The ANSS event ID is ak011djt676 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak011djt676/executive.
2011/01/08 19:33:40 59.359 -135.147 3.7 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2011/01/08 19:33:40:0 59.36 -135.15 3.7 4.0 Alaska Stations used: AK.BAL AK.BMR AK.DCPH AK.EYAK AK.PIN AK.PNL AK.RAG AT.SKAG AT.YKU2 CN.BVCY CN.DAWY CN.DLBC CN.HYT CN.PLBC CN.WHY CN.YUK5 US.WRAK Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 6.10e+21 dyne-cm Mw = 3.79 Z = 2 km Plane Strike Dip Rake NP1 180 45 90 NP2 360 45 90 Principal Axes: Axis Value Plunge Azimuth T 6.10e+21 90 145 N 0.00e+00 -0 0 P -6.10e+21 -0 270 Moment Tensor: (dyne-cm) Component Value Mxx 0.00e+00 Mxy -1.88e+14 Mxz 1.88e+14 Myy -6.10e+21 Myz -2.66e+14 Mzz 6.10e+21 -----####----- -------########------- --------############-------- --------##############-------- ---------################--------- ---------##################--------- ---------####################--------- ----------####################---------- ---------######################--------- ----------######################---------- --------########## #########---------- P --------########## T #########---------- --------########## #########---------- ---------######################--------- ----------####################---------- ---------####################--------- ---------##################--------- ---------################--------- --------##############-------- --------############-------- -------########------- -----####----- Global CMT Convention Moment Tensor: R T P 6.10e+21 1.88e+14 2.66e+14 1.88e+14 0.00e+00 1.88e+14 2.66e+14 1.88e+14 -6.10e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110108193340/index.html |
STK = 0 DIP = 45 RAKE = 90 MW = 3.79 HS = 2.0
The NDK file is 20110108193340.ndk The waveform inversion is preferred.
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
USGS/SLU Moment Tensor Solution ENS 2011/01/08 19:33:40:0 59.36 -135.15 3.7 4.0 Alaska Stations used: AK.BAL AK.BMR AK.DCPH AK.EYAK AK.PIN AK.PNL AK.RAG AT.SKAG AT.YKU2 CN.BVCY CN.DAWY CN.DLBC CN.HYT CN.PLBC CN.WHY CN.YUK5 US.WRAK Filtering commands used: hp c 0.02 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 6.10e+21 dyne-cm Mw = 3.79 Z = 2 km Plane Strike Dip Rake NP1 180 45 90 NP2 360 45 90 Principal Axes: Axis Value Plunge Azimuth T 6.10e+21 90 145 N 0.00e+00 -0 0 P -6.10e+21 -0 270 Moment Tensor: (dyne-cm) Component Value Mxx 0.00e+00 Mxy -1.88e+14 Mxz 1.88e+14 Myy -6.10e+21 Myz -2.66e+14 Mzz 6.10e+21 -----####----- -------########------- --------############-------- --------##############-------- ---------################--------- ---------##################--------- ---------####################--------- ----------####################---------- ---------######################--------- ----------######################---------- --------########## #########---------- P --------########## T #########---------- --------########## #########---------- ---------######################--------- ----------####################---------- ---------####################--------- ---------##################--------- ---------################--------- --------##############-------- --------############-------- -------########------- -----####----- Global CMT Convention Moment Tensor: R T P 6.10e+21 1.88e+14 2.66e+14 1.88e+14 0.00e+00 1.88e+14 2.66e+14 1.88e+14 -6.10e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110108193340/index.html |
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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 0.06 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 0 45 90 3.69 0.6476 WVFGRD96 1.0 160 60 45 3.71 0.5628 WVFGRD96 2.0 0 45 90 3.79 0.6500 WVFGRD96 3.0 175 40 80 3.84 0.5841 WVFGRD96 4.0 160 30 55 3.90 0.5265 WVFGRD96 5.0 170 80 85 3.94 0.5333 WVFGRD96 6.0 170 80 85 3.92 0.5721 WVFGRD96 7.0 170 80 85 3.89 0.5912 WVFGRD96 8.0 170 80 85 3.96 0.6058 WVFGRD96 9.0 170 80 80 3.94 0.6172 WVFGRD96 10.0 170 80 80 3.92 0.6243 WVFGRD96 11.0 165 85 80 3.92 0.6264 WVFGRD96 12.0 165 85 80 3.91 0.6288 WVFGRD96 13.0 345 90 -75 3.91 0.6253 WVFGRD96 14.0 345 90 -75 3.91 0.6270 WVFGRD96 15.0 165 90 75 3.91 0.6269 WVFGRD96 16.0 165 90 80 3.91 0.6259 WVFGRD96 17.0 165 90 80 3.91 0.6241 WVFGRD96 18.0 165 90 80 3.91 0.6211 WVFGRD96 19.0 350 85 -85 3.91 0.6221 WVFGRD96 20.0 350 80 -90 3.92 0.6203 WVFGRD96 21.0 180 10 -85 3.93 0.6183 WVFGRD96 22.0 195 15 -70 3.95 0.6173 WVFGRD96 23.0 200 15 -65 3.95 0.6160 WVFGRD96 24.0 200 15 -65 3.96 0.6136 WVFGRD96 25.0 200 15 -65 3.97 0.6102 WVFGRD96 26.0 200 15 -70 3.97 0.6058 WVFGRD96 27.0 200 15 -70 3.98 0.6003 WVFGRD96 28.0 205 15 -65 3.98 0.5937 WVFGRD96 29.0 200 15 -70 3.99 0.5858 WVFGRD96 30.0 210 15 -60 4.00 0.5766 WVFGRD96 31.0 210 15 -60 4.00 0.5665 WVFGRD96 32.0 210 15 -60 4.01 0.5555 WVFGRD96 33.0 215 15 -55 4.02 0.5435 WVFGRD96 34.0 220 15 -50 4.02 0.5312 WVFGRD96 35.0 225 15 -45 4.02 0.5186 WVFGRD96 36.0 225 15 -45 4.03 0.5066 WVFGRD96 37.0 230 15 -40 4.03 0.4954 WVFGRD96 38.0 205 10 -65 4.02 0.4850 WVFGRD96 39.0 210 10 -60 4.02 0.4778 WVFGRD96 40.0 210 5 -55 4.17 0.4690 WVFGRD96 41.0 215 5 -50 4.18 0.4597 WVFGRD96 42.0 210 5 -55 4.18 0.4504 WVFGRD96 43.0 220 5 -45 4.18 0.4408 WVFGRD96 44.0 220 5 -45 4.19 0.4310 WVFGRD96 45.0 220 5 -45 4.19 0.4211 WVFGRD96 46.0 230 5 -35 4.20 0.4112 WVFGRD96 47.0 40 30 -40 4.17 0.4046 WVFGRD96 48.0 40 30 -40 4.18 0.4003 WVFGRD96 49.0 45 35 -40 4.18 0.3963
The best solution is
WVFGRD96 2.0 0 45 90 3.79 0.6500
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 0.06 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