The ANSS event ID is ak0175u3s7id and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0175u3s7id/executive.
2017/05/07 04:25:19 60.183 -151.678 67.2 5.3 Alaska
USGS/SLU Moment Tensor Solution ENS 2017/05/07 04:25:19:0 60.18 -151.68 67.2 5.3 Alaska Stations used: AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KNK AK.PWL AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K TA.Q19K Filtering commands used: cut o DIST/3.3 -50 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 = 1.20e+24 dyne-cm Mw = 5.32 Z = 82 km Plane Strike Dip Rake NP1 50 65 30 NP2 306 63 152 Principal Axes: Axis Value Plunge Azimuth T 1.20e+24 38 269 N 0.00e+00 52 86 P -1.20e+24 1 178 Moment Tensor: (dyne-cm) Component Value Mxx -1.20e+24 Mxy 6.29e+22 Mxz 1.32e+22 Myy 7.39e+23 Myz -5.85e+23 Mzz 4.60e+23 -------------- ---------------------- ---------------------------- ------------------------------ #######--------------------------# ##############-------------------### ###################--------------##### #######################-----------###### #########################--------####### ############################----########## ####### ####################-########### ####### T ###################---########## ####### #################------######### ########################----------###### ######################-------------##### ##################-----------------### ##############--------------------## #########------------------------- ------------------------------ ---------------------------- ---------- --------- ------ P ----- Global CMT Convention Moment Tensor: R T P 4.60e+23 1.32e+22 5.85e+23 1.32e+22 -1.20e+24 -6.29e+22 5.85e+23 -6.29e+22 7.39e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170507042519/index.html |
STK = 50 DIP = 65 RAKE = 30 MW = 5.32 HS = 82.0
The NDK file is 20170507042519.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 2017/05/07 04:25:19:0 60.18 -151.68 67.2 5.3 Alaska Stations used: AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KNK AK.PWL AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K TA.Q19K Filtering commands used: cut o DIST/3.3 -50 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 = 1.20e+24 dyne-cm Mw = 5.32 Z = 82 km Plane Strike Dip Rake NP1 50 65 30 NP2 306 63 152 Principal Axes: Axis Value Plunge Azimuth T 1.20e+24 38 269 N 0.00e+00 52 86 P -1.20e+24 1 178 Moment Tensor: (dyne-cm) Component Value Mxx -1.20e+24 Mxy 6.29e+22 Mxz 1.32e+22 Myy 7.39e+23 Myz -5.85e+23 Mzz 4.60e+23 -------------- ---------------------- ---------------------------- ------------------------------ #######--------------------------# ##############-------------------### ###################--------------##### #######################-----------###### #########################--------####### ############################----########## ####### ####################-########### ####### T ###################---########## ####### #################------######### ########################----------###### ######################-------------##### ##################-----------------### ##############--------------------## #########------------------------- ------------------------------ ---------------------------- ---------- --------- ------ P ----- Global CMT Convention Moment Tensor: R T P 4.60e+23 1.32e+22 5.85e+23 1.32e+22 -1.20e+24 -6.29e+22 5.85e+23 -6.29e+22 7.39e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170507042519/index.html |
Regional Moment Tensor (Mwr) Moment 9.588e+16 N-m Magnitude 5.3 Mwr Depth 77.0 km Percent DC 93 % Half Duration – Catalog US Data Source US3 Contributor US3 Nodal Planes Plane Strike Dip Rake NP1 314 70 155 NP2 53 66 22 Principal Axes Axis Value Plunge Azimuth T 9.751e+16 N-m 32 273 N -0.334e+16 N-m 58 98 P -9.416e+16 N-m 2 4 |
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 -50 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 2.0 110 55 -50 4.53 0.2516 WVFGRD96 4.0 305 85 -25 4.55 0.2732 WVFGRD96 6.0 125 85 25 4.62 0.2900 WVFGRD96 8.0 125 85 30 4.69 0.2993 WVFGRD96 10.0 130 80 25 4.73 0.2975 WVFGRD96 12.0 215 70 -10 4.77 0.2971 WVFGRD96 14.0 215 70 -5 4.80 0.2990 WVFGRD96 16.0 215 70 -5 4.83 0.2978 WVFGRD96 18.0 35 70 -10 4.85 0.3034 WVFGRD96 20.0 40 75 -5 4.88 0.3118 WVFGRD96 22.0 40 75 0 4.90 0.3210 WVFGRD96 24.0 40 75 0 4.92 0.3312 WVFGRD96 26.0 40 75 5 4.94 0.3431 WVFGRD96 28.0 40 75 5 4.96 0.3546 WVFGRD96 30.0 220 75 5 4.99 0.3692 WVFGRD96 32.0 220 80 -5 5.00 0.3827 WVFGRD96 34.0 220 80 -5 5.02 0.3976 WVFGRD96 36.0 220 80 -5 5.05 0.4103 WVFGRD96 38.0 45 80 15 5.07 0.4258 WVFGRD96 40.0 50 75 30 5.14 0.4553 WVFGRD96 42.0 50 75 25 5.16 0.4640 WVFGRD96 44.0 50 70 30 5.19 0.4711 WVFGRD96 46.0 50 70 30 5.20 0.4809 WVFGRD96 48.0 50 70 30 5.22 0.4911 WVFGRD96 50.0 50 70 30 5.23 0.5056 WVFGRD96 52.0 50 70 30 5.24 0.5170 WVFGRD96 54.0 50 70 30 5.25 0.5303 WVFGRD96 56.0 50 70 30 5.26 0.5431 WVFGRD96 58.0 50 70 30 5.27 0.5540 WVFGRD96 60.0 50 70 30 5.28 0.5637 WVFGRD96 62.0 50 70 30 5.28 0.5716 WVFGRD96 64.0 50 70 30 5.29 0.5789 WVFGRD96 66.0 50 70 30 5.29 0.5844 WVFGRD96 68.0 50 70 30 5.30 0.5903 WVFGRD96 70.0 50 70 30 5.30 0.5938 WVFGRD96 72.0 50 70 30 5.30 0.5968 WVFGRD96 74.0 50 65 30 5.31 0.5989 WVFGRD96 76.0 50 65 30 5.31 0.6007 WVFGRD96 78.0 50 65 30 5.31 0.6013 WVFGRD96 80.0 50 70 30 5.32 0.6024 WVFGRD96 82.0 50 65 30 5.32 0.6028 WVFGRD96 84.0 50 70 30 5.33 0.6011 WVFGRD96 86.0 50 65 30 5.33 0.6001 WVFGRD96 88.0 50 65 25 5.33 0.6005 WVFGRD96 90.0 50 65 25 5.33 0.5991 WVFGRD96 92.0 50 65 25 5.33 0.5959 WVFGRD96 94.0 50 65 25 5.34 0.5938 WVFGRD96 96.0 50 65 30 5.34 0.5934 WVFGRD96 98.0 50 65 30 5.35 0.5912
The best solution is
WVFGRD96 82.0 50 65 30 5.32 0.6028
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 -50 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