The ANSS event ID is ak0152w8vj95 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0152w8vj95/executive.
2015/03/04 03:39:05 60.931 -145.812 14.9 4 Alaska
USGS/SLU Moment Tensor Solution ENS 2015/03/04 03:39:05:0 60.93 -145.81 14.9 4.0 Alaska Stations used: AK.BPAW AK.CCB AK.FID AK.GHO AK.GLI AK.HDA AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MDM AK.RAG AK.RND AK.SAW AK.SCM AK.SSN AK.TGL AK.TRF AK.WRH AT.PMR IM.IL31 IU.COLA TA.K27K 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 0.06 n 3 Best Fitting Double Couple Mo = 1.30e+22 dyne-cm Mw = 4.01 Z = 28 km Plane Strike Dip Rake NP1 139 81 -155 NP2 45 65 -10 Principal Axes: Axis Value Plunge Azimuth T 1.30e+22 11 270 N 0.00e+00 63 158 P -1.30e+22 24 5 Moment Tensor: (dyne-cm) Component Value Mxx -1.08e+22 Mxy -8.67e+20 Mxz -4.86e+21 Myy 1.25e+22 Myz -2.81e+21 Mzz -1.73e+21 -------------- ----------- -------- -------------- P ----------- ##------------- ------------ #####--------------------------### #######-------------------------#### ##########----------------------###### ############--------------------######## #############-------------------######## ################---------------########### # #############-------------############ # T ###############----------############# # ################-------############### #####################---################ ######################-################# ###################-----############## ################---------########### ###########---------------######## #####---------------------#### ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -1.73e+21 -4.86e+21 2.81e+21 -4.86e+21 -1.08e+22 8.67e+20 2.81e+21 8.67e+20 1.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150304033905/index.html |
STK = 45 DIP = 65 RAKE = -10 MW = 4.01 HS = 28.0
The NDK file is 20150304033905.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 2015/03/04 03:39:05:0 60.93 -145.81 14.9 4.0 Alaska Stations used: AK.BPAW AK.CCB AK.FID AK.GHO AK.GLI AK.HDA AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MDM AK.RAG AK.RND AK.SAW AK.SCM AK.SSN AK.TGL AK.TRF AK.WRH AT.PMR IM.IL31 IU.COLA TA.K27K 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 0.06 n 3 Best Fitting Double Couple Mo = 1.30e+22 dyne-cm Mw = 4.01 Z = 28 km Plane Strike Dip Rake NP1 139 81 -155 NP2 45 65 -10 Principal Axes: Axis Value Plunge Azimuth T 1.30e+22 11 270 N 0.00e+00 63 158 P -1.30e+22 24 5 Moment Tensor: (dyne-cm) Component Value Mxx -1.08e+22 Mxy -8.67e+20 Mxz -4.86e+21 Myy 1.25e+22 Myz -2.81e+21 Mzz -1.73e+21 -------------- ----------- -------- -------------- P ----------- ##------------- ------------ #####--------------------------### #######-------------------------#### ##########----------------------###### ############--------------------######## #############-------------------######## ################---------------########### # #############-------------############ # T ###############----------############# # ################-------############### #####################---################ ######################-################# ###################-----############## ################---------########### ###########---------------######## #####---------------------#### ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -1.73e+21 -4.86e+21 2.81e+21 -4.86e+21 -1.08e+22 8.67e+20 2.81e+21 8.67e+20 1.25e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150304033905/index.html |
Regional Moment Tensor (Mwr) Moment 1.645e+15 N-m Magnitude 4.08 Depth 33.0 km Percent DC 91% Half Duration – Catalog AK (ak11521877) Data Source US3 Contributor US3 Nodal Planes Plane Strike Dip Rake NP1 141 83 -160 NP2 48 70 -8 Principal Axes Axis Value Plunge Azimuth T 1.607 9 273 N 0.072 69 160 P -1.679 19 6 |
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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 230 90 0 3.47 0.2864 WVFGRD96 2.0 50 85 10 3.59 0.3829 WVFGRD96 3.0 230 90 0 3.64 0.4231 WVFGRD96 4.0 50 85 0 3.68 0.4494 WVFGRD96 5.0 45 75 -10 3.71 0.4727 WVFGRD96 6.0 45 80 -15 3.74 0.4962 WVFGRD96 7.0 45 80 -15 3.77 0.5200 WVFGRD96 8.0 45 75 -20 3.81 0.5427 WVFGRD96 9.0 45 75 -10 3.82 0.5536 WVFGRD96 10.0 45 70 -10 3.84 0.5672 WVFGRD96 11.0 45 70 -10 3.85 0.5790 WVFGRD96 12.0 45 70 -20 3.87 0.5980 WVFGRD96 13.0 45 70 -20 3.88 0.6092 WVFGRD96 14.0 45 70 -20 3.89 0.6184 WVFGRD96 15.0 45 70 -15 3.90 0.6274 WVFGRD96 16.0 45 65 -15 3.91 0.6352 WVFGRD96 17.0 45 65 -15 3.92 0.6437 WVFGRD96 18.0 45 65 -15 3.93 0.6512 WVFGRD96 19.0 45 65 -15 3.94 0.6579 WVFGRD96 20.0 45 65 -15 3.95 0.6637 WVFGRD96 21.0 45 65 -10 3.96 0.6683 WVFGRD96 22.0 45 65 -10 3.97 0.6721 WVFGRD96 23.0 45 65 -10 3.97 0.6752 WVFGRD96 24.0 45 65 -10 3.98 0.6772 WVFGRD96 25.0 45 65 -10 3.99 0.6781 WVFGRD96 26.0 45 65 -10 4.00 0.6787 WVFGRD96 27.0 45 65 -10 4.00 0.6794 WVFGRD96 28.0 45 65 -10 4.01 0.6796 WVFGRD96 29.0 45 65 -10 4.02 0.6784 WVFGRD96 30.0 45 65 -10 4.03 0.6762 WVFGRD96 31.0 45 65 -10 4.03 0.6734 WVFGRD96 32.0 45 65 -10 4.04 0.6696 WVFGRD96 33.0 45 65 -10 4.05 0.6650 WVFGRD96 34.0 45 65 -10 4.06 0.6592 WVFGRD96 35.0 45 70 -10 4.07 0.6548 WVFGRD96 36.0 45 70 -10 4.07 0.6505 WVFGRD96 37.0 45 70 -10 4.08 0.6452 WVFGRD96 38.0 45 70 -10 4.09 0.6395 WVFGRD96 39.0 45 70 -5 4.11 0.6338 WVFGRD96 40.0 45 65 -10 4.15 0.6301 WVFGRD96 41.0 45 65 -10 4.16 0.6311 WVFGRD96 42.0 45 65 -10 4.17 0.6307 WVFGRD96 43.0 45 65 -10 4.17 0.6293 WVFGRD96 44.0 45 65 -10 4.18 0.6274 WVFGRD96 45.0 45 65 -10 4.19 0.6256 WVFGRD96 46.0 45 65 -10 4.19 0.6230 WVFGRD96 47.0 45 70 -10 4.20 0.6207 WVFGRD96 48.0 45 70 -10 4.21 0.6185 WVFGRD96 49.0 45 70 -10 4.21 0.6164
The best solution is
WVFGRD96 28.0 45 65 -10 4.01 0.6796
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 0.06 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