The ANSS event ID is ak022cjvjpsi and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak022cjvjpsi/executive.
2022/09/30 23:23:35 60.456 -152.764 13.1 4.1 Alaska
USGS/SLU Moment Tensor Solution ENS 2022/09/30 23:23:35:0 60.46 -152.76 13.1 4.1 Alaska Stations used: AK.BPAW AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.HOM AK.J19K AK.J20K AK.K20K AK.L17K AK.L19K AK.L20K AK.M16K AK.MCK AK.N15K AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K AK.PPLA AK.PWL AK.Q19K AK.R18K AK.RND AK.SKN AK.SSN AK.SWD AV.ACH AV.ILS AV.PLBL AV.PLK3 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 0.10 n 3 Best Fitting Double Couple Mo = 1.72e+22 dyne-cm Mw = 4.09 Z = 15 km Plane Strike Dip Rake NP1 50 85 -45 NP2 145 45 -173 Principal Axes: Axis Value Plunge Azimuth T 1.72e+22 26 106 N 0.00e+00 45 225 P -1.72e+22 34 357 Moment Tensor: (dyne-cm) Component Value Mxx -1.07e+22 Mxy -3.14e+21 Mxz -9.84e+21 Myy 1.28e+22 Myz 6.88e+21 Mzz -2.11e+21 -------------- ---------------------- #----------- ------------- ##----------- P -------------- ###------------ --------------## ####---------------------------##### #####-------------------------######## ######-----------------------########### ######---------------------############# ########------------------################ ########----------------################## #########-------------#################### ##########----------############### #### ##########------################## T ### ###########---#################### ### ###################################### #######-----######################## ###----------##################### --------------################ ----------------############ ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -2.11e+21 -9.84e+21 -6.88e+21 -9.84e+21 -1.07e+22 3.14e+21 -6.88e+21 3.14e+21 1.28e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220930232335/index.html |
STK = 50 DIP = 85 RAKE = -45 MW = 4.09 HS = 15.0
The NDK file is 20220930232335.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 1.0 30 45 -85 3.63 0.1657 WVFGRD96 2.0 30 45 -85 3.79 0.2286 WVFGRD96 3.0 250 65 -5 3.73 0.2077 WVFGRD96 4.0 245 80 45 3.79 0.2235 WVFGRD96 5.0 55 90 -55 3.83 0.2629 WVFGRD96 6.0 235 90 55 3.85 0.3035 WVFGRD96 7.0 240 85 50 3.88 0.3379 WVFGRD96 8.0 235 90 55 3.95 0.3640 WVFGRD96 9.0 245 75 50 3.98 0.3926 WVFGRD96 10.0 245 75 45 4.01 0.4152 WVFGRD96 11.0 240 80 45 4.02 0.4313 WVFGRD96 12.0 240 80 45 4.04 0.4423 WVFGRD96 13.0 235 85 45 4.06 0.4488 WVFGRD96 14.0 50 85 -45 4.07 0.4522 WVFGRD96 15.0 50 85 -45 4.09 0.4524 WVFGRD96 16.0 50 85 -45 4.10 0.4491 WVFGRD96 17.0 50 75 -45 4.11 0.4432 WVFGRD96 18.0 50 75 -45 4.12 0.4352 WVFGRD96 19.0 50 80 -50 4.13 0.4251 WVFGRD96 20.0 225 80 -50 4.15 0.4154 WVFGRD96 21.0 230 85 -50 4.16 0.4065 WVFGRD96 22.0 230 80 -50 4.17 0.3973 WVFGRD96 23.0 225 70 -55 4.18 0.3879 WVFGRD96 24.0 230 70 -60 4.18 0.3774 WVFGRD96 25.0 230 70 -60 4.19 0.3662 WVFGRD96 26.0 230 70 -60 4.19 0.3540 WVFGRD96 27.0 230 75 -60 4.20 0.3408 WVFGRD96 28.0 235 75 -60 4.20 0.3277 WVFGRD96 29.0 235 75 -60 4.21 0.3135
The best solution is
WVFGRD96 15.0 50 85 -45 4.09 0.4524
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