Location

2008/10/18 02:27:38 36.1450 -114.5810 10.0 3.30 Arizona

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for

Focal Mechanism

 SLU Moment Tensor Solution
 2008/10/18 02:27:38  36.1450  -114.5810  10.0  3.30 Arizona
 
 Best Fitting Double Couple
    Mo = 1.64e+21 dyne-cm
    Mw = 3.41 
    Z  = 7 km
     Plane   Strike  Dip  Rake
      NP1      205    81   150
      NP2      300    60    10
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.64e+21     27     158
     N   0.00e+00     59      11
     P  -1.64e+21     14     256



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.03e+21
       Mxy    -8.06e+20
       Mxz    -5.27e+20
       Myy    -1.27e+21
       Myz     6.28e+20
       Mzz     2.47e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ###################---              
              ####################--------           
             ####################----------          
           #####################-------------        
          -------------########---------------       
         -------------------##-----------------      
        ---------------------###----------------     
        --------------------#######-------------     
       --------------------##########------------    
       -------------------#############----------    
       ------------------################--------    
       --   ------------###################------    
        - P ------------####################----     
        -   -----------######################---     
         -------------#######################--      
          -----------#########################       
           ----------###########   ##########        
             -------############ T ########          
              ------############   #######           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.47e+20  -5.27e+20  -6.28e+20 
 -5.27e+20   1.03e+21   8.06e+20 
 -6.28e+20   8.06e+20  -1.27e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081018022738/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 = 300
      DIP = 60
     RAKE = 10
       MW = 3.41
       HS = 7.0

The waveform inversion is preferred. This is a marginal solution. The depth, moment magnitude and pressure and tension axes are fairly robust, but the nodal plane dips can vary.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 SLU Moment Tensor Solution
 2008/10/18 02:27:38  36.1450  -114.5810  10.0  3.30 Arizona
 
 Best Fitting Double Couple
    Mo = 1.64e+21 dyne-cm
    Mw = 3.41 
    Z  = 7 km
     Plane   Strike  Dip  Rake
      NP1      205    81   150
      NP2      300    60    10
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.64e+21     27     158
     N   0.00e+00     59      11
     P  -1.64e+21     14     256



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.03e+21
       Mxy    -8.06e+20
       Mxz    -5.27e+20
       Myy    -1.27e+21
       Myz     6.28e+20
       Mzz     2.47e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ###################---              
              ####################--------           
             ####################----------          
           #####################-------------        
          -------------########---------------       
         -------------------##-----------------      
        ---------------------###----------------     
        --------------------#######-------------     
       --------------------##########------------    
       -------------------#############----------    
       ------------------################--------    
       --   ------------###################------    
        - P ------------####################----     
        -   -----------######################---     
         -------------#######################--      
          -----------#########################       
           ----------###########   ##########        
             -------############ T ########          
              ------############   #######           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.47e+20  -5.27e+20  -6.28e+20 
 -5.27e+20   1.03e+21   8.06e+20 
 -6.28e+20   8.06e+20  -1.27e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081018022738/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
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   295    75   -20   3.22 0.4412
WVFGRD96    1.0   295    75   -15   3.23 0.4639
WVFGRD96    2.0   295    75   -20   3.30 0.5279
WVFGRD96    3.0   295    70   -15   3.33 0.5445
WVFGRD96    4.0   295    70   -15   3.35 0.5508
WVFGRD96    5.0   300    60    10   3.39 0.5529
WVFGRD96    6.0   300    60    10   3.40 0.5553
WVFGRD96    7.0   300    60    10   3.41 0.5554
WVFGRD96    8.0   300    55    10   3.44 0.5548
WVFGRD96    9.0   300    60    10   3.44 0.5512
WVFGRD96   10.0   300    60    10   3.45 0.5473
WVFGRD96   11.0   300    60    10   3.46 0.5426
WVFGRD96   12.0   300    60    10   3.47 0.5371
WVFGRD96   13.0   295    65   -10   3.47 0.5316
WVFGRD96   14.0   300    75    25   3.48 0.5261
WVFGRD96   15.0   300    75    25   3.48 0.5260
WVFGRD96   16.0   300    75    20   3.49 0.5249
WVFGRD96   17.0   300    75    20   3.50 0.5241
WVFGRD96   18.0   300    75    20   3.51 0.5222
WVFGRD96   19.0   300    75    20   3.52 0.5193
WVFGRD96   20.0   300    75    20   3.52 0.5156
WVFGRD96   21.0   300    75    20   3.53 0.5118
WVFGRD96   22.0   300    75    20   3.54 0.5074
WVFGRD96   23.0   300    75    20   3.55 0.5022
WVFGRD96   24.0   300    75    15   3.55 0.4965
WVFGRD96   25.0   300    75    15   3.56 0.4907
WVFGRD96   26.0   295    90    20   3.57 0.4848
WVFGRD96   27.0   295    90    20   3.57 0.4789
WVFGRD96   28.0   295    90    20   3.58 0.4729
WVFGRD96   29.0   295    90    20   3.59 0.4668
WVFGRD96   30.0   295    90    20   3.60 0.4606
WVFGRD96   31.0   295    90    20   3.60 0.4540
WVFGRD96   32.0   115    85   -20   3.61 0.4476
WVFGRD96   33.0   295    90    20   3.62 0.4405
WVFGRD96   34.0   115    85   -15   3.63 0.4340
WVFGRD96   35.0   115    85   -15   3.64 0.4266
WVFGRD96   36.0   295    90    15   3.65 0.4192
WVFGRD96   37.0   295    90    15   3.67 0.4118
WVFGRD96   38.0   295    90    15   3.68 0.4036
WVFGRD96   39.0   300    80    15   3.69 0.3953
WVFGRD96   40.0   300    75    20   3.72 0.3853
WVFGRD96   41.0   300    75    20   3.73 0.3789
WVFGRD96   42.0   300    75    20   3.74 0.3724
WVFGRD96   43.0   300    75    20   3.74 0.3665
WVFGRD96   44.0   300    75    20   3.75 0.3607
WVFGRD96   45.0   300    75    20   3.76 0.3549
WVFGRD96   46.0   300    75    20   3.76 0.3491
WVFGRD96   47.0   300    75    15   3.77 0.3433
WVFGRD96   48.0   300    75    15   3.77 0.3377
WVFGRD96   49.0   300    75    15   3.78 0.3322
WVFGRD96   50.0   300    75    15   3.78 0.3268

The best solution is

WVFGRD96    7.0   300    60    10   3.41 0.5554

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
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=Sat Oct 18 09:36:54 CDT 2008

Last Changed 2008/10/18