Location

2007/04/27 15:42:50 37.10N 115.27W 0 3.8 Nevada

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

 SLU Moment Tensor Solution
 2007/04/27 15:42:50 37.10N 115.27W 0 3.8 Nevada
 
 Best Fitting Double Couple
    Mo = 6.76e+21 dyne-cm
    Mw = 3.82 
    Z  = 10 km
     Plane   Strike  Dip  Rake
      NP1      168    80   170
      NP2      260    80    10
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   6.76e+21     14     124
     N   0.00e+00     76     305
     P  -6.76e+21      0     214



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.63e+21
       Mxy    -6.09e+21
       Mxz    -8.86e+20
       Myy     2.23e+21
       Myz     1.33e+21
       Mzz     4.02e+20
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 #######---------------              
              ##########------------------           
             ###########-------------------          
           #############---------------------        
          ##############----------------------       
         ###############-----------------------      
        ################------------------------     
        #################-----------------------     
       ##################----####################    
       #############-----########################    
       #######------------#######################    
       ##-----------------#######################    
        ------------------######################     
        -------------------#####################     
         ------------------##############   ###      
          ------------------############# T ##       
           ------------------############   #        
             ----------------##############          
              --   -----------############           
                 P ------------########              
                     -----------###                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  4.02e+20  -8.86e+20  -1.33e+21 
 -8.86e+20  -2.63e+21   6.09e+21 
 -1.33e+21   6.09e+21   2.23e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070427154250/index.html
        

      STK = 260
      DIP = 80
     RAKE = 10
       MW = 3.82
       HS = 10

The two solutions are compatible. The waveform inversion is the preferred solution.

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    75    70   -35   3.46 0.2789
WVFGRD96    1.0    85    85   -10   3.45 0.3135
WVFGRD96    2.0    75    75   -20   3.61 0.4842
WVFGRD96    3.0    80    75   -15   3.67 0.5585
WVFGRD96    4.0    80    75   -15   3.71 0.6019
WVFGRD96    5.0   260    75     5   3.73 0.6253
WVFGRD96    6.0   260    75     5   3.75 0.6393
WVFGRD96    7.0   260    80     5   3.77 0.6472
WVFGRD96    8.0   260    80    10   3.79 0.6513
WVFGRD96    9.0   260    80    10   3.81 0.6545
WVFGRD96   10.0   260    80    10   3.82 0.6555
WVFGRD96   11.0   260    80    10   3.84 0.6545
WVFGRD96   12.0   260    80     5   3.85 0.6522
WVFGRD96   13.0   260    80     5   3.86 0.6487
WVFGRD96   14.0   260    80    10   3.87 0.6449
WVFGRD96   15.0   260    80     5   3.88 0.6406
WVFGRD96   16.0   260    80     5   3.89 0.6353
WVFGRD96   17.0   260    80     5   3.90 0.6287
WVFGRD96   18.0   260    85    10   3.92 0.6212
WVFGRD96   19.0   260    85    10   3.93 0.6135
WVFGRD96   20.0   260    85    10   3.94 0.6049
WVFGRD96   21.0   260    80    10   3.95 0.5960
WVFGRD96   22.0   260    80    10   3.96 0.5868
WVFGRD96   23.0   260    80    15   3.98 0.5767
WVFGRD96   24.0   260    80    10   3.98 0.5658
WVFGRD96   25.0   260    80    10   3.99 0.5543
WVFGRD96   26.0   260    80    15   4.00 0.5422
WVFGRD96   27.0   260    80    15   4.00 0.5301
WVFGRD96   28.0    80    80   -10   4.00 0.5184
WVFGRD96   29.0    80    80   -10   4.01 0.5076

The best solution is

WVFGRD96   10.0   260    80    10   3.82 0.6555

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 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=     259.99
  DIP=      75.00
 RAKE=      24.99
  
             OR
  
  STK=     163.11
  DIP=      65.91
 RAKE=     163.53
 
 
DEPTH = 8.0 km
 
Mw = 3.87
Best Fit 0.8966 - 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
TPNV      259   89 iP_X
FUR       244  159 iP_D
SHO       214  161 iP_D
GRA       267  187 -12345
MPM       240  230 eP_-
GMR       188  259 -12345
CWC       255  261 -12345
TIN       270  263 -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.

The velocity model used for the search is a modified Utah model .

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 distributiuon

Sta Az(deg)    Dist(km)   
TPNV	  259	   89
FUR	  244	  159
SHO	  214	  161
GRA	  267	  187
TUQ	  198	  194
MPM	  240	  230
GSC	  215	  243
GMR	  188	  259
CWC	  255	  261
TIN	  270	  263
HEC	  201	  270
DAN	  182	  273
LRL	  231	  282
RRX	  213	  292
MLAC	  282	  321
IRM	  178	  327
PDM	  162	  327
ISA	  242	  329
ADO	  215	  344
BEL	  191	  350
VES	  249	  369
SVD	  207	  371
BC3	  183	  383
BFS	  215	  384
CHF	  220	  395
DUG	   31	  405
MWC	  219	  407
DGR	  203	  414
OSI	  230	  416
DEC	  222	  420
PLM	  200	  440
GLA	  175	  451
JCS	  196	  461
SMM	  246	  469
WVOR	  335	  659
HLID	    6	  721
AHID	   28	  723
RRI2	   25	  771
TPAW	   26	  799
SNOW	   27	  804
ANMO	  104	  829
BMO	  349	  878
COR	  323	 1071
MSO	    5	 1086
MNTX	  120	 1088
AMTX	   97	 1249
NEW	  354	 1249
KSU1	   77	 1649
ECSD	   59	 1742

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 velocity model used for the waveform fit is a modified Utah model .

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 0.12 0.25 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

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Fri Apr 27 14:05:52 CDT 2007