The ANSS event ID is us6000i1v7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us6000i1v7/executive.
2022/07/12 16:09:45 54.449 -161.483 35.0 3.8 Alaska
USGS/SLU Moment Tensor Solution ENS 2022/07/12 16:09:45:0 54.45 -161.48 35.0 3.8 Alaska Stations used: AK.CHN AK.FALS AT.SDPT AV.PS4A AV.S14K AV.WESP 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 = 5.13e+21 dyne-cm Mw = 3.74 Z = 29 km Plane Strike Dip Rake NP1 72 58 116 NP2 210 40 55 Principal Axes: Axis Value Plunge Azimuth T 5.13e+21 66 32 N 0.00e+00 22 238 P -5.13e+21 10 144 Moment Tensor: (dyne-cm) Component Value Mxx -2.67e+21 Mxy 2.74e+21 Mxz 2.32e+21 Myy -1.47e+21 Myz 4.95e+20 Mzz 4.14e+21 -------------- ---------------####### -------------############### -----------################### -----------####################### ----------########################## ----------############################ ---------############ ################ --------############# T ###############- ---------############# ##############--- --------#############################----- -------#############################------ -------##########################--------- ------#######################----------- #-----####################-------------- ####-##############------------------- ####-------------------------------- ####------------------------------ ##---------------------- --- ##--------------------- P -- -------------------- -------------- Global CMT Convention Moment Tensor: R T P 4.14e+21 2.32e+21 -4.95e+20 2.32e+21 -2.67e+21 -2.74e+21 -4.95e+20 -2.74e+21 -1.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220712160945/index.html |
STK = 210 DIP = 40 RAKE = 55 MW = 3.74 HS = 29.0
The NDK file is 20220712160945.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 1.0 85 45 -90 3.18 0.2738 WVFGRD96 2.0 85 45 -90 3.30 0.3489 WVFGRD96 3.0 85 45 -95 3.35 0.3107 WVFGRD96 4.0 305 65 -45 3.28 0.2244 WVFGRD96 5.0 140 30 -25 3.28 0.2151 WVFGRD96 6.0 145 30 -15 3.29 0.2421 WVFGRD96 7.0 150 30 -15 3.30 0.2693 WVFGRD96 8.0 145 25 -20 3.38 0.2953 WVFGRD96 9.0 150 30 -10 3.39 0.3258 WVFGRD96 10.0 155 30 -5 3.41 0.3546 WVFGRD96 11.0 155 30 -5 3.43 0.3822 WVFGRD96 12.0 180 35 10 3.46 0.4092 WVFGRD96 13.0 185 35 20 3.48 0.4374 WVFGRD96 14.0 190 35 30 3.50 0.4644 WVFGRD96 15.0 190 40 30 3.53 0.4923 WVFGRD96 16.0 195 40 35 3.55 0.5198 WVFGRD96 17.0 200 45 50 3.58 0.5483 WVFGRD96 18.0 200 45 45 3.60 0.5769 WVFGRD96 19.0 200 45 45 3.61 0.6043 WVFGRD96 20.0 205 45 55 3.63 0.6304 WVFGRD96 21.0 205 45 50 3.65 0.6530 WVFGRD96 22.0 205 45 50 3.66 0.6767 WVFGRD96 23.0 205 45 50 3.68 0.6983 WVFGRD96 24.0 205 45 50 3.69 0.7172 WVFGRD96 25.0 210 40 55 3.70 0.7345 WVFGRD96 26.0 210 40 55 3.71 0.7494 WVFGRD96 27.0 210 40 55 3.72 0.7609 WVFGRD96 28.0 210 40 55 3.73 0.7680 WVFGRD96 29.0 210 40 55 3.74 0.7700 WVFGRD96 30.0 210 40 55 3.75 0.7663 WVFGRD96 31.0 215 35 60 3.76 0.7589 WVFGRD96 32.0 210 35 55 3.76 0.7482 WVFGRD96 33.0 210 35 50 3.77 0.7332 WVFGRD96 34.0 210 35 50 3.78 0.7146 WVFGRD96 35.0 210 35 50 3.78 0.6936 WVFGRD96 36.0 205 35 45 3.79 0.6712 WVFGRD96 37.0 205 35 45 3.79 0.6486 WVFGRD96 38.0 205 35 45 3.79 0.6274 WVFGRD96 39.0 205 35 50 3.80 0.6071 WVFGRD96 40.0 210 30 50 3.90 0.5790 WVFGRD96 41.0 210 30 50 3.91 0.5593 WVFGRD96 42.0 210 30 55 3.91 0.5390 WVFGRD96 43.0 210 30 55 3.91 0.5202 WVFGRD96 44.0 205 30 50 3.92 0.5021 WVFGRD96 45.0 205 30 50 3.92 0.4860 WVFGRD96 46.0 205 30 50 3.92 0.4704 WVFGRD96 47.0 205 30 50 3.92 0.4564 WVFGRD96 48.0 205 30 50 3.93 0.4432 WVFGRD96 49.0 200 30 45 3.93 0.4312
The best solution is
WVFGRD96 29.0 210 40 55 3.74 0.7700
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