Location

2007/08/18 13:16:30 38.07 -113.31 7.0 3.9 Utah

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
 2007/08/18 13:16:30 38.07 -113.31 7.0 3.9 Utah
 
 Best Fitting Double Couple
    Mo = 3.76e+21 dyne-cm
    Mw = 3.65 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1       65    75   -75
      NP2      199    21   -134
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.76e+21     28     143
     N   0.00e+00     14     241
     P  -3.76e+21     57     355



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     7.71e+20
       Mxy    -1.30e+21
       Mxz    -2.96e+21
       Myy     1.04e+21
       Myz     1.10e+21
       Mzz    -1.82e+21
                                                     
                                                     
                                                     
                                                     
                     #######-------                  
                 ######----------------              
              ######----------------------           
             #####-------------------------          
           #####-----------------------------        
          #####-----------   -----------------       
         #####------------ P -----------------#      
        #####-------------   ----------------###     
        ####-------------------------------#####     
       #####----------------------------#########    
       ####---------------------------###########    
       ####------------------------##############    
       ####--------------------##################    
        ###----------------#####################     
        ####----------##########################     
         ###---################################      
          ---#######################   #######       
           ---###################### T ######        
             --#####################   ####          
              --##########################           
                 -#####################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.82e+21  -2.96e+21  -1.10e+21 
 -2.96e+21   7.71e+20   1.30e+21 
 -1.10e+21   1.30e+21   1.04e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070818131630/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 = 65
      DIP = 75
     RAKE = -75
       MW = 3.65
       HS = 9

Both techniques give the same solution. The waveform inversion is preferred. This could not have been done without the EarthScope Transportable Array stations. Data processing was complicated by the 18 second surface waves from a Pacific Event.

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 0.05 n 3
lp c 0.15 n 3
br c 0.045 0.055 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   240    45   -90   3.21 0.2913
WVFGRD96    1.0    40    50    65   3.17 0.2005
WVFGRD96    2.0    55    45    85   3.38 0.2665
WVFGRD96    3.0    20    85   -60   3.40 0.2890
WVFGRD96    4.0    65    80   -80   3.45 0.3607
WVFGRD96    5.0    65    75   -80   3.48 0.4123
WVFGRD96    6.0    60    70   -80   3.51 0.4487
WVFGRD96    7.0    65    70   -75   3.53 0.4722
WVFGRD96    8.0    65    70   -80   3.63 0.4887
WVFGRD96    9.0    65    70   -75   3.65 0.4913
WVFGRD96   10.0   220    45    65   3.70 0.4842
WVFGRD96   11.0   220    45    65   3.72 0.4722
WVFGRD96   12.0   220    50    65   3.73 0.4510
WVFGRD96   13.0   220    55    60   3.74 0.4259
WVFGRD96   14.0   235    75    60   3.74 0.4061
WVFGRD96   15.0   235    75    60   3.76 0.3891
WVFGRD96   16.0   240    75    65   3.77 0.3719
WVFGRD96   17.0   240    75    65   3.78 0.3546
WVFGRD96   18.0   240    75    65   3.79 0.3358
WVFGRD96   19.0   240    75    60   3.81 0.3162
WVFGRD96   20.0   260    80    65   3.81 0.2974
WVFGRD96   21.0   265    80    65   3.83 0.2826
WVFGRD96   22.0   255    85    65   3.83 0.2698
WVFGRD96   23.0   260    85    65   3.84 0.2580
WVFGRD96   24.0    75    90   -65   3.84 0.2482
WVFGRD96   25.0   255    90    65   3.85 0.2395
WVFGRD96   26.0   255    90    70   3.86 0.2314
WVFGRD96   27.0    75    85   -70   3.86 0.2247
WVFGRD96   28.0   195    20    35   3.85 0.2141
WVFGRD96   29.0   195    20    35   3.85 0.2125

The best solution is

WVFGRD96    9.0    65    70   -75   3.65 0.4913

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 0.05 n 3
lp c 0.15 n 3
br c 0.045 0.055 n 4 p 2
Figure 3. Waveform comparison for depth of 8 km
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.

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=      70.51
  DIP=      61.97
 RAKE=     -68.12
  
             OR
  
  STK=     210.00
  DIP=      35.00
 RAKE=    -124.99
 
 
DEPTH = 11.0 km
 
Mw = 3.76
Best Fit 0.8403 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   First motion
S14A      161   37 -12345
R13A      282   59 -12345
CCUT      177   61 -12345
S13A      222   73 -12345
R15A       80   91 -12345
S15A      118   94 -12345
Q14A        1  102 -12345
T14A      170  114 -12345
Q13A      328  116 -12345
R12A      284  118 -12345
T13A      205  128 -12345
Q15A       38  131 -12345
T15A      145  142 -12345
S12A      250  145 -12345
S16A      104  155 -12345
P13A      338  166 -12345
P14A        7  170 -12345
Q12A      309  171 -12345
P15A       28  189 -12345
R11A      279  202 -12345
U12A      209  211 -12345

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

Data preparation

Digital data were collected, instrument response removed and traces converted to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.


Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distribution

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
S13A	  222	   73
Q14A	    1	  102
T14A	  170	  114
Q13A	  328	  116
R12A	  284	  118
T13A	  205	  128
Q15A	   38	  131
T15A	  145	  142
S12A	  250	  145
S16A	  104	  155
Q12A	  310	  162
P13A	  338	  166
P14A	    7	  170
U14A	  176	  184
P15A	   28	  189
T11A	  242	  192
U13A	  198	  193
T12A	  220	  195
T16A	  127	  200
R11A	  280	  202
U15A	  153	  203
U12A	  209	  211
S11A	  258	  220
Q11A	  294	  222
P16A	   39	  223
S17A	  102	  226
R17A	   80	  230
TMU	   55	  233
NLU	   29	  236
T17A	  118	  251
U11A	  226	  260
P11A	  308	  269
P17A	   54	  272
U17A	  124	  285
Q18A	   66	  299
P18A	   56	  317
LDF	  205	  369
GSC	  225	  435
ISA	  241	  527
AHID	   20	  553
WCN	  286	  568
GLA	  193	  575
MWC	  226	  600
CMB	  271	  613
OSI	  233	  614
REDW	   19	  623
BEK	  290	  633
LOHW	   20	  656
MOOW	   19	  666
BAR	  207	  671
IMW	   17	  676
FLWY	   18	  702
DLMT	    4	  809
BMO	  338	  817
HUMO	  305	  952

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.05 n 3
lp c 0.15 n 3
br c 0.045 0.055 n 4 p 2

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

Appendix A


Spectra fit plots to each station

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=Wed Apr 1 07:09:33 CDT 2009

Last Changed 2007/08/18