The ANSS event ID is ak019by0dpof and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019by0dpof/executive.
2019/09/17 01:09:55 61.720 -150.890 63.1 5.2 Alaska
USGS/SLU Moment Tensor Solution ENS 2019/09/17 01:09:55:0 61.72 -150.89 63.1 5.2 Alaska Stations used: AK.DHY AK.FID AK.FIRE AK.GHO AK.GLI AK.KNK AK.KTH AK.MCK AK.PPLA AK.PWL AK.RC01 AK.RND AK.SCM AK.SKN AK.SLK AK.SSN AK.TRF AT.PMR AV.ILSW AV.RED AV.SPU AV.STLK TA.K20K TA.L19K TA.M22K TA.N19K TA.O22K 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 = 8.51e+23 dyne-cm Mw = 5.22 Z = 76 km Plane Strike Dip Rake NP1 178 80 -113 NP2 65 25 -25 Principal Axes: Axis Value Plunge Azimuth T 8.51e+23 31 287 N 0.00e+00 23 182 P -8.51e+23 50 62 Moment Tensor: (dyne-cm) Component Value Mxx -2.34e+22 Mxy -3.15e+23 Mxz -8.59e+22 Myy 2.99e+23 Myz -7.31e+23 Mzz -2.76e+23 ######-------- #########------------- ############---------------- #############----------------- ###############------------------- ################-------------------- ################---------------------- ##### #########---------- ---------# ##### T #########---------- P ---------# ###### #########---------- ---------## ##################----------------------## ##################---------------------### ##################---------------------### ##################-------------------### ##################------------------#### #################----------------##### ################---------------##### -###############------------###### --############---------####### ------#######----########### -----------########### -------####### Global CMT Convention Moment Tensor: R T P -2.76e+23 -8.59e+22 7.31e+23 -8.59e+22 -2.34e+22 3.15e+23 7.31e+23 3.15e+23 2.99e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190917010955/index.html |
STK = 65 DIP = 25 RAKE = -25 MW = 5.22 HS = 76.0
The NDK file is 20190917010955.ndk The waveform inversion is preferred.
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
USGS/SLU Moment Tensor Solution ENS 2019/09/17 01:09:55:0 61.72 -150.89 63.1 5.2 Alaska Stations used: AK.DHY AK.FID AK.FIRE AK.GHO AK.GLI AK.KNK AK.KTH AK.MCK AK.PPLA AK.PWL AK.RC01 AK.RND AK.SCM AK.SKN AK.SLK AK.SSN AK.TRF AT.PMR AV.ILSW AV.RED AV.SPU AV.STLK TA.K20K TA.L19K TA.M22K TA.N19K TA.O22K 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 = 8.51e+23 dyne-cm Mw = 5.22 Z = 76 km Plane Strike Dip Rake NP1 178 80 -113 NP2 65 25 -25 Principal Axes: Axis Value Plunge Azimuth T 8.51e+23 31 287 N 0.00e+00 23 182 P -8.51e+23 50 62 Moment Tensor: (dyne-cm) Component Value Mxx -2.34e+22 Mxy -3.15e+23 Mxz -8.59e+22 Myy 2.99e+23 Myz -7.31e+23 Mzz -2.76e+23 ######-------- #########------------- ############---------------- #############----------------- ###############------------------- ################-------------------- ################---------------------- ##### #########---------- ---------# ##### T #########---------- P ---------# ###### #########---------- ---------## ##################----------------------## ##################---------------------### ##################---------------------### ##################-------------------### ##################------------------#### #################----------------##### ################---------------##### -###############------------###### --############---------####### ------#######----########### -----------########### -------####### Global CMT Convention Moment Tensor: R T P -2.76e+23 -8.59e+22 7.31e+23 -8.59e+22 -2.34e+22 3.15e+23 7.31e+23 3.15e+23 2.99e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190917010955/index.html |
Regional Moment Tensor (Mwr) Moment 7.928e+16 N-m Magnitude 5.20 Mwr Depth 75.0 km Percent DC 97% Half Duration - Catalog US Data Source US 3 Contributor US 3 Nodal Planes Plane Strike Dip Rake NP1 181 79 -114 NP2 67 26 -26 Principal Axes Axis Value Plunge Azimuth T 7.988e+16 N-m 30 290 N -0.122e+16 N-m 23 186 P -7.867e+16 N-m 50 64 |
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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 355 60 60 4.35 0.1340 WVFGRD96 4.0 325 60 -30 4.40 0.1465 WVFGRD96 6.0 250 60 20 4.45 0.1672 WVFGRD96 8.0 250 60 20 4.55 0.1896 WVFGRD96 10.0 60 70 -15 4.59 0.2026 WVFGRD96 12.0 60 70 -15 4.63 0.2121 WVFGRD96 14.0 60 70 -10 4.67 0.2170 WVFGRD96 16.0 65 70 -10 4.69 0.2203 WVFGRD96 18.0 65 70 -10 4.72 0.2229 WVFGRD96 20.0 65 70 -10 4.74 0.2269 WVFGRD96 22.0 65 70 -5 4.76 0.2320 WVFGRD96 24.0 65 65 0 4.78 0.2371 WVFGRD96 26.0 65 65 5 4.80 0.2421 WVFGRD96 28.0 65 60 5 4.82 0.2464 WVFGRD96 30.0 65 60 5 4.84 0.2516 WVFGRD96 32.0 70 60 20 4.86 0.2594 WVFGRD96 34.0 70 60 20 4.87 0.2675 WVFGRD96 36.0 70 60 20 4.89 0.2760 WVFGRD96 38.0 70 60 20 4.92 0.2818 WVFGRD96 40.0 55 45 -20 5.00 0.2903 WVFGRD96 42.0 60 45 -20 5.02 0.2953 WVFGRD96 44.0 60 40 -25 5.04 0.3006 WVFGRD96 46.0 60 40 -25 5.05 0.3080 WVFGRD96 48.0 60 35 -25 5.07 0.3191 WVFGRD96 50.0 60 35 -25 5.09 0.3292 WVFGRD96 52.0 60 35 -25 5.10 0.3385 WVFGRD96 54.0 60 35 -25 5.12 0.3484 WVFGRD96 56.0 65 35 -25 5.13 0.3584 WVFGRD96 58.0 60 30 -30 5.14 0.3671 WVFGRD96 60.0 65 30 -25 5.16 0.3755 WVFGRD96 62.0 65 30 -25 5.17 0.3836 WVFGRD96 64.0 65 30 -25 5.18 0.3913 WVFGRD96 66.0 65 30 -25 5.19 0.3961 WVFGRD96 68.0 65 30 -25 5.19 0.4012 WVFGRD96 70.0 65 25 -25 5.21 0.4038 WVFGRD96 72.0 65 25 -25 5.21 0.4074 WVFGRD96 74.0 65 25 -25 5.22 0.4076 WVFGRD96 76.0 65 25 -25 5.22 0.4082 WVFGRD96 78.0 65 25 -25 5.23 0.4068 WVFGRD96 80.0 70 25 -20 5.23 0.4046 WVFGRD96 82.0 70 25 -20 5.24 0.4018 WVFGRD96 84.0 70 25 -20 5.24 0.3976 WVFGRD96 86.0 70 25 -20 5.24 0.3954 WVFGRD96 88.0 75 20 -15 5.25 0.3913
The best solution is
WVFGRD96 76.0 65 25 -25 5.22 0.4082
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 0.10 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