1999/06/30 15:27:32 40.64 111.58 3.5 Utah
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 1999/06/30 15:27:32 40.64 111.58 3.5 Utah Best Fitting Double Couple Mo = 2.37e+21 dynecm Mw = 3.55 Z = 15 km Plane Strike Dip Rake NP1 155 50 85 NP2 327 40 96 Principal Axes: Axis Value Plunge Azimuth T 2.37e+21 5 241 N 0.00e+00 4 332 P 2.37e+21 84 100 Moment Tensor: (dynecm) Component Value Mxx 5.37e+20 Mxy 9.93e+20 Mxz 5.30e+19 Myy 1.79e+21 Myz 4.28e+20 Mzz 2.33e+21 ############## ################## ################## ################ ################ ################ ################# ################# ################ ################### ########## ######## ########### P ####### ########### ####### ################# ################## ############## T ############### ############### ############### ################ ############### ############## Harvard Convention Moment Tensor: R T F 2.33e+21 5.30e+19 4.28e+20 5.30e+19 5.37e+20 9.93e+20 4.28e+20 9.93e+20 1.79e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/19990630152732/index.html 
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.

STK = 155 DIP = 50 RAKE = 85 MW = 3.55 HS = 15
The waveform solution is preferred. The surfacewave is preferred. Rake is least well determined.
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 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 150 45 90 3.22 0.5305 WVFGRD96 1.0 25 85 5 3.29 0.4799 WVFGRD96 2.0 25 85 5 3.36 0.4883 WVFGRD96 3.0 25 80 15 3.42 0.4356 WVFGRD96 4.0 285 65 35 3.50 0.4530 WVFGRD96 5.0 50 5 10 3.48 0.6011 WVFGRD96 6.0 75 5 15 3.49 0.6901 WVFGRD96 7.0 95 5 35 3.48 0.7337 WVFGRD96 8.0 105 5 50 3.51 0.7551 WVFGRD96 9.0 130 10 70 3.51 0.7658 WVFGRD96 10.0 145 20 85 3.53 0.7750 WVFGRD96 11.0 325 65 85 3.54 0.7864 WVFGRD96 12.0 340 35 90 3.57 0.8020 WVFGRD96 13.0 165 50 80 3.57 0.8155 WVFGRD96 14.0 165 50 80 3.56 0.8207 WVFGRD96 15.0 155 50 85 3.55 0.8226 WVFGRD96 16.0 150 50 90 3.55 0.8195 WVFGRD96 17.0 160 50 75 3.56 0.8140 WVFGRD96 18.0 160 50 75 3.57 0.8063 WVFGRD96 19.0 170 50 70 3.58 0.7951
The best solution is
WVFGRD96 15.0 155 50 85 3.55 0.8226
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 observedpredicted 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 0.12 0.25 n 4 p 2

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= 342.22 DIP= 48.36 RAKE= 71.12 OR STK= 134.99 DIP= 45.00 RAKE= 109.99 DEPTH = 18.0 km Mw = 3.58 Best Fit 0.9255  PT axis plot gives solutions with FIT greater than FIT90
The Pwave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion DUG 245 116 iP_C AHID 9 239 iP_X ELK 273 310 eP_X KNB 195 417 eP_X BMN 269 479 eP_X
Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surfacewave 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 Lovewave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.

Pressuretension axis trends. Since the surfacewave 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 Taxes 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 0180 degrees are sampled. 
Sta Az(deg) Dist(km) DUG 245 116 AHID 9 239 ELK 273 310 KNB 195 417 BMN 269 479 WUAZ 178 569 TPNV 226 577 MNV 249 616 DAC 229 714
Since the analysis of the surfacewave radiation patterns uses only spectral amplitudes and because the surfavewave radiation patterns have a 180 degree symmetry, each surfacewave solution consists of four possible focal mechanisms corresponding to the interchange of the P and Taxes and a roation of the mechanism by 180 degrees. To select one mechanism, Pwave first motion can be used. This was not possible in this case because all the Pwave first motions were emergent ( a feature of the Pwave 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, Rradial 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 0.12 0.25 n 4 p 2
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 100200 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 cmsec) 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: