The ANSS event ID is ak021eh9cns3 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak021eh9cns3/executive.
2021/11/11 18:58:50 59.639 -153.123 110.4 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2021/11/11 18:58:50:0 59.64 -153.12 110.4 4.0 Alaska Stations used: AK.CNP AK.N18K AK.N19K AK.O18K AK.O19K AK.Q19K AK.SKN AK.SLK AK.SSN AV.ACH AV.ILS AV.RED AV.SPCP AV.STLK II.KDAK 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 0.08 n 3 Best Fitting Double Couple Mo = 1.66e+22 dyne-cm Mw = 4.08 Z = 130 km Plane Strike Dip Rake NP1 35 80 20 NP2 301 70 169 Principal Axes: Axis Value Plunge Azimuth T 1.66e+22 21 260 N 0.00e+00 68 61 P -1.66e+22 7 167 Moment Tensor: (dyne-cm) Component Value Mxx -1.51e+22 Mxy 6.16e+21 Mxz 8.41e+20 Myy 1.31e+22 Myz -5.92e+21 Mzz 1.94e+21 -------------- ---------------------- --------------------------## --------------------------#### ---------------------------####### #######--------------------######### ##############-------------########### ###################--------############# ######################----############## ########################################## #########################----############# ### #################--------########### ### T ################-----------######### ## ###############--------------###### ##################-----------------##### ################-------------------### #############----------------------# ###########----------------------- #######----------------------- ####------------------------ -------------- ----- ---------- P - Global CMT Convention Moment Tensor: R T P 1.94e+21 8.41e+20 5.92e+21 8.41e+20 -1.51e+22 -6.16e+21 5.92e+21 -6.16e+21 1.31e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211111185850/index.html |
STK = 35 DIP = 80 RAKE = 20 MW = 4.08 HS = 130.0
The NDK file is 20211111185850.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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 60.0 30 70 0 3.91 0.5495 WVFGRD96 62.0 30 70 0 3.92 0.5578 WVFGRD96 64.0 30 70 0 3.93 0.5628 WVFGRD96 66.0 30 75 0 3.93 0.5688 WVFGRD96 68.0 30 75 0 3.94 0.5751 WVFGRD96 70.0 30 75 0 3.94 0.5815 WVFGRD96 72.0 30 75 5 3.94 0.5861 WVFGRD96 74.0 30 75 5 3.95 0.5923 WVFGRD96 76.0 35 75 15 3.96 0.5977 WVFGRD96 78.0 35 75 15 3.96 0.6058 WVFGRD96 80.0 35 75 15 3.97 0.6134 WVFGRD96 82.0 35 75 15 3.98 0.6206 WVFGRD96 84.0 35 75 15 3.98 0.6266 WVFGRD96 86.0 35 75 20 3.98 0.6336 WVFGRD96 88.0 35 75 20 3.99 0.6394 WVFGRD96 90.0 35 75 20 3.99 0.6459 WVFGRD96 92.0 35 75 20 4.00 0.6505 WVFGRD96 94.0 35 75 20 4.01 0.6556 WVFGRD96 96.0 35 75 20 4.01 0.6600 WVFGRD96 98.0 35 80 20 4.01 0.6637 WVFGRD96 100.0 35 80 20 4.02 0.6686 WVFGRD96 102.0 35 80 20 4.02 0.6720 WVFGRD96 104.0 35 80 20 4.03 0.6760 WVFGRD96 106.0 35 80 20 4.03 0.6785 WVFGRD96 108.0 35 80 20 4.04 0.6805 WVFGRD96 110.0 35 80 20 4.04 0.6841 WVFGRD96 112.0 35 80 20 4.05 0.6860 WVFGRD96 114.0 35 80 20 4.05 0.6868 WVFGRD96 116.0 35 80 20 4.05 0.6885 WVFGRD96 118.0 210 90 -20 4.04 0.6792 WVFGRD96 120.0 210 90 -20 4.05 0.6805 WVFGRD96 122.0 210 90 -20 4.05 0.6807 WVFGRD96 124.0 35 80 20 4.07 0.6908 WVFGRD96 126.0 210 90 -20 4.06 0.6832 WVFGRD96 128.0 210 90 -20 4.06 0.6838 WVFGRD96 130.0 35 80 20 4.08 0.6917 WVFGRD96 132.0 210 90 -20 4.07 0.6844 WVFGRD96 134.0 35 80 20 4.08 0.6892 WVFGRD96 136.0 210 90 -20 4.08 0.6840 WVFGRD96 138.0 35 80 20 4.09 0.6864
The best solution is
WVFGRD96 130.0 35 80 20 4.08 0.6917
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 0.08 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