The ANSS event ID is ak020a5v6uly and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020a5v6uly/executive.
2020/08/08 22:54:20 59.770 -153.525 142.1 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2020/08/08 22:54:20:0 59.77 -153.52 142.1 4.0 Alaska Stations used: AK.CAPN AK.CNP AK.GHO AK.HOM AK.L18K AK.L19K AK.L22K AK.M20K AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K AK.PWL AK.Q19K AK.RC01 AK.SKN AK.SLK AK.SWD AT.PMR AV.ACH AV.ILSW AV.SPU AV.STLK II.KDAK TA.M16K TA.M17K TA.M22K TA.N17K TA.O15K TA.O16K 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 = 2.11e+22 dyne-cm Mw = 4.15 Z = 158 km Plane Strike Dip Rake NP1 313 74 143 NP2 55 55 20 Principal Axes: Axis Value Plunge Azimuth T 2.11e+22 37 269 N 0.00e+00 50 113 P -2.11e+22 12 8 Moment Tensor: (dyne-cm) Component Value Mxx -1.98e+22 Mxy -2.37e+21 Mxz -4.51e+21 Myy 1.31e+22 Myz -1.07e+22 Mzz 6.79e+21 -------- --- ------------ P ------- --------------- ---------- #----------------------------- ########-------------------------- ############-----------------------# ###############---------------------## ###################-----------------#### #####################--------------##### ########################-----------####### ###### #################--------######## ###### T ###################-----######### ###### ####################--########### ############################--########## ##########################-----######### ######################---------####### ##################-------------##### ############-------------------### ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 6.79e+21 -4.51e+21 1.07e+22 -4.51e+21 -1.98e+22 2.37e+21 1.07e+22 2.37e+21 1.31e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200808225420/index.html |
STK = 55 DIP = 55 RAKE = 20 MW = 4.15 HS = 158.0
The NDK file is 20200808225420.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 90.0 60 45 25 4.06 0.4632 WVFGRD96 92.0 55 50 20 4.07 0.4807 WVFGRD96 94.0 55 50 20 4.08 0.4978 WVFGRD96 96.0 55 50 20 4.08 0.5114 WVFGRD96 98.0 55 50 20 4.08 0.5232 WVFGRD96 100.0 55 50 20 4.09 0.5325 WVFGRD96 102.0 55 50 20 4.09 0.5409 WVFGRD96 104.0 55 50 20 4.09 0.5503 WVFGRD96 106.0 55 50 20 4.09 0.5588 WVFGRD96 108.0 55 50 20 4.10 0.5654 WVFGRD96 110.0 55 50 20 4.10 0.5706 WVFGRD96 112.0 55 50 20 4.10 0.5750 WVFGRD96 114.0 55 50 20 4.10 0.5810 WVFGRD96 116.0 55 50 20 4.10 0.5867 WVFGRD96 118.0 55 50 20 4.11 0.5911 WVFGRD96 120.0 55 50 20 4.11 0.5931 WVFGRD96 122.0 55 50 20 4.11 0.5957 WVFGRD96 124.0 55 50 20 4.11 0.5996 WVFGRD96 126.0 55 55 20 4.12 0.6022 WVFGRD96 128.0 55 55 20 4.12 0.6026 WVFGRD96 130.0 55 55 20 4.12 0.6058 WVFGRD96 132.0 55 55 20 4.12 0.6072 WVFGRD96 134.0 55 55 20 4.12 0.6073 WVFGRD96 136.0 55 55 20 4.13 0.6099 WVFGRD96 138.0 55 55 20 4.13 0.6105 WVFGRD96 140.0 55 55 20 4.13 0.6100 WVFGRD96 142.0 55 55 20 4.13 0.6121 WVFGRD96 144.0 55 55 20 4.13 0.6122 WVFGRD96 146.0 55 55 20 4.13 0.6115 WVFGRD96 148.0 55 55 20 4.14 0.6132 WVFGRD96 150.0 55 55 20 4.14 0.6123 WVFGRD96 152.0 55 55 20 4.14 0.6138 WVFGRD96 154.0 55 55 20 4.14 0.6142 WVFGRD96 156.0 55 55 20 4.14 0.6129 WVFGRD96 158.0 55 55 20 4.15 0.6148 WVFGRD96 160.0 55 55 20 4.15 0.6134 WVFGRD96 162.0 55 55 20 4.15 0.6139 WVFGRD96 164.0 55 55 20 4.15 0.6128 WVFGRD96 166.0 55 55 20 4.15 0.6120 WVFGRD96 168.0 55 55 20 4.16 0.6119
The best solution is
WVFGRD96 158.0 55 55 20 4.15 0.6148
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