The ANSS event ID is ak01971eyw1l and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01971eyw1l/executive.
2019/06/02 23:08:49 69.501 -144.592 14.3 3.7 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/06/02 23:08:49:0 69.50 -144.59 14.3 3.7 Alaska Stations used: AK.FYU TA.C24K TA.C26K TA.C27K TA.D23K TA.D24K TA.D25K TA.D27M TA.D28M TA.E23K TA.E24K TA.E25K TA.E27K TA.E28M TA.E29M TA.F24K TA.F25K TA.F26K TA.F28M TA.F30M TA.G23K TA.G24K TA.G26K TA.G29M TA.H24K TA.H27K TA.TOLK 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 0.07 n 3 Best Fitting Double Couple Mo = 3.76e+21 dyne-cm Mw = 3.65 Z = 12 km Plane Strike Dip Rake NP1 205 85 -25 NP2 297 65 -174 Principal Axes: Axis Value Plunge Azimuth T 3.76e+21 14 254 N 0.00e+00 65 14 P -3.76e+21 21 158 Moment Tensor: (dyne-cm) Component Value Mxx -2.55e+21 Mxy 2.08e+21 Mxz 9.30e+20 Myy 2.83e+21 Myz -1.29e+21 Mzz -2.76e+20 -------------- -------------------### ---------------------####### ---------------------######### ----------------------############ ----------------------############## ################------################ #####################-################## #####################----############### #####################--------############# ####################-----------########### ###################--------------######### ## #############-----------------####### # T #############-------------------#### # ############---------------------### ##############-----------------------# ############------------------------ ##########------------------------ ########----------- -------- ######------------ P ------- ###------------ ---- -------------- Global CMT Convention Moment Tensor: R T P -2.76e+20 9.30e+20 1.29e+21 9.30e+20 -2.55e+21 -2.08e+21 1.29e+21 -2.08e+21 2.83e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190602230849/index.html |
STK = 205 DIP = 85 RAKE = -25 MW = 3.65 HS = 12.0
The NDK file is 20190602230849.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 115 90 -20 3.26 0.3216 WVFGRD96 2.0 110 65 -15 3.37 0.3911 WVFGRD96 3.0 110 85 -35 3.43 0.4162 WVFGRD96 4.0 295 50 0 3.48 0.4504 WVFGRD96 5.0 295 50 0 3.50 0.4797 WVFGRD96 6.0 295 55 0 3.52 0.5024 WVFGRD96 7.0 205 80 -30 3.54 0.5314 WVFGRD96 8.0 200 75 -35 3.59 0.5614 WVFGRD96 9.0 200 75 -30 3.61 0.5814 WVFGRD96 10.0 200 75 -30 3.62 0.5936 WVFGRD96 11.0 200 75 -25 3.63 0.5997 WVFGRD96 12.0 205 85 -25 3.65 0.6009 WVFGRD96 13.0 200 80 -25 3.66 0.5993 WVFGRD96 14.0 200 80 -25 3.67 0.5954 WVFGRD96 15.0 200 80 -20 3.67 0.5878 WVFGRD96 16.0 200 80 -20 3.68 0.5776 WVFGRD96 17.0 200 80 -20 3.69 0.5646 WVFGRD96 18.0 200 80 -20 3.70 0.5501 WVFGRD96 19.0 200 80 -20 3.70 0.5341 WVFGRD96 20.0 200 80 -20 3.71 0.5178 WVFGRD96 21.0 15 70 -20 3.70 0.5046 WVFGRD96 22.0 15 70 -20 3.70 0.4921 WVFGRD96 23.0 15 70 -20 3.70 0.4789 WVFGRD96 24.0 15 70 -20 3.71 0.4663 WVFGRD96 25.0 15 70 -20 3.71 0.4539 WVFGRD96 26.0 15 70 -20 3.72 0.4423 WVFGRD96 27.0 20 75 -15 3.73 0.4312 WVFGRD96 28.0 20 70 -10 3.74 0.4218 WVFGRD96 29.0 20 70 -10 3.75 0.4137
The best solution is
WVFGRD96 12.0 205 85 -25 3.65 0.6009
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 0.07 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