The ANSS event ID is ak012gthgzoe and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak012gthgzoe/executive.
2012/12/31 11:38:28 61.560 -141.142 10.9 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2012/12/31 11:38:28:0 61.56 -141.14 10.9 4.0 Alaska Stations used: AK.BCP AK.DCPH AK.DHY AK.DIV AK.DOT AK.FYU AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM AK.PPD AK.RIDG AK.SCRK AK.WRH CN.DAWY CN.WHY IU.COLA Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.10 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.72e+22 dyne-cm Mw = 4.09 Z = 10 km Plane Strike Dip Rake NP1 245 85 65 NP2 144 25 168 Principal Axes: Axis Value Plunge Azimuth T 1.72e+22 44 130 N 0.00e+00 25 247 P -1.72e+22 35 356 Moment Tensor: (dyne-cm) Component Value Mxx -7.76e+21 Mxy -3.61e+21 Mxz -1.36e+22 Myy 5.06e+21 Myz 7.05e+21 Mzz 2.70e+21 -------------- ---------------------- #----------- ------------- #------------ P -------------- ##------------- ---------------- ##---------------------------------- ###---------------------------------## ###-----------------------------######## ###------------------------############# ####--------------------################## ####----------------###################### #####-----------########################## #####-------############################## #####---################### ########## #####-##################### T ########## #-----#################### ######### ------############################## -------########################### -------####################### ---------################### ----------###########- -------------- Global CMT Convention Moment Tensor: R T P 2.70e+21 -1.36e+22 -7.05e+21 -1.36e+22 -7.76e+21 3.61e+21 -7.05e+21 3.61e+21 5.06e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121231113828/index.html |
STK = 245 DIP = 85 RAKE = 65 MW = 4.09 HS = 10.0
The NDK file is 20121231113828.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 3 br c 0.10 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 75 45 -95 3.84 0.4283 WVFGRD96 1.0 50 45 -80 3.80 0.3904 WVFGRD96 2.0 220 45 -90 3.92 0.4956 WVFGRD96 3.0 250 55 -50 3.96 0.4398 WVFGRD96 4.0 60 90 -70 4.07 0.4229 WVFGRD96 5.0 240 90 70 4.08 0.5268 WVFGRD96 6.0 60 90 -70 4.07 0.5829 WVFGRD96 7.0 240 90 65 4.05 0.6107 WVFGRD96 8.0 60 90 -70 4.11 0.6198 WVFGRD96 9.0 60 90 -65 4.09 0.6260 WVFGRD96 10.0 245 85 65 4.09 0.6275 WVFGRD96 11.0 60 90 -60 4.08 0.6263 WVFGRD96 12.0 60 90 -60 4.08 0.6245 WVFGRD96 13.0 245 80 60 4.09 0.6228 WVFGRD96 14.0 250 75 60 4.10 0.6195 WVFGRD96 15.0 250 75 60 4.10 0.6153 WVFGRD96 16.0 250 75 60 4.10 0.6099 WVFGRD96 17.0 250 75 60 4.11 0.6034 WVFGRD96 18.0 250 75 60 4.11 0.5960 WVFGRD96 19.0 250 75 60 4.12 0.5878 WVFGRD96 20.0 255 70 60 4.14 0.5790 WVFGRD96 21.0 255 70 65 4.15 0.5701 WVFGRD96 22.0 260 70 65 4.16 0.5595 WVFGRD96 23.0 260 70 65 4.17 0.5484 WVFGRD96 24.0 260 70 65 4.18 0.5364 WVFGRD96 25.0 260 70 65 4.18 0.5237 WVFGRD96 26.0 260 70 65 4.19 0.5101 WVFGRD96 27.0 265 70 70 4.20 0.4959 WVFGRD96 28.0 355 35 25 4.19 0.4827 WVFGRD96 29.0 265 70 70 4.21 0.4677
The best solution is
WVFGRD96 10.0 245 85 65 4.09 0.6275
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 br c 0.10 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