The ANSS event ID is ak0126vtr8t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0126vtr8t/executive.
2012/01/04 16:25:37 59.076 -138.025 9.9 4.2 Alaska
USGS/SLU Moment Tensor Solution ENS 2012/01/04 16:25:37:0 59.08 -138.02 9.9 4.2 Alaska Stations used: AK.BCP AK.BESE AK.JIS AK.PAX AK.PNL AT.SKAG AT.YKU2 CN.DAWY CN.DLBC CN.HYT CN.PLBC CN.WHY CN.YUK7 Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.31e+22 dyne-cm Mw = 4.28 Z = 15 km Plane Strike Dip Rake NP1 295 65 40 NP2 185 54 149 Principal Axes: Axis Value Plunge Azimuth T 3.31e+22 45 155 N 0.00e+00 44 322 P -3.31e+22 6 58 Moment Tensor: (dyne-cm) Component Value Mxx 4.22e+21 Mxy -2.10e+22 Mxz -1.69e+22 Myy -2.05e+22 Myz 3.93e+21 Mzz 1.63e+22 #######------- #########------------- ##########------------------ ##########-------------------- ###########--------------------- ###########---------------------- P ------------###------------------- - ------------#########------------------- ------------#############--------------- ------------#################------------- ------------####################---------- ------------######################-------- ------------########################------ -----------#########################---- -----------###########################-- ----------############ ############# ----------########### T ############ ---------########### ########### --------###################### --------#################### ------################ ----########## Global CMT Convention Moment Tensor: R T P 1.63e+22 -1.69e+22 -3.93e+21 -1.69e+22 4.22e+21 2.10e+22 -3.93e+21 2.10e+22 -2.05e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120104162537/index.html |
STK = 295 DIP = 65 RAKE = 40 MW = 4.28 HS = 15.0
The NDK file is 20120104162537.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 115 35 -90 4.08 0.3606 WVFGRD96 1.0 115 40 -90 4.11 0.3674 WVFGRD96 2.0 110 55 -45 4.14 0.3689 WVFGRD96 3.0 115 65 -40 4.13 0.3635 WVFGRD96 4.0 115 65 -40 4.14 0.3638 WVFGRD96 5.0 305 75 50 4.16 0.3911 WVFGRD96 6.0 295 75 50 4.16 0.4241 WVFGRD96 7.0 300 65 50 4.18 0.4559 WVFGRD96 8.0 300 65 50 4.19 0.4850 WVFGRD96 9.0 300 65 45 4.20 0.5081 WVFGRD96 10.0 300 65 50 4.23 0.5309 WVFGRD96 11.0 295 65 40 4.24 0.5474 WVFGRD96 12.0 295 65 40 4.25 0.5601 WVFGRD96 13.0 295 65 40 4.26 0.5684 WVFGRD96 14.0 295 65 40 4.27 0.5724 WVFGRD96 15.0 295 65 40 4.28 0.5729 WVFGRD96 16.0 295 65 40 4.29 0.5703 WVFGRD96 17.0 295 65 40 4.30 0.5648 WVFGRD96 18.0 295 65 40 4.31 0.5563 WVFGRD96 19.0 295 65 40 4.32 0.5454 WVFGRD96 20.0 295 65 40 4.35 0.5372 WVFGRD96 21.0 295 65 40 4.36 0.5226 WVFGRD96 22.0 295 65 40 4.36 0.5064 WVFGRD96 23.0 295 65 40 4.37 0.4889 WVFGRD96 24.0 295 65 40 4.37 0.4698 WVFGRD96 25.0 295 65 40 4.38 0.4496 WVFGRD96 26.0 110 65 30 4.39 0.4282 WVFGRD96 27.0 110 65 30 4.39 0.4121 WVFGRD96 28.0 110 65 30 4.40 0.3968 WVFGRD96 29.0 110 65 30 4.40 0.3823
The best solution is
WVFGRD96 15.0 295 65 40 4.28 0.5729
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 0.10 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