The ANSS event ID is ak0233l8vwxa and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0233l8vwxa/executive.
2023/03/19 15:06:27 59.609 -151.907 65.4 5.4 Alaska
USGS/SLU Moment Tensor Solution ENS 2023/03/19 15:06:27:0 59.61 -151.91 65.4 5.4 Alaska Stations used: AK.BRLK AK.CAPN AK.DIV AK.FIRE AK.GHO AK.KLU AK.KNK AK.L20K AK.N19K AK.O18K AK.O19K AK.PWL AK.Q19K AK.RC01 AK.SAW AK.SLK AK.SWD AT.PMR AV.ACH AV.PLK3 AV.RED AV.STLK II.KDAK Filtering commands used: cut o DIST/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 1.53e+24 dyne-cm Mw = 5.39 Z = 70 km Plane Strike Dip Rake NP1 45 75 40 NP2 303 52 161 Principal Axes: Axis Value Plunge Azimuth T 1.53e+24 38 271 N 0.00e+00 48 62 P -1.53e+24 15 169 Moment Tensor: (dyne-cm) Component Value Mxx -1.38e+24 Mxy 2.46e+23 Mxz 3.88e+23 Myy 8.87e+23 Myz -8.17e+23 Mzz 4.92e+23 -------------- ---------------------- ---------------------------- -----------------------------# --###########------------------### ####################-----------##### ########################-------####### ############################--########## #############################-########## ####### ###################----######### ####### T #################-------######## ####### ###############----------####### ########################------------###### #####################---------------#### ###################------------------### ###############---------------------## ############-----------------------# ########-------------------------- ###--------------------------- ---------------- --------- ------------- P ------ --------- -- Global CMT Convention Moment Tensor: R T P 4.92e+23 3.88e+23 8.17e+23 3.88e+23 -1.38e+24 -2.46e+23 8.17e+23 -2.46e+23 8.87e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230319150627/index.html |
STK = 45 DIP = 75 RAKE = 40 MW = 5.39 HS = 70.0
The NDK file is 20230319150627.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/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 120 45 -35 4.63 0.1950 WVFGRD96 4.0 135 75 20 4.62 0.2099 WVFGRD96 6.0 135 70 20 4.67 0.2254 WVFGRD96 8.0 35 65 -30 4.75 0.2446 WVFGRD96 10.0 35 65 -25 4.77 0.2594 WVFGRD96 12.0 35 65 -25 4.80 0.2684 WVFGRD96 14.0 40 75 -25 4.82 0.2766 WVFGRD96 16.0 40 75 -25 4.85 0.2850 WVFGRD96 18.0 40 75 -20 4.87 0.2925 WVFGRD96 20.0 40 75 -20 4.89 0.2996 WVFGRD96 22.0 40 75 -20 4.91 0.3056 WVFGRD96 24.0 220 75 -10 4.93 0.3112 WVFGRD96 26.0 220 75 -10 4.95 0.3169 WVFGRD96 28.0 225 80 15 4.97 0.3243 WVFGRD96 30.0 40 75 -15 5.00 0.3296 WVFGRD96 32.0 40 75 -10 5.02 0.3358 WVFGRD96 34.0 220 80 -10 5.04 0.3400 WVFGRD96 36.0 40 80 15 5.08 0.3506 WVFGRD96 38.0 40 85 15 5.11 0.3621 WVFGRD96 40.0 45 70 30 5.18 0.3803 WVFGRD96 42.0 45 75 30 5.20 0.3940 WVFGRD96 44.0 45 70 30 5.23 0.4070 WVFGRD96 46.0 45 70 30 5.25 0.4197 WVFGRD96 48.0 45 75 30 5.26 0.4315 WVFGRD96 50.0 45 75 35 5.29 0.4427 WVFGRD96 52.0 45 75 35 5.30 0.4541 WVFGRD96 54.0 45 75 35 5.32 0.4650 WVFGRD96 56.0 45 75 35 5.33 0.4746 WVFGRD96 58.0 45 75 35 5.34 0.4826 WVFGRD96 60.0 45 75 35 5.35 0.4894 WVFGRD96 62.0 45 75 35 5.36 0.4947 WVFGRD96 64.0 45 75 35 5.36 0.4990 WVFGRD96 66.0 45 75 35 5.37 0.5027 WVFGRD96 68.0 45 75 35 5.38 0.5044 WVFGRD96 70.0 45 75 40 5.39 0.5054 WVFGRD96 72.0 45 75 35 5.38 0.5053 WVFGRD96 74.0 45 75 35 5.39 0.5039 WVFGRD96 76.0 45 75 35 5.39 0.5023 WVFGRD96 78.0 45 75 35 5.39 0.5000 WVFGRD96 80.0 45 75 35 5.39 0.4967 WVFGRD96 82.0 45 75 35 5.39 0.4934 WVFGRD96 84.0 45 75 35 5.39 0.4895 WVFGRD96 86.0 45 75 35 5.40 0.4860 WVFGRD96 88.0 45 75 35 5.40 0.4819 WVFGRD96 90.0 45 75 30 5.39 0.4773 WVFGRD96 92.0 45 75 30 5.39 0.4734 WVFGRD96 94.0 45 75 30 5.39 0.4696 WVFGRD96 96.0 45 80 30 5.39 0.4663 WVFGRD96 98.0 45 80 30 5.39 0.4623
The best solution is
WVFGRD96 70.0 45 75 40 5.39 0.5054
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/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 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