The ANSS event ID is ak022esj2eua and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak022esj2eua/executive.
2022/11/18 00:03:09 61.350 -150.168 42.9 4.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/11/18 00:03:09:0 61.35 -150.17 42.9 4.9 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CNP AK.CRQ AK.CUT AK.EYAK AK.FID
AK.FIRE AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN AK.HOM AK.I21K
AK.I23K AK.ISLE AK.J25K AK.K20K AK.K24K AK.KLU AK.KNK
AK.KTH AK.L20K AK.L22K AK.L26K AK.MCAR AK.MCK AK.MLY
AK.N18K AK.N19K AK.NEA2 AK.O18K AK.P17K AK.P23K AK.PAX
AK.POKR AK.PWL AK.RAG AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK
AK.SKN AK.SWD AK.TGL AK.VRDI AK.WRH AT.MENT AT.PMR AV.ILS
AV.RED AV.STLK IM.IL31 IU.COLA
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 0.06 n 3
Best Fitting Double Couple
Mo = 2.92e+23 dyne-cm
Mw = 4.91
Z = 54 km
Plane Strike Dip Rake
NP1 190 80 -75
NP2 313 18 -146
Principal Axes:
Axis Value Plunge Azimuth
T 2.92e+23 33 267
N 0.00e+00 15 7
P -2.92e+23 53 118
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.25e+22
Mxy 5.34e+22
Mxz 5.89e+22
Myy 1.19e+23
Myz -2.59e+23
Mzz -9.64e+22
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---##########-########
-##############-------######
################---------#####
#################------------#####
##################--------------####
##################----------------####
###################-----------------####
##################-------------------###
###################--------------------###
###### ##########--------------------###
###### T #########---------------------###
###### #########---------- ---------##
#################---------- P ---------#
################----------- --------##
###############----------------------#
##############---------------------#
#############--------------------#
###########-------------------
##########------------------
#######---------------
###-----------
Global CMT Convention Moment Tensor:
R T P
-9.64e+22 5.89e+22 2.59e+23
5.89e+22 -2.25e+22 -5.34e+22
2.59e+23 -5.34e+22 1.19e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221118000309/index.html
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STK = 190
DIP = 80
RAKE = -75
MW = 4.91
HS = 54.0
The NDK file is 20221118000309.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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 165 40 90 4.25 0.2620
WVFGRD96 4.0 155 45 75 4.33 0.2430
WVFGRD96 6.0 300 50 30 4.30 0.2225
WVFGRD96 8.0 85 25 5 4.35 0.2443
WVFGRD96 10.0 90 30 15 4.37 0.2863
WVFGRD96 12.0 90 30 15 4.39 0.3255
WVFGRD96 14.0 85 30 5 4.41 0.3618
WVFGRD96 16.0 80 25 -5 4.44 0.3960
WVFGRD96 18.0 75 25 -15 4.46 0.4299
WVFGRD96 20.0 75 25 -15 4.49 0.4616
WVFGRD96 22.0 70 25 -20 4.52 0.4905
WVFGRD96 24.0 70 20 -20 4.54 0.5190
WVFGRD96 26.0 70 20 -20 4.56 0.5466
WVFGRD96 28.0 70 20 -25 4.59 0.5740
WVFGRD96 30.0 60 15 -35 4.60 0.6010
WVFGRD96 32.0 60 15 -35 4.62 0.6276
WVFGRD96 34.0 55 15 -45 4.64 0.6520
WVFGRD96 36.0 190 80 -100 4.66 0.6732
WVFGRD96 38.0 190 80 -80 4.66 0.6919
WVFGRD96 40.0 190 80 -85 4.82 0.7044
WVFGRD96 42.0 190 80 -80 4.83 0.7153
WVFGRD96 44.0 190 80 -80 4.85 0.7254
WVFGRD96 46.0 190 80 -80 4.86 0.7341
WVFGRD96 48.0 190 80 -80 4.87 0.7404
WVFGRD96 50.0 190 80 -80 4.89 0.7443
WVFGRD96 52.0 190 80 -75 4.90 0.7469
WVFGRD96 54.0 190 80 -75 4.91 0.7472
WVFGRD96 56.0 190 80 -75 4.92 0.7444
WVFGRD96 58.0 185 80 -80 4.93 0.7406
WVFGRD96 60.0 185 80 -80 4.94 0.7346
WVFGRD96 62.0 185 80 -80 4.95 0.7269
WVFGRD96 64.0 185 80 -80 4.96 0.7171
WVFGRD96 66.0 185 80 -80 4.96 0.7065
WVFGRD96 68.0 185 80 -80 4.97 0.6935
WVFGRD96 70.0 185 85 -80 4.97 0.6802
WVFGRD96 72.0 185 85 -80 4.98 0.6677
WVFGRD96 74.0 185 85 -80 4.99 0.6550
WVFGRD96 76.0 40 5 -50 4.99 0.6399
WVFGRD96 78.0 40 5 -50 5.00 0.6282
WVFGRD96 80.0 45 5 -40 5.00 0.6148
WVFGRD96 82.0 50 5 -35 5.01 0.6013
WVFGRD96 84.0 15 5 -70 5.01 0.5832
WVFGRD96 86.0 150 0 60 5.01 0.5663
WVFGRD96 88.0 65 5 -20 5.02 0.5569
WVFGRD96 90.0 65 5 -20 5.02 0.5408
WVFGRD96 92.0 70 5 -15 5.02 0.5252
WVFGRD96 94.0 70 5 -15 5.02 0.5084
WVFGRD96 96.0 75 5 -15 5.02 0.4911
WVFGRD96 98.0 70 5 -20 5.02 0.4747
The best solution is
WVFGRD96 54.0 190 80 -75 4.91 0.7472
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 0.06 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