2006/10/03 00:07:37 44.33N 68.17WW 5 3.9 Maine
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SLU Moment Tensor Solution 2006/10/03 00:07:37 44.33N 68.17WW 5 3.9 Maine Best Fitting Double Couple Mo = 7.16e+21 dyne-cm Mw = 3.87 Z = 2 km Plane Strike Dip Rake NP1 166 55 93 NP2 340 35 85 Principal Axes: Axis Value Plunge Azimuth T 7.16e+21 79 90 N 0.00e+00 3 344 P -7.16e+21 10 254 Moment Tensor: (dyne-cm) Component Value Mxx -5.54e+20 Mxy -1.88e+21 Mxz 3.54e+20 Myy -6.15e+21 Myz 2.47e+21 Mzz 6.70e+21 -#------------ ----########---------- ------############---------- -------###############-------- --------#################--------- ---------###################-------- ----------####################-------- ----------######################-------- ----------#######################------- ------------########### ########-------- ------------########### T #########------- ------------########### #########------- - ---------######################------- P ---------######################------ ----------#####################------ -------------####################----- -------------##################----- -------------#################---- ------------###############--- -------------############--- ------------########-- ----------#### Harvard Convention Moment Tensor: R T F 6.70e+21 3.54e+20 -2.47e+21 3.54e+20 -5.54e+20 1.88e+21 -2.47e+21 1.88e+21 -6.15e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20061003000737/index.html |
STK = 340 DIP = 35 RAKE = 85 MW = 3.87 HS = 2
The waveform inversion is preferred. The surface-wave spectral amplitude solution is consistent with the waveform inversion.
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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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 0.03 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 0.5 150 65 70 3.76 0.4454 WVFGRD96 1.0 350 25 85 3.83 0.4573 WVFGRD96 2.0 340 35 85 3.87 0.4593 WVFGRD96 3.0 295 45 25 3.93 0.4184 WVFGRD96 4.0 100 55 -20 3.88 0.3694 WVFGRD96 5.0 100 50 -20 3.88 0.3734 WVFGRD96 6.0 100 55 -25 3.87 0.3787 WVFGRD96 7.0 95 50 -30 3.88 0.3841 WVFGRD96 8.0 95 55 -35 3.88 0.3889 WVFGRD96 9.0 95 55 -35 3.88 0.3924 WVFGRD96 10.0 95 55 -35 3.92 0.3916 WVFGRD96 11.0 95 50 -35 3.93 0.3935 WVFGRD96 12.0 95 50 -35 3.94 0.3941 WVFGRD96 13.0 95 50 -35 3.95 0.3932 WVFGRD96 14.0 330 85 60 3.81 0.3917 WVFGRD96 15.0 335 85 60 3.82 0.3907 WVFGRD96 16.0 335 85 60 3.83 0.3886 WVFGRD96 17.0 260 45 -75 4.01 0.3876 WVFGRD96 18.0 255 50 -80 4.01 0.3874 WVFGRD96 19.0 255 50 -80 4.02 0.3856
The best solution is
WVFGRD96 2.0 340 35 85 3.87 0.4593
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 0.03 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. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 7.24 DIP= 48.36 RAKE= 108.88 OR STK= 159.99 DIP= 45.00 RAKE= 70.00 DEPTH = 1.0 km Mw = 3.79 Best Fit 0.8836 - 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 GGN 50 139 eP_X FFD 252 295 eP_X WES 231 334 eP_X HNH 260 337 eP_X A11 335 361 eP_- A16 339 378 eP_X LMQ 336 395 eP_- QUA2 238 408 eP_X MNT 289 450 eP_X GAC 287 596 eP_X PAL 234 598 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.
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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) GGN 50 139 FFD 252 295 WES 231 334 HNH 260 337 A11 335 361 A16 338 378 LMQ 336 395 QUA2 238 408 MNT 289 450 ICQ 6 582 GAC 287 596 PAL 234 598 KGNO 272 664 MEDO 265 838 VLDQ 304 854 SADO 277 873 TORO 269 900 HSMO 295 954 ACTO 269 957 KLBO 281 958 BRCO 274 1059 MALO 310 1082 TIMO 299 1108 SCHQ 4 1173 OTRO 307 1207 MSNO 314 1210 PLIO 261 1211 KAPO 303 1230 BLO 256 1626 USIN 253 1777 SIUC 255 1910 SLM 259 1942 RWWY 278 3168 LOHW 284 3369 SNOW 284 3386
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 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 0.03 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 CUS 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: