The ANSS event ID is ak0155xidfbx and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0155xidfbx/executive.
2015/05/09 10:15:49 61.516 -146.573 19.9 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2015/05/09 10:15:49:0 61.52 -146.57 19.9 4.0 Alaska Stations used: AK.BARN AK.BMR AK.BWN AK.CRQ AK.CTG AK.CUT AK.DOT AK.FID AK.GHO AK.GLB AK.GLI AK.GRNC AK.HIN AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MLY AK.PAX AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AK.VRDI AK.WAT3 AK.WAT4 AK.WAT5 AK.WRH AK.YAH AT.PMR TA.I23K TA.M24K TA.N25K TA.POKR Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 2.51e+22 dyne-cm Mw = 4.20 Z = 44 km Plane Strike Dip Rake NP1 55 80 89 NP2 240 10 95 Principal Axes: Axis Value Plunge Azimuth T 2.51e+22 55 324 N 0.00e+00 1 55 P -2.51e+22 35 146 Moment Tensor: (dyne-cm) Component Value Mxx -6.09e+21 Mxy 3.90e+21 Mxz 1.93e+22 Myy -2.47e+21 Myz -1.36e+22 Mzz 8.56e+21 -------------- ----################-- ----######################-- --############################ --################################ --################################-- --########### #################----- --############ T ###############-------- -############# #############---------- --###########################------------- -##########################--------------- -#######################------------------ -####################--------------------- ##################---------------------- -##############------------------------- ##########---------------------------- ######------------------ --------- ----------------------- P -------- --------------------- ------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 8.56e+21 1.93e+22 1.36e+22 1.93e+22 -6.09e+21 -3.90e+21 1.36e+22 -3.90e+21 -2.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150509101549/index.html |
STK = 240 DIP = 10 RAKE = 95 MW = 4.20 HS = 44.0
The NDK file is 20150509101549.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 2015/05/09 10:15:49:0 61.52 -146.57 19.9 4.0 Alaska Stations used: AK.BARN AK.BMR AK.BWN AK.CRQ AK.CTG AK.CUT AK.DOT AK.FID AK.GHO AK.GLB AK.GLI AK.GRNC AK.HIN AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MLY AK.PAX AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AK.VRDI AK.WAT3 AK.WAT4 AK.WAT5 AK.WRH AK.YAH AT.PMR TA.I23K TA.M24K TA.N25K TA.POKR Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 n 3 Best Fitting Double Couple Mo = 2.51e+22 dyne-cm Mw = 4.20 Z = 44 km Plane Strike Dip Rake NP1 55 80 89 NP2 240 10 95 Principal Axes: Axis Value Plunge Azimuth T 2.51e+22 55 324 N 0.00e+00 1 55 P -2.51e+22 35 146 Moment Tensor: (dyne-cm) Component Value Mxx -6.09e+21 Mxy 3.90e+21 Mxz 1.93e+22 Myy -2.47e+21 Myz -1.36e+22 Mzz 8.56e+21 -------------- ----################-- ----######################-- --############################ --################################ --################################-- --########### #################----- --############ T ###############-------- -############# #############---------- --###########################------------- -##########################--------------- -#######################------------------ -####################--------------------- ##################---------------------- -##############------------------------- ##########---------------------------- ######------------------ --------- ----------------------- P -------- --------------------- ------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 8.56e+21 1.93e+22 1.36e+22 1.93e+22 -6.09e+21 -3.90e+21 1.36e+22 -3.90e+21 -2.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150509101549/index.html |
Regional Moment Tensor (Mwr) Moment 2.617e+15 N-m Magnitude 4.21 Depth 43.0 km Percent DC 79% Half Duration – Catalog AK (ak11588180) Data Source US3 Contributor US3 Nodal Planes Plane Strike Dip Rake NP1 243° 11° 99° NP2 54° 79° 88° Principal Axes Axis Value Plunge Azimuth T 2.461 56° 322° N 0.289 2° 55° P -2.750 34° 146° |
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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 2.0 230 45 90 3.59 0.2946 WVFGRD96 4.0 230 45 95 3.66 0.2319 WVFGRD96 6.0 165 70 -15 3.67 0.2292 WVFGRD96 8.0 160 60 -25 3.73 0.2545 WVFGRD96 10.0 50 80 80 3.70 0.2931 WVFGRD96 12.0 45 80 70 3.72 0.3394 WVFGRD96 14.0 50 75 75 3.75 0.3854 WVFGRD96 16.0 45 75 65 3.79 0.4292 WVFGRD96 18.0 50 70 70 3.82 0.4706 WVFGRD96 20.0 50 75 70 3.84 0.5071 WVFGRD96 22.0 50 75 75 3.87 0.5408 WVFGRD96 24.0 50 75 75 3.89 0.5716 WVFGRD96 26.0 50 80 80 3.92 0.5994 WVFGRD96 28.0 50 80 80 3.94 0.6288 WVFGRD96 30.0 55 80 85 3.96 0.6545 WVFGRD96 32.0 55 80 85 3.98 0.6752 WVFGRD96 34.0 55 80 85 4.00 0.6903 WVFGRD96 36.0 55 80 85 4.01 0.6990 WVFGRD96 38.0 55 80 85 4.02 0.7041 WVFGRD96 40.0 50 85 85 4.17 0.7063 WVFGRD96 42.0 55 80 90 4.19 0.7096 WVFGRD96 44.0 240 10 95 4.20 0.7098 WVFGRD96 46.0 235 10 90 4.21 0.7061 WVFGRD96 48.0 55 80 90 4.22 0.7003 WVFGRD96 50.0 55 80 90 4.23 0.6937 WVFGRD96 52.0 55 80 90 4.24 0.6844 WVFGRD96 54.0 55 80 90 4.24 0.6733 WVFGRD96 56.0 230 10 85 4.25 0.6608 WVFGRD96 58.0 50 85 85 4.25 0.6496 WVFGRD96 60.0 250 5 105 4.26 0.6374 WVFGRD96 62.0 55 85 85 4.26 0.6241 WVFGRD96 64.0 55 85 85 4.27 0.6103 WVFGRD96 66.0 55 85 85 4.27 0.5961 WVFGRD96 68.0 230 90 -80 4.27 0.5788 WVFGRD96 70.0 50 85 80 4.28 0.5674 WVFGRD96 72.0 50 85 80 4.28 0.5531 WVFGRD96 74.0 50 85 80 4.29 0.5386 WVFGRD96 76.0 50 85 80 4.29 0.5246 WVFGRD96 78.0 50 85 75 4.30 0.5114 WVFGRD96 80.0 45 85 70 4.30 0.5005 WVFGRD96 82.0 45 85 70 4.30 0.4906 WVFGRD96 84.0 45 85 70 4.31 0.4811 WVFGRD96 86.0 45 85 70 4.31 0.4715 WVFGRD96 88.0 45 85 70 4.31 0.4618 WVFGRD96 90.0 45 85 65 4.32 0.4520 WVFGRD96 92.0 220 90 -60 4.32 0.4315 WVFGRD96 94.0 40 85 60 4.32 0.4222 WVFGRD96 96.0 40 90 55 4.33 0.4088 WVFGRD96 98.0 180 65 -85 4.29 0.3960
The best solution is
WVFGRD96 44.0 240 10 95 4.20 0.7098
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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.07 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