The ANSS event ID is ak019ez5u73j and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019ez5u73j/executive.
2019/11/22 01:07:58 61.334 -149.981 46.6 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/11/22 01:07:58:0 61.33 -149.98 46.6 4.0 Alaska Stations used: AK.FIRE AK.GHO AK.PWL AK.RC01 AK.SAW AK.SKN AK.SSN AT.PMR AV.STLK TA.M22K 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 0.15 n 3 Best Fitting Double Couple Mo = 9.89e+21 dyne-cm Mw = 3.93 Z = 38 km Plane Strike Dip Rake NP1 190 85 -80 NP2 306 11 -153 Principal Axes: Axis Value Plunge Azimuth T 9.89e+21 39 271 N 0.00e+00 10 9 P -9.89e+21 49 111 Moment Tensor: (dyne-cm) Component Value Mxx -5.34e+20 Mxy 1.32e+21 Mxz 1.81e+21 Myy 2.22e+21 Myz -9.42e+21 Mzz -1.69e+21 ----------#### --############---##### -###############--------#### #################----------### ##################-------------### ###################---------------## ###################-----------------## ####################------------------## ####################-------------------# ####### ##########--------------------## ####### T ##########--------------------## ####### ##########--------- ---------# ###################---------- P ---------# ##################---------- --------# ##################---------------------# ################---------------------- ###############--------------------- ##############-------------------- ############------------------ ###########----------------- #######--------------- ###----------- Global CMT Convention Moment Tensor: R T P -1.69e+21 1.81e+21 9.42e+21 1.81e+21 -5.34e+20 -1.32e+21 9.42e+21 -1.32e+21 2.22e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191122010758/index.html |
STK = 190 DIP = 85 RAKE = -80 MW = 3.93 HS = 38.0
The NDK file is 20191122010758.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 0.15 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 10 70 30 3.31 0.1637 WVFGRD96 4.0 5 85 -40 3.42 0.1857 WVFGRD96 6.0 275 60 -5 3.52 0.2267 WVFGRD96 8.0 280 65 10 3.66 0.2675 WVFGRD96 10.0 275 65 0 3.73 0.2902 WVFGRD96 12.0 280 70 10 3.79 0.2909 WVFGRD96 14.0 100 60 15 3.80 0.2874 WVFGRD96 16.0 100 55 10 3.82 0.2854 WVFGRD96 18.0 100 50 10 3.83 0.2822 WVFGRD96 20.0 105 45 15 3.85 0.2807 WVFGRD96 22.0 110 45 25 3.87 0.2829 WVFGRD96 24.0 145 15 50 3.88 0.2908 WVFGRD96 26.0 5 80 95 3.90 0.3059 WVFGRD96 28.0 5 80 95 3.91 0.3151 WVFGRD96 30.0 5 80 95 3.92 0.3164 WVFGRD96 32.0 150 10 50 3.93 0.3280 WVFGRD96 34.0 140 10 40 3.94 0.3481 WVFGRD96 36.0 195 90 -80 3.95 0.3596 WVFGRD96 38.0 190 85 -80 3.93 0.3616 WVFGRD96 40.0 65 5 -35 4.05 0.3538 WVFGRD96 42.0 190 85 -85 4.06 0.3555 WVFGRD96 44.0 185 80 -85 4.06 0.3536 WVFGRD96 46.0 185 80 -85 4.07 0.3556 WVFGRD96 48.0 -15 10 -110 4.07 0.3567 WVFGRD96 50.0 -5 10 -95 4.08 0.3538 WVFGRD96 52.0 0 10 -90 4.09 0.3550 WVFGRD96 54.0 -10 15 -95 4.10 0.3508 WVFGRD96 56.0 0 15 -85 4.11 0.3469 WVFGRD96 58.0 85 30 -25 4.13 0.3423 WVFGRD96 60.0 85 30 -30 4.14 0.3393 WVFGRD96 62.0 85 30 -30 4.15 0.3374 WVFGRD96 64.0 175 70 -85 4.14 0.3321 WVFGRD96 66.0 175 70 -90 4.14 0.3284 WVFGRD96 68.0 170 70 -90 4.15 0.3249 WVFGRD96 70.0 5 20 -80 4.15 0.3212 WVFGRD96 72.0 80 35 -45 4.21 0.3199 WVFGRD96 74.0 80 35 -45 4.21 0.3172 WVFGRD96 76.0 80 35 -45 4.22 0.3138 WVFGRD96 78.0 80 40 -40 4.23 0.3102 WVFGRD96 80.0 80 40 -40 4.23 0.3058 WVFGRD96 82.0 80 40 -45 4.25 0.3027 WVFGRD96 84.0 110 30 10 4.18 0.3010 WVFGRD96 86.0 110 30 10 4.18 0.2986 WVFGRD96 88.0 110 30 10 4.18 0.2981 WVFGRD96 90.0 110 30 10 4.18 0.2930 WVFGRD96 92.0 110 30 10 4.19 0.2937 WVFGRD96 94.0 100 10 15 4.17 0.2909 WVFGRD96 96.0 55 10 -30 4.17 0.2827 WVFGRD96 98.0 75 40 -50 4.28 0.2760
The best solution is
WVFGRD96 38.0 190 85 -80 3.93 0.3616
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 0.15 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