2001/02/23 21:43:50 38.73N 112.56W 1 4.1 Utah
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.
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STK = 195
DIP = 40
RAKE = -85
MW = 4.24
HS = 10
The waveform inversion mechanism is preferred because of the low level of the spectral amplitudes. However the waveforms inversion is dominated by the station at the shortest distance. The depths and moments agree.
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 0.2 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 0.5 115 45 80 4.05 0.3438
WVFGRD96 1.0 255 90 5 3.83 0.2874
WVFGRD96 2.0 95 50 60 4.09 0.4268
WVFGRD96 3.0 90 50 60 4.09 0.4741
WVFGRD96 4.0 75 70 25 4.08 0.5382
WVFGRD96 5.0 75 70 20 4.12 0.5942
WVFGRD96 6.0 245 80 -30 4.12 0.6343
WVFGRD96 7.0 235 60 -40 4.17 0.6637
WVFGRD96 8.0 200 40 -80 4.23 0.6950
WVFGRD96 9.0 195 40 -85 4.24 0.7300
WVFGRD96 10.0 195 40 -85 4.24 0.7368
WVFGRD96 11.0 195 40 -85 4.24 0.7228
WVFGRD96 12.0 205 40 -70 4.25 0.7052
WVFGRD96 13.0 205 40 -65 4.25 0.6886
WVFGRD96 14.0 205 40 -65 4.25 0.6735
WVFGRD96 15.0 205 40 -65 4.26 0.6585
WVFGRD96 16.0 190 45 -85 4.24 0.6441
WVFGRD96 17.0 210 45 -55 4.26 0.6269
WVFGRD96 18.0 10 40 -85 4.24 0.6082
WVFGRD96 19.0 265 65 -85 4.22 0.5969
The best solution is
WVFGRD96 10.0 195 40 -85 4.24 0.7368
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 0.12 0.2 n 4 p 2
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| 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= 195.50
DIP= 61.97
RAKE= -68.12
OR
STK= 334.99
DIP= 35.00
RAKE= -124.99
DEPTH = 11.0 km
Mw = 4.12
Best Fit 0.8598 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion MVU 130 39 iP_C CTU 17 229 eP_X ELK 315 321 eP_X HWUT 15 331 eP_- HLID 344 559 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.
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| 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. |
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| 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) MVU 130 39 CTU 17 229 ELK 315 321 HWUT 14 331 WUAZ 163 372 TPNV 240 380 TPH 261 414 AHID 15 465 MNV 268 488 BW06 28 515 DAC 240 522 HLID 344 559 ISCO 77 611 ISA 239 626 WVOR 311 659 LKWY 15 673 CMB 266 688 MOD 301 746 BOZ 5 772
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:
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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
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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.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
