2000/11/19 12:54:50 40.49N 119.51W 5 4.3 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

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Preferred Solution

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

      STK = 65
      DIP = 65
     RAKE = -25
       MW = 3.93
       HS = 11

UC Berkeley Solution

      STK = 54
      DIP = 43
     RAKE = -43
       MW = 4.0
       HS = 4

Based on ORV, WDC waveform inversion in 0.02 - 0.05 Hz band


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 3
lp c 0.10 3
br c 0.1 0.2 n 8 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   280    45    60   3.52 0.2310
WVFGRD96    1.0    75    85    -5   3.46 0.2559
WVFGRD96    2.0    75    85    -5   3.62 0.4040
WVFGRD96    3.0    75    90    -5   3.69 0.4785
WVFGRD96    4.0   255    90     5   3.74 0.5336
WVFGRD96    5.0    65    60   -20   3.82 0.5794
WVFGRD96    6.0    65    60   -25   3.85 0.6236
WVFGRD96    7.0    65    60   -25   3.88 0.6555
WVFGRD96    8.0    60    55   -30   3.92 0.6796
WVFGRD96    9.0    60    55   -30   3.93 0.6914
WVFGRD96   10.0    65    60   -25   3.93 0.6969
WVFGRD96   11.0    65    60   -25   3.94 0.6968
WVFGRD96   12.0    65    65   -20   3.94 0.6953
WVFGRD96   13.0    65    65   -20   3.95 0.6910
WVFGRD96   14.0    65    65   -20   3.96 0.6836
WVFGRD96   15.0    65    70   -15   3.97 0.6770
WVFGRD96   16.0    65    70   -15   3.98 0.6683
WVFGRD96   17.0   250    70   -10   4.00 0.6591
WVFGRD96   18.0   250    75   -10   4.01 0.6483
WVFGRD96   19.0   250    75   -10   4.02 0.6381

The best solution is

WVFGRD96   10.0    65    60   -25   3.93 0.6969

The mechanism correspond to the best fit is
Figure 1. Wavefrom 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 3
lp c 0.10 3
br c 0.1 0.2 n 8 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


  STK=     225.52
  DIP=      61.98
 RAKE=     111.88
  STK=       5.00
  DIP=      35.00
 RAKE=      55.00
DEPTH = 9.0 km
Mw = 4.04
Best Fit 0.8901 - P-T axis plot gives solutions with FIT greater than FIT90

Surface-Wave Focal Mechanism

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   First motion
MOD       337  170 iP_C
ORV       239  199 eP_X
WVOR       18  228 iP_D
MNV       152  257 eP_X
WDC       273  257 eP_X
CMB       196  283 eP_X
KCC       177  352 eP_X
ELK        84  363 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)   
MOD	  337	  170
ORV	  239	  199
WVOR	   18	  228
MNV	  152	  257
WDC	  273	  257
CMB	  196	  283
YBH	  298	  302
BDM	  217	  347
CVS	  228	  348
HOPS	  243	  348
KCC	  177	  352
ELK	   84	  363
BRIB	  220	  365
WENL	  212	  373
BKS	  220	  374
BRK	  220	  375
ARC	  278	  389
MHC	  209	  395
JRSC	  216	  416
SAO	  203	  447
TPNV	  143	  488
PKD	  190	  513
DUG	   91	  570
HAWA	  360	  656
HWUT	   77	  679
AHID	   67	  745
BOZ	   46	  860
NEW	   12	  885

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 3
lp c 0.10 3
br c 0.1 0.2 n 8 p 2


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.


Dr. Harley Benz, USGS, provided the USGS USNSN digital data.

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 Jun 8 14:38:22 CDT 2005