The moment tensor inversion using the ANSS location was significantly offset from the location of other events in the region. P- and S-wave first arrivals and p-wave first motions were manually read, and the event was located using the WUs model woth the program elocate. The results are in the file elocate.txt. The manual location is about 30 km to the SSW of the posted ANSS solution. Interestingly, there is a much better waveforms fit and the azimuthal plot indicates that the location is good.
The SLU relocation is used to document this event. The ANSS solution is provided for reference. Note the companion first motion plot shown below is not very informative because of the paucity of data.
2025/03/17 05:25:58 69.505 -144.608 4.1 4.1 Alaska
The ANSS event ID is ak0253hrz0l8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0253hrz0l8/executive.
2025/03/17 05:25:54 69.858 -143.840 14.9 4.1 Alaska
USGS/SLU Moment Tensor Solution ENS 2025/03/17 05:25:58:0 69.50 -144.61 4.1 4.1 Alaska Stations used: AK.C26K AK.C27K AK.COLD AK.D25K AK.E24K AK.E25K AK.E27K AK.FYU AK.G23K 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 0.07 n 3 Best Fitting Double Couple Mo = 8.61e+21 dyne-cm Mw = 3.89 Z = 10 km Plane Strike Dip Rake NP1 190 80 20 NP2 96 70 169 Principal Axes: Axis Value Plunge Azimuth T 8.61e+21 21 55 N 0.00e+00 68 216 P -8.61e+21 7 322 Moment Tensor: (dyne-cm) Component Value Mxx -2.76e+21 Mxy 7.66e+21 Mxz 9.03e+20 Myy 1.75e+21 Myz 2.97e+21 Mzz 1.01e+21 ----------#### ------------######### - P -----------############# -- -----------############## -----------------############ ## ------------------############ T ### ------------------############# #### -------------------##################### ------------------###################### -------------------####################### ##-----------------####################### #####-------------######################## ##########--------#######################- ##################################------ ################------------------------ ###############----------------------- ##############---------------------- #############--------------------- ###########------------------- ##########------------------ #######--------------- ###----------- Global CMT Convention Moment Tensor: R T P 1.01e+21 9.03e+20 -2.97e+21 9.03e+20 -2.76e+21 -7.66e+21 -2.97e+21 -7.66e+21 1.75e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250317052558/index.html |
STK = 190 DIP = 80 RAKE = 20 MW = 3.89 HS = 10.0
The NDK file is 20250317052558.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 2025/03/17 05:25:58:0 69.50 -144.61 4.1 4.1 Alaska Stations used: AK.C26K AK.C27K AK.COLD AK.D25K AK.E24K AK.E25K AK.E27K AK.FYU AK.G23K 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 0.07 n 3 Best Fitting Double Couple Mo = 8.61e+21 dyne-cm Mw = 3.89 Z = 10 km Plane Strike Dip Rake NP1 190 80 20 NP2 96 70 169 Principal Axes: Axis Value Plunge Azimuth T 8.61e+21 21 55 N 0.00e+00 68 216 P -8.61e+21 7 322 Moment Tensor: (dyne-cm) Component Value Mxx -2.76e+21 Mxy 7.66e+21 Mxz 9.03e+20 Myy 1.75e+21 Myz 2.97e+21 Mzz 1.01e+21 ----------#### ------------######### - P -----------############# -- -----------############## -----------------############ ## ------------------############ T ### ------------------############# #### -------------------##################### ------------------###################### -------------------####################### ##-----------------####################### #####-------------######################## ##########--------#######################- ##################################------ ################------------------------ ###############----------------------- ##############---------------------- #############--------------------- ###########------------------- ##########------------------ #######--------------- ###----------- Global CMT Convention Moment Tensor: R T P 1.01e+21 9.03e+20 -2.97e+21 9.03e+20 -2.76e+21 -7.66e+21 -2.97e+21 -7.66e+21 1.75e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250317052558/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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 10 85 0 3.51 0.3975 WVFGRD96 2.0 10 75 10 3.63 0.5059 WVFGRD96 3.0 10 70 15 3.69 0.5460 WVFGRD96 4.0 190 75 10 3.73 0.5842 WVFGRD96 5.0 190 75 15 3.76 0.6297 WVFGRD96 6.0 190 80 15 3.80 0.6695 WVFGRD96 7.0 190 80 15 3.82 0.7032 WVFGRD96 8.0 190 80 20 3.86 0.7289 WVFGRD96 9.0 190 80 20 3.88 0.7443 WVFGRD96 10.0 190 80 20 3.89 0.7497 WVFGRD96 11.0 190 85 20 3.91 0.7495 WVFGRD96 12.0 190 85 20 3.92 0.7453 WVFGRD96 13.0 190 85 15 3.93 0.7363 WVFGRD96 14.0 10 90 -20 3.94 0.7195 WVFGRD96 15.0 190 80 15 3.94 0.7145 WVFGRD96 16.0 190 80 15 3.95 0.7041 WVFGRD96 17.0 190 80 15 3.95 0.6918 WVFGRD96 18.0 190 90 15 3.96 0.6794 WVFGRD96 19.0 190 90 15 3.97 0.6666 WVFGRD96 20.0 190 90 15 3.98 0.6537 WVFGRD96 21.0 190 90 15 3.98 0.6414 WVFGRD96 22.0 190 90 15 3.99 0.6286 WVFGRD96 23.0 190 90 15 3.99 0.6159 WVFGRD96 24.0 190 85 15 3.99 0.6056 WVFGRD96 25.0 190 85 15 4.00 0.5958 WVFGRD96 26.0 10 90 -10 4.00 0.5857 WVFGRD96 27.0 190 85 10 4.01 0.5797 WVFGRD96 28.0 190 85 10 4.01 0.5729 WVFGRD96 29.0 190 85 10 4.02 0.5659
The best solution is
WVFGRD96 10.0 190 80 20 3.89 0.7497
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 0.07 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