20050306 0617 49.730L 47.7528 -69.7321 13.27km 5.1MS GSC
Arrival time list Geological Survey Canada:
USGS 2005/06/06 06:17:49 47.750N 69.730W 18 5.4 Quebec
Arrival time list US Geological Survey
USGS Felt map for this earthquake
USGS Felt reports page for Eastern Canada
We present three solutions: SLU wavefrom inversion (preferred), Lamont-Doherty Waveform Solution, and the surface-wave radiation pattern solution.
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:
The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 185 35 90 4.23 0.4660
WVFGRD96 1.0 175 35 90 4.31 0.5324
WVFGRD96 2.0 115 70 10 4.47 0.4812
WVFGRD96 3.0 185 80 50 4.28 0.4592
WVFGRD96 4.0 190 75 50 4.31 0.5128
WVFGRD96 5.0 190 70 50 4.34 0.5633
WVFGRD96 6.0 200 60 55 4.42 0.6097
WVFGRD96 7.0 200 60 55 4.45 0.6518
WVFGRD96 8.0 200 65 70 4.46 0.6883
WVFGRD96 9.0 210 55 65 4.55 0.7186
WVFGRD96 10.0 215 60 75 4.63 0.7404
WVFGRD96 11.0 355 25 80 4.55 0.7606
WVFGRD96 12.0 350 30 80 4.58 0.7663
WVFGRD96 13.0 350 30 80 4.60 0.7568
WVFGRD96 14.0 165 55 85 4.61 0.7168
WVFGRD96 15.0 330 70 -70 4.52 0.6775
WVFGRD96 16.0 325 70 -75 4.53 0.6684
WVFGRD96 17.0 305 75 -85 4.56 0.6563
WVFGRD96 18.0 310 75 -80 4.56 0.6391
WVFGRD96 19.0 305 80 -80 4.57 0.6207
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 0.20 3
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Lamont-Doherty Waveform inversion moment tensor
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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NODAL PLANES
STK= 192.78
DIP= 61.98
RAKE= 101.17
OR
STK= 349.97
DIP= 30.00
RAKE= 69.99
DEPTH = 20.0 km
Mw = 4.81
Best Fit 0.7788 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion LMQ 244 50 iP_D ICQ 42 268 iP_C GGN 142 368 iP_+ MNT 231 389 iP_D LMN 118 431 iP_C GAC 245 495 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. |
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| 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. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. 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) LMQ 244 50 ICQ 42 268 GGN 142 368 MNT 232 389 LMN 118 431 GAC 245 495 NCB 221 546 ACCN 214 575 HRV 194 601 VLDQ 278 602 KGNO 236 654 SADO 249 797 SCHQ 13 814 PAL 205 820 BRNJ 208 875 DRLN 75 918 KAPO 286 961 MVL 214 1012 ERPA 236 1023 SSPA 222 1024 ALLY 234 1067 SDMD 214 1090 MCWV 226 1212 CBN 213 1229 AAM 246 1252 ACSO 237 1350 BLO 241 1655 JFWS 259 1691 WCI 238 1710 FRB 2 1784 USIN 240 1818 ULM 288 1925 SIUC 242 1939 SLM 246 1942 GOGA 221 1966 FVM 245 2000 FCC 316 2019 CCM 246 2051 MPH 237 2184 LRAL 228 2186 DWPF 209 2404 MIAR 242 2465 CBKS 259 2611 AMTX 252 3009 BW06 275 3141 BOZ 282 3176 HWUT 274 3346 HLID 279 3467 NEW 289 3482
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:
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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
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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.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
