2005/09/16 15:09:44 39.03N 119.56W 5. 4.2
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2005/09/16 15:09:44 39.03N 119.56W 5. 4.2 Best Fitting Double Couple Mo = 1.38e+22 dyne-cm Mw = 4.06 Z = 12 km Plane Strike Dip Rake NP1 200 65 -50 NP2 317 46 -144 Principal Axes: Axis Value Plunge Azimuth T 1.38e+22 11 262 N 0.00e+00 36 0 P -1.38e+22 52 158 Moment Tensor: (dyne-cm) Component Value Mxx -4.22e+21 Mxy 3.56e+21 Mxz 5.85e+21 Myy 1.23e+22 Myz -5.10e+21 Mzz -8.10e+21 -------------- ---------------####### ---###----------############ ##############-############### ################----############## ################-------############# ################----------############ ################-------------########### ###############---------------########## ###############------------------######### ###############-------------------######## # ##########--------------------######## # T ##########---------------------####### #########-----------------------##### ############---------- ----------##### ###########---------- P ----------#### #########----------- ----------### ########------------------------## #######----------------------- ######---------------------- ###------------------- -------------- Harvard Convention Moment Tensor: R T F -8.10e+21 5.85e+21 5.10e+21 5.85e+21 -4.22e+21 -3.56e+21 5.10e+21 -3.56e+21 1.23e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20050916150944/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 = 65 RAKE = -50 MW = 4.06 HS = 12
The wave-form inversion is preferred. The surface-wave spectral amplitude solution agrees well with the waveform inversion.
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 15 50 40 3.67 0.3393 WVFGRD96 1.0 360 70 20 3.69 0.3414 WVFGRD96 2.0 360 70 15 3.79 0.3798 WVFGRD96 3.0 170 65 -35 3.89 0.3730 WVFGRD96 4.0 175 75 -50 3.91 0.4309 WVFGRD96 5.0 190 80 -55 3.89 0.4852 WVFGRD96 6.0 195 80 -55 3.91 0.5296 WVFGRD96 7.0 200 80 -50 3.93 0.5647 WVFGRD96 8.0 195 70 -55 3.99 0.5917 WVFGRD96 9.0 195 70 -55 4.01 0.6162 WVFGRD96 10.0 195 65 -55 4.03 0.6327 WVFGRD96 11.0 195 65 -55 4.05 0.6432 WVFGRD96 12.0 200 65 -50 4.06 0.6471 WVFGRD96 13.0 200 65 -50 4.07 0.6462 WVFGRD96 14.0 205 70 -45 4.08 0.6410 WVFGRD96 15.0 205 70 -45 4.09 0.6324 WVFGRD96 16.0 205 70 -45 4.10 0.6193 WVFGRD96 17.0 210 75 -40 4.11 0.6037 WVFGRD96 18.0 205 75 -40 4.11 0.5856 WVFGRD96 19.0 205 75 -40 4.12 0.5654
The best solution is
WVFGRD96 12.0 200 65 -50 4.06 0.6471
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= 197.52 DIP= 54.06 RAKE= -52.55 OR STK= 324.98 DIP= 50.00 RAKE= -129.99 DEPTH = 12.0 km Mw = 4.13 Best Fit 0.8999 - 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 118 139 iP_C TPH 117 230 eP_X SAO 214 301 iP_D WDC 305 307 eP_- DAC 150 352 eP_- ISA 165 386 iP_C WVOR 11 386 eP_X GSC 149 481 eP_X HUMO 325 489 eP_-
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) MNV 118 139 TPH 117 230 SAO 214 301 WDC 305 307 DAC 150 352 ISA 165 386 WVOR 11 386 GSC 149 481 HUMO 325 489 MWC 165 551 HWUT 65 737 BAR 159 752 GLA 146 789 AHID 57 824 LON 349 877 BW06 61 939 TTW 351 978 MSO 26 980 GNW 346 984 NEW 10 1045 TUC 131 1089 LAO 48 1380 MNTX 118 1521 LTX 122 1813 KSU1 82 1982 FVM 83 2534 WVT 87 2807 NHSC 88 3580
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 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: