2006/10/08 02:48:27 46.85N 121.60W 4 4.5 Washington
USGS Felt map for this earthquake
USGS Felt reports page for Pacific Northwest US
SLU Moment Tensor Solution 2006/10/08 02:48:27 46.85N 121.60W 4 4.5 Washington Best Fitting Double Couple Mo = 3.67e+22 dyne-cm Mw = 4.31 Z = 12 km Plane Strike Dip Rake NP1 145 74 127 NP2 255 40 25 Principal Axes: Axis Value Plunge Azimuth T 3.67e+22 47 94 N 0.00e+00 36 314 P -3.67e+22 20 208 Moment Tensor: (dyne-cm) Component Value Mxx -2.50e+22 Mxy -1.47e+22 Mxz 9.20e+21 Myy 9.67e+21 Myz 2.39e+22 Mzz 1.53e+22 -------------- ---------------------- ##-------------------------- ####-------------------------- ######--------#############------- ########--######################---- ########--##########################-- ######------###########################- #####-------############################ ####----------############################ ###-------------############## ######### ##---------------############# T ######### #-----------------############ ######### ------------------###################### --------------------#################### --------------------################## ---------------------############### -------- -----------############ ------ P -------------######## ----- ---------------##### ---------------------- -------------- Harvard Convention Moment Tensor: R T F 1.53e+22 9.20e+21 -2.39e+22 9.20e+21 -2.50e+22 1.47e+22 -2.39e+22 1.47e+22 9.67e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20061008024827/index.html |
Fault Plane Parameters for 06100802482c Fault Choice 1 Fault Choice 2 Strike(deg) 120.0 277.0 Dip(deg) 65.0 27.0 Rake(deg) 100.2 69.3 Fault Type reverse reverse |
STK = 255 DIP = 40 RAKE = 25 MW = 4.31 HS = 12
Both techniques give similar results in terms of source depth and moment magnitude. The common feature is a very shallowly dipping nodal plane to the SE. The waveform inversion is used for defining the source parameters. Much processing time was required to define an appropriate pass band for the wave form inversion. In this case the surface wave technique was more robust. Using higher frequencies for the waveform inversion introduced ringing surface waves into the data, which was hard to model, in that the orientations of the P and T axes were very sensitive to the time shifts in the fitting process. The final solution is consistent with short distance first motions.
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 0.02 n 3 lp c 0.05 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 115 55 65 3.99 0.1253 WVFGRD96 1.0 95 60 45 3.98 0.1181 WVFGRD96 2.0 270 55 35 4.06 0.1385 WVFGRD96 3.0 255 45 15 4.12 0.1409 WVFGRD96 4.0 250 30 10 4.21 0.1556 WVFGRD96 5.0 250 30 10 4.23 0.1740 WVFGRD96 6.0 250 30 10 4.25 0.1886 WVFGRD96 7.0 255 30 20 4.26 0.2028 WVFGRD96 8.0 260 30 30 4.30 0.2129 WVFGRD96 9.0 255 35 25 4.30 0.2222 WVFGRD96 10.0 260 35 35 4.31 0.2287 WVFGRD96 11.0 260 35 35 4.31 0.2310 WVFGRD96 12.0 255 40 25 4.31 0.2331 WVFGRD96 13.0 255 40 25 4.31 0.2325 WVFGRD96 14.0 245 45 0 4.30 0.2316 WVFGRD96 15.0 245 45 0 4.31 0.2312 WVFGRD96 16.0 240 50 -15 4.31 0.2307 WVFGRD96 17.0 240 55 -15 4.31 0.2300 WVFGRD96 18.0 240 55 -15 4.32 0.2288 WVFGRD96 19.0 240 55 -15 4.32 0.2271 WVFGRD96 20.0 240 60 -15 4.33 0.2257 WVFGRD96 21.0 240 55 -15 4.36 0.2214 WVFGRD96 22.0 240 55 -15 4.37 0.2192 WVFGRD96 23.0 240 55 -15 4.37 0.2167 WVFGRD96 24.0 240 55 -15 4.38 0.2142 WVFGRD96 25.0 240 55 -10 4.38 0.2113 WVFGRD96 26.0 240 55 -10 4.39 0.2084 WVFGRD96 27.0 240 55 -10 4.39 0.2053 WVFGRD96 28.0 240 55 -10 4.40 0.2022 WVFGRD96 29.0 240 60 -10 4.41 0.1990
The best solution is
WVFGRD96 12.0 255 40 25 4.31 0.2331
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 n 3 lp c 0.05 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= 138.98 DIP= 71.25 RAKE= 113.86 OR STK= 264.98 DIP= 30.00 RAKE= 39.99 DEPTH = 9.0 km Mw = 4.36 Best Fit 0.8325 - 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 HOOD 181 170 eP_- MEGW 250 186 iP_D OFR 301 243 eP_X PGC 326 244 eP_+ HEBO 223 247 eP_- HNB 345 279 eP_X HOPB 3 283 eP_X COR 208 285 iP_D TOLO 217 307 eP_X PNT 28 312 eP_- EUO 201 334 eP_- TAKO 210 397 iP_D BMO 122 401 eP_- LLLB 357 419 eP_X SLEB 27 543 eP_X DLMT 100 715 iP_D
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) HOOD 181 170 MEGW 250 186 OFR 301 243 PGC 326 244 HEBO 223 247 HNB 345 279 HOPB 3 283 COR 208 285 TOLO 217 307 PNT 28 312 EUO 201 334 TAKO 210 397 BMO 122 401 LLLB 357 419 DBO 198 435 SLEB 27 543 MOD 169 560 YBH 189 576 WALA 64 625 JCC 197 698 WDC 186 701 DLMT 100 715 ORV 179 811 HOPS 188 881 IMW 108 897 RRI2 112 897 HVU 126 901 FLWY 106 906 TPAW 110 916 MOOW 108 918 EDM 37 922 REDW 111 930 SNOW 110 932 LOHW 109 935 AHID 115 946 CMB 174 985 DUG 133 1025 CTU 127 1046 JRSC 183 1051 BW06 111 1054 MPU 130 1107 PKD 176 1215 SRU 130 1245 RWWY 111 1282 FNBB 356 1344 DLBC 339 1409 ULM 69 1928 CBKS 109 1990 FCC 44 2256 UALR 108 2787 USIN 97 2937
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 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 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: