The ANSS event ID is ak022c4o9ui7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak022c4o9ui7/executive.
2022/09/21 04:02:30 62.927 -150.781 106.3 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2022/09/21 04:02:30:0 62.93 -150.78 106.3 4.1 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.DHY AK.GHO AK.H22K AK.I21K
AK.J19K AK.J20K AK.K20K AK.KNK AK.KTH AK.L19K AK.L20K
AK.MCK AK.MLY AK.NEA2 AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND
AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN 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 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.50e+22 dyne-cm
Mw = 4.05
Z = 108 km
Plane Strike Dip Rake
NP1 50 81 87
NP2 250 10 110
Principal Axes:
Axis Value Plunge Azimuth
T 1.50e+22 54 316
N 0.00e+00 3 50
P -1.50e+22 36 143
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.68e+21
Mxy 2.23e+21
Mxz 1.07e+22
Myy -1.13e+21
Myz -9.25e+21
Mzz 4.81e+21
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----###############---
---#######################--
--###########################-
--##############################-#
--##############################----
-########## #################-------
--########## T ###############----------
-########### ##############-----------
-###########################--------------
-#########################----------------
-#######################------------------
-####################---------------------
##################----------------------
################------------------------
############-------------- ---------
########----------------- P --------
####-------------------- -------
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Global CMT Convention Moment Tensor:
R T P
4.81e+21 1.07e+22 9.25e+21
1.07e+22 -3.68e+21 -2.23e+21
9.25e+21 -2.23e+21 -1.13e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220921040230/index.html
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STK = 250
DIP = 10
RAKE = 110
MW = 4.05
HS = 108.0
The NDK file is 20220921040230.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 30 40 -105 3.16 0.1881
WVFGRD96 4.0 255 75 -45 3.19 0.2036
WVFGRD96 6.0 90 75 35 3.23 0.2204
WVFGRD96 8.0 160 50 -25 3.32 0.2413
WVFGRD96 10.0 175 55 20 3.36 0.2560
WVFGRD96 12.0 175 60 25 3.40 0.2658
WVFGRD96 14.0 175 60 25 3.44 0.2680
WVFGRD96 16.0 180 60 30 3.46 0.2678
WVFGRD96 18.0 180 60 30 3.49 0.2666
WVFGRD96 20.0 350 65 15 3.52 0.2666
WVFGRD96 22.0 190 70 25 3.55 0.2697
WVFGRD96 24.0 205 75 25 3.60 0.2721
WVFGRD96 26.0 205 80 25 3.62 0.2729
WVFGRD96 28.0 5 60 0 3.61 0.2745
WVFGRD96 30.0 0 65 -20 3.63 0.2789
WVFGRD96 32.0 355 60 -20 3.65 0.2876
WVFGRD96 34.0 355 60 -20 3.67 0.2977
WVFGRD96 36.0 350 70 10 3.67 0.3135
WVFGRD96 38.0 350 70 15 3.70 0.3277
WVFGRD96 40.0 350 60 15 3.77 0.3445
WVFGRD96 42.0 180 65 35 3.80 0.3438
WVFGRD96 44.0 345 65 -5 3.82 0.3431
WVFGRD96 46.0 180 65 35 3.83 0.3416
WVFGRD96 48.0 175 60 25 3.84 0.3456
WVFGRD96 50.0 175 50 20 3.85 0.3585
WVFGRD96 52.0 175 50 20 3.86 0.3701
WVFGRD96 54.0 175 45 20 3.88 0.3821
WVFGRD96 56.0 170 45 15 3.89 0.3928
WVFGRD96 58.0 160 45 20 3.93 0.4034
WVFGRD96 60.0 30 80 55 3.94 0.4156
WVFGRD96 62.0 35 80 60 3.96 0.4284
WVFGRD96 64.0 35 80 60 3.96 0.4400
WVFGRD96 66.0 35 80 60 3.97 0.4514
WVFGRD96 68.0 35 80 60 3.98 0.4644
WVFGRD96 70.0 35 80 60 3.99 0.4781
WVFGRD96 72.0 40 85 80 4.01 0.4958
WVFGRD96 74.0 40 85 80 4.02 0.5127
WVFGRD96 76.0 230 5 90 4.03 0.5238
WVFGRD96 78.0 235 5 100 4.03 0.5383
WVFGRD96 80.0 240 5 100 4.04 0.5500
WVFGRD96 82.0 45 85 85 4.04 0.5644
WVFGRD96 84.0 245 10 110 4.03 0.5749
WVFGRD96 86.0 45 80 85 4.04 0.5870
WVFGRD96 88.0 245 10 110 4.04 0.5953
WVFGRD96 90.0 45 80 85 4.04 0.6040
WVFGRD96 92.0 45 80 85 4.04 0.6131
WVFGRD96 94.0 245 10 110 4.05 0.6200
WVFGRD96 96.0 245 10 105 4.05 0.6245
WVFGRD96 98.0 45 80 85 4.05 0.6296
WVFGRD96 100.0 45 80 85 4.05 0.6334
WVFGRD96 102.0 45 80 85 4.05 0.6355
WVFGRD96 104.0 250 10 110 4.05 0.6388
WVFGRD96 106.0 45 80 85 4.05 0.6398
WVFGRD96 108.0 250 10 110 4.05 0.6402
WVFGRD96 110.0 45 80 85 4.05 0.6396
WVFGRD96 112.0 50 80 85 4.05 0.6384
WVFGRD96 114.0 250 10 110 4.06 0.6383
WVFGRD96 116.0 50 80 85 4.05 0.6357
WVFGRD96 118.0 250 10 110 4.06 0.6342
WVFGRD96 120.0 50 80 85 4.05 0.6317
WVFGRD96 122.0 50 80 85 4.05 0.6278
WVFGRD96 124.0 50 80 85 4.05 0.6254
WVFGRD96 126.0 50 80 85 4.05 0.6222
WVFGRD96 128.0 50 80 85 4.05 0.6189
WVFGRD96 130.0 250 10 110 4.06 0.6166
WVFGRD96 132.0 250 10 110 4.06 0.6115
WVFGRD96 134.0 250 10 110 4.06 0.6070
WVFGRD96 136.0 50 80 85 4.05 0.6015
WVFGRD96 138.0 50 80 85 4.05 0.5973
WVFGRD96 140.0 50 80 85 4.05 0.5918
WVFGRD96 142.0 50 80 85 4.05 0.5877
WVFGRD96 144.0 50 80 85 4.05 0.5818
WVFGRD96 146.0 250 10 110 4.05 0.5766
WVFGRD96 148.0 50 80 85 4.05 0.5722
The best solution is
WVFGRD96 108.0 250 10 110 4.05 0.6402
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 0.10 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