The ANSS event ID is ak0199gznnwd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0199gznnwd/executive.
2019/07/25 18:39:04 66.310 -157.332 5.3 3.3 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/07/25 18:39:04:0 66.31 -157.33 5.3 3.3 Alaska Stations used: AK.BPAW AK.MLY TA.B20K TA.B21K TA.C19K TA.D19K TA.D20K TA.D22K TA.E18K TA.E19K TA.E21K TA.E22K TA.F17K TA.F19K TA.F20K TA.F21K TA.G16K TA.G18K TA.G19K TA.G21K TA.G23K TA.H17K TA.H18K TA.H19K TA.H21K TA.H23K TA.I17K TA.I20K TA.I21K TA.I23K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K 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 0.10 n 3 Best Fitting Double Couple Mo = 1.53e+21 dyne-cm Mw = 3.39 Z = 11 km Plane Strike Dip Rake NP1 290 90 -150 NP2 200 60 0 Principal Axes: Axis Value Plunge Azimuth T 1.53e+21 21 61 N 0.00e+00 60 290 P -1.53e+21 21 159 Moment Tensor: (dyne-cm) Component Value Mxx -8.52e+20 Mxy 1.02e+21 Mxz 7.19e+20 Myy 8.52e+20 Myz 2.62e+20 Mzz 0.00e+00 -------------- --------------######## ---------------############# --------------################ ---------------################### ---------------################ ## --------------################## T ### ##-------------################## #### #######-------########################## #############-############################ #############-----######################## #############-----------################## ############-----------------############# ###########----------------------####### ###########---------------------------## ##########---------------------------- #########--------------------------- ########-------------------------- ######------------- -------- #####------------- P ------- ###------------ ---- -------------- Global CMT Convention Moment Tensor: R T P 0.00e+00 7.19e+20 -2.62e+20 7.19e+20 -8.52e+20 -1.02e+21 -2.62e+20 -1.02e+21 8.52e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190725183904/index.html |
STK = 200 DIP = 60 RAKE = 0 MW = 3.39 HS = 11.0
The NDK file is 20190725183904.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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.
![]() |
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.
![]() |
|
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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 195 85 0 2.94 0.3211 WVFGRD96 2.0 195 70 -5 3.07 0.3922 WVFGRD96 3.0 195 60 -10 3.15 0.4189 WVFGRD96 4.0 195 50 -5 3.19 0.4572 WVFGRD96 5.0 195 50 -5 3.22 0.4995 WVFGRD96 6.0 195 55 -5 3.25 0.5354 WVFGRD96 7.0 200 55 5 3.27 0.5626 WVFGRD96 8.0 200 55 5 3.33 0.5858 WVFGRD96 9.0 200 55 5 3.35 0.5995 WVFGRD96 10.0 200 55 5 3.37 0.6036 WVFGRD96 11.0 200 60 0 3.39 0.6042 WVFGRD96 12.0 200 60 5 3.40 0.5989 WVFGRD96 13.0 200 60 5 3.42 0.5900 WVFGRD96 14.0 200 60 5 3.43 0.5778 WVFGRD96 15.0 200 60 5 3.44 0.5629 WVFGRD96 16.0 195 65 -5 3.46 0.5464 WVFGRD96 17.0 200 65 5 3.47 0.5290 WVFGRD96 18.0 200 65 5 3.48 0.5110 WVFGRD96 19.0 200 60 0 3.48 0.4934 WVFGRD96 20.0 200 60 0 3.49 0.4761 WVFGRD96 21.0 200 60 0 3.50 0.4579 WVFGRD96 22.0 200 60 0 3.51 0.4393 WVFGRD96 23.0 200 60 0 3.51 0.4211 WVFGRD96 24.0 200 60 0 3.51 0.4027 WVFGRD96 25.0 200 60 0 3.52 0.3853 WVFGRD96 26.0 200 60 0 3.52 0.3689 WVFGRD96 27.0 200 60 0 3.52 0.3553 WVFGRD96 28.0 200 60 0 3.53 0.3438 WVFGRD96 29.0 200 60 0 3.53 0.3345
The best solution is
WVFGRD96 11.0 200 60 0 3.39 0.6042
The mechanism corresponding to the best fit is
![]() |
|
The best fit as a function of depth is given in the following figure:
![]() |
|
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 0.10 n 3
![]() |
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. |
![]() |
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