Location

Location ANSS

The ANSS event ID is nn00773931 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00773931/executive.

2020/09/15 16:33:36 38.160 -117.962 11.6 4.2 Nevada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/09/15 16:33:36:0  38.16 -117.96  11.6 4.2 Nevada
 
 Stations used:
   BK.DANT BK.HATC BK.LEGD BK.MHC BK.OVRO BK.SUTB BK.WELL 
   BK.WINE CI.BFS CI.CCC CI.CLC CI.CWC CI.FUR CI.GRA CI.GSC 
   CI.HAR CI.ISA CI.LRL CI.LUC2 CI.MWC CI.OSI CI.PASC CI.RAG 
   CI.SHO CI.TIN CI.TPO GS.MCA04 IM.NV31 LB.BMN LB.TPH NC.AFD 
   NC.BBGB NC.LDH NC.MDPB NN.BEK NN.CMK6 NN.GMN NN.GWY NN.KVN 
   NN.LHV NN.PAH NN.PIO NN.PNT NN.PRN NN.PYM2 NN.Q09A NN.QSM 
   NN.REDF NN.SHP NN.WAK NN.WDEM NN.WLDB NN.YER SN.HEL US.ELK 
   US.TPNV UU.CCUT UU.LCMT UU.PSUT UU.SZCU 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       70    85   -15
   NP2      161    75   -175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21      7     117
    N   0.00e+00     74     232
    P  -7.76e+21     14      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.49e+21
       Mxy    -5.83e+21
       Mxz    -2.08e+21
       Myy     4.84e+21
       Myz     6.26e+19
       Mzz    -3.49e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ####-------------   --              
              #######------------- P -----           
             ########-------------   ------          
           ##########------------------------        
          ############------------------------       
         #############-------------------------      
        ##############------------------------##     
        ###############--------------------#####     
       ################----------------##########    
       #################----------###############    
       #################-----####################    
       #################-########################    
        ##########--------######################     
        ###---------------##################   #     
         ------------------################# T       
          ------------------################         
           ------------------################        
             -----------------#############          
              -----------------###########           
                 ----------------######              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.49e+20  -2.08e+21  -6.26e+19 
 -2.08e+21  -4.49e+21   5.83e+21 
 -6.26e+19   5.83e+21   4.84e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200915163336/index.html
        

Preferred Solution

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

      STK = 70
      DIP = 85
     RAKE = -15
       MW = 3.86
       HS = 14.0

The NDK file is 20200915163336.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
SLU
UNR
 USGS/SLU Moment Tensor Solution
 ENS  2020/09/15 16:33:36:0  38.16 -117.96  11.6 4.2 Nevada
 
 Stations used:
   BK.DANT BK.HATC BK.LEGD BK.MHC BK.OVRO BK.SUTB BK.WELL 
   BK.WINE CI.BFS CI.CCC CI.CLC CI.CWC CI.FUR CI.GRA CI.GSC 
   CI.HAR CI.ISA CI.LRL CI.LUC2 CI.MWC CI.OSI CI.PASC CI.RAG 
   CI.SHO CI.TIN CI.TPO GS.MCA04 IM.NV31 LB.BMN LB.TPH NC.AFD 
   NC.BBGB NC.LDH NC.MDPB NN.BEK NN.CMK6 NN.GMN NN.GWY NN.KVN 
   NN.LHV NN.PAH NN.PIO NN.PNT NN.PRN NN.PYM2 NN.Q09A NN.QSM 
   NN.REDF NN.SHP NN.WAK NN.WDEM NN.WLDB NN.YER SN.HEL US.ELK 
   US.TPNV UU.CCUT UU.LCMT UU.PSUT UU.SZCU 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       70    85   -15
   NP2      161    75   -175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21      7     117
    N   0.00e+00     74     232
    P  -7.76e+21     14      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.49e+21
       Mxy    -5.83e+21
       Mxz    -2.08e+21
       Myy     4.84e+21
       Myz     6.26e+19
       Mzz    -3.49e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ####-------------   --              
              #######------------- P -----           
             ########-------------   ------          
           ##########------------------------        
          ############------------------------       
         #############-------------------------      
        ##############------------------------##     
        ###############--------------------#####     
       ################----------------##########    
       #################----------###############    
       #################-----####################    
       #################-########################    
        ##########--------######################     
        ###---------------##################   #     
         ------------------################# T       
          ------------------################         
           ------------------################        
             -----------------#############          
              -----------------###########           
                 ----------------######              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.49e+20  -2.08e+21  -6.26e+19 
 -2.08e+21  -4.49e+21   5.83e+21 
 -6.26e+19   5.83e+21   4.84e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200915163336/index.html
	
Focal Mechanism

Catalog NN
Data Source NN 1
Contributor NN 1

Nodal Planes
Plane Strike Dip Rake
NP1 74 74 -18
NP2 169 72 -163

        

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude


Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.


Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 0.08 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    70    85    -5   3.35 0.2819
WVFGRD96    2.0    70    80   -10   3.49 0.3801
WVFGRD96    3.0    70    80   -15   3.55 0.4198
WVFGRD96    4.0   255    80    25   3.60 0.4583
WVFGRD96    5.0    70    90   -25   3.63 0.4949
WVFGRD96    6.0    70    85   -25   3.67 0.5386
WVFGRD96    7.0    70    85   -20   3.70 0.5797
WVFGRD96    8.0    65    75   -25   3.75 0.6188
WVFGRD96    9.0    65    75   -25   3.77 0.6520
WVFGRD96   10.0    70    80   -20   3.79 0.6793
WVFGRD96   11.0    70    80   -20   3.81 0.6988
WVFGRD96   12.0    70    80   -15   3.83 0.7132
WVFGRD96   13.0    70    85   -15   3.84 0.7223
WVFGRD96   14.0    70    85   -15   3.86 0.7262
WVFGRD96   15.0    70    85   -15   3.87 0.7255
WVFGRD96   16.0   250    90    10   3.88 0.7185
WVFGRD96   17.0    70    85   -15   3.89 0.7153
WVFGRD96   18.0   250    90    10   3.90 0.7057
WVFGRD96   19.0    70    85   -15   3.90 0.6969
WVFGRD96   20.0    70    85   -15   3.91 0.6870
WVFGRD96   21.0    70    85   -15   3.92 0.6754
WVFGRD96   22.0   250    90    15   3.93 0.6631
WVFGRD96   23.0   250    90    15   3.93 0.6511
WVFGRD96   24.0   250    90    15   3.94 0.6389
WVFGRD96   25.0    70    85   -15   3.95 0.6262
WVFGRD96   26.0   250    90    15   3.95 0.6128
WVFGRD96   27.0   250    90    15   3.96 0.5991
WVFGRD96   28.0    70    90   -15   3.96 0.5850
WVFGRD96   29.0   250    90    15   3.97 0.5708

The best solution is

WVFGRD96   14.0    70    85   -15   3.86 0.7262

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 0.08 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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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    
Last Changed Thu Apr 25 09:42:41 PM CDT 2024