2006/07/14 17:06:01 42.43N 111.54W 5. 4.0 Idaho
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2006/07/14 17:06:01 42.43N 111.54W 5. 4.0 Idaho Best Fitting Double Couple Mo = 9.77e+21 dyne-cm Mw = 3.96 Z = 10 km Plane Strike Dip Rake NP1 10 75 -70 NP2 135 25 -142 Principal Axes: Axis Value Plunge Azimuth T 9.77e+21 27 84 N 0.00e+00 19 185 P -9.77e+21 56 305 Moment Tensor: (dyne-cm) Component Value Mxx -9.66e+20 Mxy 2.25e+21 Mxz -2.23e+21 Myy 5.56e+21 Myz 7.68e+21 Mzz -4.59e+21 -----------### ---------------####### -------------------######### --------------------########## #---------------------############ #----------------------############# ##----------------------############## ##---------- ----------############### ##---------- P ----------############### ###---------- ---------########## #### ####---------------------########## T #### ####---------------------########## #### #####-------------------################## ####-------------------################# #####------------------################# #####----------------################# ######-------------################# #######-----------################ #######--------############### #########----##############- #########----####----- ####---------- Harvard Convention Moment Tensor: R T F -4.59e+21 -2.23e+21 -7.68e+21 -2.23e+21 -9.66e+20 -2.25e+21 -7.68e+21 -2.25e+21 5.56e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060714170601/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 = 10 DIP = 75 RAKE = -70 MW = 3.96 HS = 10
The waveform inversion is preferred. The surface-wave spevtral amplitude solution is in agreemnt.
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 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 175 40 90 3.59 0.2832 WVFGRD96 1.0 185 70 -25 3.48 0.2461 WVFGRD96 2.0 15 60 -45 3.67 0.2731 WVFGRD96 3.0 175 15 75 3.81 0.2648 WVFGRD96 4.0 195 85 70 3.80 0.3178 WVFGRD96 5.0 10 85 -75 3.82 0.3695 WVFGRD96 6.0 10 80 -75 3.85 0.4096 WVFGRD96 7.0 10 80 -70 3.87 0.4391 WVFGRD96 8.0 10 80 -70 3.93 0.4581 WVFGRD96 9.0 10 80 -70 3.94 0.4696 WVFGRD96 10.0 10 75 -70 3.96 0.4739 WVFGRD96 11.0 10 80 -65 3.97 0.4732 WVFGRD96 12.0 10 75 -65 3.98 0.4669 WVFGRD96 13.0 10 75 -65 3.99 0.4547 WVFGRD96 14.0 10 75 -65 4.00 0.4370 WVFGRD96 15.0 200 75 70 3.99 0.4207 WVFGRD96 16.0 200 75 75 4.00 0.4118 WVFGRD96 17.0 200 75 75 4.01 0.4026 WVFGRD96 18.0 200 75 75 4.01 0.3929 WVFGRD96 19.0 200 75 80 4.02 0.3840 WVFGRD96 20.0 200 75 85 4.03 0.3754 WVFGRD96 21.0 200 75 85 4.10 0.3678 WVFGRD96 22.0 200 75 85 4.11 0.3572 WVFGRD96 23.0 90 25 -20 4.10 0.3502 WVFGRD96 24.0 90 25 -20 4.11 0.3433 WVFGRD96 25.0 90 25 -15 4.10 0.3356 WVFGRD96 26.0 90 25 -15 4.11 0.3293 WVFGRD96 27.0 90 25 -15 4.12 0.3219 WVFGRD96 28.0 90 25 -15 4.13 0.3139 WVFGRD96 29.0 90 30 -15 4.13 0.3051
The best solution is
WVFGRD96 10.0 10 75 -70 3.96 0.4739
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 n 3 lp c 0.10 n 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= 199.99 DIP= 90.00 RAKE= 69.99 OR STK= 109.97 DIP= 20.01 RAKE= 179.99 DEPTH = 9.0 km Mw = 4.03 Best Fit 0.8872 - 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 AHID 44 52 ePgc RRI2 10 103 ePg HVU 235 125 iP_D SPU 212 146 iP_D TCU 176 146 eP_X MOOW 24 160 ePn BW06 76 167 eP_+ IMW 16 170 ePn CTU 185 194 eP_- BGU 217 209 iP_D YFT 14 232 eP_X YMR 10 253 eP_X LKWY 21 254 eP_X HLID 299 266 iP_D MPU 182 268 eP_- DUG 204 270 iP_D NLU 189 279 iP_D TMU 175 349 eP_X BOZ 359 358 eP_- ELK 240 361 eP_- RWWY 102 368 eP SRU 167 379 eP_X MVU 188 440 eP MVU 188 440 eP_X HMU 172 504 eP_X PHWY 102 520 eP_+ MSO 340 525 eP_- ISCO 118 577 eP_X WVOR 272 584 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. |
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) HVU 235 125 SPU 212 146 TCU 176 146 MOOW 24 160 BW06 76 167 IMW 16 170 CTU 185 194 BGU 217 209 YFT 14 232 YMR 10 253 HLID 299 266 MPU 182 268 DUG 204 270 NLU 190 279 BOZ 359 358 ELK 240 361 RWWY 102 368 SRU 167 379 MVU 188 440 HMU 172 504 PHWY 102 520 MSO 340 525 CCUT 197 564 ISCO 118 577 WVOR 272 584 LAO 40 634 TPH 227 684 MNV 234 716 SDCO 133 732 WUAZ 179 768 HAWA 307 774 DAC 219 860 GSC 212 914 HUMO 275 938 WDC 261 941 LON 304 946 ISA 221 961 GNW 307 1056 SAO 237 1057 MWC 214 1074 GLA 197 1080 TUC 176 1125 AMTX 132 1244 WMOK 123 1399 LTX 152 1616 EYMN 62 1685 JCT 138 1689 MIAR 113 1790 SLM 96 1848 FVM 98 1859 UALR 111 1870 SIUC 98 1969 PVMO 102 1989 MPH 106 2038 USIN 96 2085 WVT 101 2154 WCI 94 2187 PLAL 104 2193 LTL 119 2272 AAM 81 2287 ACSO 86 2392 ERPA 80 2589 SSPA 83 2798 CBN 88 2922 SDMD 85 2922 NCB 74 3013 PAL 80 3111 DWPF 111 3142 HRV 76 3258
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 n 3 lp c 0.10 n 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: