Location

Location ANSS

The ANSS event ID is nn00197036 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00197036/executive.

2007/01/24 11:30:15 37.413 -117.099 6.1 4.1 Nevada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2007/01/24 11:30:15:0  37.41 -117.10   6.1 4.1 Nevada
 
 Stations used:
   CI.CWC CI.GRA CI.ISA CI.LRL CI.MLAC CI.MPM CI.TIN TA.M08A 
   TA.M09A TA.N07B TA.N09A TA.O07A TA.O08A TA.O09A TA.P07A 
   TA.P08A TA.P09A TA.Q07A TA.Q08A TA.Q09A TA.R08A TA.R09A 
   TA.S08C TA.S09A 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      265    85   -10
   NP2      356    80   -175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21      3     311
    N   0.00e+00     79      59
    P  -7.50e+21     11     220

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.05e+21
       Mxy    -7.26e+21
       Mxz     1.33e+21
       Myy     1.28e+21
       Myz     5.29e+20
       Mzz    -2.26e+20
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ###########-----------              
               #############--------------           
             T ##############--------------          
           #   ##############----------------        
          ###################-----------------       
         #####################-----------------      
        ######################------------------     
        ######################------------------     
       #######################-------------------    
       #################------###################    
       ####--------------------##################    
       ------------------------##################    
        -----------------------#################     
        -----------------------#################     
         ----------------------################      
          ---------------------###############       
           ---   --------------##############        
             - P --------------############          
                 -------------############           
                 -------------#########              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.26e+20   1.33e+21  -5.29e+20 
  1.33e+21  -1.05e+21   7.26e+21 
 -5.29e+20   7.26e+21   1.28e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070124113015/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 265
      DIP = 85
     RAKE = -10
       MW = 3.85
       HS = 9.0

The NDK file is 20070124113015.ndk The waveform inversion is preferred.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    85    90    -5   3.53 0.4319
WVFGRD96    2.0    85    90    -5   3.64 0.5530
WVFGRD96    3.0    85    85   -10   3.69 0.6114
WVFGRD96    4.0   265    80   -10   3.73 0.6507
WVFGRD96    5.0   265    85   -10   3.76 0.6807
WVFGRD96    6.0   265    85   -10   3.78 0.7020
WVFGRD96    7.0   265    85   -10   3.81 0.7162
WVFGRD96    8.0   265    85   -10   3.83 0.7246
WVFGRD96    9.0   265    85   -10   3.85 0.7264
WVFGRD96   10.0   265    85   -10   3.86 0.7231
WVFGRD96   11.0   265    85   -10   3.87 0.7160
WVFGRD96   12.0   265    85   -10   3.89 0.7065
WVFGRD96   13.0   265    85   -10   3.89 0.6957
WVFGRD96   14.0   265    85   -10   3.90 0.6833
WVFGRD96   15.0   265    85   -10   3.91 0.6699
WVFGRD96   16.0    85    90    10   3.92 0.6541
WVFGRD96   17.0   265    85   -10   3.92 0.6430
WVFGRD96   18.0    85    90    10   3.93 0.6267
WVFGRD96   19.0    85    90    10   3.93 0.6131
WVFGRD96   20.0   265    85   -10   3.94 0.6034
WVFGRD96   21.0    85    90    10   3.95 0.5867
WVFGRD96   22.0   265    80   -10   3.95 0.5777
WVFGRD96   23.0   265    80   -10   3.96 0.5648
WVFGRD96   24.0   265    80   -10   3.96 0.5516
WVFGRD96   25.0   265    80   -10   3.97 0.5393
WVFGRD96   26.0    85    90    10   3.97 0.5250
WVFGRD96   27.0    85    90    10   3.98 0.5144
WVFGRD96   28.0    85    90    10   3.98 0.5043
WVFGRD96   29.0    85    90    10   3.99 0.4948

The best solution is

WVFGRD96    9.0   265    85   -10   3.85 0.7264

The mechanism corresponding 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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of 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=      84.99
  DIP=      85.00
 RAKE=      14.99
  
             OR
  
  STK=     353.65
  DIP=      75.06
 RAKE=     174.82
 
 
DEPTH = 9.0 km
 
Mw = 3.93
Best Fit 0.8927 - P-T axis plot gives solutions with FIT greater than FIT90

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