2003/05/29 22:52:14 38.26N 117.90W 8 4.0 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 = 270 DIP = 85 RAKE = -15 MW = 3.82 HS = 11
Both techniques yield the same solution.
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 0.03 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 0.5 270 85 25 3.39 0.3119 WVFGRD96 1.0 275 70 25 3.45 0.3482 WVFGRD96 2.0 275 65 30 3.61 0.4637 WVFGRD96 3.0 275 65 25 3.63 0.5761 WVFGRD96 4.0 270 80 20 3.65 0.6524 WVFGRD96 5.0 270 75 10 3.67 0.6999 WVFGRD96 6.0 90 90 10 3.75 0.7413 WVFGRD96 7.0 270 90 -10 3.77 0.7855 WVFGRD96 8.0 265 80 -25 3.80 0.8155 WVFGRD96 9.0 270 85 -20 3.81 0.8338 WVFGRD96 10.0 270 85 -15 3.81 0.8444 WVFGRD96 11.0 270 85 -15 3.82 0.8480 WVFGRD96 12.0 270 80 -20 3.83 0.8467 WVFGRD96 13.0 270 80 -20 3.85 0.8407 WVFGRD96 14.0 270 80 -15 3.84 0.8278 WVFGRD96 15.0 270 75 -20 3.85 0.8172 WVFGRD96 16.0 270 75 -20 3.87 0.7964 WVFGRD96 17.0 270 80 -10 3.87 0.7799 WVFGRD96 18.0 265 75 -10 3.87 0.7615 WVFGRD96 19.0 265 70 -10 3.86 0.7444
The best solution is
WVFGRD96 11.0 270 85 -15 3.82 0.8480
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 0.03 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= 94.99 DIP= 84.99 RAKE= 19.99 OR STK= 3.18 DIP= 70.09 RAKE= 174.68 DEPTH = 11.0 km Mw = 3.85 Best Fit 0.8753 - 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 MNV 311 29 iP_C TIN 192 137 eP_- CMB 264 219 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) MLAC 230 108 TIN 192 137 CWC 185 203 TPNV 134 206 CMB 264 219 ISA 190 293 GSC 163 343 ELK 39 359 SCZ 240 361 OSI 191 411 MWC 182 448 MOD 334 453 SBC 202 454 PAS 183 457 DAN 150 461 TOV 191 463 DJJ 186 464 USC 184 472 WDC 304 475 NEE 141 482 DUG 62 489 RPV 185 503 KNW 168 516 DGR 171 518 RDM 169 522 CRY 168 531 WMC 168 532 SND 167 535 BZN 168 541 CIA 185 541 FRD 167 542 TRO 165 542 PLM 170 552 LVA2 167 558 YBH 315 563 SNCC 195 575 YAQ 166 582 JCS 168 586 MONP 167 610 BAR 169 629 GLA 153 641 WUAZ 116 656 HWUT 54 657 HLID 25 658 PIN 339 665 DBO 322 704 NE71 165 751 AHID 47 763 NE72 168 839 BW06 52 866 HAWA 352 914 TUC 134 924 LKWY 39 939 LON 342 996 SDCO 89 1090 TTW 345 1093 GNW 340 1109
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 0.03 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: