Location

2001/01/18 15:12:34 37.3490 -117.1390 4.3 3.80 Nevada

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2001/01/18 15:12:34:0  37.35 -117.14   4.3 3.8 Nevada
 
 Stations used:
   BK.CMB CI.CHF CI.DJJ CI.GSC CI.ISA CI.MLAC CI.SLA CI.TIN 
   CI.VCS CI.VTV LB.BMN LB.DAC LB.TPH TS.PAS US.TPNV 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 6 km
  Plane   Strike  Dip  Rake
   NP1       70    65   -30
   NP2      174    63   -152
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21      1     122
    N   0.00e+00     52     214
    P  -7.76e+21     38      31

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.29e+21
       Mxy    -5.62e+21
       Mxz    -3.32e+21
       Myy     4.26e+21
       Myz    -1.82e+21
       Mzz    -2.97e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #######---------------              
              #########-------------------           
             #########---------------------          
           ###########-----------   ---------        
          ###########------------ P ----------       
         ############------------   -----------      
        #############--------------------------#     
        #############-------------------------##     
       ##############-----------------------#####    
       ##############---------------------#######    
       ##############-------------------#########    
       ##############----------------############    
        ##############-----------###############     
        ##############-------###################     
         ----#######---#####################         
          -------------##################### T       
           -------------####################         
             ------------##################          
              ------------################           
                 ----------############              
                     --------######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.97e+21  -3.32e+21   1.82e+21 
 -3.32e+21  -1.29e+21   5.62e+21 
  1.82e+21   5.62e+21   4.26e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20010118151234/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 = 70
      DIP = 65
     RAKE = -30
       MW = 3.86
       HS = 6.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2001/01/18 15:12:34:0  37.35 -117.14   4.3 3.8 Nevada
 
 Stations used:
   BK.CMB CI.CHF CI.DJJ CI.GSC CI.ISA CI.MLAC CI.SLA CI.TIN 
   CI.VCS CI.VTV LB.BMN LB.DAC LB.TPH TS.PAS US.TPNV 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 6 km
  Plane   Strike  Dip  Rake
   NP1       70    65   -30
   NP2      174    63   -152
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21      1     122
    N   0.00e+00     52     214
    P  -7.76e+21     38      31

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.29e+21
       Mxy    -5.62e+21
       Mxz    -3.32e+21
       Myy     4.26e+21
       Myz    -1.82e+21
       Mzz    -2.97e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #######---------------              
              #########-------------------           
             #########---------------------          
           ###########-----------   ---------        
          ###########------------ P ----------       
         ############------------   -----------      
        #############--------------------------#     
        #############-------------------------##     
       ##############-----------------------#####    
       ##############---------------------#######    
       ##############-------------------#########    
       ##############----------------############    
        ##############-----------###############     
        ##############-------###################     
         ----#######---#####################         
          -------------##################### T       
           -------------####################         
             ------------##################          
              ------------################           
                 ----------############              
                     --------######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.97e+21  -3.32e+21   1.82e+21 
 -3.32e+21  -1.29e+21   5.62e+21 
  1.82e+21   5.62e+21   4.26e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20010118151234/index.html
	

Waveform Inversion

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:

hp c 0.02 n 3
lp c 0.06 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5    75    60   -45   3.66 0.4189
WVFGRD96    1.0    75    60   -40   3.68 0.4375
WVFGRD96    2.0    75    60   -40   3.77 0.5470
WVFGRD96    3.0    70    55   -45   3.82 0.5793
WVFGRD96    4.0   240    50   -60   3.88 0.5899
WVFGRD96    5.0    70    65   -35   3.85 0.5897
WVFGRD96    6.0    70    65   -30   3.86 0.5899
WVFGRD96    7.0    70    65   -30   3.87 0.5855
WVFGRD96    8.0    70    65   -30   3.89 0.5800
WVFGRD96    9.0   265    75    30   3.87 0.5687
WVFGRD96   10.0   265    70    30   3.88 0.5679
WVFGRD96   11.0   265    70    25   3.88 0.5676
WVFGRD96   12.0   265    70    25   3.89 0.5659
WVFGRD96   13.0   265    70    25   3.90 0.5640
WVFGRD96   14.0   265    65    25   3.92 0.5613
WVFGRD96   15.0   265    65    25   3.93 0.5582
WVFGRD96   16.0   265    65    25   3.93 0.5549
WVFGRD96   17.0   265    65    20   3.93 0.5513
WVFGRD96   18.0   265    65    20   3.94 0.5474
WVFGRD96   19.0   265    65    20   3.95 0.5438
WVFGRD96   20.0   265    65    20   3.96 0.5397
WVFGRD96   21.0   265    60    20   3.97 0.5349
WVFGRD96   22.0   265    60    20   3.98 0.5305
WVFGRD96   23.0   265    60    20   3.99 0.5265
WVFGRD96   24.0   265    55    20   4.00 0.5228
WVFGRD96   25.0   265    55    20   4.01 0.5191
WVFGRD96   26.0   265    55    20   4.02 0.5151
WVFGRD96   27.0   265    55    20   4.02 0.5106
WVFGRD96   28.0   265    50    15   4.03 0.5058
WVFGRD96   29.0   255    50   -20   4.04 0.5033

The best solution is

WVFGRD96    6.0    70    65   -30   3.86 0.5899

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.06 n 3
br c 0.12 0.25 n 4 p 2
Figure 3. Waveform comparison for selected depth
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.

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS 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:

DATE=Fri May 8 16:43:33 CDT 2009

Last Changed 2001/01/18