The ANSS event ID is ak023640kxsy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023640kxsy/executive.
2023/05/13 03:53:58 61.052 -151.144 16.1 4.6 Alaska
USGS/SLU Moment Tensor Solution ENS 2023/05/13 03:53:58:0 61.05 -151.14 16.1 4.6 Alaska Stations used: AK.BMR AK.BPAW AK.CAPN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.EYAK AK.GHO AK.GLI AK.HDA AK.J20K AK.K24K AK.KNK AK.L19K AK.L22K AK.MCK AK.MLY AK.N19K AK.NEA2 AK.O18K AK.O19K AK.P16K AK.P17K AK.PAX AK.PWL AK.Q19K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AK.VRDI AK.WAT6 AK.WRH AT.PMR AV.ACH AV.P19K AV.RED AV.SPCP AV.STLK 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 0.08 n 3 Best Fitting Double Couple Mo = 2.34e+22 dyne-cm Mw = 4.18 Z = 23 km Plane Strike Dip Rake NP1 235 60 55 NP2 109 45 135 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 59 93 N 0.00e+00 30 254 P -2.34e+22 9 349 Moment Tensor: (dyne-cm) Component Value Mxx -2.21e+22 Mxy 3.83e+21 Mxz -4.01e+21 Myy 5.47e+21 Myz 1.10e+22 Mzz 1.66e+22 -- P --------- ------ ------------- ---------------------------- ------------------------------ --------------------------######## ---------------------############### ------------------#################### ----------------######################## #-------------########################## ##----------############################## ###--------################ ############ ####-----################## T ############ ######--################### ############ #####-################################## #####----############################### ###-------###########################- #------------####################--- -------------------######--------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.66e+22 -4.01e+21 -1.10e+22 -4.01e+21 -2.21e+22 -3.83e+21 -1.10e+22 -3.83e+21 5.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230513035358/index.html |
STK = 235 DIP = 60 RAKE = 55 MW = 4.18 HS = 23.0
The NDK file is 20230513035358.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 2023/05/13 03:53:58:0 61.05 -151.14 16.1 4.6 Alaska Stations used: AK.BMR AK.BPAW AK.CAPN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.EYAK AK.GHO AK.GLI AK.HDA AK.J20K AK.K24K AK.KNK AK.L19K AK.L22K AK.MCK AK.MLY AK.N19K AK.NEA2 AK.O18K AK.O19K AK.P16K AK.P17K AK.PAX AK.PWL AK.Q19K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AK.VRDI AK.WAT6 AK.WRH AT.PMR AV.ACH AV.P19K AV.RED AV.SPCP AV.STLK 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 0.08 n 3 Best Fitting Double Couple Mo = 2.34e+22 dyne-cm Mw = 4.18 Z = 23 km Plane Strike Dip Rake NP1 235 60 55 NP2 109 45 135 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 59 93 N 0.00e+00 30 254 P -2.34e+22 9 349 Moment Tensor: (dyne-cm) Component Value Mxx -2.21e+22 Mxy 3.83e+21 Mxz -4.01e+21 Myy 5.47e+21 Myz 1.10e+22 Mzz 1.66e+22 -- P --------- ------ ------------- ---------------------------- ------------------------------ --------------------------######## ---------------------############### ------------------#################### ----------------######################## #-------------########################## ##----------############################## ###--------################ ############ ####-----################## T ############ ######--################### ############ #####-################################## #####----############################### ###-------###########################- #------------####################--- -------------------######--------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.66e+22 -4.01e+21 -1.10e+22 -4.01e+21 -2.21e+22 -3.83e+21 -1.10e+22 -3.83e+21 5.47e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230513035358/index.html |
Regional Moment Tensor (Mwr) Moment 2.667e+15 N-m Magnitude 4.22 Mwr Depth 23.0 km Percent DC 75% Half Duration - Catalog US Data Source US 3 Contributor US 3 Nodal Planes Plane Strike Dip Rake NP1 224 56 30 NP2 117 65 142 Principal Axes Axis Value Plunge Azimuth T 2.828e+15 N-m 43 77 N -0.358e+15 N-m 46 268 P -2.470e+15 N-m 6 172 |
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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 80 40 90 3.67 0.1570 WVFGRD96 2.0 80 40 90 3.82 0.2147 WVFGRD96 3.0 260 50 90 3.85 0.1815 WVFGRD96 4.0 10 45 0 3.84 0.1645 WVFGRD96 5.0 5 35 0 3.88 0.1913 WVFGRD96 6.0 5 30 0 3.89 0.2211 WVFGRD96 7.0 5 35 5 3.91 0.2468 WVFGRD96 8.0 10 25 10 3.98 0.2668 WVFGRD96 9.0 10 30 15 4.00 0.2907 WVFGRD96 10.0 255 75 75 4.00 0.3141 WVFGRD96 11.0 255 70 80 4.02 0.3341 WVFGRD96 12.0 255 70 75 4.03 0.3508 WVFGRD96 13.0 255 70 75 4.05 0.3643 WVFGRD96 14.0 250 70 70 4.06 0.3748 WVFGRD96 15.0 250 70 70 4.07 0.3829 WVFGRD96 16.0 250 70 70 4.08 0.3883 WVFGRD96 17.0 245 55 60 4.10 0.3946 WVFGRD96 18.0 245 55 60 4.12 0.4040 WVFGRD96 19.0 235 60 55 4.13 0.4115 WVFGRD96 20.0 235 60 55 4.14 0.4176 WVFGRD96 21.0 235 60 55 4.16 0.4214 WVFGRD96 22.0 235 60 55 4.17 0.4240 WVFGRD96 23.0 235 60 55 4.18 0.4243 WVFGRD96 24.0 235 60 55 4.19 0.4227 WVFGRD96 25.0 235 60 55 4.19 0.4190 WVFGRD96 26.0 235 60 55 4.20 0.4132 WVFGRD96 27.0 235 60 55 4.21 0.4059 WVFGRD96 28.0 235 60 55 4.21 0.3967 WVFGRD96 29.0 230 65 55 4.22 0.3867
The best solution is
WVFGRD96 23.0 235 60 55 4.18 0.4243
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 0.08 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