           ########--------------------------        
          ######-------------------------###--       
         #####---------------------------#####-      
        ####----------------------------#######-     
        ###----------------------------#########     
       ###-----------   --------------###########    
       ##------------ P -------------############    
       ##------------   ------------#############    
       #--------------------------###############    
        -------------------------###############     
        -----------------------#################     
         --------------------##################      
          -----------------###################       
           -------------#############   #####        
             --------################ T ###          
              #######################   ##           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.47e+21  -1.42e+21  -2.45e+21 
 -1.42e+21   2.66e+21   1.87e+21 
 -2.45e+21   1.87e+21   8.10e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200515181032/index.html
	

Magnitudes

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

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    75    50   -60   3.39 0.3966
WVFGRD96    2.0    75    55   -60   3.49 0.4616
WVFGRD96    3.0   275    30   -25   3.58 0.4598
WVFGRD96    4.0   275    25   -25   3.62 0.5127
WVFGRD96    5.0   240    20   -75   3.63 0.5672
WVFGRD96    6.0   255    25   -55   3.63 0.6097
WVFGRD96    7.0   255    30   -55   3.64 0.6353
WVFGRD96    8.0   245    25   -70   3.71 0.6605
WVFGRD96    9.0   255    30   -60   3.71 0.6623
WVFGRD96   10.0   255    30   -60   3.71 0.6559
WVFGRD96   11.0   265    35   -45   3.71 0.6414
WVFGRD96   12.0   265    35   -45   3.70 0.6246
WVFGRD96   13.0   270    35   -40   3.70 0.6045
WVFGRD96   14.0   275    40   -30   3.70 0.5845
WVFGRD96   15.0   275    40   -30   3.71 0.5653
WVFGRD96   16.0   280    40   -25   3.71 0.5467
WVFGRD96   17.0   290    55    30   3.72 0.5337
WVFGRD96   18.0   290    55    30   3.73 0.5197
WVFGRD96   19.0   290    55    30   3.73 0.5060
WVFGRD96   20.0   290    55    30   3.74 0.4931
WVFGRD96   21.0   290    55    30   3.74 0.4783
WVFGRD96   22.0   290    55    30   3.75 0.4660
WVFGRD96   23.0   290    50    30   3.75 0.4542
WVFGRD96   24.0   290    50    30   3.76 0.4433
WVFGRD96   25.0   290    50    30   3.76 0.4331
WVFGRD96   26.0   290    50    30   3.77 0.4230
WVFGRD96   27.0   290    50    35   3.77 0.4140
WVFGRD96   28.0   290    50    35   3.78 0.4056
WVFGRD96   29.0   290    50    35   3.78 0.3975

The best solution is

WVFGRD96    9.0   255    30   -60   3.71 0.6623

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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:

Assuming only a mislocation, the time shifts are fit to a functional form:

 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 Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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:

Last Changed Fri May 15 13:49:43 CDT 2020