The ANSS event ID is ak0193yei6yy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0193yei6yy/executive.
2019/03/27 10:56:07 66.321 -157.243 4.9 3.5 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/03/27 10:56:07:0 66.32 -157.24 4.9 3.5 Alaska Stations used: AK.COLD AK.GCSA TA.C18K TA.D19K TA.D20K TA.E18K TA.E19K TA.E20K TA.E23K TA.F15K TA.F17K TA.F18K TA.F19K TA.F21K TA.G15K TA.G17K TA.G18K TA.G19K TA.G21K TA.G23K TA.H16K TA.H17K TA.H19K TA.H20K TA.H21K TA.H22K TA.H23K TA.I20K TA.J19K TA.J20K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 1.95e+21 dyne-cm Mw = 3.46 Z = 10 km Plane Strike Dip Rake NP1 165 80 -25 NP2 260 65 -169 Principal Axes: Axis Value Plunge Azimuth T 1.95e+21 10 214 N 0.00e+00 63 325 P -1.95e+21 25 120 Moment Tensor: (dyne-cm) Component Value Mxx 8.89e+20 Mxy 1.58e+21 Mxz 9.60e+19 Myy -6.07e+20 Myz -8.27e+20 Mzz -2.82e+20 --############ ------################ ---------################### ----------#################### ------------###################### -------------####################### ---------------####################### ----------------#####------------####### -------------###-----------------------# ---------#########------------------------ ------############------------------------ ---################----------------------- -##################----------------------- ###################--------------------- ###################------------- ----- ###################------------ P ---- ###################----------- --- ##################---------------- ### ###########------------- ## T ############----------- #############------- ############-- Global CMT Convention Moment Tensor: R T P -2.82e+20 9.60e+19 8.27e+20 9.60e+19 8.89e+20 -1.58e+21 8.27e+20 -1.58e+21 -6.07e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190327105607/index.html |
STK = 165 DIP = 80 RAKE = -25 MW = 3.46 HS = 10.0
The NDK file is 20190327105607.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 +60 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 1.0 345 85 0 3.04 0.3399 WVFGRD96 2.0 165 90 0 3.17 0.4568 WVFGRD96 3.0 165 90 20 3.23 0.5011 WVFGRD96 4.0 160 75 -30 3.30 0.5330 WVFGRD96 5.0 160 70 -30 3.34 0.5686 WVFGRD96 6.0 160 70 -30 3.37 0.5948 WVFGRD96 7.0 160 70 -30 3.40 0.6116 WVFGRD96 8.0 160 70 -30 3.44 0.6255 WVFGRD96 9.0 165 80 -25 3.44 0.6299 WVFGRD96 10.0 165 80 -25 3.46 0.6321 WVFGRD96 11.0 165 85 -25 3.48 0.6291 WVFGRD96 12.0 345 90 20 3.48 0.6225 WVFGRD96 13.0 345 90 20 3.50 0.6163 WVFGRD96 14.0 350 80 20 3.52 0.6092 WVFGRD96 15.0 350 80 20 3.53 0.6020 WVFGRD96 16.0 350 75 20 3.54 0.5932 WVFGRD96 17.0 350 75 20 3.55 0.5841 WVFGRD96 18.0 350 75 15 3.56 0.5737 WVFGRD96 19.0 350 75 15 3.57 0.5630 WVFGRD96 20.0 350 75 15 3.57 0.5514 WVFGRD96 21.0 345 75 -10 3.57 0.5394 WVFGRD96 22.0 345 75 -10 3.58 0.5292 WVFGRD96 23.0 345 75 -10 3.59 0.5183 WVFGRD96 24.0 345 75 -10 3.59 0.5062 WVFGRD96 25.0 345 75 -15 3.60 0.4950 WVFGRD96 26.0 345 75 -15 3.60 0.4832 WVFGRD96 27.0 345 75 -15 3.61 0.4705 WVFGRD96 28.0 345 80 -15 3.61 0.4575 WVFGRD96 29.0 345 80 -15 3.61 0.4470
The best solution is
WVFGRD96 10.0 165 80 -25 3.46 0.6321
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 +60 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