2003/06/08 10:14:54 41.22N 116.36W 5 3.8 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 = 75
      DIP = 80
     RAKE = 85
       MW = 3.79
       HS = 11.0

The waveform fits are excellent. However there is not much sensitivity to a particular solution. The surface-wave spectral amplitude data focus on an almost pure dip-slip solution which is characterized by poor Love wave generation. The surface-wave solution does fit the observed waveforms. The surface-wave solution is preferred.

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
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   325    50    90   3.27 0.4074
WVFGRD96    1.0   260    85    10   3.35 0.4180
WVFGRD96    2.0   145    45    90   3.44 0.4596
WVFGRD96    3.0   170    75    15   3.55 0.4377
WVFGRD96    4.0   170    70    25   3.57 0.4302
WVFGRD96    5.0   175    65    40   3.61 0.4728
WVFGRD96    6.0   175    60    40   3.63 0.5264
WVFGRD96    7.0   175    60    40   3.65 0.5765
WVFGRD96    8.0   175    55    35   3.72 0.6159
WVFGRD96    9.0   175    50    35   3.74 0.6560
WVFGRD96   10.0   175    45    30   3.77 0.6871
WVFGRD96   11.0   175    45    30   3.78 0.7073
WVFGRD96   12.0   175    45    30   3.80 0.7168
WVFGRD96   13.0   170    40    20   3.81 0.7191
WVFGRD96   14.0   170    40    20   3.83 0.7164
WVFGRD96   15.0   170    40    20   3.84 0.7079
WVFGRD96   16.0   170    35    20   3.85 0.6958
WVFGRD96   17.0   175    25    20   3.84 0.6816
WVFGRD96   18.0   175    25    20   3.85 0.6660
WVFGRD96   19.0   175    25    20   3.85 0.6500

The best solution is

WVFGRD96   13.0   170    40    20   3.81 0.7191

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 3
lp c 0.10 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


  STK=      75.00
  DIP=      80.00
 RAKE=      84.99
  STK=     281.75
  DIP=      11.18
 RAKE=     116.32
DEPTH = 11.0 km
Mw = 3.79
Best Fit 0.8963 - 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
BMN       220  114 iP_D
HLID       31  306 iP_C
DUG       110  321 eP_X
MOD       284  338 eP_X
MNV       207  346 iP_D
TPH       192  357 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)   
BMN	  220	  114
HLID	   31	  306
DUG	  110	  321
MOD	  284	  338
MNV	  207	  346
TPH	  192	  357
HWUT	   82	  403
MLAC	  209	  452
AHID	   67	  468
PIN	  309	  469
TPNV	  179	  474
CMB	  226	  494
WDC	  264	  526
CWC	  196	  551
DAC	  191	  559
BW06	   71	  589
LKWY	   51	  612
HOOD	  318	  626
HAWA	  337	  629
ISA	  197	  644
MSO	   16	  653
GSC	  184	  658
SAO	  223	  662
TOLO	  304	  724
DAN	  173	  736
VTV	  187	  744
HEBO	  309	  747
LON	  326	  754
OSI	  197	  762
WUAZ	  144	  768
MWC	  191	  791
PAS	  192	  801
DJJ	  194	  811
TOV	  196	  813
USC	  193	  817
MEGW	  315	  825
KNW	  182	  834
TTW	  331	  835
RDM	  183	  844
RPV	  193	  849
WMC	  182	  849
CRY	  182	  850
SND	  182	  851
TRO	  180	  854
BZN	  182	  858
FRD	  182	  858
SMER	  185	  865
GNW	  326	  873
LVA2	  181	  874
PLM	  183	  874
CIA	  192	  887
YAQ	  180	  894
JCS	  181	  903
HWB	  184	  911
GLA	  171	  917
MONP	  180	  924
BAR	  182	  948
NE70	  174	  981
NE71	  178	 1058
NE72	  179	 1151
NE80	  162	 1244

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


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 Sat Jun 18 11:47:06 CDT 2005