The ANSS event ID is uw10569358 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/uw10569358/executive.
2002/06/29 14:36:04 45.335 -121.686 4.9 4.5 Oregon
USGS/SLU Moment Tensor Solution ENS 2002/06/29 14:36:04:0 45.33 -121.69 4.9 4.5 Oregon Stations used: LB.BMN UO.EUO US.HLID US.NEW US.WVOR YS.GPSS1 Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 2.19e+22 dyne-cm Mw = 4.16 Z = 10 km Plane Strike Dip Rake NP1 141 76 164 NP2 235 75 15 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 21 98 N 0.00e+00 69 279 P -2.19e+22 0 188 Moment Tensor: (dyne-cm) Component Value Mxx -2.11e+22 Mxy -5.65e+21 Mxz -8.80e+20 Myy 1.82e+22 Myz 7.29e+21 Mzz 2.83e+21 -------------- ---------------------- #--------------------------- ###--------------------------- #####----------------------------- #######----------------------####### #########----------------############# ###########------------################# ############--------#################### ##############----######################## ########################################## ##############---################### ### ###########-------################## T ### #########----------################ ## #######--------------################### ####------------------################ ##--------------------############## ------------------------########## ------------------------###### --------------------------## ------- ------------ --- P -------- Global CMT Convention Moment Tensor: R T P 2.83e+21 -8.80e+20 -7.29e+21 -8.80e+20 -2.11e+22 5.65e+21 -7.29e+21 5.65e+21 1.82e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020629143604/index.html |
STK = 235 DIP = 75 RAKE = 15 MW = 4.16 HS = 10.0
The NDK file is 20020629143604.ndk The waveform inversion is preferred.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 220 50 -35 3.95 0.5065 WVFGRD96 1.0 220 50 -35 3.98 0.5358 WVFGRD96 2.0 220 50 -35 4.06 0.6532 WVFGRD96 3.0 40 60 -40 4.07 0.6904 WVFGRD96 4.0 50 75 -25 4.05 0.7187 WVFGRD96 5.0 50 80 -20 4.05 0.7414 WVFGRD96 6.0 55 85 -15 4.09 0.7582 WVFGRD96 7.0 55 85 -15 4.11 0.7709 WVFGRD96 8.0 55 85 -15 4.14 0.7794 WVFGRD96 9.0 55 90 -15 4.15 0.7813 WVFGRD96 10.0 235 75 15 4.16 0.7834 WVFGRD96 11.0 235 75 10 4.18 0.7796 WVFGRD96 12.0 235 75 10 4.19 0.7722 WVFGRD96 13.0 235 80 20 4.21 0.7633 WVFGRD96 14.0 235 80 20 4.22 0.7526 WVFGRD96 15.0 235 80 20 4.23 0.7394 WVFGRD96 16.0 235 80 20 4.24 0.7240 WVFGRD96 17.0 235 80 20 4.25 0.7065 WVFGRD96 18.0 230 85 20 4.25 0.6875 WVFGRD96 19.0 230 85 25 4.26 0.6683 WVFGRD96 20.0 230 85 25 4.27 0.6486 WVFGRD96 21.0 230 85 25 4.28 0.6267 WVFGRD96 22.0 230 85 30 4.29 0.6049 WVFGRD96 23.0 230 85 30 4.30 0.5832 WVFGRD96 24.0 40 90 -40 4.31 0.5594 WVFGRD96 25.0 225 85 40 4.32 0.5387 WVFGRD96 26.0 40 90 -45 4.34 0.5174 WVFGRD96 27.0 40 90 -45 4.34 0.4968 WVFGRD96 28.0 220 90 50 4.36 0.4750 WVFGRD96 29.0 40 90 -50 4.37 0.4543
The best solution is
WVFGRD96 10.0 235 75 15 4.16 0.7834
The mechanism corresponding 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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3
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Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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 check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01 Model after 8 iterations ISOTROPIC KGS FLAT EARTH 1-D CONSTANT VELOCITY LINE08 LINE09 LINE10 LINE11 H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS 1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00 6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00 13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00 19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00 0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00