The ANSS event ID is ak0169byh1zh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0169byh1zh/executive.
2016/07/21 14:07:16 61.625 -150.321 60.2 4.1 Alaska
USGS/SLU Moment Tensor Solution ENS 2016/07/21 14:07:16:0 61.62 -150.32 60.2 4.1 Alaska Stations used: AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CUT AK.DIV AK.FID AK.FIRE AK.GHO AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.TRF AT.PMR AV.ILSW TA.K20K TA.M19K TA.M20K TA.M22K TA.N19K TA.O19K TA.O22K Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 2.11e+22 dyne-cm Mw = 4.15 Z = 64 km Plane Strike Dip Rake NP1 60 85 45 NP2 325 45 173 Principal Axes: Axis Value Plunge Azimuth T 2.11e+22 34 293 N 0.00e+00 45 65 P -2.11e+22 26 184 Moment Tensor: (dyne-cm) Component Value Mxx -1.48e+22 Mxy -6.32e+21 Mxz 1.21e+22 Myy 1.22e+22 Myz -8.49e+21 Mzz 2.60e+21 -------------- ---------------------- ############---------------- ################-------------- #####################------------- ########################------------ ###########################-------#### ###### ####################----####### ###### T ############################### ####### ##################----########## #########################--------######### ######################------------######## ###################---------------######## ##############--------------------###### ###########-----------------------###### ######---------------------------##### #-------------------------------#### -------------------------------### ------------- ------------## ------------ P ------------# --------- ---------- -------------- Global CMT Convention Moment Tensor: R T P 2.60e+21 1.21e+22 8.49e+21 1.21e+22 -1.48e+22 6.32e+21 8.49e+21 6.32e+21 1.22e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160721140716/index.html |
STK = 60 DIP = 85 RAKE = 45 MW = 4.15 HS = 64.0
The NDK file is 20160721140716.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 2016/07/21 14:07:16:0 61.62 -150.32 60.2 4.1 Alaska Stations used: AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CUT AK.DIV AK.FID AK.FIRE AK.GHO AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.TRF AT.PMR AV.ILSW TA.K20K TA.M19K TA.M20K TA.M22K TA.N19K TA.O19K TA.O22K Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 2.11e+22 dyne-cm Mw = 4.15 Z = 64 km Plane Strike Dip Rake NP1 60 85 45 NP2 325 45 173 Principal Axes: Axis Value Plunge Azimuth T 2.11e+22 34 293 N 0.00e+00 45 65 P -2.11e+22 26 184 Moment Tensor: (dyne-cm) Component Value Mxx -1.48e+22 Mxy -6.32e+21 Mxz 1.21e+22 Myy 1.22e+22 Myz -8.49e+21 Mzz 2.60e+21 -------------- ---------------------- ############---------------- ################-------------- #####################------------- ########################------------ ###########################-------#### ###### ####################----####### ###### T ############################### ####### ##################----########## #########################--------######### ######################------------######## ###################---------------######## ##############--------------------###### ###########-----------------------###### ######---------------------------##### #-------------------------------#### -------------------------------### ------------- ------------## ------------ P ------------# --------- ---------- -------------- Global CMT Convention Moment Tensor: R T P 2.60e+21 1.21e+22 8.49e+21 1.21e+22 -1.48e+22 6.32e+21 8.49e+21 6.32e+21 1.22e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160721140716/index.html |
Regional Moment Tensor (Mwr) Moment 1.691e+15 N-m Magnitude 4.1 Mwr Depth 56.0 km Percent DC 56 % Half Duration – Catalog US Data Source US3 Contributor US3 Nodal Planes Plane Strike Dip Rake NP1 334 37 -168 NP2 235 83 -54 Principal Axes Axis Value Plunge Azimuth T 1.858e+15 N-m 29 296 N -0.411e+15 N-m 36 50 P -1.447e+15 N-m 41 178 |
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/3.5 -40 o DIST/3.5 +60 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 55 30 -55 3.45 0.1900 WVFGRD96 4.0 55 65 -30 3.47 0.2182 WVFGRD96 6.0 60 70 -20 3.52 0.2407 WVFGRD96 8.0 60 65 -25 3.59 0.2655 WVFGRD96 10.0 65 70 -20 3.62 0.2752 WVFGRD96 12.0 70 70 20 3.65 0.2875 WVFGRD96 14.0 70 75 20 3.67 0.2974 WVFGRD96 16.0 70 75 20 3.70 0.3058 WVFGRD96 18.0 70 75 20 3.72 0.3125 WVFGRD96 20.0 75 75 20 3.74 0.3201 WVFGRD96 22.0 75 75 25 3.76 0.3300 WVFGRD96 24.0 75 75 25 3.78 0.3402 WVFGRD96 26.0 80 75 25 3.80 0.3511 WVFGRD96 28.0 75 80 25 3.82 0.3610 WVFGRD96 30.0 75 80 25 3.84 0.3697 WVFGRD96 32.0 75 80 25 3.85 0.3771 WVFGRD96 34.0 75 80 25 3.87 0.3827 WVFGRD96 36.0 70 80 25 3.89 0.3896 WVFGRD96 38.0 70 80 20 3.92 0.4037 WVFGRD96 40.0 70 80 30 3.99 0.4100 WVFGRD96 42.0 70 80 35 4.01 0.4164 WVFGRD96 44.0 65 85 35 4.03 0.4248 WVFGRD96 46.0 65 85 35 4.05 0.4345 WVFGRD96 48.0 65 85 35 4.07 0.4426 WVFGRD96 50.0 65 85 35 4.08 0.4490 WVFGRD96 52.0 240 90 -40 4.10 0.4532 WVFGRD96 54.0 65 85 40 4.10 0.4598 WVFGRD96 56.0 65 85 40 4.11 0.4632 WVFGRD96 58.0 60 85 40 4.13 0.4666 WVFGRD96 60.0 235 90 -45 4.15 0.4657 WVFGRD96 62.0 60 85 45 4.15 0.4712 WVFGRD96 64.0 60 85 45 4.15 0.4719 WVFGRD96 66.0 60 85 45 4.16 0.4714 WVFGRD96 68.0 60 85 45 4.17 0.4700 WVFGRD96 70.0 55 85 50 4.18 0.4675 WVFGRD96 72.0 60 80 45 4.18 0.4706 WVFGRD96 74.0 60 80 45 4.19 0.4708 WVFGRD96 76.0 60 80 45 4.19 0.4684 WVFGRD96 78.0 80 20 -30 4.20 0.4661 WVFGRD96 80.0 85 20 -25 4.20 0.4660 WVFGRD96 82.0 80 15 -30 4.21 0.4657 WVFGRD96 84.0 85 15 -25 4.22 0.4649 WVFGRD96 86.0 85 15 -25 4.22 0.4644 WVFGRD96 88.0 90 15 -20 4.22 0.4632 WVFGRD96 90.0 95 15 -15 4.23 0.4621 WVFGRD96 92.0 95 15 -15 4.23 0.4609 WVFGRD96 94.0 100 15 -10 4.24 0.4589 WVFGRD96 96.0 105 15 -5 4.25 0.4592 WVFGRD96 98.0 105 10 -10 4.26 0.4636
The best solution is
WVFGRD96 64.0 60 85 45 4.15 0.4719
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/3.5 -40 o DIST/3.5 +60 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