Location

2007/09/08 07:15:40 33.673 -108.858 5.0 3.5 New Mexico

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
 2007/09/08 07:15:40 33.673 -108.858 5.0 3.5 New Mexico
 
 Best Fitting Double Couple
    Mo = 2.95e+21 dyne-cm
    Mw = 3.58 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      255    80    70
      NP2      139    22   153
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.95e+21     51     142
     N   0.00e+00     20     259
     P  -2.95e+21     32       2



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.38e+21
       Mxy    -6.24e+20
       Mxz    -2.47e+21
       Myy     4.33e+20
       Myz     8.44e+20
       Mzz     9.48e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------------   ------------           
             -------------- P -------------          
           #---------------   ---------------        
          #-----------------------------------       
         ##------------------------------------      
        ###-------------------------------------     
        ##---------------------------###########     
       ###------------------#####################    
       ###------------###########################    
       ####------################################    
       ####--####################################    
        #---####################################     
        -----####################   ############     
         -----################### T ###########      
          -----##################   ##########       
           ------############################        
             ------########################          
              --------####################           
                 ----------##########--              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  9.48e+20  -2.47e+21  -8.44e+20 
 -2.47e+21  -1.38e+21   6.24e+20 
 -8.44e+20   6.24e+20   4.33e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070908071540/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 = 255
      DIP = 80
     RAKE = 70
       MW = 3.58
       HS = 9

The waveform solution was iintially obtained using the US backbone data. This was supplemented with the Transportable Array data for completeness. The surface-wave soltuion is identical. Because of the mechanism, Love wave excitation is minimal.

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 0.13 0.20 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   235    55   -75   3.23 0.2794
WVFGRD96    1.0   240    75   -15   3.17 0.2913
WVFGRD96    2.0   240    75   -20   3.29 0.3617
WVFGRD96    3.0   240    85   -25   3.34 0.3682
WVFGRD96    4.0   240    90   -45   3.42 0.3683
WVFGRD96    5.0   250    80    65   3.50 0.3863
WVFGRD96    6.0   255    80    70   3.52 0.4140
WVFGRD96    7.0   255    80    70   3.51 0.4290
WVFGRD96    8.0   255    80    75   3.59 0.4358
WVFGRD96    9.0   255    80    70   3.58 0.4371
WVFGRD96   10.0   260    75    70   3.60 0.4344
WVFGRD96   11.0   255    80    70   3.59 0.4291
WVFGRD96   12.0   255    80    65   3.59 0.4214
WVFGRD96   13.0   240    75   -55   3.57 0.4212
WVFGRD96   14.0   240    75   -55   3.58 0.4229
WVFGRD96   15.0   240    75   -55   3.58 0.4215
WVFGRD96   16.0   240    75   -50   3.60 0.4200
WVFGRD96   17.0   240    75   -50   3.61 0.4158
WVFGRD96   18.0   240    75   -50   3.62 0.4087
WVFGRD96   19.0   245    80   -55   3.63 0.4016
WVFGRD96   20.0   245    80   -55   3.64 0.3926
WVFGRD96   21.0   245    80   -60   3.65 0.3836
WVFGRD96   22.0   245    80   -60   3.66 0.3745
WVFGRD96   23.0   250    80   -65   3.69 0.3649
WVFGRD96   24.0   250    80   -65   3.69 0.3555
WVFGRD96   25.0   250    80   -70   3.71 0.3438
WVFGRD96   26.0   250    80   -70   3.71 0.3320
WVFGRD96   27.0   250    80   -70   3.72 0.3199
WVFGRD96   28.0   250    80   -70   3.73 0.3062
WVFGRD96   29.0   250    80   -70   3.73 0.2938

The best solution is

WVFGRD96    9.0   255    80    70   3.58 0.4371

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 0.13 0.20 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=     244.99
  DIP=      75.00
 RAKE=      60.00
  
             OR
  
  STK=     130.84
  DIP=      33.23
 RAKE=     151.81
 
 
DEPTH = 10.0 km
 
Mw = 3.67
Best Fit 0.9305 - 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
Y22C       76  185 -12345
116A      246  293 -12345
WUAZ      312  308 iP_C
X15A      287  325 iP_C
115A      252  332 -12345
W15A      299  355 -12345
Z14A      266  381 -12345
X14A      284  383 iP_C
Y14A      276  385 iP_C
MVCO        5  394 -12345
W14A      295  424 -12345
Y13A      273  461 iP_D
X13A      284  470 -12345
W13A      290  489 -12345
TPNV      300  764 -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)   
Y22C	   76	  185
116A	  246	  293
WUAZ	  312	  308
X15A	  287	  325
115A	  252	  332
W15A	  299	  355
Z14A	  266	  381
X14A	  284	  383
Y14A	  276	  385
MVCO	    5	  394
W14A	  295	  424
X13A	  284	  470
W13A	  290	  489
W12A	  290	  581
V12A	  294	  594
U12A	  302	  602
V11A	  294	  648
AMTX	   76	  674
U10A	  296	  746
TPNV	  300	  764
Q12A	  320	  801
DUG	  335	  805
Q11A	  315	  838
P12A	  322	  841
S09A	  303	  880
R09A	  307	  897
O12A	  326	  900
JCT	  110	  925
N13A	  331	  927
WMOK	   80	  936
S08C	  299	  944
N12A	  327	  967
M13A	  333	  973
R08A	  304	  981
BW06	  357	 1011
M12A	  330	 1012
AHID	  350	 1028
N10A	  321	 1035
R07C	  301	 1042
M11A	  327	 1056
Q07A	  306	 1067

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 0.10 n 3
br c 0.13 0.20 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=Tue Sep 11 09:26:48 CDT 2007

Last Changed 2007/09/08