The ANSS event ID is ak0198ezhjln and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0198ezhjln/executive.
2019/07/02 20:39:03 59.091 -152.406 60.6 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/07/02 20:39:03:0 59.09 -152.41 60.6 4.0 Alaska Stations used: AK.BRLK AK.CNP AK.HOM AK.SLK AK.SWD AV.ILSW II.KDAK 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 0.08 n 3 Best Fitting Double Couple Mo = 1.55e+22 dyne-cm Mw = 4.06 Z = 66 km Plane Strike Dip Rake NP1 80 65 30 NP2 336 63 152 Principal Axes: Axis Value Plunge Azimuth T 1.55e+22 38 299 N 0.00e+00 52 116 P -1.55e+22 1 208 Moment Tensor: (dyne-cm) Component Value Mxx -9.91e+21 Mxy -1.04e+22 Mxz 3.92e+21 Myy 3.98e+21 Myz -6.45e+21 Mzz 5.93e+21 -------------- ######---------------- ###########----------------- ##############---------------- ##################---------------- ####################---------------- ###### #############---------------- ####### T ##############---------------- ####### ###############--------------- ###########################--------------# ############################-----------### #############################-------###### #############################----######### -###########################-########### -------#############---------########### ----------------------------########## ----------------------------######## ---------------------------####### -------------------------##### -- ------------------##### P ------------------## -------------- Global CMT Convention Moment Tensor: R T P 5.93e+21 3.92e+21 6.45e+21 3.92e+21 -9.91e+21 1.04e+22 6.45e+21 1.04e+22 3.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190702203903/index.html |
STK = 80 DIP = 65 RAKE = 30 MW = 4.06 HS = 66.0
The NDK file is 20190702203903.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 330 60 -30 3.30 0.3446 WVFGRD96 4.0 155 65 -20 3.39 0.3881 WVFGRD96 6.0 345 60 30 3.45 0.4255 WVFGRD96 8.0 160 75 35 3.51 0.4522 WVFGRD96 10.0 160 75 30 3.54 0.4655 WVFGRD96 12.0 160 80 30 3.57 0.4667 WVFGRD96 14.0 245 65 0 3.58 0.4710 WVFGRD96 16.0 240 65 -5 3.61 0.4844 WVFGRD96 18.0 240 65 -10 3.63 0.4953 WVFGRD96 20.0 240 65 -10 3.66 0.5052 WVFGRD96 22.0 240 65 -10 3.68 0.5157 WVFGRD96 24.0 245 70 5 3.70 0.5277 WVFGRD96 26.0 245 75 15 3.73 0.5458 WVFGRD96 28.0 245 75 10 3.74 0.5623 WVFGRD96 30.0 245 70 10 3.76 0.5761 WVFGRD96 32.0 245 75 5 3.77 0.5863 WVFGRD96 34.0 245 75 10 3.79 0.5964 WVFGRD96 36.0 245 75 10 3.81 0.6040 WVFGRD96 38.0 70 75 10 3.85 0.6169 WVFGRD96 40.0 70 70 10 3.91 0.6483 WVFGRD96 42.0 70 70 10 3.93 0.6550 WVFGRD96 44.0 75 65 20 3.96 0.6653 WVFGRD96 46.0 75 65 20 3.98 0.6792 WVFGRD96 48.0 75 65 20 3.99 0.6944 WVFGRD96 50.0 75 65 20 4.01 0.7088 WVFGRD96 52.0 75 65 20 4.02 0.7193 WVFGRD96 54.0 75 65 20 4.02 0.7293 WVFGRD96 56.0 80 60 30 4.04 0.7348 WVFGRD96 58.0 80 65 30 4.05 0.7426 WVFGRD96 60.0 80 65 30 4.05 0.7476 WVFGRD96 62.0 80 65 30 4.05 0.7490 WVFGRD96 64.0 80 65 30 4.05 0.7512 WVFGRD96 66.0 80 65 30 4.06 0.7517 WVFGRD96 68.0 80 65 30 4.06 0.7509 WVFGRD96 70.0 80 70 30 4.06 0.7488 WVFGRD96 72.0 80 70 30 4.06 0.7478 WVFGRD96 74.0 80 70 30 4.07 0.7455 WVFGRD96 76.0 80 70 30 4.07 0.7422 WVFGRD96 78.0 80 70 30 4.07 0.7388 WVFGRD96 80.0 80 70 30 4.07 0.7355 WVFGRD96 82.0 80 70 30 4.07 0.7317 WVFGRD96 84.0 80 75 30 4.08 0.7283 WVFGRD96 86.0 80 75 30 4.08 0.7248 WVFGRD96 88.0 80 75 30 4.08 0.7215 WVFGRD96 90.0 80 75 30 4.08 0.7179 WVFGRD96 92.0 80 75 30 4.08 0.7132 WVFGRD96 94.0 80 75 35 4.08 0.7090 WVFGRD96 96.0 80 75 35 4.08 0.7066 WVFGRD96 98.0 80 75 35 4.08 0.7041
The best solution is
WVFGRD96 66.0 80 65 30 4.06 0.7517
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 0.08 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