2006/07/14 09:34:46 47.00N 68.79W 5. 3.9 Maine
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SLU Moment Tensor Solution 2006/07/14 09:34:46 47.00N 68.79W 5. 3.9 Maine Best Fitting Double Couple Mo = 1.86e+21 dyne-cm Mw = 3.48 Z = 17 km Plane Strike Dip Rake NP1 38 73 108 NP2 170 25 45 Principal Axes: Axis Value Plunge Azimuth T 1.86e+21 58 333 N 0.00e+00 17 212 P -1.86e+21 25 114 Moment Tensor: (dyne-cm) Component Value Mxx 1.60e+20 Mxy 3.50e+20 Mxz 1.03e+21 Myy -1.17e+21 Myz -1.04e+21 Mzz 1.01e+21 -############# ---################### ----#######################- ---########################--- ----#########################----- ----#########################------- ----########## ############--------- -----########## T ###########----------- ----########### ##########------------ -----#######################-------------- -----######################--------------- -----#####################---------------- -----###################------------------ -----#################----------- ---- -----###############------------- P ---- -----#############-------------- --- -----##########--------------------- -----#######---------------------- -----##----------------------- ---###---------------------- #####----------------- #####--------- Harvard Convention Moment Tensor: R T F 1.01e+21 1.03e+21 1.04e+21 1.03e+21 1.60e+20 -3.50e+20 1.04e+21 -3.50e+20 -1.17e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060714093446/index.html |
STK = 170 DIP = 25 RAKE = 45 MW = 3.48 HS = 17
This is a small event that is not well recorded. The waveforms solution is preferred because of the many waveforms available from the Canadian National Seismograph Network Station. The surface-wave spectral amplitude solution is affected by the small number of observations, but is reasonably compatible with the direct 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 location of the event and the station distribution used are given in the following figure.
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The observed and predicted traces are filtered using the following gsac commands:
hp c 0.025 n 4 lp c 0.10 n 4 br c 0.12 0.25 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 40 50 -75 3.31 0.3739 WVFGRD96 1.0 40 80 -85 3.63 0.3719 WVFGRD96 2.0 125 10 -5 3.65 0.3821 WVFGRD96 3.0 130 15 5 3.55 0.4045 WVFGRD96 4.0 140 15 10 3.50 0.4204 WVFGRD96 5.0 140 20 15 3.47 0.4335 WVFGRD96 6.0 145 20 20 3.45 0.4444 WVFGRD96 7.0 160 20 40 3.44 0.4526 WVFGRD96 8.0 155 25 35 3.44 0.4613 WVFGRD96 9.0 160 25 45 3.44 0.4677 WVFGRD96 10.0 160 25 40 3.46 0.4731 WVFGRD96 11.0 160 25 40 3.46 0.4787 WVFGRD96 12.0 170 25 50 3.46 0.4830 WVFGRD96 13.0 170 25 50 3.46 0.4864 WVFGRD96 14.0 170 25 45 3.46 0.4888 WVFGRD96 15.0 170 25 45 3.47 0.4908 WVFGRD96 16.0 170 25 45 3.47 0.4917 WVFGRD96 17.0 170 25 45 3.48 0.4919 WVFGRD96 18.0 170 25 45 3.48 0.4915 WVFGRD96 19.0 170 25 45 3.49 0.4906 WVFGRD96 20.0 180 25 55 3.53 0.4892 WVFGRD96 21.0 180 25 55 3.53 0.4870 WVFGRD96 22.0 180 25 55 3.54 0.4840 WVFGRD96 23.0 180 25 50 3.55 0.4804 WVFGRD96 24.0 185 30 55 3.57 0.4761 WVFGRD96 25.0 185 30 55 3.58 0.4712 WVFGRD96 26.0 180 30 50 3.59 0.4654 WVFGRD96 27.0 190 30 60 3.60 0.4593 WVFGRD96 28.0 190 30 65 3.60 0.4524 WVFGRD96 29.0 190 30 60 3.62 0.4453
The best solution is
WVFGRD96 17.0 170 25 45 3.48 0.4919
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 0.025 n 4 lp c 0.10 n 4 br c 0.12 0.25 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= 169.99 DIP= 55.00 RAKE= 65.00 OR STK= 29.10 DIP= 42.07 RAKE= 121.11 DEPTH = 20.0 km Mw = 3.61 Best Fit 0.9173 - 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 A21 320 104 iP_D A16 300 106 eP_X A11 285 110 iP_D LMQ 298 131 iP_D A54 293 133 eP_X GGN 143 259 iP_D ICQ 21 303 eP_X LMN 111 332 iP_D MNT 248 408 iP_C NCB 234 542 eP_X HRV 205 545 eP_X KGNO 246 675 eP_X SADO 257 840 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.
The location of the event and the station distribution used for the surface-wave spectral amplitude technique are given in the following figure.
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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) A21 320 104 A16 300 106 A11 285 110 LMQ 298 131 GGN 143 259 ICQ 21 303 LMN 111 332 MNT 248 408 NCB 234 542 OTT 254 564 KGNO 246 675 VLDQ 284 687 PECO 245 726 BUKO 262 836 SADO 257 840 MALO 297 877 SCHQ 8 883 ACTO 251 960 TOBO 263 1003 SSPA 229 1017 KAPO 290 1054 SDMD 221 1066 ELFO 251 1072 EYMN 282 1710 WCI 242 1730 USIN 244 1841
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 0.025 n 4 lp c 0.10 n 4 br c 0.12 0.25 n 4 p 2
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