The ANSS event ID is ak0218d15v51 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0218d15v51/executive.
2021/07/01 02:27:22 60.584 -151.775 77.5 4.3 Alaska
USGS/SLU Moment Tensor Solution ENS 2021/07/01 02:27:22:0 60.58 -151.77 77.5 4.3 Alaska Stations used: AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KLU AK.KNK AK.L18K AK.L19K AK.L20K AK.M19K AK.M20K AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K AK.PWL AK.RC01 AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILS AV.RED AV.SPCP AV.STLK 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 0.10 n 3 Best Fitting Double Couple Mo = 6.38e+22 dyne-cm Mw = 4.47 Z = 82 km Plane Strike Dip Rake NP1 320 76 159 NP2 55 70 15 Principal Axes: Axis Value Plunge Azimuth T 6.38e+22 24 276 N 0.00e+00 65 107 P -6.38e+22 4 8 Moment Tensor: (dyne-cm) Component Value Mxx -6.16e+22 Mxy -1.48e+22 Mxz -1.73e+21 Myy 5.09e+22 Myz -2.45e+22 Mzz 1.06e+22 -------- P --- ------------ ------- #--------------------------- #####------------------------- ##########------------------------ #############---------------------## ################------------------#### ###################---------------###### ### ###############------------####### #### T ################---------########## #### ##################-----############ ##########################--############## ##########################--############## ######################------############ ###################----------########### ###############--------------######### ##########-------------------####### -----------------------------##### ----------------------------## ---------------------------# ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.06e+22 -1.73e+21 2.45e+22 -1.73e+21 -6.16e+22 1.48e+22 2.45e+22 1.48e+22 5.09e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210701022722/index.html |
STK = 55 DIP = 70 RAKE = 15 MW = 4.47 HS = 82.0
The NDK file is 20210701022722.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 320 80 20 3.56 0.1969 WVFGRD96 4.0 135 85 -20 3.67 0.2383 WVFGRD96 6.0 135 80 -20 3.74 0.2596 WVFGRD96 8.0 135 80 -20 3.82 0.2742 WVFGRD96 10.0 140 80 -20 3.85 0.2720 WVFGRD96 12.0 140 75 -20 3.89 0.2637 WVFGRD96 14.0 140 75 -20 3.91 0.2506 WVFGRD96 16.0 240 70 15 3.93 0.2435 WVFGRD96 18.0 240 70 15 3.96 0.2506 WVFGRD96 20.0 240 70 20 3.99 0.2610 WVFGRD96 22.0 240 70 20 4.02 0.2762 WVFGRD96 24.0 240 75 25 4.05 0.2934 WVFGRD96 26.0 240 75 25 4.08 0.3105 WVFGRD96 28.0 235 80 25 4.10 0.3250 WVFGRD96 30.0 235 80 20 4.11 0.3385 WVFGRD96 32.0 235 80 20 4.13 0.3518 WVFGRD96 34.0 235 80 15 4.14 0.3601 WVFGRD96 36.0 235 80 15 4.16 0.3675 WVFGRD96 38.0 235 80 10 4.19 0.3767 WVFGRD96 40.0 55 75 0 4.24 0.3886 WVFGRD96 42.0 50 75 -5 4.28 0.3942 WVFGRD96 44.0 50 75 0 4.30 0.3971 WVFGRD96 46.0 50 75 0 4.32 0.4031 WVFGRD96 48.0 50 75 0 4.34 0.4114 WVFGRD96 50.0 50 70 5 4.36 0.4213 WVFGRD96 52.0 50 70 5 4.37 0.4329 WVFGRD96 54.0 55 70 10 4.38 0.4454 WVFGRD96 56.0 55 70 10 4.39 0.4581 WVFGRD96 58.0 55 70 10 4.40 0.4687 WVFGRD96 60.0 55 70 10 4.41 0.4802 WVFGRD96 62.0 55 70 10 4.42 0.4885 WVFGRD96 64.0 55 70 10 4.43 0.4977 WVFGRD96 66.0 55 70 15 4.44 0.5035 WVFGRD96 68.0 55 70 15 4.44 0.5105 WVFGRD96 70.0 55 70 15 4.45 0.5162 WVFGRD96 72.0 55 70 15 4.45 0.5202 WVFGRD96 74.0 55 70 15 4.46 0.5232 WVFGRD96 76.0 55 70 15 4.46 0.5267 WVFGRD96 78.0 55 70 15 4.47 0.5292 WVFGRD96 80.0 55 70 15 4.47 0.5297 WVFGRD96 82.0 55 70 15 4.47 0.5299 WVFGRD96 84.0 55 70 15 4.48 0.5296 WVFGRD96 86.0 55 70 15 4.48 0.5295 WVFGRD96 88.0 55 70 15 4.48 0.5284 WVFGRD96 90.0 55 70 15 4.49 0.5279 WVFGRD96 92.0 60 65 15 4.48 0.5263 WVFGRD96 94.0 60 65 15 4.49 0.5250 WVFGRD96 96.0 60 70 15 4.49 0.5226 WVFGRD96 98.0 60 70 15 4.49 0.5203 WVFGRD96 100.0 60 70 15 4.49 0.5181 WVFGRD96 102.0 60 70 15 4.49 0.5156 WVFGRD96 104.0 60 70 15 4.50 0.5140 WVFGRD96 106.0 60 70 15 4.50 0.5116 WVFGRD96 108.0 60 70 15 4.50 0.5102
The best solution is
WVFGRD96 82.0 55 70 15 4.47 0.5299
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 0.10 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