Location

2007/01/24 11:30:15 37.41N 117.08W 3 4.0 Nevada

Region:                        NEVADA
Geographic coordinates:        37.413N, 117.079W
Magnitude:                    3.9 Ml
Depth:                        0 km
Universal Time (UTC):         24 Jan 2007  11:30:15
Time near the Epicenter:      24 Jan 2007  03:30:15
Local time in your area:      24 Jan 2007  05:30:15

Location with respect to nearby cities:
  20 km (13 miles) NE (52 degrees) of Tokop, NV
  38 km (24 miles) SSE (154 degrees) of West Spring, NV
  48 km (30 miles) NNE (29 degrees) of Scottys Castle, CA
  63 km (39 miles) NNW (333 degrees) of Beatty, NV
 214 km (133 miles) NW (310 degrees) of Las Vegas, NV


ADDITIONAL EARTHQUAKE PARAMETERS
__________________________________________________
event ID                     :  NN 00197036
version                      :  1
number of phases             :  120
rms misfit                   :  
horizontal location error    :  
vertical location error      :  
maximum azimuthal gap        :  0 degrees
distance to nearest station  :  

Flinn-Engdahl Region Number =   40

This is a computer-generated message and has not yet been reviewed by a
seismologist.
For subsequent updates, maps, and technical information, see:
http://earthquake.usgs.gov/eqcenter/recenteqsus/Quakes/nn00197036.php
or
http://earthquake.usgs.gov/

Nevada Seismological Laboratory
University
of Nevada, Reno

http://www.seismo.unr.edu

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
 2007/01/24 11:30:15 37.41N 117.08W 3 4.0 Nevada
 
 Best Fitting Double Couple
    Mo = 7.00e+21 dyne-cm
    Mw = 3.83 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      265    90   -10
      NP2      355    80   -180
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   7.00e+21      7     310
     N   0.00e+00     80      85
     P  -7.00e+21      7     220



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.20e+21
       Mxy    -6.79e+21
       Mxz     1.21e+21
       Myy     1.20e+21
       Myz    -1.06e+20
       Mzz    -2.96e+13
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ##########------------              
              ##############--------------           
              ##############---------------          
            T ###############----------------        
          #   ###############-----------------       
         #####################-----------------      
        ######################------------------     
        ######################------------------     
       #######################-------------------    
       #######################---------##########    
       ##########--------------##################    
       ------------------------##################    
        -----------------------#################     
        -----------------------#################     
         ----------------------################      
          ---------------------###############       
           --------------------##############        
             --   -------------############          
              - P -------------###########           
                  ------------#########              
                     ---------#####                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -2.96e+13   1.21e+21   1.06e+20 
  1.21e+21  -1.20e+21   6.79e+21 
  1.06e+20   6.79e+21   1.20e+21 


Details of the solution is found at

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

      STK = 265
      DIP = 90
     RAKE = -10
       MW = 3.83
       HS = 9

The waveform inversion is acceptable. The surface-wave spectral amplitude solution is virtually the same.

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   270    75    -5   3.36 0.2421
WVFGRD96    1.0   270    80     5   3.40 0.2782
WVFGRD96    2.0   270    80    10   3.58 0.4563
WVFGRD96    3.0   270    75     5   3.65 0.5474
WVFGRD96    4.0   265    75    -5   3.70 0.6113
WVFGRD96    5.0   265    75    -5   3.74 0.6530
WVFGRD96    6.0   265    80   -15   3.77 0.6776
WVFGRD96    7.0    85    90    10   3.79 0.6924
WVFGRD96    8.0    85    90    10   3.81 0.7027
WVFGRD96    9.0   265    90   -10   3.83 0.7061
WVFGRD96   10.0   265    90   -10   3.84 0.7038
WVFGRD96   11.0    85    90    10   3.86 0.6984
WVFGRD96   12.0   265    90   -10   3.87 0.6904
WVFGRD96   13.0   265    85   -10   3.87 0.6818
WVFGRD96   14.0   265    85   -10   3.88 0.6750
WVFGRD96   15.0   265    85   -10   3.89 0.6678
WVFGRD96   16.0   265    85   -10   3.90 0.6590
WVFGRD96   17.0   265    85   -10   3.91 0.6510
WVFGRD96   18.0   265    85   -10   3.92 0.6429
WVFGRD96   19.0   265    85   -10   3.93 0.6329
WVFGRD96   20.0   265    85   -10   3.94 0.6248
WVFGRD96   21.0   265    85   -10   3.95 0.6179
WVFGRD96   22.0   265    85   -10   3.96 0.6090
WVFGRD96   23.0   265    85   -10   3.97 0.6009
WVFGRD96   24.0   265    85   -10   3.97 0.5910
WVFGRD96   25.0   265    85   -10   3.98 0.5825
WVFGRD96   26.0   265    90    -5   3.98 0.5751
WVFGRD96   27.0    85    90     5   3.99 0.5691
WVFGRD96   28.0   265    90    -5   3.99 0.5647
WVFGRD96   29.0   265    90    -5   4.00 0.5583

The best solution is

WVFGRD96    9.0   265    90   -10   3.83 0.7061

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=      79.99
  DIP=      85.00
 RAKE=      14.99
  
             OR
  
  STK=     348.65
  DIP=      75.06
 RAKE=     174.82
 
 
DEPTH = 8.0 km
 
Mw = 3.90
Best Fit 0.8989 - 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
S09A      340   37 iP_C
GRA       209   53 iP_D
R09A        0   92 eP_-
S08C      276   97 iP_C
TIN       249  110 eP_-
R08A      319  138 iP_C
CWC       220  140 eP_-
MPM       194  155 iP_D
MLAC      279  157 eP_+
Q09A      357  158 eP_+
Q08A      335  177 iP_C
LRL       194  221 iP_D
Q07A      319  227 eP_+
ISA       213  231 iP_D
P09A      359  237 eP_+
P08A      341  268 eP_X
P07A      327  284 eP_X
O09A      358  306 eP_-
O08A      344  333 eP_X
O07A      333  343 eP_X
N09A      354  384 eP_X
N07B      337  408 eP_X
M09A      356  446 eP_X
M08A      346  462 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)   
TIN	  249	  110
R08A	  319	  138
CWC	  220	  140
MPM	  194	  155
MLAC	  279	  157
Q09A	  357	  158
Q08A	  335	  177
LRL	  194	  221
Q07A	  319	  227
ISA	  213	  231
P09A	  359	  238
P08A	  341	  268
P07A	  327	  284
O09A	  358	  306
O08A	  344	  333
O07A	  333	  343
N09A	  354	  384
N07B	  337	  408
M09A	  356	  446
M08A	  346	  462

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 Wed Jan 24 12:56:35 CST 2007