2006/05/18 10:16:21 44.17N 110.34W 0. 3.9 Wyoming
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SLU Moment Tensor Solution 2006/05/18 10:16:21 44.17N 110.34W 0. 3.9 Wyoming Best Fitting Double Couple Mo = 6.46e+21 dyne-cm Mw = 3.84 Z = 8 km Plane Strike Dip Rake NP1 314 71 -126 NP2 200 40 -30 Principal Axes: Axis Value Plunge Azimuth T 6.46e+21 18 70 N 0.00e+00 34 327 P -6.46e+21 50 183 Moment Tensor: (dyne-cm) Component Value Mxx -1.94e+21 Mxy 1.73e+21 Mxz 3.83e+21 Myy 5.12e+21 Myz 1.99e+21 Mzz -3.18e+21 -------------# -----------########### -----------################# ##--------#################### #########-######################## #########----####################### #########--------################# # #########-----------############### T ## ########--------------############# ## #########----------------################# ########-------------------############### ########--------------------############## ########----------------------############ #######-----------------------########## #######-------------------------######## ######----------- ------------###### #####----------- P -------------#### #####---------- -------------### ####-------------------------- ####------------------------ ##-------------------- -------------- Harvard Convention Moment Tensor: R T F -3.18e+21 3.83e+21 -1.99e+21 3.83e+21 -1.94e+21 -1.73e+21 -1.99e+21 -1.73e+21 5.12e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060518101621/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.
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STK = 200 DIP = 40 RAKE = -30 MW = 3.84 HS = 8
The waveform inversion is preferred. The surface-wave solution is compatible
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 95 30 90 3.54 0.4141 WVFGRD96 1.0 85 75 -10 3.58 0.3911 WVFGRD96 2.0 70 30 45 3.69 0.4435 WVFGRD96 3.0 65 30 55 3.72 0.5010 WVFGRD96 4.0 205 35 -10 3.72 0.5750 WVFGRD96 5.0 205 35 -15 3.75 0.6324 WVFGRD96 6.0 200 40 -25 3.78 0.6661 WVFGRD96 7.0 200 40 -30 3.80 0.6787 WVFGRD96 8.0 200 40 -30 3.84 0.6814 WVFGRD96 9.0 200 40 -30 3.85 0.6702 WVFGRD96 10.0 200 45 -25 3.86 0.6524 WVFGRD96 11.0 205 45 -15 3.86 0.6288 WVFGRD96 12.0 205 45 -15 3.87 0.6019 WVFGRD96 13.0 210 45 -15 3.88 0.5736 WVFGRD96 14.0 290 75 -45 3.90 0.5477 WVFGRD96 15.0 290 75 -40 3.91 0.5272 WVFGRD96 16.0 290 75 -40 3.92 0.5070 WVFGRD96 17.0 285 65 -40 3.94 0.4871 WVFGRD96 18.0 285 65 -40 3.94 0.4697 WVFGRD96 19.0 285 65 -40 3.95 0.4535
The best solution is
WVFGRD96 8.0 200 40 -30 3.84 0.6814
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
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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= 349.98 DIP= 65.00 RAKE= -104.99 OR STK= 202.34 DIP= 28.90 RAKE= -60.98 DEPTH = 9.0 km Mw = 3.92 Best Fit 0.8952 - 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 LKWY 353 44 ePg MOOW 215 57 ePg REDW 205 99 ePg AHID 202 168 ePn BW06 158 169 ePn HWUT 200 302 ePn HLID 260 335 ePn RWWY 136 376 ePn MSO 318 408 ePn LAO 48 427 ePn
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. |
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) LKWY 353 44 IMW 238 57 MOOW 215 57 REDW 205 99 AHID 202 168 BW06 158 169 HWUT 200 302 HLID 260 335 RWWY 136 376 MSO 318 408 LAO 48 427 PHWY 127 511 ISCO 140 624 WVOR 257 701 HAWA 292 762 TPH 223 890 MNV 228 913 LON 292 941 HUMO 265 1037 CBKS 120 1066 WDC 252 1080 GSC 212 1130 ISA 219 1172 SAO 233 1249 KSU1 112 1273 MWC 214 1289 GLA 199 1295 AMTX 141 1317 EYMN 67 1514 CCM 106 1737 JCT 145 1780 MIAR 121 1792 UALR 118 1861 SIUC 105 1911 NATX 130 1942 MPH 113 2012 UTMT 108 2016 USIN 102 2019 OXF 114 2090 WCI 100 2114 PLAL 111 2158
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 digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface
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: