The ANSS event ID is ak0179sf1dpj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0179sf1dpj/executive.
2017/08/01 09:35:26 61.672 -149.664 37.3 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2017/08/01 09:35:26:0 61.67 -149.66 37.3 4.0 Alaska Stations used: AK.BPAW AK.CAST AK.CRQ AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI AK.ISLE AK.KLU AK.MCAR AK.MLY AK.NEA2 AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TGL AK.TRF AT.PMR TA.M20K TA.M22K TA.M24K TA.P19K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.30e+22 dyne-cm Mw = 4.01 Z = 50 km Plane Strike Dip Rake NP1 208 68 -125 NP2 90 40 -35 Principal Axes: Axis Value Plunge Azimuth T 1.30e+22 16 323 N 0.00e+00 32 222 P -1.30e+22 53 76 Moment Tensor: (dyne-cm) Component Value Mxx 7.36e+21 Mxy -6.86e+21 Mxz 1.30e+21 Myy -6.00e+14 Myz -8.18e+21 Mzz -7.36e+21 ############## ###################--- ## ###############-------- ### T ############------------ ##### ###########--------------- ##################------------------ ##################-------------------- #################----------------------- ################------------ --------- ################------------- P ---------- ###############-------------- ---------- -#############---------------------------# --############--------------------------## --##########--------------------------## ----#######-------------------------#### -----#####-----------------------##### ----------------------------######## -------######--------############# -----######################### ----######################## -##################### ############## Global CMT Convention Moment Tensor: R T P -7.36e+21 1.30e+21 8.18e+21 1.30e+21 7.36e+21 6.86e+21 8.18e+21 6.86e+21 -6.00e+14 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170801093526/index.html |
STK = 90 DIP = 40 RAKE = -35 MW = 4.01 HS = 50.0
The NDK file is 20170801093526.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 -30 o DIST/3.3 +40 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 2.0 315 90 15 3.26 0.2639 WVFGRD96 4.0 135 90 -35 3.39 0.3032 WVFGRD96 6.0 135 90 -30 3.45 0.3321 WVFGRD96 8.0 130 90 -30 3.52 0.3434 WVFGRD96 10.0 115 55 35 3.52 0.3589 WVFGRD96 12.0 110 55 25 3.54 0.3701 WVFGRD96 14.0 105 65 -15 3.58 0.3801 WVFGRD96 16.0 105 65 -15 3.60 0.3889 WVFGRD96 18.0 105 65 -15 3.63 0.3976 WVFGRD96 20.0 100 65 -20 3.65 0.4085 WVFGRD96 22.0 100 60 -20 3.68 0.4236 WVFGRD96 24.0 100 60 -15 3.69 0.4400 WVFGRD96 26.0 95 60 -25 3.72 0.4559 WVFGRD96 28.0 95 60 -25 3.74 0.4793 WVFGRD96 30.0 95 55 -25 3.77 0.4996 WVFGRD96 32.0 95 55 -25 3.78 0.5158 WVFGRD96 34.0 95 55 -30 3.80 0.5313 WVFGRD96 36.0 95 50 -30 3.82 0.5444 WVFGRD96 38.0 95 50 -30 3.84 0.5578 WVFGRD96 40.0 95 45 -30 3.94 0.5688 WVFGRD96 42.0 95 45 -30 3.96 0.5826 WVFGRD96 44.0 95 45 -30 3.97 0.5887 WVFGRD96 46.0 95 45 -30 3.99 0.5947 WVFGRD96 48.0 90 40 -35 4.00 0.5952 WVFGRD96 50.0 90 40 -35 4.01 0.5973 WVFGRD96 52.0 90 40 -35 4.02 0.5954 WVFGRD96 54.0 90 40 -35 4.02 0.5911 WVFGRD96 56.0 90 40 -35 4.03 0.5890 WVFGRD96 58.0 90 40 -35 4.04 0.5858 WVFGRD96 60.0 90 40 -35 4.04 0.5798 WVFGRD96 62.0 95 40 -30 4.05 0.5745 WVFGRD96 64.0 95 40 -30 4.05 0.5691 WVFGRD96 66.0 95 40 -30 4.05 0.5641 WVFGRD96 68.0 95 40 -30 4.06 0.5582 WVFGRD96 70.0 100 40 -25 4.06 0.5528 WVFGRD96 72.0 100 40 -25 4.06 0.5472 WVFGRD96 74.0 100 40 -25 4.07 0.5409 WVFGRD96 76.0 100 40 -25 4.07 0.5355 WVFGRD96 78.0 95 40 -30 4.07 0.5274 WVFGRD96 80.0 95 40 -30 4.07 0.5198 WVFGRD96 82.0 95 40 -30 4.07 0.5089 WVFGRD96 84.0 105 60 -5 4.08 0.4985 WVFGRD96 86.0 105 60 -5 4.08 0.4959 WVFGRD96 88.0 100 55 -10 4.08 0.4941 WVFGRD96 90.0 100 55 -10 4.09 0.4899 WVFGRD96 92.0 100 55 -10 4.09 0.4783 WVFGRD96 94.0 100 50 -10 4.08 0.4633 WVFGRD96 96.0 100 40 -20 4.08 0.4500 WVFGRD96 98.0 100 40 -20 4.08 0.4373
The best solution is
WVFGRD96 50.0 90 40 -35 4.01 0.5973
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 +40 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