The ANSS event ID is ak008e7a9oaz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak008e7a9oaz/executive.
2008/11/04 15:46:34 66.344 -157.977 4.9 4.3 Alaska
USGS/SLU Moment Tensor Solution ENS 2008/11/04 15:46:34:0 66.34 -157.98 4.9 4.3 Alaska Stations used: AK.BPAW AK.COLD AK.MCK AK.PPLA AK.TNA AK.TRF IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 2.11e+22 dyne-cm Mw = 4.15 Z = 10 km Plane Strike Dip Rake NP1 345 80 30 NP2 249 61 168 Principal Axes: Axis Value Plunge Azimuth T 2.11e+22 28 211 N 0.00e+00 59 2 P -2.11e+22 13 114 Moment Tensor: (dyne-cm) Component Value Mxx 8.77e+21 Mxy 1.47e+22 Mxz -5.64e+21 Myy -1.24e+22 Myz -8.77e+21 Mzz 3.61e+21 --############ -------############### -----------################# -------------################# ----------------################## -----------------################### -------------------##---------------## ----------------#####------------------- ------------#########------------------- ----------#############------------------- --------###############------------------- ------##################------------------ ----####################------------------ --######################---------------- -#######################----------- -- #######################----------- P - ######### ###########---------- ######## T ###########------------ ###### ###########---------- ###################--------- ################------ ############-- Global CMT Convention Moment Tensor: R T P 3.61e+21 -5.64e+21 8.77e+21 -5.64e+21 8.77e+21 -1.47e+22 8.77e+21 -1.47e+22 -1.24e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081104154634/index.html |
STK = 345 DIP = 80 RAKE = 30 MW = 4.15 HS = 10.0
The NDK file is 20081104154634.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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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:
hp c 0.02 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 155 65 -40 4.00 0.6103 WVFGRD96 1.0 150 60 -45 4.03 0.6127 WVFGRD96 2.0 150 60 -45 4.09 0.6686 WVFGRD96 3.0 150 65 -50 4.11 0.6713 WVFGRD96 4.0 340 90 45 4.08 0.6955 WVFGRD96 5.0 160 90 -40 4.07 0.7209 WVFGRD96 6.0 345 80 35 4.08 0.7437 WVFGRD96 7.0 345 80 35 4.10 0.7565 WVFGRD96 8.0 350 75 40 4.14 0.7662 WVFGRD96 9.0 345 85 30 4.13 0.7727 WVFGRD96 10.0 345 80 30 4.15 0.7759 WVFGRD96 11.0 160 90 -25 4.15 0.7709 WVFGRD96 12.0 345 85 25 4.16 0.7713 WVFGRD96 13.0 160 90 -25 4.16 0.7648 WVFGRD96 14.0 160 90 -25 4.17 0.7585 WVFGRD96 15.0 160 85 -20 4.18 0.7521 WVFGRD96 16.0 345 75 20 4.20 0.7494 WVFGRD96 17.0 160 85 -20 4.20 0.7376 WVFGRD96 18.0 345 75 20 4.22 0.7377 WVFGRD96 19.0 345 80 20 4.23 0.7286 WVFGRD96 20.0 345 80 20 4.23 0.7171 WVFGRD96 21.0 345 80 20 4.24 0.7034 WVFGRD96 22.0 345 80 20 4.25 0.6885 WVFGRD96 23.0 345 80 20 4.26 0.6719 WVFGRD96 24.0 160 90 -20 4.26 0.6520 WVFGRD96 25.0 160 90 -20 4.27 0.6341 WVFGRD96 26.0 160 90 -20 4.27 0.6156 WVFGRD96 27.0 340 90 20 4.28 0.5972 WVFGRD96 28.0 160 90 -20 4.29 0.5784 WVFGRD96 29.0 160 90 -20 4.29 0.5595 WVFGRD96 30.0 160 90 -20 4.30 0.5416 WVFGRD96 31.0 160 85 -20 4.31 0.5249 WVFGRD96 32.0 160 85 -20 4.31 0.5093 WVFGRD96 33.0 160 85 -20 4.32 0.4946 WVFGRD96 34.0 160 90 -20 4.33 0.4818 WVFGRD96 35.0 70 75 -15 4.34 0.4740 WVFGRD96 36.0 70 75 -15 4.35 0.4735 WVFGRD96 37.0 70 75 -15 4.37 0.4731 WVFGRD96 38.0 70 75 -15 4.39 0.4720 WVFGRD96 39.0 70 75 -15 4.41 0.4695 WVFGRD96 40.0 70 70 -20 4.45 0.4565 WVFGRD96 41.0 70 70 -20 4.46 0.4504 WVFGRD96 42.0 70 70 -20 4.47 0.4431 WVFGRD96 43.0 70 70 -20 4.48 0.4353 WVFGRD96 44.0 70 70 -20 4.49 0.4272 WVFGRD96 45.0 70 75 -20 4.49 0.4192 WVFGRD96 46.0 70 75 -20 4.50 0.4109 WVFGRD96 47.0 70 75 -20 4.51 0.4023 WVFGRD96 48.0 70 75 -20 4.52 0.3935 WVFGRD96 49.0 70 75 -15 4.52 0.3844 WVFGRD96 50.0 70 75 -15 4.53 0.3761
The best solution is
WVFGRD96 10.0 345 80 30 4.15 0.7759
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
hp c 0.02 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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