The ANSS event ID is ak017apifhie and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017apifhie/executive.
2017/08/21 10:43:33 62.058 -149.555 57.2 4.3 Alaska
USGS/SLU Moment Tensor Solution ENS 2017/08/21 10:43:33:0 62.06 -149.55 57.2 4.3 Alaska Stations used: AK.CAST AK.CUT AK.DHY AK.DIV AK.GHO AK.GLI AK.HIN AK.KLU AK.KNK AK.KTH AK.MCK AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M20K TA.M22K TA.O22K Filtering commands used: cut o DIST/3.5 -30 o DIST/3.5 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.52e+22 dyne-cm Mw = 4.37 Z = 64 km Plane Strike Dip Rake NP1 215 50 -75 NP2 12 42 -107 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 4 294 N 0.00e+00 11 25 P -4.52e+22 78 186 Moment Tensor: (dyne-cm) Component Value Mxx 5.72e+21 Mxy -1.71e+22 Mxz 1.05e+22 Myy 3.73e+22 Myz -1.90e+21 Mzz -4.30e+22 #############- ##################---- ##################-----##### ###############----------##### ###############-------------###### #############---------------####### T ###########------------------####### #########--------------------######## ###########---------------------######## ###########----------------------######### ##########-----------------------######### #########---------- -----------######### #########---------- P -----------######### #######----------- ----------######### #######-----------------------########## ######-----------------------######### #####----------------------######### ####--------------------########## ##-------------------######### ##----------------########## -------------######### -----######### Global CMT Convention Moment Tensor: R T P -4.30e+22 1.05e+22 1.90e+21 1.05e+22 5.72e+21 1.71e+22 1.90e+21 1.71e+22 3.73e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170821104333/index.html |
STK = 215 DIP = 50 RAKE = -75 MW = 4.37 HS = 64.0
The NDK file is 20170821104333.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.
![]() |
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/3.5 -30 o DIST/3.5 +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 2.0 215 45 95 3.60 0.3016 WVFGRD96 4.0 345 25 15 3.62 0.2661 WVFGRD96 6.0 20 90 -75 3.65 0.3333 WVFGRD96 8.0 355 25 40 3.74 0.3635 WVFGRD96 10.0 205 70 85 3.78 0.3820 WVFGRD96 12.0 205 65 90 3.81 0.3821 WVFGRD96 14.0 5 30 65 3.83 0.3724 WVFGRD96 16.0 340 25 50 3.85 0.3581 WVFGRD96 18.0 335 25 45 3.87 0.3468 WVFGRD96 20.0 330 25 40 3.89 0.3350 WVFGRD96 22.0 60 70 -50 3.92 0.3289 WVFGRD96 24.0 60 70 -50 3.94 0.3261 WVFGRD96 26.0 80 75 35 3.97 0.3313 WVFGRD96 28.0 80 75 35 3.99 0.3331 WVFGRD96 30.0 75 80 35 4.00 0.3320 WVFGRD96 32.0 80 70 30 4.01 0.3251 WVFGRD96 34.0 55 55 -55 4.03 0.3646 WVFGRD96 36.0 205 45 -95 4.07 0.4240 WVFGRD96 38.0 30 45 -90 4.11 0.4904 WVFGRD96 40.0 25 50 -95 4.22 0.5789 WVFGRD96 42.0 25 50 -95 4.25 0.5972 WVFGRD96 44.0 20 50 -100 4.28 0.6134 WVFGRD96 46.0 25 45 -95 4.29 0.6304 WVFGRD96 48.0 25 45 -95 4.31 0.6500 WVFGRD96 50.0 215 45 -80 4.32 0.6695 WVFGRD96 52.0 215 45 -80 4.33 0.6844 WVFGRD96 54.0 220 50 -70 4.35 0.6977 WVFGRD96 56.0 220 50 -70 4.36 0.7105 WVFGRD96 58.0 220 50 -70 4.36 0.7207 WVFGRD96 60.0 215 50 -75 4.36 0.7274 WVFGRD96 62.0 215 50 -75 4.37 0.7314 WVFGRD96 64.0 215 50 -75 4.37 0.7319 WVFGRD96 66.0 215 50 -75 4.37 0.7313 WVFGRD96 68.0 220 55 -70 4.38 0.7307 WVFGRD96 70.0 215 55 -75 4.38 0.7296 WVFGRD96 72.0 215 55 -75 4.38 0.7249 WVFGRD96 74.0 215 55 -75 4.38 0.7213 WVFGRD96 76.0 215 55 -75 4.38 0.7172 WVFGRD96 78.0 215 55 -75 4.38 0.7101 WVFGRD96 80.0 215 55 -75 4.38 0.7037 WVFGRD96 82.0 215 55 -75 4.38 0.6953 WVFGRD96 84.0 215 55 -75 4.38 0.6872 WVFGRD96 86.0 215 55 -75 4.38 0.6789 WVFGRD96 88.0 215 55 -70 4.39 0.6710 WVFGRD96 90.0 215 55 -70 4.39 0.6632 WVFGRD96 92.0 215 55 -70 4.39 0.6538 WVFGRD96 94.0 215 60 -75 4.39 0.6472 WVFGRD96 96.0 215 60 -75 4.39 0.6410 WVFGRD96 98.0 215 60 -75 4.39 0.6355
The best solution is
WVFGRD96 64.0 215 50 -75 4.37 0.7319
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/3.5 -30 o DIST/3.5 +40 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