The ANSS event ID is ak02431au8tv and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02431au8tv/executive.
2024/03/06 09:02:05 62.232 -148.200 29.5 4.1 Alaska
USGS/SLU Moment Tensor Solution ENS 2024/03/06 09:02:05:0 62.23 -148.20 29.5 4.1 Alaska Stations used: AK.BAE AK.CUT AK.DIV AK.FID AK.GHO AK.GLI AK.HARP AK.KLU AK.KNK AK.L22K AK.MCK AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AT.PMR Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.60e+22 dyne-cm Mw = 4.21 Z = 58 km Plane Strike Dip Rake NP1 210 50 -80 NP2 15 41 -102 Principal Axes: Axis Value Plunge Azimuth T 2.60e+22 5 293 N 0.00e+00 8 24 P -2.60e+22 81 173 Moment Tensor: (dyne-cm) Component Value Mxx 3.31e+21 Mxy -9.19e+21 Mxz 4.74e+21 Myy 2.19e+22 Myz -2.40e+21 Mzz -2.52e+22 #############- ##################-### ################-------##### ##############-----------##### ##############--------------###### ############-----------------###### T ###########------------------####### #########--------------------######## ###########----------------------####### ###########-----------------------######## ##########------------------------######## ##########---------- ----------######### #########----------- P ----------######### ########----------- ---------######### ########-----------------------######### ######-----------------------######### #####----------------------######### #####-------------------########## ###------------------######### ###---------------########## ------------########## -----######### Global CMT Convention Moment Tensor: R T P -2.52e+22 4.74e+21 2.40e+21 4.74e+21 3.31e+21 9.19e+21 2.40e+21 9.19e+21 2.19e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240306090205/index.html |
STK = 210 DIP = 50 RAKE = -80 MW = 4.21 HS = 58.0
The NDK file is 20240306090205.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/3.5 -40 o DIST/3.5 +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 30 45 90 3.42 0.2118 WVFGRD96 4.0 185 35 40 3.46 0.2111 WVFGRD96 6.0 0 45 35 3.47 0.2277 WVFGRD96 8.0 5 45 45 3.58 0.2499 WVFGRD96 10.0 0 50 40 3.61 0.2647 WVFGRD96 12.0 0 65 -45 3.63 0.2663 WVFGRD96 14.0 5 70 -50 3.66 0.2657 WVFGRD96 16.0 70 90 -40 3.67 0.2672 WVFGRD96 18.0 230 50 -55 3.74 0.2873 WVFGRD96 20.0 230 50 -55 3.77 0.3091 WVFGRD96 22.0 230 50 -55 3.81 0.3224 WVFGRD96 24.0 65 65 -30 3.80 0.3293 WVFGRD96 26.0 65 65 -30 3.82 0.3345 WVFGRD96 28.0 65 70 -40 3.84 0.3390 WVFGRD96 30.0 65 65 -35 3.86 0.3543 WVFGRD96 32.0 60 60 -40 3.88 0.3845 WVFGRD96 34.0 60 60 -40 3.90 0.4096 WVFGRD96 36.0 45 50 -65 3.94 0.4327 WVFGRD96 38.0 45 50 -70 3.96 0.4560 WVFGRD96 40.0 40 50 -75 4.08 0.5063 WVFGRD96 42.0 205 40 -95 4.12 0.5148 WVFGRD96 44.0 205 40 -95 4.14 0.5133 WVFGRD96 46.0 30 45 -85 4.15 0.5121 WVFGRD96 48.0 210 45 -85 4.17 0.5177 WVFGRD96 50.0 210 50 -85 4.18 0.5291 WVFGRD96 52.0 210 50 -85 4.19 0.5382 WVFGRD96 54.0 210 50 -80 4.21 0.5434 WVFGRD96 56.0 210 50 -80 4.21 0.5462 WVFGRD96 58.0 210 50 -80 4.21 0.5468 WVFGRD96 60.0 210 50 -80 4.21 0.5447 WVFGRD96 62.0 210 50 -80 4.21 0.5411 WVFGRD96 64.0 210 50 -80 4.21 0.5369 WVFGRD96 66.0 215 55 -70 4.23 0.5372 WVFGRD96 68.0 215 55 -70 4.23 0.5370 WVFGRD96 70.0 215 55 -70 4.23 0.5345 WVFGRD96 72.0 215 55 -70 4.22 0.5308 WVFGRD96 74.0 215 55 -70 4.22 0.5270 WVFGRD96 76.0 220 60 -60 4.25 0.5229 WVFGRD96 78.0 220 60 -60 4.24 0.5196 WVFGRD96 80.0 220 60 -60 4.24 0.5168 WVFGRD96 82.0 220 60 -60 4.24 0.5134 WVFGRD96 84.0 220 65 -55 4.26 0.5097 WVFGRD96 86.0 220 65 -55 4.26 0.5065 WVFGRD96 88.0 220 65 -55 4.26 0.5029 WVFGRD96 90.0 220 65 -55 4.26 0.5002 WVFGRD96 92.0 220 65 -55 4.26 0.4963 WVFGRD96 94.0 220 65 -55 4.26 0.4926 WVFGRD96 96.0 220 70 -55 4.27 0.4897 WVFGRD96 98.0 220 70 -55 4.27 0.4871
The best solution is
WVFGRD96 58.0 210 50 -80 4.21 0.5468
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/3.5 -40 o DIST/3.5 +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