2004/08/26 23:11:37 64.76N 86.28W 18 5.0 Canada
http://www.seismo.nrcan.gc.ca/nedb/bull_sol_e.php?solid=20040826.2311004 Arrival time list
USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
SLU Moment Tensor Solution
2004/08/26 23:11:37 64.76N 86.28W 18 5.0 Canada
Best Fitting Double Couple
Mo = 2.57e+22 dyne-cm
Mw = 4.24
Z = 23 km
Plane Strike Dip Rake
NP1 155 60 80
NP2 354 31 107
Principal Axes:
Axis Value Plunge Azimuth
T 2.57e+22 73 40
N 0.00e+00 9 160
P -2.57e+22 14 252
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.54e+20
Mxy -5.91e+21
Mxz 7.37e+21
Myy -2.10e+22
Myz 1.05e+22
Mzz 2.19e+22
#########-----
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----------########### ##########------
------------########## T ##########-------
------------########## ##########-------
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-- ---------#####################-------
- P ----------####################------
- -----------###################------
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Harvard Convention
Moment Tensor:
R T F
2.19e+22 7.37e+21 -1.05e+22
7.37e+21 -9.54e+20 5.91e+21
-1.05e+22 5.91e+21 -2.10e+22
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/NEW/20040826231137/index.html
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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 = 155
DIP = 60
RAKE = 80
MW = 4.24
HS = 23
The waveform solution is preferred. The surface-wave solution admitted two likely possibilities differing in strike by about 90 degrees. The surface-wave soltuions elected provides a better fit to the waveforms
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 0.5 335 50 -90 3.91 0.4943
WVFGRD96 1.0 155 40 -85 3.96 0.5241
WVFGRD96 2.0 150 45 -85 4.07 0.5530
WVFGRD96 3.0 150 50 -80 4.11 0.4384
WVFGRD96 4.0 320 65 -55 4.07 0.4175
WVFGRD96 5.0 340 80 50 4.02 0.4317
WVFGRD96 6.0 305 10 -25 4.14 0.4605
WVFGRD96 7.0 305 10 -25 4.14 0.4866
WVFGRD96 8.0 300 10 -30 4.14 0.5093
WVFGRD96 9.0 300 10 -30 4.14 0.5276
WVFGRD96 10.0 300 10 -30 4.18 0.5423
WVFGRD96 11.0 300 10 -30 4.18 0.5597
WVFGRD96 12.0 305 10 -25 4.19 0.5743
WVFGRD96 13.0 305 10 -25 4.20 0.5859
WVFGRD96 14.0 240 30 -10 4.09 0.5992
WVFGRD96 15.0 240 30 -10 4.10 0.6165
WVFGRD96 16.0 240 30 -10 4.12 0.6309
WVFGRD96 17.0 240 30 -10 4.13 0.6424
WVFGRD96 18.0 245 30 0 4.14 0.6515
WVFGRD96 19.0 320 20 70 4.15 0.6606
WVFGRD96 20.0 335 20 85 4.19 0.6669
WVFGRD96 21.0 335 20 85 4.20 0.6713
WVFGRD96 22.0 155 65 85 4.22 0.6741
WVFGRD96 23.0 155 60 80 4.24 0.6757
WVFGRD96 24.0 155 60 80 4.24 0.6743
WVFGRD96 25.0 155 55 80 4.26 0.6710
WVFGRD96 26.0 155 55 80 4.27 0.6658
WVFGRD96 27.0 155 55 80 4.27 0.6580
WVFGRD96 28.0 155 55 80 4.28 0.6481
WVFGRD96 29.0 155 55 80 4.29 0.6371
WVFGRD96 30.0 150 50 70 4.30 0.6266
WVFGRD96 31.0 150 50 70 4.31 0.6169
WVFGRD96 32.0 150 50 70 4.32 0.6065
WVFGRD96 33.0 145 50 70 4.32 0.5956
The best solution is
WVFGRD96 23.0 155 60 80 4.24 0.6757
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= 149.99
DIP= 54.99
RAKE= 64.99
OR
STK= 9.11
DIP= 42.07
RAKE= 121.11
DEPTH = 19.0 km
Mw = 4.29
Best Fit 0.8234 - 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 FCC 215 785 eP_X FRB 89 866 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. 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) FCC 215 785 FRB 89 866 RES 347 1156 YKW1 273 1411 YKW3 273 1414 YKW4 273 1423 SCHQ 127 1544 ULM 204 1713 KAPO 171 1721 VLDQ 160 1917 EDM 243 1994 FNBB 268 2013 INK 302 2086 DGMT 219 2107 BMBC 260 2165 SADO 165 2271 MNT 154 2283 KGNO 160 2368 DLBC 273 2376 DRLN 122 2420 NCB 156 2436 JFWS 188 2447 WHY 282 2468 LMN 138 2483 DAWY 293 2490 ACCN 156 2510 ERPA 168 2554 LLLB 250 2591 BESE 278 2596 NEW 240 2596 BINY 160 2597 ALLY 168 2604 MSO 233 2605 PNT 245 2606 BOZ 228 2635 HRV 152 2649 DCPH 281 2720 PNL 283 2739 PAL 157 2763 FOR 157 2779 COLD 304 2792 HARP 292 2834 TTW 245 2842 BLO 180 2850 BW06 222 2855 LTH 241 2859 SQM 247 2866 HAWA 241 2870 OPC 248 2880 SDMD 163 2889 BMR 289 2896 AHID 225 2906 MCK 297 2921 SLM 187 2921 LON 244 2933 KSU1 198 2936 DIV 290 2938 HLID 230 2942 OCWA 248 2946 WCI 180 2954 EYAK 289 2974 USIN 182 2985 BPAW 298 2987 FVM 187 2994 TRF 297 2995 SAW 293 3010 CBN 164 3014 KTH 297 3016 SIUC 185 3018 CBKS 204 3020 HWUT 224 3039 BLA 170 3092 PPLA 297 3109 RC01 292 3117 PVMO 186 3164 UTMT 184 3168 SWD 290 3169 WVT 183 3189 WVOR 235 3213 DUG 225 3225 SDCO 212 3263 MPH 186 3308 PLAL 183 3317 DBO 242 3334 BMN 231 3357 UALR 190 3363 OXF 185 3374 MIAR 192 3399 WMOK 200 3447 LRAL 181 3531 NHSC 170 3547 WDC 238 3550 MNV 231 3592 TPH 229 3594 TPNV 227 3674 CMB 234 3716 NATX 193 3717 SAO 234 3884 JCT 200 3928 PAS 228 4028
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:
