Location

2007/02/28 11:47:41 41.06N 114.77W 5 3.5 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
 2007/02/28 11:47:41 41.06N 114.77W 5 3.5 Nevada
 
 Best Fitting Double Couple
    Mo = 2.95e+21 dyne-cm
    Mw = 3.58 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1       60    70   -25
      NP2      159    67   -158
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.95e+21      2     110
     N   0.00e+00     58     204
     P  -2.95e+21     32      19



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.58e+21
       Mxy    -1.60e+21
       Mxz    -1.28e+21
       Myy     2.38e+21
       Myz    -3.15e+20
       Mzz    -8.02e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ###-------------------              
              ######-----------   --------           
             #######----------- P ---------          
           ########------------   -----------        
          ##########--------------------------       
         ###########-------------------------##      
        ############------------------------####     
        ############----------------------######     
       ##############-------------------#########    
       ##############-----------------###########    
       ###############--------------#############    
       ################----------################    
        ###############-------################       
        ################---################### T     
         #############---#####################       
          #####-----------####################       
           -----------------#################        
             ----------------##############          
              -----------------###########           
                 ----------------######              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -8.02e+20  -1.28e+21   3.15e+20 
 -1.28e+21  -1.58e+21   1.60e+21 
  3.15e+20   1.60e+21   2.38e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070228114741/index.html
        

      STK = 60
      DIP = 70
     RAKE = -25
       MW = 3.58
       HS = 12

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 0.10 n 3
br c 0.12 0.2 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   105    45    90   3.09 0.1984
WVFGRD96    1.0   175    85    -5   3.02 0.1883
WVFGRD96    2.0   175    85     0   3.18 0.2729
WVFGRD96    3.0    65    75    20   3.29 0.3171
WVFGRD96    4.0    65    75    15   3.34 0.3570
WVFGRD96    5.0    65    90    25   3.39 0.3927
WVFGRD96    6.0   240    75   -25   3.44 0.4326
WVFGRD96    7.0    60    65   -30   3.49 0.4779
WVFGRD96    8.0    60    65   -30   3.53 0.5123
WVFGRD96    9.0    60    65   -30   3.55 0.5359
WVFGRD96   10.0    60    65   -25   3.56 0.5476
WVFGRD96   11.0    60    70   -25   3.57 0.5533
WVFGRD96   12.0    60    70   -25   3.58 0.5545
WVFGRD96   13.0    60    70   -25   3.59 0.5525
WVFGRD96   14.0    60    70   -25   3.59 0.5474
WVFGRD96   15.0    60    70   -25   3.60 0.5399
WVFGRD96   16.0    65    75   -20   3.61 0.5329
WVFGRD96   17.0    65    75   -20   3.61 0.5265
WVFGRD96   18.0    65    75   -20   3.62 0.5187
WVFGRD96   19.0    65    75   -20   3.63 0.5105
WVFGRD96   20.0   245    80   -10   3.62 0.5072
WVFGRD96   21.0   250    75   -20   3.63 0.5043
WVFGRD96   22.0   250    75   -20   3.64 0.5001
WVFGRD96   23.0   250    75   -20   3.65 0.4966
WVFGRD96   24.0   250    75   -20   3.65 0.4916
WVFGRD96   25.0   250    75   -20   3.66 0.4861
WVFGRD96   26.0   250    75   -20   3.67 0.4826
WVFGRD96   27.0   240    75     5   3.69 0.4787
WVFGRD96   28.0   250    80    20   3.69 0.4766
WVFGRD96   29.0   250    80    20   3.70 0.4749

The best solution is

WVFGRD96   12.0    60    70   -25   3.58 0.5545

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 0.2 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=      54.63
  DIP=      67.47
 RAKE=     -44.10
  
             OR
  
  STK=     164.99
  DIP=      50.00
 RAKE=    -149.99
 
 
DEPTH = 11.0 km
 
Mw = 3.70
Best Fit 0.8953 - 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
N12A      223   32 iP_D
M12A      344   41 iP_C
ELK       228   52 iP_D
N13A      115   54 iP_C
M13A       57   61 iP_C
N11A      252   85 iP_C
O12A      178   88 eP_-
M11A      296   94 iP_C
O11A      216  127 iP_D
BGU        95  148 iP_C
N10A      256  150 iP_C
M10A      290  156 iP_C
O10A      240  169 iP_C
P12A      184  177 iP_C
P11A      207  187 eP_X
DUG       119  192 eP_X
SPU        81  197 eP_X
P10A      222  215 eP_-
Q12A      181  224 iP_C
M09A      281  227 eP_+
O09A      245  227 eP_X
N09A      265  232 eP_+
Q11A      197  257 iP_D
P09A      231  261 eP_X
HLID        6  280 iP_D
N08A      265  284 eP_X
O08A      254  298 eP_X
M08A      279  305 eP_X
SRU       119  423 eP_X

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)   
ELK	  228	   52
O11A	  216	  127
BGU	   95	  148
N10A	  256	  150
O10A	  240	  169
P12A	  184	  177
P11A	  207	  187
DUG	  119	  192
SPU	   81	  197
P10A	  222	  215
Q12A	  181	  224
M09A	  281	  227
O09A	  245	  227
N09A	  265	  232
CTU	   98	  259
P09A	  231	  261
HLID	    6	  280
N08A	  265	  284
M08A	  279	  305
P08A	  243	  319
L08A	  294	  322
O07A	  255	  361
Q08A	  229	  363
P07A	  246	  388
J08A	  311	  398
R08A	  224	  415
Q07A	  237	  417
SRU	  119	  423
S09A	  210	  426
O06A	  258	  438
J07A	  306	  454
P06A	  252	  461
BMO	  335	  469
BW06	   65	  473
TPNV	  196	  474
M06C	  274	  479
R06C	  236	  489
S08C	  218	  492
R07C	  229	  493
ELFS	  266	  504
U12A	  178	  514
R05C	  242	  523
P05C	  250	  533
O05C	  259	  534
M05C	  276	  535
O04C	  263	  539
HATC	  269	  563
V11A	  186	  582
V12A	  181	  592
K04A	  289	  602
R04C	  242	  613
Q04C	  248	  615
T06C	  225	  620
W12A	  181	  639
MSO	    6	  645
W14A	  167	  666
S04C	  237	  689
G05A	  313	  706
X13A	  173	  723
F05A	  317	  761
V03C	  227	  793
H03A	  303	  803
Y14A	  168	  806
Y13A	  174	  809
F04A	  314	  821
Z14A	  169	  870
E03A	  314	  932
116A	  163	  982
B05A	  327	  988
B04A	  321	 1040
A05A	  329	 1052
A04A	  326	 1056

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 0.2 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 Mar 2 20:10:32 CST 2007