The ANSS event ID is ak0196hcswgq and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0196hcswgq/executive.
2019/05/21 10:12:28 59.137 -152.997 69.7 3.8 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/05/21 10:12:28:0 59.14 -153.00 69.7 3.8 Alaska Stations used: AK.BRLK AK.CNP AK.HOM AK.SWD II.KDAK TA.N19K TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K TA.Q20K Filtering commands used: cut o DIST/3.3 -50 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 = 1.02e+22 dyne-cm Mw = 3.94 Z = 78 km Plane Strike Dip Rake NP1 85 75 35 NP2 345 56 162 Principal Axes: Axis Value Plunge Azimuth T 1.02e+22 35 310 N 0.00e+00 52 105 P -1.02e+22 12 211 Moment Tensor: (dyne-cm) Component Value Mxx -4.32e+21 Mxy -7.72e+21 Mxz 4.87e+21 Myy 1.38e+21 Myz -2.60e+21 Mzz 2.93e+21 ###----------- ##########------------ ###############------------- #################------------- #####################------------- ###### ##############------------- ####### T ###############------------- ######## ################------------- ###########################------------- #############################------------- ##############################----------## ##############################------###### ---##########################-############ -----------------------------########### -----------------------------########### ----------------------------########## ---------------------------######### --------------------------######## ---- ----------------####### --- P ----------------###### ---------------#### -------------# Global CMT Convention Moment Tensor: R T P 2.93e+21 4.87e+21 2.60e+21 4.87e+21 -4.32e+21 7.72e+21 2.60e+21 7.72e+21 1.38e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190521101228/index.html |
STK = 85 DIP = 75 RAKE = 35 MW = 3.94 HS = 78.0
The NDK file is 20190521101228.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 -50 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 2.0 100 50 -100 3.17 0.3055 WVFGRD96 4.0 160 80 35 3.14 0.3455 WVFGRD96 6.0 165 65 30 3.23 0.3906 WVFGRD96 8.0 165 65 30 3.31 0.4143 WVFGRD96 10.0 160 75 30 3.34 0.4169 WVFGRD96 12.0 160 75 30 3.38 0.4066 WVFGRD96 14.0 240 60 -25 3.42 0.3950 WVFGRD96 16.0 240 60 -25 3.46 0.4009 WVFGRD96 18.0 255 65 20 3.48 0.4075 WVFGRD96 20.0 260 75 25 3.52 0.4241 WVFGRD96 22.0 255 65 25 3.53 0.4439 WVFGRD96 24.0 255 65 25 3.55 0.4654 WVFGRD96 26.0 250 65 20 3.57 0.4860 WVFGRD96 28.0 250 70 15 3.57 0.5021 WVFGRD96 30.0 70 80 -20 3.59 0.5201 WVFGRD96 32.0 75 90 -25 3.62 0.5367 WVFGRD96 34.0 75 90 -20 3.63 0.5476 WVFGRD96 36.0 235 65 -30 3.68 0.5555 WVFGRD96 38.0 230 65 -25 3.70 0.5631 WVFGRD96 40.0 255 75 40 3.75 0.5756 WVFGRD96 42.0 255 70 40 3.77 0.5685 WVFGRD96 44.0 80 70 40 3.81 0.5694 WVFGRD96 46.0 80 75 40 3.83 0.5738 WVFGRD96 48.0 80 75 40 3.84 0.5787 WVFGRD96 50.0 80 75 40 3.85 0.5847 WVFGRD96 52.0 80 65 35 3.85 0.5957 WVFGRD96 54.0 80 70 35 3.86 0.6078 WVFGRD96 56.0 80 70 35 3.87 0.6233 WVFGRD96 58.0 80 70 35 3.88 0.6340 WVFGRD96 60.0 80 70 35 3.88 0.6443 WVFGRD96 62.0 80 70 35 3.89 0.6516 WVFGRD96 64.0 85 70 35 3.91 0.6599 WVFGRD96 66.0 85 70 35 3.91 0.6672 WVFGRD96 68.0 85 70 35 3.91 0.6698 WVFGRD96 70.0 85 70 35 3.92 0.6748 WVFGRD96 72.0 85 70 35 3.92 0.6775 WVFGRD96 74.0 85 70 35 3.93 0.6764 WVFGRD96 76.0 85 75 35 3.94 0.6796 WVFGRD96 78.0 85 75 35 3.94 0.6811 WVFGRD96 80.0 85 75 35 3.94 0.6810 WVFGRD96 82.0 85 75 35 3.95 0.6796 WVFGRD96 84.0 85 75 35 3.95 0.6777 WVFGRD96 86.0 85 75 35 3.95 0.6771 WVFGRD96 88.0 85 75 35 3.95 0.6763 WVFGRD96 90.0 85 75 30 3.95 0.6752 WVFGRD96 92.0 85 75 30 3.95 0.6727 WVFGRD96 94.0 85 75 30 3.96 0.6713 WVFGRD96 96.0 85 75 30 3.96 0.6690 WVFGRD96 98.0 85 75 30 3.96 0.6661
The best solution is
WVFGRD96 78.0 85 75 35 3.94 0.6811
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 -50 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