Location

2010/01/04 16:24:02 37.587 -113.042 3.7 4.10 Utah

I used the program elocate from Computer Programs in Seismology together with the WUS velocity model givne below to locate the earthquake. The purpose was to get take-off angles to compare first motion polarities to the nodal planes predicted using the waveform inversion solution. The elocate.out.txt file contains the results of running this program.

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  2010/01/04 16:24:03:0  37.59 -113.04   3.7 4.1 Utah
 
 Stations used:
   CI.GLA CI.ISA II.PFO LB.MVU LB.TPH TA.O20A TA.R11A TA.S22A 
   TA.U22A TA.V22A TA.W18A TA.Y12C US.DUG US.MVCO US.TPNV 
   US.WUAZ UU.CCUT UU.KNB UU.MPU UU.SPU 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.76e+21 dyne-cm
  Mw = 3.82 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      355    85   -175
   NP2      265    85    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.76e+21      0     130
    N   0.00e+00     83      40
    P  -6.76e+21      7     220

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.06e+21
       Mxy    -6.62e+21
       Mxz     6.29e+20
       Myy     1.17e+21
       Myz     5.34e+20
       Mzz    -1.02e+20
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ##########------------              
              #############---------------           
             ###############---------------          
           #################-----------------        
          ##################------------------       
         ###################-------------------      
        #####################-------------------     
        #####################-------------------     
       ######################--------------------    
       #################-----####################    
       ####------------------####################    
       ----------------------####################    
        ----------------------##################     
        ----------------------##################     
         ---------------------#################      
          --------------------#############          
           -------------------############# T        
             --   ------------#############          
              - P ------------############           
                  ------------#########              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.02e+20   6.29e+20  -5.34e+20 
  6.29e+20  -1.06e+21   6.62e+21 
 -5.34e+20   6.62e+21   1.17e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100104162403/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 = 265
      DIP = 85
     RAKE = -5
       MW = 3.82
       HS = 9.0

The waveform inversion is preferred. This solution required adding a microseism filter. The Rayleigh waves had very low amplitudes. Depths less than 6 km are rejected on the basis of fit.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/01/04 16:24:03:0  37.59 -113.04   3.7 4.1 Utah
 
 Stations used:
   CI.GLA CI.ISA II.PFO LB.MVU LB.TPH TA.O20A TA.R11A TA.S22A 
   TA.U22A TA.V22A TA.W18A TA.Y12C US.DUG US.MVCO US.TPNV 
   US.WUAZ UU.CCUT UU.KNB UU.MPU UU.SPU 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.76e+21 dyne-cm
  Mw = 3.82 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      355    85   -175
   NP2      265    85    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.76e+21      0     130
    N   0.00e+00     83      40
    P  -6.76e+21      7     220

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.06e+21
       Mxy    -6.62e+21
       Mxz     6.29e+20
       Myy     1.17e+21
       Myz     5.34e+20
       Mzz    -1.02e+20
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ##########------------              
              #############---------------           
             ###############---------------          
           #################-----------------        
          ##################------------------       
         ###################-------------------      
        #####################-------------------     
        #####################-------------------     
       ######################--------------------    
       #################-----####################    
       ####------------------####################    
       ----------------------####################    
        ----------------------##################     
        ----------------------##################     
         ---------------------#################      
          --------------------#############          
           -------------------############# T        
             --   ------------#############          
              - P ------------############           
                  ------------#########              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.02e+20   6.29e+20  -5.34e+20 
  6.29e+20  -1.06e+21   6.62e+21 
 -5.34e+20   6.62e+21   1.17e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100104162403/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 0.10 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   275    85     0   3.29 0.2392
WVFGRD96    1.0    95    90     0   3.36 0.2860
WVFGRD96    2.0   270    90     0   3.57 0.5614
WVFGRD96    3.0   270    85     0   3.65 0.6862
WVFGRD96    4.0    90    90     0   3.70 0.7605
WVFGRD96    5.0   265    85    -5   3.73 0.8060
WVFGRD96    6.0   265    85    -5   3.76 0.8294
WVFGRD96    7.0   265    85    -5   3.78 0.8409
WVFGRD96    8.0   265    85   -10   3.81 0.8476
WVFGRD96    9.0   265    85    -5   3.82 0.8483
WVFGRD96   10.0   265    85    -5   3.84 0.8451
WVFGRD96   11.0   265    85    -5   3.85 0.8413
WVFGRD96   12.0   265    85    -5   3.87 0.8364
WVFGRD96   13.0   265    85    10   3.87 0.8296
WVFGRD96   14.0   265    85    10   3.89 0.8256
WVFGRD96   15.0   265    85    10   3.89 0.8190
WVFGRD96   16.0   265    85    10   3.90 0.8140
WVFGRD96   17.0   265    85     5   3.91 0.8078
WVFGRD96   18.0    80    85    -5   3.92 0.7983
WVFGRD96   19.0    80    85    -5   3.93 0.7929
WVFGRD96   20.0   265    85     5   3.94 0.7904
WVFGRD96   21.0   265    85    10   3.94 0.7853
WVFGRD96   22.0    80    85   -10   3.95 0.7780
WVFGRD96   23.0   265    85    10   3.96 0.7749
WVFGRD96   24.0   265    85     5   3.97 0.7713
WVFGRD96   25.0    80    85    -5   3.97 0.7638
WVFGRD96   26.0    80    85   -10   3.97 0.7609
WVFGRD96   27.0   265    85     5   3.99 0.7597
WVFGRD96   28.0    80    85   -10   3.98 0.7531
WVFGRD96   29.0   265    85     5   4.00 0.7535

The best solution is

WVFGRD96    9.0   265    85    -5   3.82 0.8483

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 0.10 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=Mon Jan 4 14:42:10 CST 2010

Last Changed 2010/01/04