Location

2007/11/05 21:48:00 39.36 -111.64 1.0 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
 2007/11/05 21:48:00 39.36 -111.64 1.0 3.9 Utah
 
 Best Fitting Double Couple
    Mo = 5.50e+21 dyne-cm
    Mw = 3.76 
    Z  = 15 km
     Plane   Strike  Dip  Rake
      NP1       23    67   -101
      NP2      230    25   -65
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   5.50e+21     22     121
     N   0.00e+00     10      27
     P  -5.50e+21     66     273



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.27e+21
       Mxy    -2.05e+21
       Mxz    -1.10e+21
       Myy     2.54e+21
       Myz     3.67e+21
       Mzz    -3.82e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ###################---              
              ##########-------------###--           
             #######-----------------######          
           #######-------------------########        
          ######---------------------#########       
         #####----------------------###########      
        #####-----------------------############     
        ####------------------------############     
       ####---------   ------------##############    
       ####--------- P -----------###############    
       ###----------   -----------###############    
       ###-----------------------################    
        ##----------------------################     
        ##---------------------##########   ####     
         #--------------------########### T ###      
          #------------------############   ##       
           -----------------#################        
             -------------#################          
              -----------#################           
                 -----#################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -3.82e+21  -1.10e+21  -3.67e+21 
 -1.10e+21   1.27e+21   2.05e+21 
 -3.67e+21   2.05e+21   2.54e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20071105214800/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 = 230
      DIP = 25
     RAKE = -65
       MW = 3.76
       HS = 15

The solutions from the two techniques are in agreement. The waveform inversion is preferred.

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 0.07 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   210    40    90   3.37 0.2931
WVFGRD96    1.0   210    45   -90   3.38 0.2549
WVFGRD96    2.0   210    50   -90   3.50 0.3127
WVFGRD96    3.0    35    35   -80   3.55 0.2561
WVFGRD96    4.0   280    15    -5   3.60 0.2969
WVFGRD96    5.0   270    10   -20   3.61 0.3710
WVFGRD96    6.0   255    10   -40   3.61 0.4295
WVFGRD96    7.0   250    15   -45   3.62 0.4747
WVFGRD96    8.0   240    15   -55   3.70 0.5058
WVFGRD96    9.0   235    15   -60   3.71 0.5445
WVFGRD96   10.0   230    20   -65   3.73 0.5766
WVFGRD96   11.0   225    20   -70   3.73 0.6010
WVFGRD96   12.0   230    25   -70   3.75 0.6198
WVFGRD96   13.0   230    25   -70   3.76 0.6317
WVFGRD96   14.0   230    25   -70   3.76 0.6371
WVFGRD96   15.0   230    25   -65   3.76 0.6376
WVFGRD96   16.0   230    25   -65   3.77 0.6342
WVFGRD96   17.0   230    25   -65   3.77 0.6274
WVFGRD96   18.0   230    25   -65   3.78 0.6184
WVFGRD96   19.0   230    25   -65   3.78 0.6073
WVFGRD96   20.0   230    25   -65   3.79 0.5947
WVFGRD96   21.0   230    25   -65   3.80 0.5820
WVFGRD96   22.0   235    25   -60   3.80 0.5670
WVFGRD96   23.0   240    30   -55   3.81 0.5512
WVFGRD96   24.0   240    30   -55   3.82 0.5351
WVFGRD96   25.0   240    30   -50   3.82 0.5187
WVFGRD96   26.0   240    30   -50   3.82 0.5018
WVFGRD96   27.0   240    30   -55   3.83 0.4844
WVFGRD96   28.0   240    30   -55   3.83 0.4668
WVFGRD96   29.0   240    30   -50   3.83 0.4488

The best solution is

WVFGRD96   15.0   230    25   -65   3.76 0.6376

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 0.07 n 3
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=      25.00
  DIP=      64.99
 RAKE=    -105.00
  
             OR
  
  STK=     237.37
  DIP=      28.91
 RAKE=     -60.97
 
 
DEPTH = 14.0 km
 
Mw = 3.85
Best Fit 0.8943 - 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
P16A      357   28 -12345
P15A      293   60 -12345
Q16A      140   64 -12345
MPU         0   73 -12345
Q15A      238   75 -12345
P17A       81   79 -12345
O16A        7   95 -12345
SRU       106  100 -12345
O15A      325  124 -12345
P18A       76  124 -12345
P14A      282  126 -12345
R17A      142  132 -12345
Q18A      102  133 -12345
DUG       313  137 -12345
R15A      204  139 -12345
Q14A      254  147 -12345
N16A        6  170 -12345
S16A      179  182 -12345
R18A      125  186 -12345
S17A      159  205 -12345
BGU       326  210 -12345
Q19A      102  210 -12345
N14A      322  212 -12345
M16A        0  217 -12345
O13A      294  218 -12345
S18A      142  234 -12345
R19A      119  238 -12345
S13A      225  277 -12345
S19A      129  282 -12345
T18A      147  291 -12345
L14A      336  326 -12345
ELK       298  343 -12345
L13A      328  360 -12345
MVCO      130  364 -12345
AHID        7  381 -12345
WUAZ      177  427 -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)   
O15A	  326	  124
P18A	   76	  124
P14A	  282	  126
R17A	  142	  132
Q18A	  102	  133
DUG	  313	  137
JLU	    7	  139
R15A	  204	  139
Q14A	  254	  147
CTU	  356	  148
R14A	  226	  168
N16A	    6	  170
O18A	   54	  172
HMU	  153	  176
S16A	  179	  182
N15A	  336	  186
R18A	  125	  186
N17A	   21	  189
S15A	  199	  197
S17A	  159	  205
BGU	  326	  210
Q19A	  102	  210
N14A	  322	  212
M16A	    0	  217
O13A	  294	  218
S14A	  217	  222
SPU	  343	  227
S18A	  142	  234
R19A	  119	  238
R13A	  238	  241
M15A	  344	  243
N18A	   42	  246
T15A	  194	  268
S13A	  225	  277
M14A	  329	  279
S19A	  129	  282
HVU	  341	  285
T14A	  207	  285
T18A	  147	  291
L16A	    3	  295
L15A	  348	  300
L14A	  336	  326
T13A	  218	  327
ELK	  298	  343
L13A	  328	  360
MVCO	  130	  364
K14A	  340	  377
K15A	  349	  377
AHID	    7	  381
K16A	    1	  386
WUAZ	  177	  427
TPNV	  238	  485
RLMT	   16	  669
WVOR	  302	  681
BOZ	    0	  698
BMO	  324	  769
LAO	   27	  926

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 0.02 n 3
lp c 0.07 n 3

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=Mon Nov 5 20:59:27 CST 2007

Last Changed 2007/11/05