Location

2008/02/21 15:34:25 41.132 -114.907 5.0 3.70 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  2008/02/21 15:34:25:4  41.13 -114.91   5.0 3.7 Nevada
 
 Stations used:
   TA.L12A TA.L13A TA.M11A TA.M13A TA.M14A TA.N11A TA.N12A 
   TA.N13A US.ELK 
 
 Filtering commands used:
   hp c 0.04 n 3
   lp c 0.12 n 3
 
 Best Fitting Double Couple
  Mo = 4.32e+21 dyne-cm
  Mw = 3.69 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      187    51   -98
   NP2       20    40   -80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.32e+21      5     283
    N   0.00e+00      6     192
    P  -4.32e+21     82      53

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.80e+20
       Mxy    -9.76e+20
       Mxz    -2.87e+20
       Myy     4.01e+21
       Myz    -8.90e+20
       Mzz    -4.19e+21
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 #########-----------##              
              ##########--------------####           
             ##########----------------####          
           ###########------------------#####        
          ###########--------------------#####       
         ###########---------------------######      
          #########----------------------#######     
        T #########----------------------#######     
          ########----------   ----------########    
       ###########---------- P ----------########    
       ###########----------   ----------########    
       ###########----------------------#########    
        ##########----------------------########     
        ##########---------------------#########     
         ##########-------------------#########      
          #########------------------#########       
           #########---------------##########        
             #######-------------##########          
              #######----------###########           
                 ######-----###########              
                     #--###########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.19e+21  -2.87e+20   8.90e+20 
 -2.87e+20   1.80e+20   9.76e+20 
  8.90e+20   9.76e+20   4.01e+21 


Details of the solution is found at

http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20080221153425/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 = 20
      DIP = 40
     RAKE = -80
       MW = 3.69
       HS = 10.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2008/02/21 15:34:25:4  41.13 -114.91   5.0 3.7 Nevada
 
 Stations used:
   TA.L12A TA.L13A TA.M11A TA.M13A TA.M14A TA.N11A TA.N12A 
   TA.N13A US.ELK 
 
 Filtering commands used:
   hp c 0.04 n 3
   lp c 0.12 n 3
 
 Best Fitting Double Couple
  Mo = 4.32e+21 dyne-cm
  Mw = 3.69 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      187    51   -98
   NP2       20    40   -80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.32e+21      5     283
    N   0.00e+00      6     192
    P  -4.32e+21     82      53

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.80e+20
       Mxy    -9.76e+20
       Mxz    -2.87e+20
       Myy     4.01e+21
       Myz    -8.90e+20
       Mzz    -4.19e+21
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 #########-----------##              
              ##########--------------####           
             ##########----------------####          
           ###########------------------#####        
          ###########--------------------#####       
         ###########---------------------######      
          #########----------------------#######     
        T #########----------------------#######     
          ########----------   ----------########    
       ###########---------- P ----------########    
       ###########----------   ----------########    
       ###########----------------------#########    
        ##########----------------------########     
        ##########---------------------#########     
         ##########-------------------#########      
          #########------------------#########       
           #########---------------##########        
             #######-------------##########          
              #######----------###########           
                 ######-----###########              
                     #--###########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.19e+21  -2.87e+20   8.90e+20 
 -2.87e+20   1.80e+20   9.76e+20 
  8.90e+20   9.76e+20   4.01e+21 


Details of the solution is found at

http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20080221153425/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 0.04 n 3
lp c 0.12 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    0.5    10    50   -90   3.27 0.2432
WVFGRD96    1.0   235    70   -20   3.20 0.1552
WVFGRD96    2.0   200    40   -75   3.43 0.2071
WVFGRD96    3.0    70    25    -5   3.45 0.2237
WVFGRD96    4.0    75    25   -15   3.48 0.2918
WVFGRD96    5.0     5    75    65   3.52 0.3438
WVFGRD96    6.0    60    35   -40   3.53 0.3751
WVFGRD96    7.0    60    40   -45   3.56 0.4009
WVFGRD96    8.0    30    40   -70   3.66 0.4204
WVFGRD96    9.0    25    40   -75   3.68 0.4368
WVFGRD96   10.0    20    40   -80   3.69 0.4409
WVFGRD96   11.0    30    45   -65   3.70 0.4350
WVFGRD96   12.0    30    45   -65   3.71 0.4239
WVFGRD96   13.0   220    50   -50   3.74 0.4069
WVFGRD96   14.0   220    50   -50   3.75 0.3900
WVFGRD96   15.0   220    55   -50   3.76 0.3713
WVFGRD96   16.0   220    55   -50   3.77 0.3511
WVFGRD96   17.0   220    55   -45   3.78 0.3310
WVFGRD96   18.0   215    55   -50   3.78 0.3126
WVFGRD96   19.0   220    60   -45   3.79 0.2934
WVFGRD96   20.0   220    60   -45   3.80 0.2782
WVFGRD96   21.0   215    60   -50   3.81 0.2669
WVFGRD96   22.0   215    60   -50   3.81 0.2541
WVFGRD96   23.0   260    40   -15   3.83 0.2428
WVFGRD96   24.0   260    40   -15   3.84 0.2367
WVFGRD96   25.0   260    40   -15   3.85 0.2333
WVFGRD96   26.0   255    45   -20   3.86 0.2358
WVFGRD96   27.0   255    40   -20   3.87 0.2431
WVFGRD96   28.0   255    40   -20   3.89 0.2512
WVFGRD96   29.0   255    40   -25   3.90 0.2570

The best solution is

WVFGRD96   10.0    20    40   -80   3.69 0.4409

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 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

hp c 0.04 n 3
lp c 0.12 n 3
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.

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

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 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:

DATE=Sun Aug 26 16:54:27 CDT 2012

Last Changed 2008/02/21