The ANSS event ID is ak023cv4juze and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023cv4juze/executive.
2023/10/07 03:48:46 62.358 -150.914 72.0 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/10/07 03:48:46:0 62.36 -150.91 72.0 3.9 Alaska
Stations used:
AK.CAST AK.GHO AK.KNK AK.KTH AK.L20K AK.L22K AK.M20K
AK.PPLA AK.RC01 AK.SAW AK.SKN AK.WAT6 AT.PMR AV.STLK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 9.89e+21 dyne-cm
Mw = 3.93
Z = 72 km
Plane Strike Dip Rake
NP1 37 75 103
NP2 175 20 50
Principal Axes:
Axis Value Plunge Azimuth
T 9.89e+21 58 325
N 0.00e+00 13 213
P -9.89e+21 29 116
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.40e+20
Mxy 1.72e+21
Mxz 5.44e+21
Myy -5.21e+21
Myz -6.30e+21
Mzz 4.87e+21
--############
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---#########################--------
----######### #############---------
----########## T ############-----------
----########## ###########------------
----########################--------------
----######################----------------
----#####################-----------------
----####################------------------
----#################----------- -----
----################------------ P -----
----#############-------------- ----
----##########----------------------
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---#------------------------
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Global CMT Convention Moment Tensor:
R T P
4.87e+21 5.44e+21 6.30e+21
5.44e+21 3.40e+20 -1.72e+21
6.30e+21 -1.72e+21 -5.21e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231007034846/index.html
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STK = 175
DIP = 20
RAKE = 50
MW = 3.93
HS = 72.0
The NDK file is 20231007034846.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 0.04 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 5 30 -100 3.25 0.2801
WVFGRD96 4.0 195 65 -80 3.30 0.2815
WVFGRD96 6.0 200 75 -65 3.28 0.2762
WVFGRD96 8.0 330 30 50 3.37 0.2999
WVFGRD96 10.0 330 30 50 3.39 0.3166
WVFGRD96 12.0 20 70 -50 3.41 0.3188
WVFGRD96 14.0 300 50 -30 3.41 0.3327
WVFGRD96 16.0 50 65 55 3.45 0.3461
WVFGRD96 18.0 145 50 40 3.49 0.3570
WVFGRD96 20.0 150 50 40 3.52 0.3678
WVFGRD96 22.0 160 35 30 3.56 0.3812
WVFGRD96 24.0 160 35 30 3.59 0.4017
WVFGRD96 26.0 160 35 30 3.61 0.4188
WVFGRD96 28.0 155 35 30 3.63 0.4297
WVFGRD96 30.0 155 35 30 3.64 0.4338
WVFGRD96 32.0 160 35 35 3.65 0.4383
WVFGRD96 34.0 160 35 35 3.66 0.4443
WVFGRD96 36.0 160 35 35 3.67 0.4475
WVFGRD96 38.0 160 35 35 3.68 0.4489
WVFGRD96 40.0 165 30 40 3.79 0.4519
WVFGRD96 42.0 155 30 20 3.80 0.4622
WVFGRD96 44.0 155 25 20 3.82 0.4973
WVFGRD96 46.0 155 25 20 3.84 0.5290
WVFGRD96 48.0 155 25 20 3.85 0.5594
WVFGRD96 50.0 155 25 25 3.86 0.5872
WVFGRD96 52.0 160 25 30 3.87 0.6104
WVFGRD96 54.0 160 25 30 3.88 0.6311
WVFGRD96 56.0 160 25 30 3.89 0.6486
WVFGRD96 58.0 160 25 30 3.89 0.6624
WVFGRD96 60.0 170 20 45 3.90 0.6740
WVFGRD96 62.0 170 20 45 3.91 0.6829
WVFGRD96 64.0 170 20 45 3.91 0.6900
WVFGRD96 66.0 175 20 50 3.92 0.6931
WVFGRD96 68.0 175 20 50 3.92 0.6951
WVFGRD96 70.0 175 20 50 3.92 0.6954
WVFGRD96 72.0 175 20 50 3.93 0.6965
WVFGRD96 74.0 175 20 50 3.93 0.6942
WVFGRD96 76.0 175 20 50 3.93 0.6909
WVFGRD96 78.0 180 20 55 3.94 0.6906
WVFGRD96 80.0 180 20 55 3.94 0.6866
WVFGRD96 82.0 180 20 55 3.94 0.6823
WVFGRD96 84.0 180 20 55 3.94 0.6801
WVFGRD96 86.0 180 20 55 3.95 0.6754
WVFGRD96 88.0 185 20 60 3.95 0.6711
WVFGRD96 90.0 185 20 60 3.95 0.6674
WVFGRD96 92.0 185 20 60 3.96 0.6610
WVFGRD96 94.0 185 20 60 3.96 0.6579
WVFGRD96 96.0 185 20 60 3.96 0.6523
WVFGRD96 98.0 185 20 60 3.96 0.6479
The best solution is
WVFGRD96 72.0 175 20 50 3.93 0.6965
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 0.04 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