The ANSS event ID is ak0209vq2cuz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0209vq2cuz/executive.
2020/08/02 09:27:15 63.368 -155.375 5.7 3.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2020/08/02 09:27:15:0 63.37 -155.38 5.7 3.7 Alaska Stations used: AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.H21K AK.H22K AK.HDA AK.I21K AK.I23K AK.J17K AK.J19K AK.J20K AK.K20K AK.KNK AK.KTH AK.L18K AK.L19K AK.L22K AK.M20K AK.MCK AK.MLY AK.N18K AK.N19K AK.NEA2 AK.O18K AK.O19K AK.PPLA AK.SKN AK.TRF AK.WRH AV.ILSW AV.SPU IU.COLA TA.F19K TA.F20K TA.F21K TA.G16K TA.G19K TA.H19K TA.I17K TA.I20K TA.J16K TA.L14K TA.L16K TA.M16K TA.M17K TA.N15K TA.N17K 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 = 2.48e+21 dyne-cm Mw = 3.53 Z = 13 km Plane Strike Dip Rake NP1 155 80 30 NP2 59 61 168 Principal Axes: Axis Value Plunge Azimuth T 2.48e+21 28 21 N 0.00e+00 59 172 P -2.48e+21 13 284 Moment Tensor: (dyne-cm) Component Value Mxx 1.55e+21 Mxy 1.20e+21 Mxz 8.32e+20 Myy -1.97e+21 Myz 9.00e+20 Mzz 4.25e+20 ############## --#################### ------############ ####### -------############ T ######## ----------########### ########## -----------######################### -------------#######################-- - ----------######################---- - P -----------####################----- -- ------------##################------- ------------------################-------- -------------------#############---------- -------------------###########------------ -------------------########------------- --------------------####---------------- -------------------#------------------ -------------#######---------------- #####################------------- ####################---------- ####################-------- ###################--- ############## Global CMT Convention Moment Tensor: R T P 4.25e+20 8.32e+20 -9.00e+20 8.32e+20 1.55e+21 -1.20e+21 -9.00e+20 -1.20e+21 -1.97e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200802092715/index.html |
STK = 155 DIP = 80 RAKE = 30 MW = 3.53 HS = 13.0
The NDK file is 20200802092715.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 1.0 150 90 0 3.04 0.3586 WVFGRD96 2.0 330 90 0 3.17 0.4387 WVFGRD96 3.0 155 80 30 3.25 0.4387 WVFGRD96 4.0 150 90 -45 3.31 0.4734 WVFGRD96 5.0 150 90 -45 3.33 0.5132 WVFGRD96 6.0 330 90 40 3.35 0.5450 WVFGRD96 7.0 155 80 35 3.38 0.5769 WVFGRD96 8.0 330 90 -40 3.44 0.5983 WVFGRD96 9.0 330 90 -35 3.46 0.6186 WVFGRD96 10.0 155 80 35 3.48 0.6361 WVFGRD96 11.0 155 80 30 3.50 0.6467 WVFGRD96 12.0 155 80 30 3.51 0.6526 WVFGRD96 13.0 155 80 30 3.53 0.6533 WVFGRD96 14.0 155 75 30 3.54 0.6493 WVFGRD96 15.0 155 75 30 3.56 0.6415 WVFGRD96 16.0 155 75 25 3.57 0.6314 WVFGRD96 17.0 155 75 25 3.58 0.6186 WVFGRD96 18.0 155 75 25 3.59 0.6032 WVFGRD96 19.0 155 75 30 3.60 0.5867 WVFGRD96 20.0 155 75 30 3.61 0.5689 WVFGRD96 21.0 155 75 30 3.62 0.5502 WVFGRD96 22.0 155 75 30 3.62 0.5306 WVFGRD96 23.0 155 75 30 3.63 0.5104 WVFGRD96 24.0 155 75 30 3.64 0.4902 WVFGRD96 25.0 155 75 30 3.64 0.4690 WVFGRD96 26.0 155 75 30 3.64 0.4485 WVFGRD96 27.0 155 80 35 3.65 0.4274 WVFGRD96 28.0 155 80 35 3.65 0.4075 WVFGRD96 29.0 155 80 35 3.65 0.3870
The best solution is
WVFGRD96 13.0 155 80 30 3.53 0.6533
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