2003/06/08 10:14:54 41.22N 116.36W 5 3.8 Nevada
Arrival time list
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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.
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 3The 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
The best fit as a function of depth is given in the following figure:
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
|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.|
NODAL PLANES STK= 75.00 DIP= 80.00 RAKE= 84.99 OR 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
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 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 .
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.
|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.|
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
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
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.
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.
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: