Location

1997/05/16 01:23:20 40.58N 114.99W 5 4.4 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
 1997/05/16 01:23:20 40.58N 114.99W 5 4.4 Nevada
 
 Best Fitting Double Couple
    Mo = 1.22e+22 dyne-cm
    Mw = 3.99 
    Z  = 11 km
     Plane   Strike  Dip  Rake
      NP1       30    50   -100
      NP2      225    41   -78
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.22e+22      5     127
     N   0.00e+00      8      36
     P  -1.22e+22     81     247



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     4.35e+21
       Mxy    -5.92e+21
       Mxz     1.36e+20
       Myy     7.45e+21
       Myz     2.48e+21
       Mzz    -1.18e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ####################----#---           
             ##############-------------###          
           #############----------------#####        
          ###########--------------------#####       
         ##########----------------------######      
        #########-----------------------########     
        ########------------------------########     
       ########-------------------------#########    
       #######----------   ------------##########    
       ######----------- P ------------##########    
       ######-----------   -----------###########    
        ####-------------------------###########     
        ####------------------------############     
         ###----------------------#############      
          ##---------------------#########   #       
           #-------------------########### T         
             ----------------#############           
              ------------################           
                 ----##################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.18e+22   1.36e+20  -2.48e+21 
  1.36e+20   4.35e+21   5.92e+21 
 -2.48e+21   5.92e+21   7.45e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/19970516012320/index.html
        
Stations used: BW06(Z), HWUT(T,R), DUG, KNB,
               LDS, CMB, ORV, BMN(Z), MIN(Z),
               WDC, YBH, WVOR(Z), ELK(R,Z)


         OSU MOMENT TENSOR SOLUTION
http://quakes.oce.orst.edu/moment-tensor/1997/solutions/970516_0123.ap.20_30s9km
   
   Event: northern Nevada
     Date/Time: 97/ 5/16  1:23:20
     Lat./Lon.:   40.580 -114.990
         mb/Ms:    4.4      0.0

   Depth (km):   9.0    Stations Used: 13
   Moment Tensor: Scale = 10**21 dyn cm
     Component  Value    Component  Value
        Mxx     2.275       Myy     7.516
        Mxy    -5.819       Myz     0.043
        Mxz    -1.258       Mzz    -9.791
   Source Composition:
     (Type/%)  DC/ 76  CLVD/ 24   Iso/  0
   Principal Axes:
       Axis     Value    Plunge   Azimuth
         T     11.302         2       123
         N     -1.360         7       213
         P     -9.942        83        17
   Best Fitting Double-Couple:
       Mo = 1.06E+22 dyn cm     Mw = 3.98
       Plane   Strike      Rake       Dip
        NP1        40       -81        47
        NP2       206      -100        43
                                              
                                              
                   #######                    
             ##################-              
          ###############----------           
        #############---------------#         
      #############-----------------###       
     ############-------------------####      
    ############--------------------#####     
   ###########----------------------######    
   ##########-----------------------######    
  ##########---------   -----------########   
  ##########--------- P -----------########   
  #########----------   ----------#########   
  #########----------------------##########   
  ########----------------------###########   
   #######---------------------###########    
   #######-------------------#############    
    ######-----------------###########        
     ######--------------############# T      
      #####-----------################        
        ####------###################         
          --#######################           
             ###################              
                   #######                    
                                              
   Lower Hemisphere Equal Area Projection

	

      STK = 30
      DIP = 50
     RAKE = -100
       MW = 3.99
       HS = 11.0

The surface-wave is preferred because of its fit to waveforms.

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
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    35   -90   3.44 0.3010
WVFGRD96    1.0   275    65    10   3.48 0.3012
WVFGRD96    2.0   275    55    10   3.62 0.3894
WVFGRD96    3.0   275    50     5   3.67 0.4946
WVFGRD96    4.0   270    50   -10   3.70 0.5647
WVFGRD96    5.0   270    50   -15   3.71 0.6105
WVFGRD96    6.0   270    55   -20   3.74 0.6378
WVFGRD96    7.0   260    45   -50   3.77 0.6684
WVFGRD96    8.0   260    45   -55   3.81 0.7032
WVFGRD96    9.0   250    45   -70   3.85 0.7367
WVFGRD96   10.0   250    45   -70   3.86 0.7564
WVFGRD96   11.0   260    45   -60   3.86 0.7654
WVFGRD96   12.0   230    40   -55   3.87 0.7699
WVFGRD96   13.0   230    40   -55   3.88 0.7664
WVFGRD96   14.0   235    40   -45   3.89 0.7543
WVFGRD96   15.0   235    40   -50   3.90 0.7408
WVFGRD96   16.0   240    40   -40   3.90 0.7217
WVFGRD96   17.0    45    60   -55   3.91 0.7015
WVFGRD96   18.0    45    65   -55   3.90 0.6841
WVFGRD96   19.0    45    65   -50   3.92 0.6681
WVFGRD96   20.0    45    70   -50   3.91 0.6552
WVFGRD96   21.0    40    70   -60   3.95 0.6423
WVFGRD96   22.0    45    75   -55   3.95 0.6324
WVFGRD96   23.0    45    75   -55   3.96 0.6251
WVFGRD96   24.0    40    75   -60   3.97 0.6167
WVFGRD96   25.0    40    75   -65   3.98 0.6066
WVFGRD96   26.0    45    80   -60   3.98 0.5981
WVFGRD96   27.0    45    80   -60   3.99 0.5875
WVFGRD96   28.0    50    90   -60   3.98 0.5775
WVFGRD96   29.0   230    85    60   3.98 0.5692

The best solution is

WVFGRD96   12.0   230    40   -55   3.87 0.7699

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
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=      30.00
  DIP=      49.99
 RAKE=    -100.00
  
             OR
  
  STK=     225.34
  DIP=      41.03
 RAKE=     -78.31
 
 
DEPTH = 11.0 km
 
Mw = 3.99
Best Fit 0.9180 - 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
ELK       311   28 iP_C
BMN       266  190 eP_+
DUG       102  190 eP_+
HWUT       67  310 eP_X
TPH       215  338 iP_D
MNV       230  362 iP_D
WVOR      305  368 eP_X
LDS       158  397 eP_X
TPNV      196  418 iP_D
KNB       154  438 iP_+
BW06       60  513 eP_X
WDC       272  639 -12345
YBH       284  661 -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)   
BMN	  266	  190
DUG	  102	  190
HWUT	   67	  310
TPH	  215	  338
MNV	  230	  362
WVOR	  305	  368
LDS	  158	  397
TPNV	  196	  418
KNB	  154	  438
MLAC	  227	  467
BW06	   60	  513
DAC	  206	  529
CWC	  211	  533
CMB	  240	  544
SCZ	  234	  712
OSI	  208	  740
KNW	  192	  777
RDM	  193	  789
DGR	  194	  790
SND	  191	  793
CRY	  192	  794
FRD	  191	  799
ISCO	   93	  803
COR	  306	  814
LVA2	  190	  814
NEW	  350	  871
ANMO	  127	  977
TUC	  156	  992
CCM	   90	 2061
SLM	   88	 2130
WVT	   93	 2415

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

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 Nov 29 15:10:07 CST 2006