The ANSS event ID is ak023dtvt3ys and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023dtvt3ys/executive.
2023/10/28 05:11:57 58.985 -136.474 7.2 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/28 05:11:57:0 58.99 -136.47 7.2 3.5 Alaska
Stations used:
AK.BARN AK.BESE AK.GRNC AK.JIS AK.KIAG AK.LOGN AK.MESA
AK.PIN AK.PNL AK.R32K AK.RKAV AK.S31K AK.S32K AK.SAMH
AK.U33K AK.WAX AT.MENT AT.SIT AT.SKAG AV.EDCR AV.EDES
AV.EDSO AV.WACK CN.ATLI CN.BRWY CN.HYT CN.PLBC CN.WHY
CN.YUK3 CN.YUK6 CN.YUK7 CN.YUK8 EO.KLRS NY.MAYO NY.MMPY
US.WRAK
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 = 4.03e+21 dyne-cm
Mw = 3.67
Z = 6 km
Plane Strike Dip Rake
NP1 163 51 98
NP2 330 40 80
Principal Axes:
Axis Value Plunge Azimuth
T 4.03e+21 82 117
N 0.00e+00 6 338
P -4.03e+21 5 247
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.87e+20
Mxy -1.47e+21
Mxz -1.20e+20
Myy -3.32e+21
Myz 8.64e+20
Mzz 3.91e+21
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-----------########## ##########--------
-----------########## T ###########-------
------------######### ###########-------
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P ----------#####################-----
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Global CMT Convention Moment Tensor:
R T P
3.91e+21 -1.20e+20 -8.64e+20
-1.20e+20 -5.87e+20 1.47e+21
-8.64e+20 1.47e+21 -3.32e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028051157/index.html
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STK = 330
DIP = 40
RAKE = 80
MW = 3.67
HS = 6.0
The NDK file is 20231028051157.ndk The waveform inversion is preferred.
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 1.0 110 30 0 3.64 0.4581
WVFGRD96 2.0 295 45 15 3.58 0.4772
WVFGRD96 3.0 300 50 25 3.58 0.4978
WVFGRD96 4.0 300 50 30 3.59 0.5108
WVFGRD96 5.0 320 40 65 3.66 0.5245
WVFGRD96 6.0 330 40 80 3.67 0.5267
WVFGRD96 7.0 320 45 65 3.64 0.5134
WVFGRD96 8.0 295 60 15 3.58 0.5116
WVFGRD96 9.0 295 65 15 3.58 0.5052
WVFGRD96 10.0 295 60 15 3.60 0.5000
WVFGRD96 11.0 295 65 10 3.60 0.4911
WVFGRD96 12.0 295 65 10 3.60 0.4829
WVFGRD96 13.0 295 65 10 3.61 0.4738
WVFGRD96 14.0 295 65 10 3.61 0.4660
WVFGRD96 15.0 295 65 10 3.61 0.4577
WVFGRD96 16.0 295 65 10 3.62 0.4486
WVFGRD96 17.0 290 70 5 3.63 0.4396
WVFGRD96 18.0 290 70 5 3.63 0.4305
WVFGRD96 19.0 290 70 5 3.64 0.4213
WVFGRD96 20.0 290 65 -5 3.65 0.4117
WVFGRD96 21.0 290 65 -5 3.65 0.4037
WVFGRD96 22.0 295 70 20 3.66 0.3975
WVFGRD96 23.0 295 70 20 3.66 0.3903
WVFGRD96 24.0 295 70 20 3.66 0.3827
WVFGRD96 25.0 295 70 20 3.67 0.3752
WVFGRD96 26.0 295 65 20 3.67 0.3681
WVFGRD96 27.0 295 65 20 3.68 0.3608
WVFGRD96 28.0 290 75 25 3.69 0.3536
WVFGRD96 29.0 290 75 25 3.69 0.3472
The best solution is
WVFGRD96 6.0 330 40 80 3.67 0.5267
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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00