The ANSS event ID is ak018em0mbpl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018em0mbpl/executive.
2018/11/14 06:25:02 64.774 -150.917 18.0 3.8 Alaska
USGS/SLU Moment Tensor Solution ENS 2018/11/14 06:25:02:0 64.77 -150.92 18.0 3.8 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.DHY AK.GHO AK.HDA AK.KTH AK.MCK AK.MLY AK.NEA2 AK.RND AK.SCRK AK.SKN AK.TRF AK.WRH AT.MENT IM.IL31 IU.COLA TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K TA.F19K TA.F20K TA.F21K TA.F24K TA.F26K TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H23K TA.H24K TA.I21K TA.I23K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L TA.K17K TA.K20K TA.K27K TA.L18K TA.L19K TA.M19K TA.M20K TA.M24K TA.POKR TA.TOLK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 4.47e+21 dyne-cm Mw = 3.70 Z = 12 km Plane Strike Dip Rake NP1 35 80 15 NP2 302 75 170 Principal Axes: Axis Value Plunge Azimuth T 4.47e+21 18 259 N 0.00e+00 72 68 P -4.47e+21 3 168 Moment Tensor: (dyne-cm) Component Value Mxx -4.12e+21 Mxy 1.64e+21 Mxz 9.39e+18 Myy 3.73e+21 Myz -1.32e+21 Mzz 3.95e+20 -------------- ---------------------- --------------------------## --------------------------#### ---------------------------####### #####----------------------######### ###########----------------########### ################-----------############# ####################------############## ########################--################ #########################-################ ## ##################------############# ## T #################---------########### # ################------------######## ##################----------------###### ################-------------------### #############----------------------# ###########----------------------- #######----------------------- ####------------------------ -------------- ----- ---------- P - Global CMT Convention Moment Tensor: R T P 3.95e+20 9.39e+18 1.32e+21 9.39e+18 -4.12e+21 -1.64e+21 1.32e+21 -1.64e+21 3.73e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181114062502/index.html |
STK = 35 DIP = 80 RAKE = 15 MW = 3.70 HS = 12.0
The NDK file is 20181114062502.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 35 90 0 3.24 0.3343 WVFGRD96 2.0 35 90 5 3.43 0.5723 WVFGRD96 3.0 215 90 -5 3.49 0.6548 WVFGRD96 4.0 215 90 -10 3.53 0.6963 WVFGRD96 5.0 215 90 -25 3.57 0.7218 WVFGRD96 6.0 35 85 25 3.59 0.7479 WVFGRD96 7.0 35 80 20 3.61 0.7692 WVFGRD96 8.0 35 80 25 3.65 0.7856 WVFGRD96 9.0 35 80 20 3.66 0.7971 WVFGRD96 10.0 35 80 20 3.68 0.8064 WVFGRD96 11.0 35 80 20 3.69 0.8130 WVFGRD96 12.0 35 80 15 3.70 0.8182 WVFGRD96 13.0 215 90 -15 3.71 0.8154 WVFGRD96 14.0 215 90 -15 3.72 0.8172 WVFGRD96 15.0 215 90 -15 3.74 0.8164 WVFGRD96 16.0 215 90 -10 3.75 0.8151 WVFGRD96 17.0 215 90 -10 3.76 0.8113 WVFGRD96 18.0 215 90 -10 3.77 0.8062 WVFGRD96 19.0 35 85 10 3.78 0.8017 WVFGRD96 20.0 35 85 15 3.79 0.7933 WVFGRD96 21.0 35 85 15 3.80 0.7835 WVFGRD96 22.0 215 90 -10 3.81 0.7709 WVFGRD96 23.0 35 85 15 3.81 0.7599 WVFGRD96 24.0 35 85 15 3.82 0.7467 WVFGRD96 25.0 35 85 15 3.83 0.7321 WVFGRD96 26.0 35 85 15 3.83 0.7173 WVFGRD96 27.0 215 90 -15 3.84 0.7023 WVFGRD96 28.0 215 90 -15 3.84 0.6874 WVFGRD96 29.0 35 90 15 3.85 0.6735
The best solution is
WVFGRD96 12.0 35 80 15 3.70 0.8182
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 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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