The ANSS event ID is ak020er5qnyf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020er5qnyf/executive.
2020/11/16 17:18:16 59.725 -138.923 2.3 4.2 Alaska
USGS/SLU Moment Tensor Solution ENS 2020/11/16 17:18:16:0 59.72 -138.92 2.3 4.2 Alaska Stations used: AK.BARN AK.BESE AK.BMR AK.CRQ AK.DIV AK.GLB AK.GRNC AK.KLU AK.L26K AK.LOGN AK.M26K AK.M27K AK.MCAR AK.PIN AK.R32K AK.S31K AK.TGL AK.VRDI AT.MENT AT.SKAG CN.HYT CN.WHY TA.L27K TA.L29M TA.M29M TA.M30M TA.M31M TA.N30M TA.N31M TA.O28M TA.O29M TA.O30N TA.P29M TA.P32M TA.P33M 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 = 2.19e+22 dyne-cm Mw = 4.16 Z = 5 km Plane Strike Dip Rake NP1 100 60 90 NP2 280 30 90 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 75 10 N 0.00e+00 -0 100 P -2.19e+22 15 190 Moment Tensor: (dyne-cm) Component Value Mxx -1.84e+22 Mxy -3.24e+21 Mxz 1.08e+22 Myy -5.71e+20 Myz 1.90e+21 Mzz 1.89e+22 -------------- ---------------------- ---------------------------- -------#############---------- -----######################------- ---###########################------ --################################---- -###################################---- ################### ################-- -################### T #################-- ---################# #################-- -----####################################- -------##################################- ---------##############################- --------------#####################----- -------------------------------------- ------------------------------------ ---------------------------------- ------------------------------ --------- ---------------- ------ P ------------- -- --------- Global CMT Convention Moment Tensor: R T P 1.89e+22 1.08e+22 -1.90e+21 1.08e+22 -1.84e+22 3.24e+21 -1.90e+21 3.24e+21 -5.71e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201116171816/index.html |
STK = 100 DIP = 60 RAKE = 90 MW = 4.16 HS = 5.0
The NDK file is 20201116171816.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 65 80 60 4.12 0.5623 WVFGRD96 2.0 70 75 55 4.10 0.5803 WVFGRD96 3.0 90 65 80 4.14 0.6000 WVFGRD96 4.0 100 60 90 4.17 0.6527 WVFGRD96 5.0 100 60 90 4.16 0.6743 WVFGRD96 6.0 280 35 90 4.16 0.6667 WVFGRD96 7.0 280 35 90 4.14 0.6402 WVFGRD96 8.0 280 35 90 4.13 0.6044 WVFGRD96 9.0 230 70 -30 4.08 0.5881 WVFGRD96 10.0 230 70 -30 4.10 0.5803 WVFGRD96 11.0 230 70 -30 4.11 0.5754 WVFGRD96 12.0 230 70 -30 4.11 0.5678 WVFGRD96 13.0 230 70 -30 4.11 0.5582 WVFGRD96 14.0 230 70 -30 4.12 0.5472 WVFGRD96 15.0 230 70 -30 4.12 0.5351 WVFGRD96 16.0 230 65 -25 4.13 0.5231 WVFGRD96 17.0 230 65 -25 4.14 0.5116 WVFGRD96 18.0 230 65 -25 4.14 0.4993 WVFGRD96 19.0 50 65 -25 4.15 0.4898 WVFGRD96 20.0 45 60 -25 4.17 0.4758 WVFGRD96 21.0 230 70 -30 4.17 0.4613 WVFGRD96 22.0 225 65 -30 4.17 0.4493 WVFGRD96 23.0 50 65 -20 4.19 0.4399 WVFGRD96 24.0 50 65 -20 4.19 0.4293 WVFGRD96 25.0 60 55 15 4.20 0.4224 WVFGRD96 26.0 60 55 15 4.21 0.4156 WVFGRD96 27.0 60 55 15 4.21 0.4090 WVFGRD96 28.0 60 55 15 4.22 0.4020 WVFGRD96 29.0 60 60 15 4.23 0.3952
The best solution is
WVFGRD96 5.0 100 60 90 4.16 0.6743
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